Jeffrey McKee, Erica Chambers, and Julie Guseman, “Human Population Density and Growth Validated as Extinction Threats to Mammal and Bird Species,” Human Ecology, vol. 41, iss. 5 (October 2013), pp. 773–78. DOI: https://doi.org/10.1007/s10745-013-9586-8. This paper was collaboratively assessed.
Both the density and growth of human populations are strongly correlated with increases in the number of threatened bird and mammal species, this paper concludes. The authors predict that the average country in which human population is growing will experience a 3.3 percent increase in the number of threatened species from 2010 to 2020. Countries with declining human populations between 2010 and 2015 can expect a 2.5 reduction in the number of threatened species on average during that period, according to the analysis. (The term “threatened” applies to animal or plant species likely to become endangered—vulnerable to imminent extinction—due to loss of habitat or diminishing numbers of individuals.)
Countries with declining human populations between 2010 and 2015 can expect a 2.5 reduction in the number of threatened species on average during that period.
The paper ambitiously aims to “test the veracity of the hypothesis that there is a . . . causal link between human population density and threats to species of mammals and birds.” The authors provide a graph of the number of new threatened species that their model predicts by 2050 in each of the 114 countries for which data were available
The researchers worked with national population data from the U.S. Census Bureau as well as lists of threatened birds and mammals from the United Nations and two conservation organizations for the countries assessed. Islands and very small countries were excluded from the analysis out of concern that these data would skew the study results artificially toward a population impact on species survival. The authors conceded limits of their methodology and model, but argued that these best suited the available data.
The model tests were based on comparing changes in numbers of threatened species in each country from 2000 to 2010 and seeing how these correlated both with human population size and rates of human population growth in each country over the decade. Although the relationships were scarcely linear, they came closer than other tested correlations, such as with gross domestic product (GDP). Moreover the authors found no “threshold effect” from growing human population density; even countries with significant numbers of threatened bird and mammal species saw increases in these numbers as human populations rose. The strong correlations identified “establish a degree of certainty that human population density is a key ultimate cause, and probably in many places a proximate cause [i.e., the one closest or most direct] of species of mammals and birds becoming threatened with extinction,” the authors conclude.
“Yet human population density is demonstrably at the core of extinction threats to mammals and birds.”
Key quotes: “There is no doubt that a multitude of factors go into diminishing the availability of resources that mammals and birds need to survive as viable species . . . . Yet human population density is demonstrably at the core of extinction threats to mammals and birds. . . . The data reveal that if future conservation efforts are to be sustainable, they must not leave human population density out of consideration, and indeed should include them in the forefront.”
FPESA assessments (seven total) were uniformly positive about the paper and its relevance to the primary FPESA hypothesis. Some assessors noted that its accuracy depends on the reliability of the species and population data. (The accuracy of the data on threatened species is less certain than data on national human populations. Species richness among birds and mammals is nonetheless among the most closely monitored indicators in biology.) Several assessments suggested that the focus on the national level or the exclusion of islands and small states are potential weaknesses. Two assessors expressed disappointment that no data were used (or at least described) that would suggest how per capita economic activity or consumption correlate with threatened species. One assessor wondered about the impact of corruption and human mobility. No assessment challenged the paper’s basic conclusion.
This paper stands as strong evidence that current human population density and growth undermine the chances of survival for terrestrial bird and mammal species.
Overall assessment: Assuming reasonable data accuracy, the authors have identified strong correlations that are hard to explain without considering human population density and growth to be directly or indirectly threatening the survival of bird and mammal species. The fact that both the density and growth of human populations strongly correlated with increases in threatened species from 2000 to 2010 adds to the paper’s strength. It is hardly plausible that threats to animal survival are causing human population density and growth. Nor is it easy to imagine some third force simultaneously increasing both human population and the number of threatened animal species. Until better and more geographically detailed data on population and other factors are available, this paper stands as strong evidence that current human population density and growth undermine the chances of survival for terrestrial bird and mammal species.
The following annotation and assessment is excerpted from the forthcoming report of the Family Planning and Environmental Sustainability Assessment (FPESA) project. The report will contain selected annotations from articles reviewed for potential relevance to the hypothesis that family planning supports environmental sustainability. The annotations are written by project director Robert Engelman with input from members of the project team and, where indicated, members of the project’s international network of research assessors.