Humans and their predecessors have accelerated the loss of Earth’s biodiversity into a sixth global mass extinction. Paleontologically, it is a new phenomenon for a mass extinction to be attributed to a single species, but the evolution and expansion of humans has created unique circumstances. Whereas behaviors associated with the human enterprise can be tied to specific extinctions, the global pattern of biodiversity loss is clearly linked to the extraordinary growth of our population’s size and density. The prospects for continued losses of plant and animal species remain likely if the growth of the human population goes unabated.

Keywords: Mass extinction, human population density, Pleistocene, Holocene, conservation

1. Introduction

It was 150 years ago this month when Charles Darwin published On the Origin of Species by Means of Natural Selection. In that great work, one of Darwin’s many, he postulated what has since become modern biology’s understanding of the origins of biodiversity by means of natural selection. He put natural processes as the key to biological innovation. Yet, Darwin (1859: 317) also wrote: "On the theory of natural selection the extinction of old forms and the production of new and improved forms are intimately connected together." In other words, extinction is a normal part of the evolutionary process that generates biodiversity. On the other hand, today we see worldwide extinction writ large for at least the sixth time since life as we know it began on the planet we call "home." Observable to conservation biologists, and gauged by paleontologists, there is little doubt that the contemporary loss of species biodiversity has been accelerating into a mass extinction, and the threats continue to mount from human agencies. It can also be argued that biodiversity is being diminished at genetic and population levels, and as Darwin (1859: 319-320) presciently noted, "rarity precedes extinction; and we know that this has been the progress of events with those animals which have been exterminated, either locally or wholly, through man’s agency."

Estimates of contemporary extinction rates vary. Such estimates range from a loss of 0.6% of all species per decade, up to 30% per decade (Stork, 1997). Most estimates hover around a loss of 8% of all species per decade. Any one of those estimates constitutes a massive and rapid extinction event. To imagine the impact of those numbers, we can add a temporal perspective. Let’s assume that the lowest estimate (0.6%) is correct. Using a biodiversity estimate of 12.5 million species on earth, that would mean a loss of about 21 species every day! Even if we had only a net loss of one species every day, the extinction rate remains too high: all life forms would be extinct in less than 33,000 years – a mere fragment of geological time. The basic principle behind this stress on our planet was summed up just over 200 years ago, even as the very concept of “extinction” was still under debate (Mayr, 1982).

“Through the animal and vegetable kingdoms Nature has scattered the seeds of life abroad with the most profuse and liberal hand; but has been comparatively sparing in the room and the nourishment necessary to rear them. The germs of existence contained in this earth, if they could freely develop themselves, would fill millions of worlds in the course of a few thousand years. Necessity, that imperious, all-pervading law of nature, restrains them within the prescribed bounds. The race of plants and the race of animals shrink under this great restrictive law; and man cannot by any efforts of reason escape from it.” -Malthus (1758: 2).

The words and works of economist Thomas Robert Malthus (1758) greatly influenced Darwin in his formulation of the hypothesis of evolution through natural selection (Darwin, 1859). Malthus had been mainly focused on human population growth, but the ability of any population to overgrow finite resources, and the checks and balances that necessarily ensue, has become a fundamental tenet of modern biology.

Resources are finite even on a planet-wide basis for our growing human population, and that has consequences beyond the adaptations of our own species to the unsustainability of others. Currently the human population has a net gain of over 213,000 people per day (U.S. Census Bureau). Meanwhile other plants and animals are going extinct at a rate that can only be estimated; but of known mammal and bird species, 15.3% are threatened with extinction (IUCN, 2009). The question then becomes if this pair of distinct phenomena are related, either directly or indirectly. Does the Malthusian “great restrictive law” accelerate extinctions of other species as our human population continues to rapidly grow? Such questions can be addressed from a variety of perspectives, both past and present.

2. Human Predecessors, Humans, and Mammalian Extinctions of the Pleistocene and Holocene

Paleontologists have the tools to assess the impact of humans and their antecedents on biodiversity from a long term perspective. It becomes clear that in Africa, starting about 1.8 million years ago, following the origin of our evolutionary predecessor known as Homo erectus, large African mammal species started to go extinct at an increased rate (McKee1995, 2001, 2003a; Behrensmeyer et al. 1997), and biodiversity of mammals declined. Klein (2000) further documented the correlation of the expansion of our genus across other continents, with the extinction of other entire genera. It is most noticeable in North America, with the extinction of 27 species comprising 8 genera since the arrival and expansion of modern human beings. Although many have argued, and some still argue, that the end of the last glacial phase and associated environmental changes accelerated the demise of North American megafauna, Alroy (2001) makes a cogent argument that the growing human population on the continent was a more proximal cause.

It is not just in North America that the Holocene witnessed increased extinction rates of mammals. In both East Africa and South Africa, prior to 1.8 million years ago when the mammalian biodiversity deficit began, every 100,000 years we would see the extinction of four large mammals, balanced out by the evolutionary origin of four large mammals (McKee 1995, 2001, 2003a). But in the past 10,000 years, southern Africa alone has seen the extinction of at least 16 mammal species, 9 of which have gone extinct in historic times (Klein 2000). It is that magnitude of extinction rate increase that warrants the claim of a sixth mass extinction in the world today.

Yet it is still not clear that ancestral and modern human population growth alone were the main drivers for the correlated extinctions in Africa and beyond. For example, the origin of Homo erectus not only brought increased pre-human population size (as judged by the rapid spread of the species in Africa and western Asia), but it also brought an increased body size (Ruff & Walker 1993) which would require more resources, and an increased reliance on hunting over scavenging and foraging. It is thus difficult to dissect out the relative effects of population growth versus niche expansion.

The questions of our distant past bring us to more contemporary issues. Was it the sheer quantity of humans and their predecessors that led to the dramatic increases in mammalian biodiversity loss, or was it behavioral changes, first hunting, much later agriculture, that diminished finite resources for other living beings? This translates into the contemporary question of the mass extinction we are now continuing to witness. Is it the rapidly growing size of the human population, our human behavior, or both, that are responsible for the current demise of other species of animals and plants? A further option, that we are not culpable in the least, seems to be off the table already.

3. Contemporary Human Population Growth, Behavior, and Animal Extinction

There are sound theoretical reasons and considerable evidence suggesting that a close relationship exists between human population size and biodiversity losses. Increases in population size and density have caused rapid landscape and ecological changes that restrict the habitats of wild plants and animals. Contemporary terminal extinctions are difficult to document, but analyses of the population size effect can be made on the basis of known “threats” to extant species. Again, the species is a convenient unit of analysis, though genetic and ecosystem biodiversity are also important variables to consider.

In order to explore a broader view of current trends, McKee et al. (2004) analyzed data on threatened species per nation, comprising critically endangered, endangered, and vulnerable species of mammals and birds from the IUCN Red List (based on threats in 2000). Data from continental nations, excluding exceptionally small nations, was also compiled on human population densities and “species richness” – defined for our analyses as the number of known mammal and bird species per unit area. A stepwise multiple regression analysis determined a mathematical model that explained 88% of the variability in current threats to mammal and bird species per country on the basis of just two variables: human population density and species richness (Fig. 1). Clearly “species richness” is not the root cause of the threats – these diverse ecosystems persisted through climatic changes and ecosystem shifts over many thousands of years. That leaves the other variable in the equation, human population density, as the likely factor leading to globally increased species threats. In short, the greater concentration of species set the stage for human impact to be more intense.

Figure 1. Predicted versus observed threats to mammal and bird species by nation as a function of human population density per nation and species richness. All variables were log transformed.

One must ask if our increased population density is the root cause, or a spurious correlation that masks the proximal effects of human behavior. Certainly, one can assume, there must be some effect from what many ecologists often refer to as the “ecological footprint” – the effect each individual or group has in terms of resource consumption (Wackernagel and Rees 1996; Chambers et al. 2000). This is manifested in many ways – fuel consumption, deforestation, habitat destruction and fragmentation, fresh water usage, global warming, pollution, and more. There are direct correlates of the “ecological footprint” with depletion of both renewable and nonrenewable resources. Are they also related to biodiversity losses?

Such questions can be teased from the global data by adding variables to the model and testing further hypotheses. One measure of some aspects of the “ecological footprint,” for which data are generally available, is per capita Gross National Product (Purchasing Power Parity, www.unesco.org) . Previously, McKee (2003b) found that whereas there is a strong correlation between species threats and human density, the threat has virtually no correlation with per capita GNP. The effects of affluence on threatened species originally appeared to be overshadowed by our sheer numbers.

In order to address these issues further, McKee and Chambers (in press) re-analyzed the data, again looking at GNP, but this time considering GNP per unit area. An interesting, albeit complex, picture emerged. A strong and statistically significant positive correlation between GNP and threatened species came into focus (R² =0.443, p<0.001). This correlation is evident in the scattergram of Fig. 2. By comparison, human population density alone was a slightly lesser predictor of species threats (R²=0.402, p<0.001).

Figure 2. The relation of threatened mammal and bird species by nation (per area) and Gross National Product (per area).

On the other hand, a stepwise multiple regression analysis in which GNP per unit area was added to the variables of the McKee et al. (2004) model, left us with the same model: human population density and species richness were the better predictors, to the exclusion of GNP. Part of the reason for this counterintuitive result is that GNP is positively correlated with species richness (R²=0.445, p<0.001). Perhaps the high primary productivity of these areas drives animal diversity as well as economics – but from a statistical perspective, the overlap of GNP and species richness explains some of the same variability in contemporary threats to species of mammals and birds.

Given the archaeological association of the origins of agriculture and extinctions of many mammal species during the Holocene, it is also instructive to look at contemporary correlations between agricultural land use and species threats. We found a statistically significant positive correlation (R²=0.187, p<0.001). This correlation is weaker than that of either GNP or population density. Then again, population density and percentage of land devoted to agriculture are correlated as well (R²=0.654, p<0.001). Thus the question arises as to whether the correlation reflects the direct effect of agriculture usurping the resources of other species, or is agriculture simply a mediator of a human population density effect?

Adding agricultural land use into the stepwise multiple regression analysis, we found that it does add a small but statistically significant component to the model predicting nation by nation species threats (McKee & Chambers, in press.) It explains some of the variability that other variables, including GNP per unit area, do not, thereby increasing the predictability of the model from 88% to 89%.

In summary, numerically speaking, once all of the variables used in this analysis are taken into account, species richness, human population density, and agricultural land use are the best combined predictors of threats to species of mammals and birds. GNP per unit area, while strongly correlated with species threats, does not add to the predictive ability of the model. These results, combined with long-term observations of the human impact on mammal species, led us to argue that human population density is a primary cause of biodiversity losses, in a large part mediated by agricultural land use, and thus is a key factor that must be addressed in order to reduce future threats to Earth’s biodiversity.

4. Discussion

In order to stem the dramatic rate of biodiversity loss, the issue of human over-population must be at the forefront of conservation strategies. Does this mean that traditional conservation is unimportant or that we need not concern ourselves with the magnitude of our ecological footprints? Absolutely not. Clearly conservation is more important than ever, as human behavior is a mediator our population’s effects on other species of plants and animals. If all of the peoples of the earth accelerated to a modern western style of living, then certainly the treats to biodiversity would be greater.

The point is that even if we all live a much more modest lifestyle and utilize significantly fewer resources, Earth’s biodiversity would still be under threat from our accumulating numbers alone. Without addressing the human over-population issue, all other measures will eventually be for naught.

The model based on human population density and species richness alone is very telling. Using the regression model with projected population sizes of each nation, we found that, all else being equal, the number of threatened species in the average nation is expected to increase 7% by 2020, and 14% by 2050, as forecast by continued human population growth alone (McKee et al. 2004).

How much of a reduction in human population would it take before every country in our data base demonstrated a reduction in threats to at least one or more species of mammals or birds? The answer is that a reduction to 57% of the global population in 2000, i.e. to approximately 3.4 billion, would accomplish this goal (McKee 2003b). As that is not likely to happen soon, threats to biodiversity will clearly mount, making human population reduction through declining birth rates an urgent priority.

In Darwin’s (1859) oft cited conclusion to On the Origin of Species, he noted the “grandeur” of the evolutionary view of life. Yet the grandeur is being severely diminished by a single species that has experienced unparalleled success in mastering nature. It is imperative that such mastery move toward greater human stewardship and responsibility, including responsible rates of reproduction.