THE DYNAMICS OF PLANT POPULATIONS: DOES THE HISTORY OF INDIVIDUALS MATTER?

Abstract

Historical events have been used to explain a wide range of phenomena including geographical distributions of species, community diversity, and population structure. At the level of individuals, historical effects in which past conditions influence future performance are particularly likely to occur in long‐lived organisms that store resources between seasons and that form organs months or years before their elaboration. Such carry‐over mechanisms have been documented in several perennial plant species, but the implications for population processes are poorly known. In this study, I examine how the history of individuals influences their future performance, population dynamics, and life cycle structure in the long‐lived herb Lathyrus vernus. Overall effects of plant history on population dynamics, in terms of growth rate, reproductive values, stable stage distribution, and elasticities, are examined by comparing an ordinary first‐order matrix model with a second‐order matrix model. In the latter, not only the present state of individuals, but also their past state is allowed to influence future fate.

The results demonstrate that the history of individuals is sometimes important in models of population dynamics. Plant size change over a one‐year period was negatively correlated among time intervals. Addition of the previous year's stage in population models shifted the growth rate from positive (λ = 1.010) to negative (λ = 0.986) and increased the proportion of small established individuals in the stable stage distribution. If historical effects are due to a capacity to buffer environmental variation and regain size or state, as in L. vernus, then recruitment contributes less and stasis more to population growth than suggested by ahistorical models. The presence of historical effects at the level of individuals, in any form, may have important consequences for population development and should be included in any interpretation of the life‐cycle structure.