Lecture 16: Population Limits

Reading: Economy of Nature, pp. 340-349.

Causes for equilibrium need not be constant in time nor the same at all possible densities. Density dependence may not yield an equilibrium if the environment changes and a given species may exhibit different density dependence in different environments.

European Spruce Sawfly (Diprion hercyniae):
            favorable weather conditions, death rate is density dependent and birth rate is density independent
                        increased death rate with increased density due to increased rates of attack by insect parasites and diseases

            unfavorable weather conditions, birth rate may remain density independent, but death rate increases and also becomes density independent (populations become unstable under these conditions)

Jumping Plant Lice (Cardiaspina albitextura):
            low density, death rate is density dependent and birth rate is density independent
                    increased death rate with increased density due to increased rates of attack by insect parasites and bird predators
            high density, birth rate is density dependent and death rate is density independent
                    reduced fecundity with increased density, food competition

Human Population Growth Limitation

Although world human population considered in aggregate is growing exponentially, not all human populations are growing at the same rate.

Per capita growth rates (r) and exponential doubling times vary widely among different countries and regions. Current population size alone is not a good predictor of doubling time, nor is population density. The data given below are from 1993, population sizes are in millions, fertility rate is the average number of children per woman. Replacement fertility is 2.10. (data source: Worldwatch Database. Worldwatch Institute, 1996)

Countries that presently have relatively low fertility rates (<= 2.00) and relatively small annual rates of growth (<= 1%) did not previously exhibit such slow growth characteristics. The process of change from rapid population growth to slow population growth, in human populations, is called demographic transition.

Demographic transition is the process of change from populations with both high birth and death rates (that are at or near equilibrium) to a situation in which both birth and death rates are low (and at or near equilibrium).

Historically, the process of demographic transition involves an initial drop in death rates followed by a later drop in birth rates. The disparity between birth and death rates during transition results in a period of very rapid population growth. All presently industrialized countries (economically developed or more developed countries) have gone through demographic transition, and all other countries (less developed, shaded area of the graph) are presently in the transition (rapid growth) phase of demographic transition (after Keyfitz, 1990, p 67, Fig. 6.5).

Causes for decreases in death rate:
            nutrition improvements
            public health sanitation (water and sewage) improvements
            medical advances

Causes for decreases in birth rate:
            socioeconomic change, economic development, industrialization
            government commitment to family planning (contraception) increases
            infant mortality decreases
            social status of women increases
            education opportunities for women increase

Humans exhibit Type I survivorship, this means that most mortality occurs late in life, after reproduction. Mortality that follows reproduction may have relatively little effect on absolute population growth rates, so even high death rates (or poor life expectancy) compared to industrialized countries have little effect on population growth if birth rates are also high.

High birth rate (rapid population growth, less developed) countries typically have a broader based age structure pyramid than do low birth rate (slow population growth, industrialized) countries (Ricklefs, 1996, p 332, Fig. 15.5).

The skew toward younger (pre-reproductive) age classes in the age structure of less developed countries makes absolute population growth continue even as individual fecundity is decreasing. Similarly, large population size makes it possible for absolute population growth to occur even if the per capita rate of growth is near zero.

Although, all human populations are changing toward equilibrium, the absolute population growth that will occur could exceed the carrying capacity of earth for humans. How is human carrying capacity evaluated?

Critical limiting resources for humans:
            Food, Land, Water, Raw Materials, Energy Resources

Evidence of impending critical resource limitation:

Food production and trading trends

(data sources: Worldwatch Database. Worldwatch Institute, 1996)

(data source: Worldwatch Database. Worldwatch Institute, 1996)

Water use trends

(data source: Worldwatch Database. Worldwatch Institute, 1996)

Land degradation trends

Human-Induced Land Degradation, Cumulative 1945 - 1991 (million hectares)
(data source: Worldwatch Database. Worldwatch Institute, 1996)

Causes for land degradation
            Erosion
            Over grazing
            Over cultivation
            Over irrigation (salinization)

Annual Sediment Loads in Selected Rivers

Raw Materials: Evidence of declining supplies is not seen in price trends.

(data sources: Worldwatch Database. Worldwatch Institute, 1996)

Copyright 1999 Northern Arizona University ALL RIGHTS RESERVED