Population Size

From R.L.McConnell & Daniel C. Abel, Environmental Issues. Measuring, Analyzing and Evaluating

There are four variables which govern changes in population size.
  • births
  • deaths
  • immigration
  • emigration
A population gains individuals by birth and immigration and loses individuals by death and emigration.

Biotic Potential

Populations vary in their capacity to grow. The maximum rate at which a population can increase when resources are unlimited and environmental conditions are ideal is termed the population's biotic potential. Each species will have a different biotic potential due to variations in
  • the species' reproductive span (how long an individual is capable of reproducing)
  • the frequency of reproduction (how often an individual can reproduce)
  • "litter size" (how many offspring are born each time)
  • survival rate (how many offspring survive to reproductive age)
There are always limits to population growth in nature. Populations cannot grow exponentially indefinitely. Exploding populations always reach a size limit imposed by the shortage of one or more factors such as water, space, and nutrients or by adverse conditions such as disease, drought and temperature extremes. The factors which act jointly to limit a population's growth are termed the environmental resistance. The interplay of biotic potential and density-dependent environmental resistance keeps a population in balance.

Carrying Capacity

For a given region, carrying capacity is the maximum number of individuals of a given species that an area's resources can sustain indefinitely without significantly depleting or degrading those resources. Determining the carrying capacities for most organisms is fairly straightforward. For humans carrying capacity is much more complicated. The definition is expanded to include not degrading our cultural and social environments and not harming the physical environment in ways that would adversely affect future generations.

For populations which grow exponentially, growth starts out slowly, enters a rapid growth phase and then levels off when the carrying capacity for that species has been reached. The size of the population then fluctuates slightly above or below the carrying capacity. Reproductive lag time may cause the population to overshoot the carrying capacity temporarily. Reproductive lag time is the time required for the birth rate to decline and the death rate to increase in response to resource limits. In this scenario, the population will suffer a crash or dieback to a lower level near the carrying capacity unless a large number of individuals can emigrate to an area with more favorable conditions. An area's carrying capacity is not static. The carrying capacity may be lowered by resource destruction and degradation during an overshoot period or extended through technological and social changes.

An example of dieback occurred in Ireland after a fungus infection destroyed the potato crop in 1845. During this potato famine approximately 1 million people died and 3 million people emigrated to other countries. Increased food production due to improved agricultural practices, control of many diseases by modern medicine and the use of energy to make historically uninhabitable areas of Earth inhabitable are examples of things which can extend carrying capacity. The question is how long will we be able to keep increasing our population on a planet with finite size and resources?

Population Impact

Homo sapiens is a species possessing a diversity of individual needs. Thus, sub-populations will have different requirements and different impacts on the environment. For example 100 million vegetarians will have a significantly different environmental impact than 100 million meat-eaters. This can be demonstrated by comparing the affect on water supplies by both sub-populations. About 1000 tons of water are needed to produce 1 ton of grain. Almost 40% of all grain is used in meat and poultry production. Add to this the amount of water that goes into the production of meat, and you can see that meat comsumption places more stress on global water supplies than grain consumption.

Although it is difficult to determine the exact human carrying capacity for a country as large and diverse as the U.S., an estimate of carrying capacity is essential for those whose jobs involve the development of policies to ensure that the environment is able to support life into the future.

Consider the information in the list below and decide if the item is an indication that humans may have exceeded their carrying capacity in some regions.

   The Los Angeles riots of 1992
   Closure of the Georges Banks fishing grounds
   The 1.9% per capita annual decline of arable land in developing countries during the 1980's
   Closure of Murrells Inset, South Carolina to shell fishing after heavy rains
   The loss of 70,000 km2 of cropland yearly due to nutrient depletion
   1.7% annual decline in rainforest acreage in Equador during the 1980's
   The existence of 33,000 potential Superfund toxic contamination sites
   Projected exhaustion of fossil fuels by the middle of the next century
   Using groundwater in the U.S. 25% faster than aquifers are recharged
   High unemployment in Flint, Michigan
   Crowded roads and sociopathic driving behavior
   Deteriorating infrastructure (e.g., bridges and roads
   Massacres in Rwanda, one of Africa's most densely populated countries
   Issuance of a fishery management plan by the National Marine Fisheries Service to allow threatened species of sharks to recover
  1. Discuss the choices you made with the rest of the class via the forum on Moodle. Don't be afraid to voice your opinions and to comment on the posts of your classmates. One thing about opinions, everyone has one! Justify yours.
  2. Post other evidence that you can think of that supports the claim that the U.S., or any part of the U.S., has exceeded its carrying capacity.