Necrobiome Framework for Bridging Decomposition Ecology of Autotrophically and Heterotrophically Derived Organic Matter

Decomposition contributes to global ecosystem function by contributing to nutrient recycling, energy flow, and limiting biomass accumulation. The decomposer organisms influencing this process form diverse, complex, and highly dynamic communities that often specialize on different plant or animal resources. Despite performing the same net role, there is a need to conceptually synthesize information on the structure and function of decomposer communities across the spectrum of dead plant and animal resources. A lack of synthesis has limited cross-disciplinary learning and research in important areas of ecosystem and community ecology. Here we expound on the “necrobiome” concept and develop a framework describing the decomposer communities and their interactions associated with plant and animal resource types within multiple ecosystems. We outline the biotic structure and ecological functions of the necrobiome, along with how the necrobiome fits into a broader landscape and ecosystem context. The expanded necrobiome model provides a set of perspectives on decomposer communities across resource types, and conceptually unifies plant and animal decomposer communities into the same framework, while acknowledging key differences in processes and mechanisms. This framework is intended to raise awareness among researchers, and advance the construction of explicit, mechanistic hypotheses that further our understanding of decomposer community contributions to biodiversity, the structure and function of ecosystems, global nutrient recycling and energy flow.


INTRODUCTION
Decomposer organisms are highly efficient at consuming and transforming dead organic matter, and they play a critical role in shaping ecosystem structure and function. The impact of decomposition on ecosystems is demonstrated by the relatively small accumulation of the estimated 150-200 g of carbonÁm À2 Áyr À1 (Fierer et al. 2009) generated by plant net productivity, especially considering that 90% of plant biomass is not consumed by herbivores (Cebrian 1999, Gessner et al. 2010). Early research demonstrated the importance of detritus in communities and energy flow of both terrestrial and aquatic ecosystems (e.g., Forbes 1887, Summerhayes and Elton 1923, Lindeman 1942, Teal 1962, Odum 1969, Swift et al. 1979. The detrital pool has significant impact on the structure and function of ecosystems by increasing system stability and persistence, and the fate of such "dead plant," "decaying matter," "dung," or "litter" has been described for many ecosystems (Moore et al. 2004). Dead plant biomass, defined here as autotrophically derived decomposing organic matter, is not the only form of detritus that is recycled with important ecosystem function. Another often overlooked decomposition component in ecosystems is carrion, defined here as heterotrophically derived decomposing organic matter, or the carcasses of animals and other organisms that do not harvest energy from the sun but still contribute to the global detritus budget. These two forms of decomposing biomass may also include transitional forms of decaying organic matter, such as dung or frass, excreted organic molecules and dissolved and particulate forms that reflect the once living, and are the partially digested, exuded, and egested biomass of plants and animals.
Dead organic matter of any type is defined as detritus (Petersen et al. 1918;Odum and de la Cruz 1963) but has been primarily perceived in the literature as organic matter resulting from plant death, with some, but few, instances of dead animal organic matter (or carrion) referred to as detritus (e.g., Swift et al. 1979, Moore et al. 2004). In the current literature, the majority of references to detritus describe decomposing plant biomass, reinforcing a separation with other widely ubiquitous forms of biomass with relatively higher turnover, such as carrion and dung/feces. With this understanding we propose to facilitate broader recognition of the commonalities and differences of dead organic matter of any type, defined in this paper as necromass, a term previously used in the literature (e.g., Coûteaux et al. 2005, Lomstein et al. 2012, but understood to be synonymous with the holistic definition of detritus as summarized by Moore et al. (2004).
Necromass takes a wide range of shapes, sizes, and quality (Swift et al. 1979, Carter et al. 2007). It forms the basis of many food webs and consequently has significant bottom-up importance for ecosystem function (Gessner et al. 2010). It also forms the focal point for interactions and behaviors among decomposer species comprising complex communities influencing ecosystems across temporal and spatial scales (Yang et al. 2010); and in freshwater habitats, decomposition is considered a fundamental ecosystem process that contributes to watershed biodiversity (Woodward et al. 2012). Despite the importance of decomposer communities, there exists no standard framework to conceptualize their complex and dynamic interactions across both plant and animal necromass. This lack of a framework is significant because it limits a comprehensive understanding of the community ecology of decomposition and has implications for defining and testing paradigms related to nutrient recycling, gene flow, population dynamics, and other ecosystem processes at the frontier of ecological research.
There has been a recent expansion of knowledge about both micro-and macro-scale processes structuring decomposer communities and contributing to the species interactions (e.g., Burkepile et al. 2006, Gessner et al. 2010, Wilson and Wolkovich 2011, Metcalf et al. 2016; however, much is still unknown. To date, ecological theory describing decomposition processes has included succession theory (Payne et al. 1968, meta-population theory (Hanski 1987), and aggregation and coexistence theory (Ives 1991). Carrion and dung, in particular, have been useful systems to test and develop these theories and mechanistic hypotheses having relevance to many other areas of ecology (see Box 1). However, a new synthesis is required to integrate existing knowledge with the new discoveries occurring in the animal and plant decomposition disciplines. We propose an expansion of a recent framework to facilitate this synthesis in a way that encompasses all forms, functions, and ecological dynamics of organic matter decomposition: the necrobiome (Fig. 1).
The necrobiome was originally defined as ". . .the community of species (both prokaryotic and eukaryotic) associated with decomposing remains of heterotrophic biomass, including animal carrion and human corpses" (sensu Benbow et al. 2013;Benbow et al. 2015c). While the original development of this term was focused on vertebrate carrion, we argue it can be expanded to include any form of necromass (e.g., leaves, wood, dung). We also suggest the necrobiome framework is relevant to both plant (i.e., autotrophically derived) and animal (i.e., heterotrophically derived) necromass decomposition by highlighting the fundamental similarities and differences in the processes that define decomposition. In this paper, we take a conciliatory approach to more fully develop the necrobiome framework as a unifying construct for guiding research in decomposition, regardless of how the life form was derived (Fig. 2).
All life is terminal and the remaining nutrients and energy are ultimately recycled within and across ecosystems (Polis et al. 1997). Some aspects of this ecological process are universal across all forms of necromass, such as disintegration, dispersal, and microbial activity. In contrast, other aspects, such as community dynamics and decomposition rates, and specific decomposer taxa, are distinct across different forms of necromass. The major similarities and differences in necrobiome structure and function have yet to be synthesized in detail but are influenced by the overall composition and quality of the specific necromass, and the associated decomposer species that have evolved to exploit the resource.
In this review, we provide an overview of the similarities and differences among decomposer communities associated with plant and animal necromass. We suggest plant and animal necromass research findings illustrate a broader generality to the processes structuring decomposer communities across a range of substrates that, cumulatively, have large-scale ecosystem-level effects. We synthesize the key similarities and differences among these decomposer communities and place them into a broadened conceptual model of the necrobiome. We then provide details of the biotic structure of the necrobiome, the different ecological functions each part performs, and the key interactions occurring among the decomposer organisms. Further, we explore how the necrobiome provides a useful way to conceptualize how decomposition delivers nutrients, energy, genomes, and communities across space and time with ultimate responses in ecosystem biodiversity and function. By presenting a new synthesis of decomposer communities that continues the approach developed by Moore et al. (2004), we aim to provide an integrative perspective to the key concepts used by researchers that are separated into plant or animal decomposition disciplines. Rather than continuing to consider each necromass type in isolation, we stress the importance of synthesizing this knowledge to develop a holistic understanding of nutrient cycling and food web dynamics across scales. AUTOTROPHICALLY AND HETEROTROPHICALLY DERIVED BIOMASS: STANDING STOCKS, PRODUCTION, AND TURNOVER Autotrophically derived necromass refers to the dead biomass produced from organisms that use energy from sunlight as energy (e.g., plants and algae) or chemical reactions (various prokaryotes) to produce organic compounds. Heterotrophically derived necromass, by contrast, refers to the dead biomass produced from organisms that obtain organic material through the consumption of other Box 1. Shared ecological theory across studies of plant and animal decomposition. Both plant and animal necromass has been used extensively as model systems to test and develop ecological theory, and while historically often operating independently, these areas of study are where ecologists on both sides of the decomposition "divide" have done well to learn from each other. This is perhaps due to the implicit recognition by theorists of the similarities in fundamental processes occurring during decomposition. There has been research that included both autotrophically and heterotrophically derived necromass in food web studies (Polis and Hurd 1996, Polis et al. 1997, Nakano et al. 1999, but comparisons and similarities in processes, community ecology, and mechanisms has received less attention in the literature. Community succession of carrion has a long history (M egnin 1894, Reid 1987, Michaud et al. 2015), and has many important similarities with community succession on dead plant matter, such as leaf and fruit litter and dead wood (Ulyshen 2016). Indeed, succession theory is one of the most widely used theories relating to decomposition, especially with regard to the study of the diversity and structure of arthropod communities (Richards andGoff 1997, Hobischak et al. 2006), and more recently for microbial communities , Metcalf et al. 2016. Meta-population and meta-community theory has been tested using animal carcasses and dung pads as model resource patches to simulate the focal points of insect assemblage aggregation and dispersal (Hanski 1987). This work has led to validation of competitive exclusion and dispersal limitation as mechanisms underpinning population dynamics (Hanski and Gilpin 1991) but could also be tested using decaying plant material. Aggregation and coexistence theory has been tested using fly communities at carrion (Ives 1991, Fiene et al. 2014, as well as mushrooms and rotting fruit (Heard 1998, Finn 2001. The theory predicts that if the distribution of a competitively superior species is spatially aggregated, this frees other patches to be colonized by competitively inferior species, thus leading to differences in community composition and greater overall diversity across multiple patches (Kouki andHanski 1995, Woodcock et al. 2002). Resource subsidy theory describes the spatial movement of energy and nutrients between two spatially disjunct ecosystems (Polis et al. 1997, Leroux and Loreau 2008, Gravel et al. 2010. This theory has been effective at integrating plant and animal necromass into one resource pool (Moore et al. 2004), and at demonstrating links between terrestrial and aquatic systems (Polis and Hurd 1996). For example, the mass emergence of cicadas from forest ecosystems can lead to the deposition of animal necromass into ponds, increasing pond productivity and altering their communities (Nowlin et al. 2007). Measurement of marine-derived plant and animal necromass washing up on islands has also been shown to underpin island terrestrial arthropod biomass (Polis and Hurd 1995). Landscape heterogeneity theory has emerged as a way to link patterns and processes occurring at individual patches to large-scale consequences for landscapes (Wiens 1995). For example, two patches of dead wood or carrion at different temporal decay stages or spatial locations will support a greater range of species than two of the same resource patches at the same decay stage or location. Ultimately, the state of the resource has consequences for supporting overall biodiversity (Barton et al. 2013a), driving soil nutrient heterogeneity (Harmon et al. 1986, Bump et al. 2009a, and generating novel combinations of species (Bump et al. 2009b, Barton et al. 2013b). Application of other landscape ecology principles, such as patch composition vs. configuration theory (Tscharntke et al. 2012), has the potential to yield further insights into the large-scale role of necromass in ecosystem structure and function (Barton 2015). Our necrobiome framework can act as a catalyst for further testing and synthesis of theory describing the interactions among organisms and their environment. Especially, for interactions and processes with clear temporal or spatial components in saprotrophic communities as resource substrates degrade over time, such as dispersal of insects or microbes among patches and the succession of necrophagous communities.
organisms (e.g., animals, fungi, protists, and other microeukaryotes). We focus our discussion here on the initial and intact forms of necromass exceeding 8 mm 3 in size, for which there is a richer source of literature supporting a more detailed assessment of the necrobiome. We acknowledge that all necromass ultimately becomes smaller than this size and that the decomposition of microorganisms, such as fungal necromass, is ecologically important (Fernandez and Kennedy 2018). For instance, fungi and protists produce macroscopic structures supporting unique necrobiome communities, such as giant kelp (Macrocystis pyrifera [Linnaeus]; Inglis 1989, Thunes et al. 2000. The importance of bacteria and fungi to decomposition goes beyond diversity, with reports of soil microbial biomass being an order of magnitude greater than the biomass of other soil organisms and two orders of magnitude greater than aboveground animal biomass (Fierer et al. 2009). This large amount of microbial biomass has the potential to influence ecosystem processes as it dies and becomes microbial necromass (Throckmorton et al. 2012). In fact, this microbial necromass, in combination with microbial metabolites, likely drives a significant proportion of soil organic matter formation (Kallenbach et al. 2015, Brabcov a et al. 2016, Fernandez et al. 2016. Soil fungal mycelia production alone contributes to major functional processes in terrestrial ecosystems, with production (and associated death and decomposition) reported to range from 100-300 kgÁha À1 Á yr À1 (Ekblad et al. 2013). The necrobiome of dead fungal mycelium has been reported to be quite diverse (Ekblad et al. 2013, Brabcov a et al. 2016, providing additional evidence that decomposition networks of microorganisms operate at every biological scale and contribute to complex patterns of biodiversity and associated functions in all ecosystems. However, there is much less understood of the species and processes of decomposition of fine particulate necromass and microorganisms (however, see Alldredge and Silver 1988) across ecosystems, a fruitful area of future research and review.
Based on size and composition, plant material can be divided into herbaceous (e.g., leaves) and woody forms. Animal necromass produced by vertebrates can be distinguished from invertebrate based on both size and internal or external skeletal structures. Dung, however, is FIG. 1. The necrobiome is the community of organisms associated with necromass decomposition and includes their interactions with the necromass, with each other, and with their surrounding habitat and ecosystem. This concept is general across plant, fungi, dung/frass/feces, or animal necromass types, and in both terrestrial and aquatic realms. (a) Various qualities of necromass have strong controlling effects on its associated necrobiome. (b) The decomposing necromass is host to a microbiome while alive, both internal (endonecrotic) and external (epinecrotic), and these communities begin to change after death and during the course of decomposition. The changing microbiomes use the organic matter as habitat and for energy and nutrients. As decay progresses, the organic matter and associated activities of the microbes facilitate the attraction of invertebrate and vertebrate saprophages that directly consume the necromass and the microbial communities. (c) All of these biotic components of the necrobiome interact in ways that give rise to pathways of microbial and elemental dispersal away from the decaying necromass into adjacent soil or water media, as well as to the surrounding ecosystem at both localized and broad spatial scales. (d) The necrobiome and its functions operate within a complex network of interactions that are constrained by its terrestrial or aquatic context, the regional species pool, and gradients of abiotic factors. Critically, all these aspects of the necrobiome are interlinked, and the relative importance of each component of the necrobiome, and their function, will depend on the intrinsic qualities of decaying necromass and the extrinsic controlling environmental factors (e.g., temperature, moisture, latitude).  (Reynolds and Hunter 2004) and vertebrates (Frank and Evans 1997) are known to generate large amounts of dung that play a critical role in nutrient cycling and redistribution in terrestrial and aquatic ecosystems (Wotton and Malmqvist 2001, Carline et al. 2005, Menendez et al. 2016. For the sake of space and focus, we do not elaborate on the dynamics of dung or frass; however, we encourage those interested in this topic to review the literature on the ecology and evolution of dung beetles (Hanski and Cambefort 1991) and the additional importance of feces in terrestrial ( Van der Wal et al. 2004) and aquatic (Alldredge and Silver 1988, Malmqvist 2001, Joyce et al. 2007) ecosystems. Below, we discuss in more detail the differences in the structure and composition of different forms of plant and animal necromass. To our knowledge, no efforts have been made to compare the relative production or communities associated with the decomposition of plant and animal necromass in any ecosystem. Several comparisons of biomass in forested systems, however, provide some insight into the question of how much detritus is present in a habitat at any given time. Odum (1970), for example, determined in a Puerto Rican rainforest that plants and animals made up about 58% and 0.03% of the total biomass (including soil organic matter), respectively. Leaves accounted for only about 5% of aboveground plant biomass, with the rest consisted of wood and bark. Fittkau and Klinge (1973) reported animals made up only about 0.02% of the total living biomass in an Amazonian rainforest, with <10% of that component being of vertebrate origin. In most freshwater stream ecosystems leaf litter decomposition is a key process that influences energy and nutrient flow, biodiversity, and links terrestrial and marine ecosystems through a downstream continuum (Vannote et al. 1980, Webster and Benfield 1986, Wallace et al. 1997, Gessner et al. 2010. Detritus in a variety of organic matter forms is thought to represent the dominant energy pathway in most lakes (Rich and Wetzel 1978, Mann 1988) and other lentic surface waters such as wetlands (Brinson et al. 1981), coastal ecosystems (Duggins et al. 1989), and oceans (Parsons andStrickland 1962, Alldredge andSilver 1988). The differences in standing necromass between plants and animals are probably even larger than these estimates of biomass given that animal remains typically decompose much more quickly than plants, even in some aquatic habitats (Parmenter and Lamarra 1991), making carrion a "hidden" source of energy, nutrients and biodiversity. In many cases, animal necromass will be produced more quickly given the shorter life spans of most animals compared with many plants, especially when comparing invertebrate necromass production with the woody components of plants. However, many plants shed biomass seasonally as leaves and root exudates, thus producing necromass at higher rates than the woody components that make up a large portion of the standing stock biomass. Furthermore, the total production of animal necromass per year could be expected to far exceed the standing animal necromass at any given time due to a combination of short life spans, continuous reproduction, and rapid decay rates such as those for zooplankton (e.g., Tang et al. 2014). By contrast, the production of plant necromass produced per year can be expected to be a small fraction of the standing plant necromass in many ecosystems, and this is especially true for forests where decomposition is slow and woody debris accumulates over time (Luyssaert et al. 2008). The relative importance of these differences in standing stock, production and rate of decay of plant and animal necromass is an area of inquiry that could provide broad ecological understanding of different forms or routes of energy flow and nutrient cycling in ecosystem function.
Focusing on vertebrate animals, Odum (1970) estimated that they accounted for only about 5% of total animal biomass with the remaining 95% consisting of invertebrates (see Tables 7 and 22 in Odum [1970]). Additionally, a metaanalysis by Fierer et al. (2009) determined approximately 50% of animal biomass is belowground. The fact that invertebrates are typically much shorter lived than vertebrates, usually develop to maturity more quickly, and often have multiple generations per year suggests the annual production of necromass by invertebrates far exceeds that produced by vertebrates. Indeed, Seastedt and Tate (1981) estimated the standing arthropod necromass on the forest floor of two oak-hickory forests to be up to double the estimated living arthropod biomass for the same forests. In other habitats, such as aquatic systems, standing stock biomass can be estimated for populations and communities (Waters 1966, Benke et al. 1988), but some studies also produce estimates of production, often as secondary production of macroinvertebrates (Benke et al. 1988, Huryn andWallace 2000).
Knowledge of animal biomass in aquatic systems can be derived from measures of secondary production of invertebrates (Hynes 1970, Waters 1977, Benke et al. 1988, Huryn and Wallace 2000. Secondary production includes the elaboration of heterotrophic population biomass and assumes a yield to higher trophic levels that ultimately becomes part of the necromass pool in a given ecosystem. The method employs estimates of individual organismal body mass, often using length-mass regressions (Benke et al. 1999), and densities of those organisms and how they quantitatively change over time. One such method for calculating secondary production (i.e., removal summation) assumes all biomass produced eventually dies and is equivalent to estimated production for that cohort of organisms over a given time and under steady-state conditions (Boysen-Jensen 1919, Waters 1977, Benke and Huryn 2006. Thus, secondary production estimates of invertebrates may provide quantitative insight into the contribution of animal necromass and its turnover to ecosystem energetics, much like it has been done for measuring the importance of leaf necromass on aquatic invertebrate communities (Cummins et al. 1973. Measuring living invertebrate biomass over time, however, does not allow for estimating other forms of necromass, such as dung or frass. The importance of necrobiomes in the decomposition of entire communities of organisms could be pivotal in expanding our understanding of biodiversity contributions to global carbon and nutrient cycling, as has been discussed by Moore et al. (2004). As an example, a fascinating study reported that the quality of grasshopper necromass (affected by fear of predation) mediated subsequent leaf litter decomposition in old field ecosystems, with estimated significant ecosystem-level consequences (Hawlena et al. 2012). Discovering how animal and plant necromass decomposition communities interact with each other to drive nutrient and carbon cycling could lead to paradigm shifts in ecosystem science.

INTRINSIC VARIABILITY OF NECROMASS TYPES
Major forms of necromass (e.g., carrion, dung, leaves, and wood) can be distinguished from one another in three important ways: (1) resource size, (2) nutrient availability, and (3) digestibility. First, the unit volumes of different forms of necromass vary by 5-11 orders of magnitude (Fig. 3). While the largest forms of necromass produced globally are the woody stems and collective dead organic matter shedding of large trees, vertebrate carrion is the largest form in many nonforested ecosystems (e.g., bison in grasslands, whales in oceans; Towne 2000, Smith andBaco 2003). Resource size has important implications for the decomposition process, with larger resources decomposing more slowly and involving a greater diversity of species than smaller resources. Small arthropod carcasses, for example, typically disappear within several minutes to hours, being quickly discovered and consumed by ants, wasps, or other scavengers (Fellers andFellers 1982, Young 1984, Retana et al. 1991). By contrast, the decomposition of larger carcasses typically involves a much wider variety of species and succession of decomposer communities (Jones et al. 2015, Turner et al. 2017. Large resources also have less surface area relative to volume than small resources and this limits initial accessibility by some decomposers and thus in some cases large vertebrates (e.g., elephants) decompose initially through microbial decomposition (Coe 1978). Fragmentation and penetration of necromass are both mediated by the animal community, and thus are important processes with respect to the acceleration of decomposition. Second, different forms of necromass vary in nutritional quality. The carbon:nitrogen (C:N) ratio is commonly used to indicate the nutritional quality of organic material. The C:N ratio varies greatly among the various types of necromass, being lowest for carrion and highest for wood (Fig. 3). The C:N ratio also varies widely among necromass types due largely to interspecific variability. For example, it was reported up to 16-fold differences in C:N ratios existed among 59 species of tropical wood species (Martin et al. 2014). Intra-specific variability can also be important. For example, Madritch et al. (2007) reported two-to threefold differences in C:N ratios of caterpillar frass depending on the nutritional quality of their host plants. It has also been suggested that dung beetles are limited by assimilable C and not N (Holter and Scholtz 2007).
The third dimension is digestibility (not shown in Fig. 3). For plants, digestibility is largely determined by the structure of cell walls. Due to the recalcitrance of lignocellulose (i.e., a complex of lignin, cellulose, and hemicellulose) in plant cell walls, plant matter is much more resistant to decomposition than most animal soft tissues. For example, these recalcitrant compounds dominate plant tissues, respectively accounting for 20-35%, 40-45% and 20-40% of dry wood weight (Wilson and White 1986) and are highly variable among leaf litter types that contribute to stream ecosystem function Benfield 1986, Woodward et al. 2012). Whereas a wide range of organisms can digest animal tissues, the digestion of lignin and cellulose is almost entirely limited to fungi and prokaryotes that have evolved the enzymes required to break down these compounds. This dependence on microbes to reduce autotrophically derived detritus has given rise to a number of important symbioses between wood and leaf-feeding insects and fungi (see section on symbioses in the necrobiome). Secondary plant compounds and other extractives also reduce the digestibility of dead plant matter and can have toxic effects on decomposers (K€ a€ arik 1974, Verhoeven andLiefveld 1997). Among carrion consumers, there is also specialization on recalcitrant body parts. For example, the bearded vulture eats only the bones of vertebrates (Margalida et al. 2009), whereas some tineid moths eat only the horns and hooves of ungulates (Braack 1987). Such specialization, however, involves digestive enzymes and stomach acids, and not symbioses.
Another important difference between the necromass production from plants and animals concerns the gradual process by which many perennial plants, especially woody plants, die. In forest trees, for example, young branches produced at the top cast shade on older  branches below, reducing their ability to capture sunlight and contribute to the growth of the tree. These overshadowed branches eventually die and fall to the ground, ultimately resulting in the long limbless trunks characteristic of many mature forest trees: a very long-term shedding of detritus over the lifetime of the tree. Similarly, growth in tree diameter is accompanied by the senescence and death of the oldest, innermost rings ( Fig. 2a). Not only are these tissues not living, they often become colonized by diverse assemblages of decomposers many decades before the rest of the tree fully dies and falls to the forest floor (Elton 1966). This necromass accumulation co-occurs for years to decades with living tissue, providing a unique aspect of the necrobiome concept. Wounds through the bark layer (e.g., caused by extreme weather events, toppled neighboring trees, injuries from animal activity, etc.) expose the underlying phloem and wood for colonization and mark the beginning of the decay process in living trees. Although trees possess a variety of defensive mechanisms to limit the extent to which wounded areas are colonized by microbes and insects (e.g., flooding the site with water or sap, secondary plant compounds, etc.), organisms responsible for decay often become established at these sites. The first to colonize are the more ruderal microbial taxa, such as non-basiodiomycetes and bacteria, whereas wood-rotting fungi arrive later (Boddy 2001). The proportion of dead tissue in otherwise living trees increases with tree age and the largest and oldest trees, or "veteran trees," are full of dead limbs, rot holes, and are believed to be particularly important to supporting necrobiome biodiversity (Speight 1989, Stokland et al. 2012). The decomposition of tree tissue has been shown to be a substantial source of methane (Covey et al. 2012), suggesting broader effects on global biogeochemical cycling rates and atmospheric chemistry that require future investigation.

Necrobiome structure
The necrobiome defines a relatively concentrated set of organisms where many have evolved to detect, use and ultimately congregate with decaying organic matter either as a food or habitat resource . We describe in more detail the microbial, invertebrate, and vertebrate components of the decomposer community, as well as important symbioses, interactions with soil, and abiotic factors that govern the composition and dynamics of the necrobiome (Fig. 1).

Microbial communities of the necrobiome
Structurally, the necrobiome consists of the prokaryotic and eukaryotic microbial communities (microbiome) that are internal (endonecrotic) and external (epinecrotic) components of host necromass. While given different names, these microbial communities have been reported in both living plants (Bulgarelli et al. 2013) and animals (Dillon and Dillon 2004). These microbial communities function as symbionts, commensals, pathogens, and parasites during life, and the taxa become the pioneer community of plant detritus or carrion immediately after, or prior to, death in both terrestrial (Frankland 1966, Latter andCragg 1967) and aquatic ecosystems (Preiswerk et al. 2018). The microbial community succession of carrion is rapid after death without the biotic constraints of host immunology and physiological function (Pechal et al. 2014b, Metcalf et al. 2016, which determines the taxonomic and functional changes of the microbiota of the necrobiome through decomposition (Latter and Cragg 1967, Baldy et al. 1995. In aquatic leaf litter decomposition studies in streams, microbial community structural and functional succession is related to the leaf litter species and quality (Witkamp 1966, Gessner andChauvet 1994), similar to other forms of necromass like crayfish (Procambarus versutus [Hagan]) molted exoskeletons (Aumen 1980), particulate aggregates (Grossart and Simon 1998), lake zooplankton (Tang et al. 2010), and oceanic phytoplankton (Fukami et al. 1985) and zooplankton (Tang et al. 2006) that harbor complex microbial dynamics that change with water chemistry, depth, and system circulation (Karl et al. 1988). While difficult to study, whale and other cetacean carcasses that sink to the oceanic abyss attract a high diversity of vertebrate and invertebrate scavengers (Allison et al. 1991, Jones et al. 1998, Smith and Baco 2003 and also change the microbial ecology of the local habitat in a way that is dominated by chemoautotrophic communities (Bennett et al. 1994) in a network of species interactions (Smith et al. 1998). The microbial community changes associated with the death and deep sea decomposition of whale carcasses has not, to our knowledge, been studied, but may follow successional trajectories similar to those reported for other aquatic organisms (Preiswerk et al. 2018).
Microbial succession facilitates the proliferation of existing saprophytic taxa of the once living host through the secretion of enzymes and other compounds used to convert the newly available organic matter for assimilation. The microbial ecology of decomposing necromass is likely substantially different between plant and animal forms in terrestrial and aquatic ecosystems; however, there have been no comparative studies to directly evaluate microbial succession differences of plant and animal decomposition in either terrestrial or aquatic habitats. When one descriptively compares microbial succession of plant detritus to carrion, there are interesting, and potentially functionally important, differences. For instance, the taxon richness and diversity of microbial communities were shown to increase over decomposition for beech leaves, with predominant phyla not including the Firmicutes (Purahong et al. 2016); whereas for carrion, Firmicutes often make up a significant proportion of the communities and become dominant late in decomposition when taxon richness and diversity are lowest (Pechal et al. 2014b). Interestingly, the microbial communities of decomposing fungal mycelia in soil show more similar characteristics to carrion than to leaf litter on soil, with Proteobacteria being most dominant early and being replaced by Firmicutes later in decomposition (Brabcov a et al. 2016). The microbes are inherently a part of the living organism and part of the initial communities present for decomposition are highly variable depending on the host species, health, habitat, and ecological interactions (Oh et al. 2014, Pechal et al. 2014b, Junker and Keller 2015, Metcalf et al. 2016. Using a planktonic crustacean (Daphnia magna), Preiswerk et al. (2018) followed microbial communities of the living host through initial death and subsequent decomposition, reporting that the communities were highly dynamic and revealed opportunistic microbes that could exploit the host even before death. Thus, the community assembly dynamics of necromass is likely dependent on host condition and interactions with the abiotic environment and other organisms, populations, and communities. The soil environment and the microbial communities it harbors are intimately associated with the decomposition of plant and animal necromass. When considering plant necromass, decomposition has often been thought to be a product of three hierarchically organized, interacting factors: climate, substrate quality, and the biota (Swift et al. 1979). Of these factors, climate has been assumed to be the driving force behind rates of decomposition at broad spatial scales, whereas litter quality, and to a more minor degree, the biota, play a more important role at finer spatial scales. However, such assumptions have recently been called into question (Bradford et al. 2016). For instance, the decomposition rate of woody debris is largely independent of climate and may actually be driven by its proximity to woody debris previously colonized by wood-decay fungi . This interpretation suggests that spatial proximity of decomposer organisms may be a major driver of decomposition across a landscape (Ettema and Wardle 2002).
The microbial communities in the soil and associated with the rhizosphere that decompose leaf litter also play a more pronounced role. One such phenomenon is the occurrence of "home-field advantage" (HFA), whereby decomposer communities sharing a common history with a particular resource often decompose that resource at a greater than expected rate (Gholz et al. 2000). Such a phenomenon has been found when examining the interaction between climate and microbial communities (Strickland et al. 2015, Averill et al. 2016. Although HFA has been observed across a variety of different soil decomposer communities (Ayres et al. 2009, Strickland et al. 2009a, it has been found absent in others (St John et al. 2011, Bachega et al. 2016. One explanation for this divergence is HFA only accounts for adaptation to a particular organic matter resource and does not account for the fact that some decomposer microbial communities simply decompose an array of necromass types more rapidly due to greater functional breadth (Keiser et al. 2014). That is, some microbial decomposer communities have a greater "ability" to decompose a wide range of litter substrates than do others because they have been historically exposed to chemically complex litter species. Understanding how these two metrics of microbial function drive patterns of necromass decomposition across a landscape, and how they relate to microbial community characteristics, may prove informative to predictions of decomposition rates associated with shifts in plant species or changes in necromass quality (Keiser et al. 2013, Austin et al. 2014, Martiny et al. 2017. Additionally, this HFA and "ability" framework suggests that the function of soil microbial communities is not just a product of the current input of necromass but that it may also be influenced by past inputs as well. Adding to the complex interactions between soil microbial communities, climate, and necromass quality is the inclusion of the broader soil community in our understanding of decomposition. Soil arthropods are well known for their ability to fragment litter increasing the surface area available for microbial colonization and thereby increasing litter quality as it passes through the invertebrates' gut (Seastedt 1984). Yet there is a growing awareness that trophic interactions within the soil community may also drive the rate and efficiency of decomposition (Buchkowski 2016). For example, increased soil nitrogen can lead to greater wood decaying fungal biomass and extracellular enzyme activity in the absence of a fungivore, but the presence of a fungivore may moderate fungal biomass and wood decay rates (Crowther et al. 2015). More complex trophic cascades have also been observed, such as the removal of predatory microarthropods, which led to an increase in microbivorous nematodes and a subsequent increase in microbial biomass (Santos et al. 1981). Such examples indicate that top-down drivers may play an important role in mediating decomposition (Buchkowski 2016, Hawlena and Zaguri 2016) across a variety of necromass types. The relative strengths of top-down compared to bottomup constraints on necromass decomposition may vary depending on interacting abiotic factors and climate change (Crowther et al. 2015).

Invertebrates of the necrobiome
Invertebrates play a key role in decomposition. For vertebrate carrion, numerous studies have examined the role of arthropods associated with decomposition (Early and Goff 1986, Anderson 2000, Bharti and Singh 2003, Matuszewski et al. 2011, and identified a relatively predictable community succession of this group (Payne 1965), which is often used in forensics (Byrd and Castner 2009). Early work on arthropod communities associated with carrion identified key taxa and their roles in decomposition (Fuller 1934, Bornemissza  1957). Later work on this topic examined arthropod communities associated with decomposing swine in a range of contrasting circumstances, including being buried (Payne et al. 1968), on the soil surface (Payne 1965), or submerged in water (Payne and King 1972). A key outcome of this work was the recognition of the important role arthropods play in returning nutrients to the surrounding ecosystem. Preventing arthropod colonization can result in a significant reduction in the rate of biomass removal from animal carcasses (Payne 1965) and can change the community succession of both microbes and necrophagous insects (Pechal et al. , 2014a. Additional studies have addressed other theoretical aspects of arthropod community responses to carrion resource pulses (Bilby et al. 1996, Yang 2004 or mass mortality events (Fey et al. 2015).
Research has also explored species interactions among carrion arthropods ranging from competition (Goodbrod and Goff 1990), niche partitioning (Denno and Cothran 1975), predator-prey interactions (Brundage et al. 2014), and trophic cascades responsible for variability in succession trajectories (Pechal et al. 2014a). The understanding of carrion arthropods has been facilitated by knowledge of the multiple and distinct functional roles of different taxa at carcasses (Braack 1987). For example, flies (e.g., Diptera: Calliphoridae, Sarcophagidae) are the primary arthropods responsible for consuming soft tissues of vertebrate carrion and thus are traditionally recognized as saprophages (Merritt and De Jong 2015). Certain beetles (Coleoptera) including some rove beetle species (Staphylinidae; Matuszewski et al. 2008), carrion beetles (Silphidae; Trumbo 1990, Trumbo et al. 2016, and skin/hide beetles (Dermestidae, Trogidae; Kulshrestha and Satpathy 2001,  are also saprophagous. Other important functional groups include parasitoid flies (e.g., Diptera: Tachinidae) or wasps (e.g., Hymenoptera: Braconidae; Cammack et al. 2010) and predatory beetles (e.g., Coleoptera: Staphylinidae) and ants (Hymenoptera: Formicidae; Flores et al. 2014, Barton and. Often overlooked arthropods found at vertebrate carrion are those species located in the soil (Bornemissza 1957) or in aquatic environments (Tomberlin andAdler 1998, Merritt andWallace 2010). In many instances, these arthropods rely on the resource directly as habitat or as a nutritional resource (e.g., tissue in a stream or lake, or in the case of soil, fungi, or liquids seeping into the soil). Research exploring soil or aquatic associations is limited compared with studies in above ground terrestrial environments. The ecology and evolution of dung beetles has been described and tested in detail and demonstrates close evolutionary ties of some invertebrates and necromass (Hanski andCambefort 1991, Nichols et al. 2008).
Extremely diverse assemblages of invertebrates are associated with decomposing plant material. Approximately 20-30% of all forest insect species, for example, are directly or indirectly dependent on dying or dead wood (Stokland et al. 2012). Included among these are a wide range of phloem or wood feeders (e.g., termites, beetles, wood wasps, etc.), fungus feeders (e.g., beetles, flies, true bugs, etc.), and a variety of predators (Stokland et al. 2012). There is very little overlap known between the invertebrate assemblages involved in the decomposition of animal and plant necromass, as well as between different forms of plant matter (e.g., leaf litter vs. wood; Ferro et al. 2012).
Among macroinvertebrates, earthworms and termites have by far the strongest direct accelerative effects on plant biomass decomposition (Lavelle et al. 1997, Bignell andEggleton 2000). The importance of earthworms as decomposers was illustrated by the dramatic loss of leaf litter depth following their introduction into formerly earthworm-free (due to glaciation) forests (Addison 2009). Termites are the major consumers of plant debris in soil throughout the tropics and subtropics, perhaps even exceeding fungi in importance in some areas (Liu et al. 2015, Ulyshen 2016), with the different species feeding preferentially on wood, leaf litter, humus, or soil (Donovan et al. 2001). Invertebrates consuming decaying plant material rely heavily upon microbes, which provide a variety of benefits including digesting and softening the material, neutralizing allelopathic substances, and improving nutritional quality in terrestrial and aquatic habitats (Cummins et al. 1973, Swift and Boddy 1984, Webster and Benfield 1986. Microbial biomass is itself quite nutritious relative to most forms of dead plant matter and contributes greatly to the diets of litter-or wood-feeding invertebrates, many of which are essentially fungivorous (Tanahashi et al. 2009, Mishima et al. 2016). Predators are a large proportion of the invertebrate members of the necrobiome (e.g., over one-third of saproxylic beetle species in Germany [Wende et al. 2017]) and exert strong controls on necrophagous populations.

Vertebrates of the necrobiome
No vertebrates, to our knowledge, are known to be specialist consumers of decomposing plant material but many species interact in important ways within the necrobiome of plant necromass. Many birds (e.g., woodpeckers) and mammals (e.g., aardvarks, armadillos, anteaters, echidnas, bears), for instance, are specialist or opportunistic predators of wood-feeding insects such as beetle larvae or termites. The fragmentation of plant matter caused by foraging vertebrates is assumed to have strong indirect effects on decay rates (Ulyshen 2016) but this remains untested. By contrast, vertebrates are known to be major consumers of carrion and are thus direct participants in the decomposition process in addition to any indirect effects they may have as fragmenters (DeVault et al. 2003).
Vertebrates consuming carrion are commonly classified into two categories: obligate scavengers rely entirely on carrion to meet their food requirements, whereas facultative scavengers are active predators or foragers, as well as scavengers (DeVault et al. 2003). In marine systems, there are some deep-water fish that might also be categorized as obligate scavengers (Smith and Baco 2003). The only obligate vertebrate scavengers in terrestrial systems are Old World and New World vultures (families Accipitridae and Cathartidae, respectively). Vultures have evolved several adaptations that allow for dependence on carrion, which is often an ephemeral and unpredictable food source. First and foremost, obligate scavengers (i.e., vultures) have large wing spans and other physiological mechanisms that allow them to move quickly and efficiently to search vast areas for carrion (Ruxton andHouston 2004, Shivik 2006). Vultures also have welldeveloped vision, and in some cases olfactory capabilities (e.g., turkey vulture, Cathartes aura), which allow them to find carcasses over large distances (Houston 1979, DeVault et al. 2003. Due to microbial decomposition, carrion can contain high levels of toxic compounds and thus vultures have evolved physiological adaptations in their gut that allow them to consume large volumes of these toxins Cooper 1975, Roggenbuck et al. 2014). As a group, vultures are relatively large-bodied birds and can withstand several days without food (Ruxton and Houston 2004). The amount of biomass processed by vultures and other vertebrate scavengers varies across ecosystems (Pereira et al. 2014), but historically has been greatly underestimated (DeVault et al. 2003, Wilson andWolkovich 2011). In one example, an estimated 70% of large ungulates in the Serengeti die from causes other than predation and become available to scavengers. Thus, collectively Serengeti vultures likely consume more biomass than all carnivorous mammals combined (Houston 1979), suggesting a large scale importance of carrion necromass in ecosystem structure and function. Due to their reliance on carrion, populations of obligate scavengers are inextricably linked to the availability and distribution of carrion on the landscape (Kelly et al. 2007). For some facultative scavengers the availability of carrion can influence local abundance, species distributions, or growth rates, especially in ecosystems with limited food resources (Fuglei et al. 2003, Wilmers et al. 2003, Drazen et al. 2012. For example, local Arctic fox (Alopex lagopus) populations in Norway decreased markedly following a decline in reindeer (Rangifer tarandus platyrhynchus) carcasses (Fuglei et al. 2003). In Yellowstone National Park, USA, populations of facultative scavengers have benefited from the reintroduction of grey wolves (Canis lupus), which provide a more stable subsidy of ungulate carrion (Wilmers et al. 2003). Such effects have also been reported in aquatic ecosystems, where the abundance and distribution of deep-water fish may be modulated by carrion availability (Drazen et al. 2012).
Carrion use by vertebrate scavengers is mediated by a complex suite of biotic and abiotic factors (DeVault et al. 2003, Selva et al. 2005, Mole on et al. 2015, Turner et al. 2017. As a result, use of carrion can also vary extensively within a vertebrate species. From a temporal perspective, variability in food availability and quality can be a key factor and can be a product of intra-and inter-kingdom competition (Allen et al. 2014, Beasley et al. 2015. These forms of competition demonstrate some of the ways that very different taxonomic components of the necrobiome interact to affect decomposition. For example, the activity of invertebrates and microbes is influenced by temperature, and that means that invertebrates and microbes are more active during warmer weather and will more rapidly decompose or render carcasses toxic or unpalatable for vertebrates (Janzen 1977, Burkepile et al. 2006. Consequently, vertebrate scavengers are typically most successful when temperatures are cooler and decomposition is slowed (DeVault et al. 2004a, Selva et al. 2005, Parmenter and MacMahon 2009). However, in some areas, the presence of an abundant and efficient vertebrate scavenger species can shift the balance toward vertebrates, even in warm weather (Houston 1986, DeVault et al. 2011). Competition for carrion resources also differs across habitat types (Beasley et al. 2015, Turner et al. 2017), although such differences likely reflect the composition of local vertebrate communities (DeVault et al. 2004a) and microclimates (Parmenter and MacMahon 2009).
Collectively, obligate and facultative scavengers provide a number of critical ecosystem services through the removal of animal necromass from landscapes (Mole on et al. , DeVault et al. 2016. Intact scavenging communities (especially the presence of vultures) may reduce risks of disease transmission among mammals due to rapid removal of carcasses from ecosystems (Jennelle et al. 2009, Ogada et al. 2012, Hill et al. 2018). There is a clear role of vertebrates in the decomposition of carrion and placing them in the broader necrobiome framework highlights their links with other taxonomic groups (i.e., insects and microbes) and important environmental moderators of decomposition and overall ecosystem structure and function.

Symbioses of the necrobiome
Although symbioses between invertebrates and microbes are common within necrobiomes, the nature of these interactions differs greatly between decomposers associated with autotrophically or heterotrophically derived biomass. Due to the strength of lignocellulose, dead wood is among the most recalcitrant forms of plant matter and is also characterized by some of the highest C:N ratios of any organic material (Fig. 3). Although endogenous cellulases are known from some termites (Blattodea) and may play a particularly important role in the metabolism of Termitidae, which lack the protists of "lower termites" , wood-feeding insects are mostly unable to digest lignocellulose without the enzymes produced by various endo-or ecto-symbiotic microorganisms. Endo-symbionts include a wide range of gut-dwelling bacteria, archaea, and eukarya (e.g., yeasts and protists) that have been the subjects of intense study in both termites (O'Brien andSlaytor 1982, Brune 2014) and various beetle species (Reid et al. 2011, Urbina et al. 2013). In addition to digestion, another important function performed by prokaryotes within the guts or salivary glands of many wood-feeding arthropods is nitrogen fixation, which often provides much of the nitrogen required by these organisms to complete development (Ulyshen 2015). Ecto-symbioses are common between insects and fungi associated with decomposing wood. Ambrosia beetles, for example, cultivate and consume symbiotic fungi within galleries these insects create in dead wood. These insect-fungi associations have repeatedly evolved within scolytine and platypodine Curculionidae and also exist in other beetle families, such as Lymexylidae and Erotylidae (Farrell et al. 2001, Toki et al. 2012. The bodies of these insects possess special storage structures, called mycangia, within which their fungal symbionts are transported. Mycangia also exist in wood-feeding insects that are not technically ambrosia beetles. For example, female stag beetles (Lucanidae) possess mycangia near the ends of their abdomens that contain xylose-fermenting yeasts, which are presumably added to oviposition sites to benefit larval development (Tanahashi et al. 2010). Some of the most effective forms of symbioses between insects and fungi are exhibited by leaf-cutting ants and macrotermitine termites, both of which cultivate fungal gardens within their nests to perform the digestion of plant material (Nobre et al. 2011). Macrotermitines are thought to be among the most efficient wood-consuming insects due to their symbioses with the fungi Termitomyces (Basidiomycetes: Lyophyllaceae), which both allows them to process wood more quickly and increases assimilation efficiency (Schuurman 2005, Brune and Ohkuma 2011). Other functions provided by endo-and ecto-symbionts of arthropods associated with decomposing plant material include detoxification of plant secondary compounds (Dowd 1992), defensive mutualism (Chouvenc et al. 2013), and protection from pathogens (Peterson and Scharf 2016).
Whereas symbioses within plant-based necrobiomes largely stem from the recalcitrance and nutrient deficiency of dead plant matter, those within animal-based necrobiomes appear to have arisen largely in response to intense competition with other organisms as well as threats from pathogens. Necrobiome community interactions may follow a gradient of resource quality where the most labile forms of necromass facilitate competition and the most recalcitrant forms support symbioses (Janzen 1977). Microbial competition has been demonstrated to have strong negative effects on the reproductive success of some carrion-feeding insects (Rozen et al. 2008). Although carrion-feeding insects produce a variety of antimicrobial peptides and lysozymes to minimize competition from bacteria (Jordan et al. 2015), they also harbor, within their guts, diverse and unique microbial assemblages (Kaltenpoth and Steiger 2014). The exact functions provided by these microbes remain poorly understood but reducing competition from free-living carrion microbes and other carrion-feeding animals, detoxification, and providing protection from pathogens are several potential benefits (Kaltenpoth andSteiger 2014, Trumbo et al. 2016). Adult burying beetles, for instance, are known to transmit gut bacteria to their offspring both directly through regurgitation and anal secretions applied during carcass preparation (Wang and Rozen 2017). Although the secretions produced by burying beetles have antimicrobial properties, free-living microbes appear to be reduced in favor of microbes found in the guts of these insects (Duarte et al. 2018). Rather than reducing competition from other microbes, the antimicrobial secretions produced by Nicrophorus may serve primarily to help conceal carcasses from competitor scavengers by suppressing the bacterial production of olfactory cues (Trumbo et al. 2016). While research in both plant and animal necromass decomposition has shown many examples of symbioses and cross-domain interactions, including the well-known symbioses of ruminant digestion of plant necromass (Hungate 1966), there are likely many more to be discovered.

Abiotic factors and the habitat template
Decomposition and the necrobiome constituents are constrained by abiotic factors defined by the surrounding habitat or ecosystem. Terrestrial, aquatic, and human-modified habitats each differ in their communities and climate, and this affects the rate and progression of decomposition and the species diversity that uses necromass (Beasley et al. 2012). Future studies should consider how changes in abiotic factors described below, in addition to extreme weather events, ocean acidification, and rising annual global temperatures could alter detritus availability, necrobiome dynamics, and ultimately decomposition processes in ecosystems.
Although many decomposition studies have occurred on land, the majority of Earth's surface is composed of aquatic habitat. Aquatic habitats include freshwater (e.g., streams, ponds, lakes, wetlands), marine (e.g., ocean), and brackish (e.g., estuaries) environments. The inherent physiochemical properties unique to these aquatic habitat types can substantially alter decomposition through direct or indirect abiotic interactions that constrain necrophagous physiology or mechanical ability to breakdown organic matter. Further, depending on where a resource is located within the water column, abiotic factors such as temperature, oxygen availability, carbon dioxide levels, ion concentrations, turbidity, pressure, salinity, pH, and flow dynamics will impact the rate and taphonomic changes of decomposition and determine the biodiversity of consumers that may use the decomposing organic material (Hattenschwiler et al. 2005, Wallace 2015. For example, swine carcasses placed in the deep sea fail to undergo bloat during the decomposition process at these depths because of the increased pressure within the environment (Anderson and Bell 2016). Woody debris and leaf litter in aquatic habitats, primarily in streams, have additional factors to consider that impact the abiotic conditions and ultimately the decomposition process. These include, but are not  Vol. 89,No. 1 limited to, where the wood is located within the stream (e.g., submersion, buried in the substrate, suspended); the morphology of the streambed (e.g., sand or silt, rock size); and hydrology (e.g., season flooding and/or spate events; Scherer 2004, Ruiz-Villanueva et al. 2016. Terrestrial habitats can be separated into broad biomes: tundra, boreal forest, temperate forest, desert, grassland, and tropical rain forest, with the abiotic factors of temperature, humidity, wind, solar radiation, and exposure substantially altering decomposition (Benbow et al. 2015a). These abiotic factors can further interact with landscape configuration and vegetation within each ecosystem type (Garc ıa-Palacios et al. 2013). Soil conditions specific to a habitat can alter decomposition dynamics of plants and animals due to direct and indirect associations with soil nutrients, pH, salinity, pollution or other contaminants, temperature, and moisture content (Refsnider and Janzen 2010). Further, landscape features, such as land use and cover, connectivity, and complexity can mediate consumer attraction and use of necromass resources. For example, habitats with increased biodiversity, such as tropical rainforests, have an increased species pool that can utilize a resource during decomposition when compared to habitats with slightly decreased complexity, such as an African tropical woodland (Cornaby 1974, Braack 1987. The final category of ecosystems, which may not be as apparent as the previous two groups, is anthropogenicassociated ecosystems. These habitats are comprised of human built (e.g., residential structures) or modified areas (e.g., disturbed or change habitats) that impact the decomposition of organic material and biodiversity of the necrobiome. Specifically, modification of ecosystems often leads to habitat loss and fragmentation resulting from human development (e.g., urbanization, deforestation, and monoculture farming). Habitat fragmentation typically elicits negative impacts on biodiversity (MacArthur and Wilson 1967). A recent meta-review, however, revealed approximately three-quarters of studies showed some measures of positive response by communities affected by habitat fragmentation (Fahrig 2017). Overall, the impact of humanmodification to decomposition and decomposer communities is underexplored and poorly understood.

NECROBIOME ECOLOGICAL FUNCTIONS
A key feature of our expanded necrobiome concept is the broad structural and functional template it provides for better understanding the range of similarities and differences in decomposer communities found associated with plant and animal detritus. Volatile organic compound (VOC) emissions result from tissue decomposition and microbial metabolism and are an important mechanism for the attraction of exogenous macro-saprotrophs, such as blow flies, bark beetles, and nematodes (see Table 1), from the contiguous substrate (e.g., soil) and the surrounding local habitat and ecosystem (Ma et al. 2012, Ulyshen 2016. The types of VOCs produced are determined by necromass composition, quality, and rate of decay; and they are known to affect the recruitment of exogenous macro-saprotrophs (i.e., invertebrate decomposers), in time frames ranging from minutes to years after death (Ma z dra et al. 2015, Ulyshen 2016. In leaf litter decomposition, microbial communities are known to markedly contribute to the production of associated VOC profiles (Gray et al. 2010). Ultimately, decomposition is achieved by species that directly consume necromass or produce the enzymes that catabolize compounds and degrade dead tissues. However, members of the necrobiome also affect decomposition through physical alterations to the necromass, as well as intra-and interspecific species interactions that generate complex interdependencies among organisms. Among the latter, interactions between invertebrates and microbes are particularly important. We describe below major functions performed by community members of the necrobiome that both directly and indirectly affect decomposition and compare and contrast functions occurring at plant and animal necromass.

Digestion
Many organisms can perform the primary function of digestion with respect to relatively labile animal tissues. The primary digesters at animal necromass are specialist carrion flies (Payne 1965), although there can be minor contributions from other insect groups, such as beetles or ants . Vertebrates should also be considered digesters as they will consume entire carcasses before invertebrates are able to proliferate and participate in this function (DeVault et al. 2003). For plant necromass, wood-feeding invertebrate symbionts and saprophytic fungi and microbial communities in the stomachs of ruminant mammals (Hungate 1966) are capable of completely degrading the lignocellulose of plant cell walls to perform a digestion function (Swift et al. 1979). Digestion by saprophytic fungi and microbes is performed via the production of extra-cellular enzymes in ruminants (Refsnider and Janzen 2010) and is an external process rather than internal as for insects and vertebrates.

Fragmentation/penetration
Physical alterations brought about by animals that act to fragment or tunnel into dead material can strongly affect decomposition. The activities of carrion-feeding vertebrate scavengers and vertebrates foraging for insects in dead wood act to fragment large necromass resources. Fragments decompose more quickly than intact pieces due to increased surface area and coverage of microbial biofilms, as well as improved aeration (e.g., Boddy 1983). Penetration of the outer protective layers of dead animals and plants (e.g., skin or bark) is another important physical alteration brought about by animal activity, which facilitates colonization by microbes and other organisms (Connell andSlatyer 1977, Mann et al. 1990). In animal necromass, this activity is performed by larger scavenging animals (e.g., crows, vultures, and hyenas) that pick at parts of a carcass and enable entry by insects and microbes. In decomposing logs, insects that create holes through bark and tunnel into the heartwood (Leach et al. 1937) play a particularly important role in facilitating the establishment of rot fungi. Some vertebrates, such as aardvarks (Orycteropus afer) in Africa or echidnas (Tachyglossus aculeatus) in Australia, also play a role in the fragmentation of logs when searching for ant or termite nests. Although there are many insect species that perform a fragmentation or penetration functional role in terrestrial habitats, there are fewer major groups of xylophagous macroinvertebrates in freshwater systems (Anderson et al. 1978), contributing to the slow rates of wood decomposition in these habitats. Certain molluscs perform this function in marine environments and can cause a great deal of damage to wooden boats and pilings (Nair and Saraswathy 1971). Another consequence of fragmentation can be changes in microbial community composition. The production of fine woody particulate material by many wood-boring insects is thought to favor bacterial communities Boddy 1984, Van der Wal et al. 2007), including free-living N-fixers (Ausmus 1977).

Nutrient and microbe transport and dispersal
Organisms within the necrobiome play a role in moving nutrients away from decaying necromass through both passive and biological means (Fig. 1). The microbes and elements of decaying organic matter can move by direct translocation of microbial cells or along elemental concentration gradients within the biomass and into, onto, and within contiguous substrata. This transfer is perhaps best visualized by the process of purging, whereby fluids and gases from a decaying animal escape into soil or the atmosphere. Nutrients and particles also are dispersed by mobile micro-(e.g., fungi, protists) or macro-saprotrophs (e.g., flies) that act as vectors of the organic matter, microbes, and elements. The dispersal of these materials can occur in multiple  leaching rates (Nykvist 1959a, b, 1963, Cummins et al. 1973, Kuiters and Sarink 1986) Ecologically long (weeks to years) Danell et al. (2002), Barton  Van Geel (1978), Middeldorp (1986), Barthelmes et al. (2006), Kvavadze and Kakhiani (2010), Ronkainen et al. (2013) Notes: Microbial communities include prokaryotes and microscopic protists, fungi, and eukaryotes. Temporal scales range from ecologically relevant scales related to the life history traits of decomposer species to plant and animal biomass protected from decomposer species over geological time scales of decades or centuries (e.g., peat bog mummies). ways: (1) trophic transfer where the microorganisms and elements are ingested but not assimilated, and then egested after saprotroph movement away from the necromass source; (2) trophic transfer where the microorganisms and elements are ingested and assimilated, and released after saprotroph movement and then death either through predation or decomposition; (3) external attachment to the saprotroph by physical or chemical means during movement; and (4) by being aerosolized or passively dispersed by winds, rainfall, water currents, or thermal energy (e.g., convection). Additional discussion is given below of how members of the necrobiome interact in networks to transport and disperse microbes and nutrients.

Detoxification
Non-structural secondary compounds are often present in heartwood and are known to be toxic to many microbial decomposers and insects (K€ a€ arik 1974). The abundance of these compounds can vary by as much as 50-fold among angiosperms and 20-fold among gymnosperms , and the variance is thought to be largely responsible for the differences in decay resistance observed among different wood species (Bultman and Southwell 1976). The protective effects of these and other extractives diminish over time (Bultman and Southwell 1976); however, bacteria, and fungi both play a part in neutralizing these potentially toxic compounds (Burnes et al. 2000, Dorado et al. 2000. Further, microbial symbionts of some insects associated with detritus provide detoxification function of plant secondary compounds (Dowd 1992). As far as we are aware, there are no analogous intrinsic toxins present associated with microbial communities of animal necromass that retard microbial or insect activity. However, a burying beetle, Nicrophorus vespilloides (Silphidae; Arce et al. 2012), produces antimicrobial secretions that compete with microbes on carrion. Overall, the detoxification pathways in plants suggest this function is primarily the activity of microbial components of the plant necrobiome. However, there are likely undiscovered analogous activities in animal necromass decomposition.

Predation
Predators are conspicuous and important members of the necrobiome. It is not uncommon for predators to be among the most species-rich guilds in dead wood (Hammond et al. 2001, Wende et al. 2017, as well as at carcasses . Moreover, opportunistic predation by non-predatory invertebrates is commonplace in decomposing wood. Termites are known to feed on dead insect larvae and other invertebrate carcasses encountered while foraging in wood (Thorne and Kimsey 1983), which includes cannibalizing dead or injured nestmates (Wilson 1971). Many wood-boring beetle larvae are also known to prey on other larvae encountered in dead wood (Soper and Olson 1963) and this probably relieves the limitations imposed by the low nutritional quality of wood.
For carrion, cannibalism among fly larvae may be an adaptation to intense competition rather than a nutritional requirement (e.g., the hairy maggot blow fly Chyrsomya rufifacies Macquart 1842; Baumgartner 1993). Most predatory beetles and ants at carcasses tend to be generalists and take advantage of the concentrated and abundant insects associated with carrion . Some species of predatory insect may also opportunistically scavenge on carcass tissue, possibly due to the similar stoichiometric composition and easy switching between resources. The function of predators (and parasitoids, not discussed here) in decomposition is largely restricted to moderating the abundance of other decomposer organisms, and in most cases probably does not alter the rate or pattern of decomposition to any notable degree. Predatory ants, however, have been shown to dominate the insect community and retard rates of mass loss of both wood  and animal carcasses . Indeed, Houston (1985) argued that ant communities in South America prolong carcass availability to vertebrates by feeding on dipteran larvae that otherwise would quickly consume the carcasses. Further, it is known that some invertebrate predators approach carrion specifically to prey on other organisms and not necessarily the necromass (Payne 1965).

NECROBIOME INTERACTIONS AND NETWORKS
In addition to discussing the taxonomic and functional organization of the necrobiome, we expand the necrobiome concept to also consider the complex biotic and abiotic mechanisms of species and element (e.g., nutrients, carbon) translocation and dispersal from the decaying necromass into adjacent substrates, the local habitat, and ultimately into the larger ecosystem, as originally presented by Polis and Strong (1996). We argue that the decaying biomass holds a community of microbial species that often follow elemental movement into and throughout the environment, thus providing a way to track the dispersal of species, their genomes, metabolic diversity, and function after host death and through decomposition. While understudied, the dispersal of genetic and metabolic information into the surrounding environments (Preiswerk et al. 2018) is arguably a potentially transformative way to view decomposition as an ecosystem process through multiple disciplines and areas of scientific inquiry.
Many invertebrates act as vectors of microbial digesters from one resource to another. This transfer by insects can disperse gut symbionts as well as those that passively carry microbes on their bodies (Persson et al. 2009, Strid et al. 2014 fungal gardens (Wood and Thomas 1989). Blow flies and house flies are known to carry and transmit pathogenic bacteria to and from decomposing organic matter (Greenberg and Bornsetein 1964, Greenberg et al. 1970, Greenberg 1973, Macovei and Zurek 2006, Nayduch 2017. A recent study by Weatherbee et al. (2017) showed that carrion-associated microbial taxa consumed by blow fly larvae are transferred into the gut of the larvae. Although it is not definitively known whether microbes are passed transstadially to adults in all species (Singh et al. 2015), evidence from adult flies suggests a high likelihood that some bacteria ingested from carrion are assimilated by larvae, persist into the adults, and then are dispersed into the landscape (Weatherbee et al. 2017).
In addition to vectoring microbes, many insects also carry other invertebrates from one ephemeral or isolated resource patch to another. Phoresy is common among invertebrates associated with dung and carrion (Krantz 1998), fungal fruiting structures (Fain and Ide 1976), and dead wood. Examples of saproxylic insects from which phoretic passengers have been recorded include bark beetles, click beetles, cerambycid beetles, passalid beetles, and parasitic wasps (Haack and Wilkinson 1986). As pointed out by Moser and Roton (1971), virtually all relatively large insects capable of long distance dispersal serve as hosts for one or more phoretic species. Many host species carry not just a single phoretic species but a whole community of passengers. For example, at least 18 species of mites are known to be phoretic on the bark beetle Dendroctonus frontalis (Coleoptera: Curculionidae) in the southeastern United States (Moser 1976). Although phoresy is often viewed as a form of commensalism where the phoretic species benefits and the host is unaffected, this is not always the case with reports of both negative (Lindquist 1969, Hodgkin et al. 2010) and positive impacts (Purrington and Drake 2008) on the host.
Interactions between decomposer organisms belonging to different kingdoms (or domains) are quite common and warrant separate attention. Recent evidence indicates that microscopic species interacting with higher organisms are playing a far greater role in regulating succession patterns . Interkingdom interactions between plants and microbes are prevalent, and these interactions may also transect invertebrate communities. A specific example is with Sphagnum mosses. These plant assemblages dominate peatland bog ecosystems and form a unique and extreme habitat for microbes. This moss genus is well known for its preservative properties and slow rate of decomposition, occupies one-third of land on the planet, and stores more carbon than any other single genus of plant (Turetsky 2003). Highly acidic conditions along with low concentrations of nutrients, together with specialized leaf structures with high water content, lead to very specialized microbial colonization that is unique to Sphagnum species (Opelt et al. 2007). Furthermore, Sphagnum moss species produce secondary metabolites, such as phenolics, terpenoids, and tannins, and accumulate carbohydrates, that influence microbial colonization and subsequently microbially mediated decomposition. These plant specific factors may facilitate peat accumulation through lowering of redox potential that leads to decreased microbial decomposition rates (Belyea 1996). Methanotropic bacteria within the bogs themselves are well documented with Burkholderia and Serratia to be among the most dominant bacteria colonizing Sphagnum plants (Juottonen et al. 2005). Very few fungi colonize the actual plants, as these hosts are known for their antifungal and antibacterial activity (Opelt et al. 2007). A recent study showed that bacterial isolates from two Sphagnum species had antagonistic activities against fungi, while also performing nitrogen fixation and lowering host plant ethylene levels (Knorr et al. 2015). Despite these documented antimicrobial properties, a relatively large number of fungal taxa have been identified on S. fuscum (Thormann et al. 2001). Thus, the overall functioning of peat bog ecosystems is heavily influenced by interkingdom interactions occurring among plants, bacteria, and fungi, as well as their associated chemicals associated with high volumes of necromass.
Microbes have evolved complex strategies for outcompeting one another for resources available within carrion. Although in the early stages of discovery, it is clear microbes associated with carrion play important roles in regulation of arthropod attraction, colonization, and utilization of the remains. Furthermore, recent evidence indicates within kingdom interactions (e.g., blow fly predator vs. blow fly prey) are partially governed by their associated microbiome in carrion , Weatherbee et al. 2017) and other necromass decomposition networks (Purahong et al. 2016, Deveau et al. 2018. Some compounds serve as a mechanism of communication and group responses, better known as quorum sensing (Nealson and Hastings 1979), while others serve to disrupt communication. In both cases, these processes give a competitive advantage to the producer of these compounds by reducing competition with other microbes within the environment. For example, indole, a by-product of tryptophan degradation, is a key molecule used by Escherichia coli (gut microbe of many vertebrate species) as a quorum-sensing molecule (Kim and Park 2015) and is a bacterial taxon commonly associated with vertebrate carrion (Metcalf et al. 2016). In contrast, dimethyl disulphide (DMDS) is detected during the later stages of vertebrate decomposition (Forbes and Perrault 2014) and is a quorum-quenching molecule for many Gram-negative species. In both instances, these compounds regulate blow fly attraction (Ma et al. 2012, Tomberlin et al. 2012b). In addition to being an attractant, the response of blow flies to these compounds tends to be sex and physical state (e.g., gravid vs. nongravid) specific (Brundage et al. 2017).
The roles of microbial and invertebrate activity during animal necromass decomposition also scales to  Vol. 89,No. 1 vertebrates and can affect vertebrate scavenger interactions (DeVault et al. 2003, Beasley et al. 2015. For instance, temperature is known to impact invertebrate and microbial activity (Vass 2001, Zhou andByard 2011), and was later found to affect the percentage of consumption of small-mammal carcasses by vertebrate scavengers (DeVault et al. 2004a). Although the mechanism of this interaction has not been tested, microbial and invertebrate communities are known to affect the rate of decomposition (Simmons et al. 2010) and quality of the carcass (Payne et al. 1968, 2014a, which likely influences the quality and quantity of these resources available to vertebrate scavengers. The reciprocal effect can be inferred from high vertebrate scavenging rates (DeVault et al. 2011) that eliminate these resources via consumption of fresh carcasses as they become available and therefore reducing persistence in the landscape (Jones et al. 2015). Additionally, there is some evidence that microbes of carcasses and vertebrate scavengers may have coevolved, as hyenas frequently feed on ungulate carcasses that have died from anthrax without showing disease characteristics (Villiers Pienaar 1969, Gasaway et al. 1991. There is precedent for evolved adaptations in avian scavengers for competing with carrion microbial communities (Houston 1974, Houston andCooper 1975). These cross-kingdom scavenger interactions are important to ecosystems but are often overlooked aspects of the ecology of animal decomposition (Wilson and Wolkovich 2011); however, they constitute linkages that make up food webs, indirect predator-prey effects, and necrobiome community networks. The biotic structure and interactions among necrobiome taxa inherently give rise to network structure that can be used to define the complex and dynamic nature of organic matter decomposition. Food webs are biotic networks of interacting organisms, energy, and nutrient flow and define communities and metacommunities. The complexity of food web networks has been studied in several systems (Polis and Strong 1996, Ekl€ of et al. 2013, Wende et al. 2017, and has been proposed as a useful tool for developing a systems approach to understanding ecosystems (Dunne et al. 2002). The necrobiome framework offers an opportunity to quantify a subset of overall ecosystem food web networks and test novel hypotheses related to the strength of direct and indirect interactions among species and trophic levels. Furthermore, the necrobiome approach affords the opportunity to test hypotheses and further theory with evolutionary importance of the decomposer communities, much like that argued for understanding the broader importance of scavenging in food web research (DeVault et al. 2003, Wilson andWolkovich 2011). The complexity and stability of plant and animal necromass networks likely arises, in part, from the indirect (nonphysical) interactions that involve evolved sensing, physiology and behavior associated with finding and consuming decomposing resources (Tomberlin et al. 2011) and direct interactions of the necrobiome. SPATIAL AND TEMPORAL DIMENSIONS OF THE NECROBIOME Dead wood and animal carcasses vary in size, spatial density, and temporal input and turnover, adding another layer to necrobiome dynamics. Succession and related theory (Box 1) is well documented for decomposer communities associated with plant and animal necromass (Refsnider and Janzen 2010). Yet, scaling up these localized processes and integrating them into the higher-level spatial and temporal dynamics occurring among multiple decomposer communities at ecosystem scales is a significant challenge. Indeed, very few studies have yet to document the large-scale dynamics of single sources of detritus (e.g., forest stand blow downs) beyond ontogeny-based death events of anadromous fishes (e.g., Pacific salmon, Oncorhynchus spp.), despite recent work demonstrating that large animal mass mortality events are increasing in both magnitude and frequency globally (Fey et al. 2015). Empirical studies of large-scale carrion effects include the mass emergence of cicadas (Magicicada spp.) in North American forests (Yang 2004), the annual spawning runs of salmon (e.g., Oncorhynchus spp.; Hocking and Reynolds 2011), and the mass drowning of wildebeest in the Mara River in Kenya (Subalusky et al. 2017). These studies have shown how substantial quantities of carrion can alter forest soil microbial communities (Yang 2004), change the biomass and composition of riparian plant communities (Hocking and Reynolds 2011), and subsidize 50% of the diet of fish communities (Subalusky et al. 2017). Such instances provide evidence of carrion and associated necrobiomes changing nutrient cycling in a way that alters ecosystem functioning. Examples of plant necromass altering ecosystems are more common, with litter fall and decomposition having critical role in the cycling of 100 megatonnes of carbon annually (Gessner et al. 2010). Plant litter is ubiquitous in vegetated ecosystems, and associated decomposer communities are similarly widespread (Refsnider and Janzen 2010). Similar to the spatially discrete dead trees and coarse woody debris, there are fewer instances of analogous, highly recalcitrant, structures of animal carcasses (e.g., whale bones) that provide focal points for the assemblages of distinct necrobiome communities over long (e.g., years to decades) temporal scales. However, the impact of such spatially and temporally unpredictable events on bottom-up or topdown processes are still not fully understood.
Across a landscape are many dead logs and carcasses of different sizes and decay stages, with each stage having a different necrobiome defined by its attendant species and their interactions. This variability emphasizes the among-necromass dynamics of multiple necrobiomes. Succession is well documented (see Box 1) and tells us that any single necrobiome exists only for a certain window of time (Table 1), which might range from hours to days for carcasses in warm conditions (Payne 1965, Barton and or months to years for some large fallen trees    (Brothwell et al. 2002, Chapman 2015. Decomposer organisms searching for their next resource to continue their life cycle are therefore not only looking for another log or carcass, but also a specific decay stage, thus further emphasizing the rapid temporal turnover of the necrobiome within a necromass source. For specialist decomposers in the necrobiome, their existence is entirely dependent on resource continuity through time, with continuity also dependent on spatial proximity due to dispersal limitation. Thus, the implications of highlevel among-necromass necrobiome dynamics become apparent across multiple scales in ways that include (1) the reproductive success and survival of individuals, and their subsequent dispersal and contribution to population genetics (Stokland et al. 2012); (2) competition among species, coexistence, and community diversity and heterogeneity; (3) the consumption and metabolic breakdown of necromass via microbes and saprophytes; and (4) dispersal of necromass energy, genomes, and elements through the environment as new living biomass. There are gaps in our knowledge of ecosystem-scale responses to plant and animal necromass, and many fundamental questions remain. A framework that joins patch-scale phenomena with ecosystem-scale consequences is therefore needed to fully appreciate the crossscale dynamics of the necrobiome. We synthesize in Fig. 4 previous work that has attempted to conceptualize among-carcass dynamics (Barton et al. 2013a, Benbow et al. 2015b), but which also applies conceptually to leaf litter, dead trees, and course woody debris. We suggest that the spatial and temporal dimensions of the dynamic necrobiome are best conceptualized hierarchically, with community dynamics occurring spatially and temporally within necromass sources as well as spatially and temporally among multiple sources of necromass (Fig. 4).

BUILDING THEORY IN DECOMPOSITION ECOLOGY
The necrobiome framework illustrates the links and interactions among decomposer organisms associated with defined substrates, and the key interactions with each other and the environment over a range of spatial and temporal scales. For example, microbes and necrophagous insects interact on individual carcasses and come into direct contact with one another . Similarly, phoretic mites interact directly with their insect hosts and carcasses, which provides focal points for their movement via necrophilous insects across landscapes (Perotti and Braig 2009). Yet these examples of direct contact that occur over relatively small spatial and temporal scales can have implications at much larger scales by altering metacommunity structure and the rate and pathway of nutrient and genome movement through ecosystems. The necrobiome allows for clearer conceptualization of the links between groups of species and their environmental context, and we believe this can help to develop empirical tests of ecological theory related to decomposition. For example, for many forms of necromass, especially large and recalcitrant forms, the decomposition process is characterized by a succession of species (see Box 1). Succession is, in part, guided by colonization history and priority effects as early colonizers can have facilitative or inhibitory effects on later colonizers (Connell and Slatyer 1977). In addition to direct interactions among decomposers coexisting at the same time and place, the indirect priority effects among species across time are known to be of great importance to the development of these communities as well as to their collective effects on decomposition. In Sweden, for example, Weslien et al. (2011) showed that colonization of Norway spruce (Picea abies L. Karst.) by two beetle species within one year of cutting influenced the occurrence of another beetle species a decade later in a way that was mediated through a species of wood-decaying fungi.
The examination of species traits can also benefit from the necrobiome concept by understanding what traits mediate key functions or processes. Greater general understanding of species assemblage and food web responses to decomposition is necessary to improve the transferability of findings across geographic regions with different sets of species, which is a source of variation among applied research, such as forensics, using these ecological concepts (Tomberlin et al. 2012a). For example, dispersal is a key trait that determines species ability FIG. 4. The spatial and temporal dimensions of the necrobiome among multiple necromass patches introduce an additional level of dynamics important to understanding ecosystemscale processes. (1) Each item of necromass decomposes from fresh (F) to dry (D), and the attendant necrobiome changes in diversity and composition through time.
(2) This creates time windows for colonization and dispersal by specialist decomposers and their predators as they search for the next similar decay stage. (3) Multiple necromass resources occur at any one time and generate a range of decay states with different necrobiomes. (4) This creates a shifting spatial mosaic of necrobiome patches over time, each with unique levels of diversity, composition, and types of interactions and processes. to search and colonize different forms of necromass (Tomberlin et al. 2011) but has not been widely examined among carrion-associated arthropods (Barton et al. 2013b). Additionally, the feeding traits of species are closely related to specialization and arrival dynamics (Barton and Evans 2017), can be associated with tissue quality (Ulyshen 2016), and drive competition and interactions among insects and microbial species (Burkepile et al. 2006). These complex interactions form ecological networks upon which ecosystem functions rest. There is a need for a unified ecological theory building upon the previous knowledge from plant and animal necromass across ecosystems to more comprehensively understand decomposition ecology.

IMPLICATIONS AND CONCLUSIONS
The proposed necrobiome framework provides a general template to conceptualize the structure and function of decomposer communities. A set of questions naturally arises from the necrobiome framework and its spatial and temporal dimensions. For example, what is the functional outcome for decomposition by adding or subtracting different biotic components of the necrobiome? What is the effect of altered interactions in the presence or absence of these biotic components and are the responses consistent among necromass types? Landscape consequences extend from these localized questions, such as, how does altered necrobiome structure and function lead to altered rates and pathways of genomic, element, and nutrient movement through ecosystems? Manipulative experiments have shown that excluding insects from carrion (Payne 1965, Pechal et al. 2014a) and plant matter (Heneghan et al. 1998, Wall et al. 2008, Ulyshen 2014, Stoklosa et al. 2015 resulted in changes to necromass loss rates. But what about experiments that change various functional components, or enhance fragmentation by insects, or manipulate the internal or external microbial communities, such as excluding detoxifying bacteria? Necromass varies along important gradients of nutrient quality, size, and digestibility, and decomposition progresses as a function of the attendant necrobiome and surrounding abiotic template. Clarity in our understanding of these controlling parameters means that future research on decomposition can soon be standardized across resource types. Future experimental studies that control for these parameters can then focus on manipulating key functional groups or interactions within the necrobiome to reveal the consequences for decomposition, food webs, and ecosystem services. The necrobiome concept and the further development of decomposition theory have implications to disciplines ranging from ecosystem restoration to forensics. In restoration ecology, new comparative work could examine the effects of manipulation of plant and animal necromass to return key processes critical to decomposition and nutrient cycling and broader ecosystem-scale functioning. Using the necrobiome framework, functional and biotic components can be identified that enhance or slow nutrient cycling, change community network stability, generate ecosystem heterogeneity, or add biodiversity per se. In the field of forensics, insects are the primary source of evidence used by practitioners for death cases requiring legal action. Although the dynamics of forensically important insects are rooted in ecological theory (e.g., community assembly, succession, priority effects, metacommunities, perturbation impacts, biodiversity, and patch dynamics; Benbow et al. 2015a), there must be an expansion of this basic knowledge and potential use to other constituents of the necrobiome (e.g., microbes). The necrobiome concept provides a strong conceptual basis to further both theoretical and empirical endeavors.