Population, population, and population

According to their note about themselves, “Eric R. Pianka and Laurie J. Vitt are hard core/longterm university ecologists. ... They wrote a prizewinning coffee table book ‘Lizards: Window to the Evolution of Diversity’ ... Pianka was awarded the Eminent Ecologist Award by the Ecological Society of America in 2015. Vitt is an elected member of the Brazilian Academy of Sciences.” (P. 237; page numbers without further identification refer to pages of the book under review.)

PV echo simplified versions of Malthus's thought. Experience contradicts both Malthus and PV. Contrary to PV's claim that "increased food results in production of more offspring, which in turn reduces the per capita amount of food available,"(P. 9) the global daily per capita food supply (kilocalories per person per day) rose over the decades after the Green Revolution in the early 1960s ( Fig. 1) as the global total fertility rate (TFR, defined as the number of children per woman per lifetime at then current age-specific birth rates) declined more dramatically than ever before in history (Fig. 2). Increased food allowed children to survive, and parents felt less need to have many births to assure that some children would survive. Food does not lead population in today's world, whatever food may have contributed to population growth in past millennia.
Contrary to PV's claim that if food increases, "quality of life goes down for each individual," (P. 9) life expectancy rose in every region as food supplies increased and fertility fell (Fig. 3). Net enrollment rates of primary and secondary education increased enormously (Fig. 4). These changes are tremendous improvements in the quality of life for individuals. PV claim that "… even the human component of climate change can be traced directly to population." (P. 10). Data do not entirely support the claim. In 1900, Earth had about 1.6 billion people (with a margin of uncertainty probably around 10%), and in 2000, Earth had about 6.1 billion people (with a margin of uncertainty around 2-4%). So population increased ~6.1/1.6, or 3.8-fold in the 20th century. In 1900, annual total carbon dioxide emissions were 1.95 billion tons, in 2000, 24.6 billion tons, an increase of ~24.6/1.95, or 12.6-fold (Fig. 5;Ritchie and Roser 2019). The increase in global human population numbers by less than fourfold cannot account for the increase in CO 2 emissions by more than 12-fold. Similarly, world population grew from 6.1 billion people in 2000 to 7.5 billion people in 2017, an increase of 23%, while annual total carbon dioxide emissions grew over the same interval from 24.6 billion tons to 36.2 billiontons, an increase of ~47%, more than twice the rate of increase in global population size. Economic, environmental, and cultural changes share the responsibility with demographic changes for growth in CO 2 emissions. The relationships of population size, growth, and composition to greenhouse gas emissions are subtler than this simple aggregate analysis reveals. For example, a population's age structure (proportions of old people versus young people) and distribution between urban and rural residences substantially affect energy use and CO 2 emissions. More disaggregated analyses (Cohen 2010b, O'Neill et al. 2010, Casey and Galor 2017, Lutz 2017, Scovronick et al. 2017 confirm that attributing the entire human component of climate change to changes in global population size is not defensible.

PV
PV repeat a line of argument familiar from elementary ecology texts: "If any small population of organisms is given an order of magnitude more food and space than they need, and all of their predators, diseases, and parasites are eliminated, they will reproduce and their population will increase in size exponentially until most of the food is used up and most space taken. … absolute time is irrelevant." (P. 15) PV give the example of the 29 reindeer released on St. Matthew Island off Alaska on 20 August 1944. "Herds grew exponentially (just like the human population has been growing) …" (P. 16) The 6,000 reindeer on the island by summer 1963 fell to fewer than 50 during the winter. "We are facing such an imminent population crash. Human populations have grown exponentially over the past several centuries, nearly doubling each generation. As of April 22, 2019, at 8:15 AM, the global population is 7,698,993,458 and has grown by 77,359 since midnight." (P. 17)

Comment
A continuous positive quantity, such as population size or population density (number of individuals per unit of area or volume), is defined to grow exponentially if and only if its rate of change (defined as its change per unit of time, divided by its initial size) is constant over time at some positive value. In applying this mathematical concept to real populations that consist of individuals, like reindeer or humans, it is necessary (and customary) to approximate discrete counts of numbers of individuals by continuously varying real numbers. This approximation is reasonable for large populations enumerated in millions or billions of individuals, but is less useful for small populations with stochastically fluctuating numbers of individuals. If a continuous positive quantity grows exponentially, then the time it requires to double in size (its' doubling time) is also constant. Conversely, if an increasing positive quantity has a constant doubling time, it is growing exponentially. Thus, a constant doubling time is a necessary and sufficient condition for any increasing continuous positive quantity to grow exponentially.
PV claim: "Human populations have grown exponentially over the past several centuries, nearly doubling each generation." (P. 17) On the contrary, the estimated global human population doubling times over the last five hundred years have been very far from constant (Fig. 6).
Estimated (ranges of) global human population sizes over the last 12,000 yr also do not confirm PV's claim. The global human population size at the beginning of the Holocene ca. 10,000 BCE has been estimated at 4 million people (Cohen 1995, p. 400, gives original sources) but cannot realistically be known with more precision than an order of magnitude. So let us say the population fell between 1 million and 10 million. Estimates of the global human population size at 1500 CE range from 425 million to 540 million (Cohen 1995, p. 400, gives original sources). Thus, the maximum average annual growth rate over the intervening 11,500 yr is (1/11,500) In(540/1) = 0.055%/yr, that is, just over onetwentieth of one percent per year. The minimum average growth rate over the intervening 11,500 yr is (1/11,500) In(425/10) = 0.033%/yr, that is, just under one-thirtieth of one percent per year. So, despite the 10-fold uncertainty in the initial population size 12,000 yr ago and 27% uncertainty in the population size in 1500 CE, there is less than a factor of 2 uncertainty in the average annual growth rate. The estimated range from 0.033%/yr to 0.055%/yr brackets nicely Roser's (2019) estimated growth rate of 0.04%/yr (1/25th of 1% per year) between 10,000 BCE and 1700 CE. The global population was ~2.5 billion in 1950 and 4.0 billion in 1974, giving a growth rate of ~2%/yr. This growth rate is approximately 50 times the estimated growth rates from 12,000 yr ago to 1500. The estimated growth rate of the global population currently is ~1.1%/yr, half the growth rate of the global population just half a century ago.
Despite the uncertainties in the population estimates, it is not credible that the global human population grew exponentially since the end of the last Ice Age, or over the last few centuries, or ever. Until the middle of the 1960s, the global human population grew super-exponentially, that is, with an increasing annual growth rate (apart from catastrophes like the Black Death in the 14th century CE and the Great Leap Forward in China 1959China -1961. Since the middle 1960s, the global human population has grown sub-exponentially (with a dramatically but erratically falling annual growth rate).
The weaker claim that individual regions or continents grew exponentially over some time period is not consistent with exponential global population growth unless, magically, the growth rates of all regions or continents were identical. If multiple regional populations grew exponentially, each with a different positive growth rate, the global population (the sum of exponentials) would grow superexponentially with a time-dependent growth rate increasing toward the maximum of the regional growth rates.
Contrary to PV's claim that "Human populations have grown exponentially over the past several centuries, nearly doubling each generation," (P. 17) the 20th century was, and will be, the only century in recorded human history in which the global human population doubled. In fact, it nearly quadrupled from 1.6 billion in 1900 to 6.1 billion in 2000. No earlier century had a high enough growth rate to double the population within a century, and it seems very probable that no future century will ever again have a high enough growth rate to double the population within a century. The twentieth century was and, most likely, forever will be unique in human demographic history as the only century in which global population doubled or tripled within a human lifespan. According to PV, "As of April 22, 2019, at 8:15 am, the global population is 7,698,993,458." (P. 17) This precision is pure science fiction, even if it is (very unfortunately) sponsored by the United States Census 1 and others 2 (both examples available online). A more honest statement would be that the world's population in 2019 lay somewhere between 7.5 billion and 7.9 billion, with a probable uncertainty of 2-3%. Even the percentage of uncertainty is itself uncertain. Most censuses occur once in 10 yr. Not all countries have recent censuses. Not all censuses are accurate to within 2-3%. Claims to estimate the Contrary to PV's claim that "absolute time is irrelevant," (P. 15) absolute time is enormously important to human well-being. A population that doubles in fifteen to seventeen centuries, as during most of the time from the dawn of agriculture to the European invasion of the Western Hemisphere starting ca. 1500 CE, has time to adapt economically, socially, politically, culturally, technologically, and environmentally to a very gradual, and doubtless fluctuating, increase in population size. A population that doubles in a generation, as from 1950 (2.5 billion) to 1987 (5 billion), stresses every human capacity of recognition and response. Absolute time matters.

PV
PV state: "Somehow it has become politically incorrect even to allude to overpopulation. Not wanting to face reality, people are locked in denial that such a problem could even exist. And yet, population pressures clearly underlie and drive all of the many challenges we face, from energy, food, and water shortages to political unrest and climate change." (P. 18) Again, "overpopulation … has actually become politically incorrect." (P. 19)

Comment
PV object to lack of attention to "overpopulation" but never define it. Does it mean too many of other kinds of people? Are Pianka and Vitt exemplars of "overpopulation"? Am I? Are you? What is it? How do we know it when we see it? PV do not say.
Population issues occur widely in current political arguments. Examples include international migration, population aging and Social Security, life expectancy and the opioid epidemic, abortion, contraception, unintended pregnancy, patterns of settlement in areas prone to floods or fires, and responsibilities for the care and feeding of children and the elderly.
While PV claim that "population pressures clearly underlie and drive all of the many challenges we face, from energy, food, and water shortages to political unrest and climate change," (P. 18) the quantitative evidence suggests that population size is a contributing, but not the only, factor. The analysis above showed that population growth could account for half or less of the growth of carbon dioxide emissions. Global freshwater withdrawals for agriculture, industry, and domestic uses were 671 cubic kilometers in 1901 and 3,790 cubic kilometers in 2000 (Ritchie and Roser 2018), an increase by a factor of 5.6, while global population rose by a factor of 3.8. Yes, the growth in the human population accounts for part of the increase in water withdrawals. No, population growth did not drive it all. Average water withdrawals per person increased.

Comment
In 1995, I reviewed dozens of verbal definitions of "carrying capacity," which PV never define, and more than 65 quantitative estimates of how many people the Earth could support (Cohen 1995). The estimates ranged from fewer than 1 billion to more than 1,000 billion people. Van Den Bergh et al. (2004) analyzed 69 estimates of "a limit to the world population" (also not uniquely or operationally defined). Their estimates ranged from 0.65 billion to 98 billion people. These estimates rest on diverse and often unspecified or inadequately specified assumptions and methods. PV cite neither review and take no account of the very large uncertainties of definition and calculation that surround the elusive concept of "Earth's estimated carrying capacity" (P. 19).
PV cite no data to support their two claims, first about "all ecologists" and second about "all of the existing data" when they claim that, "as all ecologists know and all of the existing data show, we are heading for a massive population crash." (P. 19) Leridon (2020, p. 4) gives a different assessment based on facts: "World population growth started slowing in the 1960s, and the trend should continue over the coming decades. The risk of 'explosion' is behind us; two-thirds of the growth expected between now and 2050 will be attributable to the current population age structure. This slowdown could accelerate if fertility in sub-Saharan Africa falls more quickly. This is not impossible, but sub-Saharan fertility is already projected to fall from 4.72 children per woman to 3.17 within 3 decades under the UN mediumvariant projection and even to 2.67 under the low variant. Unless the entire world is hit by severe disasters on an unprecedented scale, with a devastating impact on human life before 2050, it is difficult to imagine a world population much below 9 billion in the middle of this century." PV PV claim: "Food, land, and water constitute a zero sum game: per capita shares of all these commodities are decreasing continually as human populations increase. … But each year, the human population increases by nearly 100 million …" (P. 21) "Certainly, if our population continues to double in the next few decades as it has during the past few, we will finally have reached 14 billion by about the year 2050 …" (P. 22)

Comment
Food, land, and water do not constitute a zero sum game. Food per person available daily to households increased, on average, as population increased (Fig. 1). Water is indestructible except for electrolysis, but it can be polluted or it can be treated after one use to make other uses possible. Protecting the watershed of a city like New York by preserving the forests in areas around water-supply reservoirs has increased the availability and lowered the cost of water as population increases, relative to technological alternatives. China and Japan have reclaimed land from the sea since the 19th century, usually for high-end consumption, despite the considerable environmental and social costs (Shepard 2018), and Sri Lanka, Malaysia, the Philippines, Cambodia, Dubai, Qatar, Singapore, and other countries have followed the practice.
Despite PV's claim that "each year, the human population increases by nearly 100 million …" (P. 21), the truth (Leridon 2020, pp. 2-3) is that "At the global level, annual births currently outnumber deaths by 83 million (140 million births minus 57 million deaths on average over the period 2015-2020)." Based on multiple stochastic projections that model the variability of past trends in births, deaths, and migrations, the UN Population Division (2019) gives a 95% confidence interval of global population size in 2050 from 9.4 billion to 10.1 billion. The 95% confidence interval for 2100 ranges from 9.4 billion to 12.7 billion. Both are far below PV's 14 billion for 2050 (see also Leridon 2020).

PV
In a paragraph that begins "Bottom line:" in bold face plus underlining, PV write: "To increase the average qualify of life, the number of people on earth must be reduced. This presents two obvious options: (1) we can take logical steps (education, birth control, etc.), which will take several generations, or (2) resource wars, famine, and rapid spread of infectious disease will do it for us." (P. 24) "Sadly, our enormous population, now well above the level Earth can support, must soon crash, accompanied by famines and massive and widespread human misery. … The major reason China was able to turn the corner and is positioned to become the next super power in the world is because [sic] they have a police state and they could force their population to control reproduction." (P. 26)

Comment
More than half the people in the world live in countries or areas where fertility is below the level required to replace the existing population in the long run. China's TFR, averaged over 5-yr intervals, fell from 6.30 children per woman in 1965-1970 to 3.01 children per woman in 1975-1980. After this dramatic fall in fertility, China introduced its "one-child policy" in 1979 and it took effect nationally in 1980. By 1990-1995, China's TFR fell to 1.83 children per woman, well below replacement level. China ended its "one-child policy" in 2015, after 36 yr. After that policy ended, China's TFR in 2015-2020 was estimated at 1.69 children per woman, higher than Europe's 1.61 children per woman but lower than the United States' 1.78 children per woman in 2015-2020 (United Nations Population Division 2019). Forced control of reproduction explains the low TFR in neither Europe nor the USA, and probably not in China today.
Population fell by 1% or more in 27 countries between 2010 and 2019, due to both emigration and low fertility (United Nations Population Division 2019). When the government of Iran promoted rural development, education for girls, and free family planning in the late 1980s, the total fertility rate of Iran dropped in twenty years (from the early 1980s to the early 2000s) from 7.0 births per woman to below replacement (Hosseini-Chavoshi and Abbasi-Shavazi 2012). The interval required for a country's total fertility rate to drop from above six children per woman to fewer than three children per woman has dropped from 95 yr (in the 19th-century United Kingdom) to 10 yr (in Iran, 1986(in Iran, -1996 (Fig. 7). Several generations are not necessarily required for economic development, education, and voluntary contraception to lower fertility below replacement levels.
According to 2020 estimates of the CIA World Factbook 3 (available online: see section below on "Sources of credible information about human populations"), 12 countries have TFR above five children per woman, and they are all relatively low-income countries in sub-Saharan Africa. Investments in economic development, education, reproductive health, and voluntary contraception could help those countries to lower fertility to the extent desired by the people in those countries. This strong claim is backed by long-term, well-documented experimental evidence from the Matlab district of Bangladesh (Ezeh et al. 2012).

Editorial quality
This self-published book has no index. It gives several indications that it was not edited.
It is highly repetitive in scattered ways. For example, the metaphor of Ponzi schemes appears on pages 10,14,15,18,27,32, and there I stopped keeping track. On page 28, "Mitt Romney's family of five is average for Mormon families. Supreme Court Justice Scalia shrugged off his 9 children with 'I'm a Catholic' as if religion somehow justifies outrageously large families." On page 58, "When asked about his large family, Scalia's response was 'I'm a Catholic' as though that justified his breeding." On page 60, in PV's view of the end of the human species, among the last survivors "many will be Catholics or Mormons and many will carry surnames like Duggar, Romney, and Scalia!" Apart from being repetitive, a more serious problem is that such inference based on anecdote can misrepresent reality. Roser (2017) gives a data-based summary: "… in countries where more children Fig. 7. Interval (years) for TFR to fall from above 6 to below 3 children per woman per lifetime at current agespecific birth rates. Our World in Data CC BY.
survive, fertility is lower. Which religion dominates in a country has no clear relation to the fertility level-and even if it does have some importance the correlation is much weaker than that [of the fertility level] with the health of children. Countries with a majority Christian population have fertility rates as high as 6 (DR Congo) and as low as 1.25 (Portugal) children per woman. Across countries fertility rates vary within and not between religions. And what is true between countries is even more obvious for the change over time. Religious background cannot explain the rapid change in the level of fertility … In Catholic Italy the fertility declined from 2.5 [children per woman per lifetime, TFR] in 1966 to 1.2 at its lowest rate in 1997, and in Muslim Iran the fertility declined from 6.5 children per woman in 1982 to 1.8 in 2005! … it would be wrong to say that religion has no importance for the number of children women have. … everything else being equal, religious people have more children, so that religion matters for differences at the same socio-economic level. Still, the differences between religions within the same country are much smaller than the differences between different countries in different socio-economic conditions." In chapter 1 alone, references are missing for, or inconsistent with the citations to, Yong 2016 (P. 16), Hoffer 2010, Vitousek 1997, Wackernagel et al. 2002, 2004 (all four on P. 19), Trivers 1991 (P. 20), Global Footprint Network (P. 23), Lester Brown 1995 (Pp. 26 and 27), and Lincoln (P. 31).
PV write that dividing the area of Texas "by the current human population of 7.7 billion leaves each person with about 1000 square feet, a small plot the size of a big room about 33 ft × 33 ft. Sounds plausible enough, right?" (P. 22) Except that 33 × 33 ft = 1,089 square feet and the square root of 1,000 square feet is 31.62 feet.

Sources of credible information about human populations
Thanks to the Internet, an unprecedented richness of usually reliable data is freely available, along with a great deal of ill-informed nonsense, venom, cant, and rant about population. Here are some usually credible sources (Table 1).

Population problems
A useful book on world population problems would focus on today's real problems and skip apocalyptic pronouncements about imminent or inevitable collapse. Here are some real problems. (To avoid prolonging this review, I list them mostly without documentation, in the spirit of PV. Exercise for students: pick one or two sentences in the following paragraph and find relevant data and documentation among the sources above or elsewhere. Hint: start with ourworldindata.org.) Approximately 10% of people live on less than $1.90 per d, measured in dollars corrected for inflation and purchasing power parity. Economists define such low income as "extreme poverty." More than 800 million people (roughly one person in nine) are chronically hungry: They eat too few calories for normal activity and growth. Among the world's children under the age of 5 yr, 22% are stunted (excessively short for their chronological age, when compared with well-nourished children of the same age) as a result of chronic undernutrition. Refugees and migrants have the right to flee their country but not the right to enter anywhere else. An estimated 40.3 million people are enslaved (https://www.cfr.org/inter activ es/modern-slave ry/#!/secti on2/item-5). Migrants and minorities are mistreated in many countries.