Human Population Growth

 

Readings:  p. 4-5 text, plus Myers et al. (2002) Nature paper on Biodiversity hotspots

 

Before we can address specific conservation issues, it is important to understand why conservation problems exist. Species have become extinct during five major extinction events, with large numbers of families, genera and species wiped out. However, these events usually were followed by periods of radiation of taxa, resulting in new diversity.  The difference today is that factors adversely affecting are human-mediated and, secondly, 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 correspond to centres of biodiversity ['hotspots'].
 

Population growth (pg. 4-5)

 

Population modellers 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 is constant through time, while growth of the population is geometric.  We are most familiar with exponential growth from pathogenic bacteria and other microorganisms which appear to grow unhindered - that is, when there is no adverse feedback on population growth rate caused by increasing population size and its concomitant reduction in remaining resources and waste buildup:

 

dN/dt = rN and Nt = N0ert;

 

where: r is the intrinsic growth rate, t is the growth interval considered and N is population size at different times.

 

Population growth of this manner cannot continue indefinitely because even organisms as small as bacteria experience some form of feedback (e.g. a reduction in fission or birth rate, 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:

 

dN/dt = rN([K-N]/K) or  Nt = K/(1+ea-rt)

 

where: a is an integration constant to define position on curve relative to origin, and K is the environmental carrying capacity. Many species are capable of logistic growth, thought actual growth rates vary tremendously among taxa. For example, 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. Prolific bacterial growth may provide enough of an inoculum to cause food poisoning.

 

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 because of unsanitary conditions and absence of medical care.  As humans shifted from hunter-gatherer to more modern forms of agriculture, famine became less of a problem and required less manpower. 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%.
 

see human population growth

Most of these increases were due to compounding of growth and to lower death rates. One of the highest rates observed in recent years was in Kenya (4%), 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]).

 

see statistics for MDC's vs. LDC's
 

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.  Tragically, many developing countries, in Africa in particular, have exceptionally high rates of HIV infection (reported estimate of 25% of the adult Zimbabwe population). Owing to the virtual absence of therapies used to treat infected individuals, these countries are likely to experience very significant demographic and social upheaval associated with HIV/AIDS-related mortality. This tragedy will impact local and perhaps even global rates of population growth.

 
see MDC vs. LDC conflict?
 

Why do we care so much about population growth?  Simply put, each individual has an environmental ‘footprint’, the size of which depends on factors like country of residence etc.  More mouths necessarily mean a greater demand of environmental 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.  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% (Postel et al. 1996).  So, where will the water needed for these people come from? 

 

At the same time that our use of the environment increases, our adverse effects on it are also building.  For example, Vitousek et al. (1997) showed that application of nitrogenous fertilizers have increased dramatically since the 1940s, and together with other forms of human-mediated N-release, has caused a doubling of the amount of nitrogen entering the land-based N-cycle.  As an often limiting nutrient (and pollutant), this increase has a number of adverse consequences including acid rain, loss of soil nutrients (Ca, K), smog formation in cities, and eutrophication of lakes and seas.  Thus, human population growth has very profound consequences for the characteristics of our environment. 

 

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 and endemism tended to be highest in tropical countries, notably islands.  Population pressures varied from region to region, with deforestation most important in countries in Central (Costa Rica, Guatemala, Nicaragua) and South America (Columbia, Ecuador), and human population growth in eastern countries (Sri Lanka, Philippines, Taiwan, India).  Africa had high deforestation rates (Ivory Coast, Angola, Kenya) and human population growth (Nigeria).  Madagascar (Malagasy Republic) is considered of continental but not global importance. Europe and North America do not fit into any of the categories of risk.
 

See human population growth vs. biodiversity in:

1) Africa

2) Australia and Asia

3) Europe

4) North and Central America

5) South America

6) human population pressure vs. endemism rate

 

Refer to Myers et al. (2000)

 

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.

 

Myers et al. Table 1 - Hotspots

 

Remember they based their analysis on plants.  However, if we preserved these 25 hotspot areas, we would also preserve 28.5% of global bird diversity, 27.3% of mammals, 37.5% of reptiles, and 53.8% of amphibians in addition to the 44% of plants.  So, by protecting plant hotspots, we also protect other taxa.

 

See Myers et al. Table 2 - other taxa

 

What are the hotspots? 

The leading ones are:

·        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.

 

See Myers et al. (2000) Table 3 - leading hotspots

 

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).

 

See Myers et al. Table 5 - congruence

 

Overall, judging by a variety of biota (not just plants), Madagascar, the Philippines and Indonesia were the richest regions on the planet. 

 

In a follow-up paper, also published in Nature, 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 value was 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. 

 

Obviously the type of stress applied by humans will differ from place to place. In the USA, the major stresses imperiling species are, in order:

·        habitat destruction and modification,

·        nonindigenous species,

·        pollution,

·        overexploitation

·        and diseases  (Wilcove et al. 1998).

 

Sala et al. (2000) examined global ecosystems and the stresses expected to impact them over the next 100 years. Overall changes to biodiversity are expected to be led by changes in land use, climate change, nitrogen deposition (enrichment), species invasions, and increased carbon dioxide in the atmosphere.  However, the importance of different mechanisms is expected to vary tremendously across biome  types.  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.

 

See Sala et al. (2000) overall effects (Figure 1) and biome-specific cases (Figure 2).

  

Although North America, and Canada in particular, tends to have relatively low biodiversity levels and relatively low levels of habitat destruction/population growth, conservation of endangered and at risk species is still a concern.  The Ontario government has created a web site that lists all endangered, threatened, vulnerable, extirpated, and extinct species in the province. (see Endangered Species in Ontario).  Check out the area of southern deciduous forest to see what is endangered in our area. 
 

If we look at the history of biodiversity radiation and extinction (next lecture), we see that these are natural processes. Looking at the geologic record, there have been a number of time periods during which diversity has significantly expanded and later contracted.

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.

Meffe, G.K. and R.C. Carroll. 1997. 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.