Human Population Growth

1. Human Population Growth Before we can address specific conservation issues, it is important to understand why conservation problems exist. Factors adversely affecting species are largely human-mediated and are occurring at an extremely rapid and accelerating rate. Here we will address population growth of Homo sapiens, determine where growth is most focused, and identify how human growth patterns correlate with centres of biodiversity ['hotspots'].

2. Population modelers use different methods to assess population growth. Two models that you should already be familiar with are exponential growth and logistic growth. With exponential growth, the population rate of change (r) is constant through time, while growth of the population is geometric.  There is no adverse feedback on population growth rate caused by increasing population size and its concomitant reduction in remaining resources the model:

3. Exponential population growth cannot continue indefinitely because even organisms as small as bacteria experience some form of feedback (e.g. a reduction in rate of fission or an increase in death rate either because of waste build-up or resource depletion). Logistic growth adds this feedback term to the equation of growth rate: where a is an integration constant to define position on curve relative to origin

4. A bacterium growing in a egg-salad sandwich in the hot sun will divide every ~22 minutes; within 10 hours this single bacterium will have produced 1,072,200 progeny. Even so, its growth will eventually show a logistic trajectory. We can draw an analogy between prolific bacteria and human population growth, as well as to its consequences. Human population growth is affected by natality and mortality rates. Throughout our history, mortality rates have kept population growth at a relatively low exponential growth rate of about 0.002% per year. Disease and famine were particularly important.

5. As humans shifted from hunter-gatherer to more modern forms of agriculture, famine became less of a problem. Nevertheless, the population did not achieve 1 billion until around 1800; it took an additional 130 years to hit 2 billion, but only 45 years to double yet again (~1975). The world's population is currently growing at a mean rate of 1.41% per year, down from 1970 when it peaked at 2.07%.

7. Most of the increase has been due to compounding of growth and to lower death rates. One of the highest growth rates observed in recent years was in Kenya (4%/yr), but even here growth rates are coming down (from 7.7 to 6.7 kids per female). As we shall see, growth rates differ dramatically depending on whether the country is affluent (More Developed Countries [MDC]) or poor (Less Developed Countries [LDC]). AIDS and other diseases may impact growth rate statistics in many countries, though particularly in LDC's because many of the infected people are females of child-bearing age.

9. Why do we care so much about population growth? Simply put, each individual has an environmental ‘footprint’. More mouths necessarily mean a greater demand of environmental resources, but so does greater individual use of resources. For example, Postel et al. (1996) estimated that the global human population now utilizes 54% of water runoff that is geographically and temporally available. Of course, access to potable water varies tremendously on a global basis.

10. Construction of dams is projected to increase runoff available for human use by 10% over the next 30 years, but human population growth during this period could be as high as 45%. So, where will the water needed for these people come from? Vitousek et al. (1997) showed that application of nitrogenous fertilizers has increased dramatically since the 1940s; with other forms of human-mediated N-release, there has been a doubling of the nitrogen entering the land-based N-cycle. This increase has a adverse consequences including acid rain, loss of soil nutrients (Ca, K), smog, and eutrophication.

11. Thus, human population growth has very profound consequences for the characteristics of our environment. Our ecological ‘footprint’ has been calculated…

12. Sisk et al. (1994) analyzed the correspondence between two measures of population pressure (growth rate, logging rate) and two measures of biodiversity (number of species and endemism rate in mammals and butterflies).  They then identified countries that fell in the top quartile for one of each of population pressure and biodiversity. Biodiversity is measured by endemism of mammals and butterflies and also by overall species richness and endemism. Population pressure is indicated by annual growth rate and forest loss.

13. Biodiversity and endemism tended to be highest in tropical countries, notably islands. Population pressure as deforestation was most important in countries in Central (Costa Rica, Guatemala, Nicaragua) and South America (Columbia, Ecuador), and Africa (Ivory Coast, Angola, Kenya). Human population growth was highest in eastern countries (Sri Lanka, Philippines, Taiwan, India) and in Nigeria. What follows is series of tables reporting these data.

14. In each case the measure is ranked regionally and globally. Shaded (imperfectly in the scan) lines indicate countries in that top (worst) quartile.

18. In a more recent analysis of the same topic, Myers et al. (2000) reported slightly different results.  They used information on plant species, specifically, they looked at regions that contained a minimum of 0.5% (1500) of the world's plant species as endemic.  They then looked at habitat destruction rates for these regions, and only those with destruction rates >70% qualified as important and at risk.  They identified 25 regions or hotspots of biodiversity.

20. What are the leading hotspots?  Tropical Andes Sundaland (Indonesia) Madagascar, Brazil's Atlantic forest Caribbean islands Each contains at least 2% of total plant biodiversity, or a total of 20% of all plants and 16% of all mammals.  These regions are also among the world's most impacted by human activities.

21. There appeared to be pretty good correspondence between areas that were rich in plants and those rich in vertebrates. For example, areas rich in both plants and vertebrates included the Philippines and various northern African habitats, and the tropical Andes.  Low correspondence was found for The Cape region of South Africa (rich only in plants - fynbos), and SW Australia (rich in Acacia and Eucalyptus plants).

22. In a follow-up paper, Cincotta et al. (2000) showed that by 1995 more than 1.1 billion people lived in the 25 hotspots identified by Myers et al. (2000). This represented about 20% of the world’s population (12% of the world’s surface area) at that time. Population growth rate in the hotspots was 1.8% per year, much higher than the rate for the rest of the world as a whole (1.3% per year), and above that even of developing countries (1.6% per year). Human demography is thus likely to cause substantial environmental impact in these biodiversity hotspot countries.

23. Obviously the type of stress applied by humans will differ from place to place. In the USA (and, almost certainly, Canada as well) the major stresses imperiling species are (Wilcove et al. 1998), in order: habitat destruction and modification, nonindigenous species, pollution, overexploitation and diseases

24. Sala et al. (2000) examined global ecosystems and the stresses expected to impact them over the next 100 years. In streams, tropical forests and southern temperate forests land use will be the major factor effecting change.  In arctic and alpine ecosystems and boreal forests, climate change will be the leading factor.  In northern temperate forests, nitrogen deposition will be most important.  Lakes and Mediterranean regions will be most impacted by species invasions.

27. References Cincotta, R.P., J Wisnewski and R. Engelman. 2000. Human population in the biodiversity hotspots. Nature 404:990-992. Hardin, G. 1968. The tragedy of the commons. Science 162:1243-1248. Groom, M.J., G.K. Meffe and R.C. Carroll. 2005. Principles of Conservation Biology. Sinauer, Sunderland, MA. Myers, N., R. Mittermeier, C. Mittermeier, G. da Fonseca, and J. Kents. 2000. Biodiversity hotspots for conservation priorities.  Nature 403:853-858. Postel, S.L., G.C. Daily, P.R. Ehrlich. 1996. Human appropriation of renewable fresh water. Science 271:785-788. Sala, O.E., F.S. Chapin, J.J. Armesto, E. Berlow, J. Bloomfield, R. Dirzo, E. Huber-Sanwald, L.  Huenneke, R. Jackson, A. Kinzig, R. Leemans, D. Lodge, H. Mooney, M. Osterheld, N. Poff, M. Sykes, B. Walker, M. Walker, D. Hall. 2000. Global biodiversity scenarios for the year 2100. Science 287:1770- 1774. Sisk, T.D., A. Launer, K. Switky and P. Ehrlich. 1994. Identifying extinction risks. Bioscience 44:592-604. Vitousek, P.M., J. Aber, R. Howarth, G. Likens, P. Matson, D. Schindler, W. Schlesinger and D. Tilman. 1997. Human alteration of the global nitrogen cycle: causes and consequences. Issues in Ecology 1: 1-15. Wilcove, et al. 1998. Quantifying threats to imperiled species in the United States. Bioscience 48:607-615.