Population Ecology

In 1798, the English political economist and demographer Thomas Malthus authored an essay projecting that the human population was growing at such a rapid rate that, if not brought under control, it would lead to mass starvation and poverty. Happily, the agricultural revolution produced food far faster than the production of mouths to feed. Nevertheless, the matter of a rising population became a major issue in the United States in the early 1920s. In response to a severely restrictive immigration law, Alfred Lotka, a Polish immigrant to the United States, brought his mathematical insights to biology. In an influential 1925 article, he demonstrated that the observed rise in population resulted from a disproportionately large number of individuals who had immigrated in previous decades and were now in their peak reproductive years. Restricting the number of immigrants, he argued, would lead to a population decline.

A population refers to members of the same species in a particular geographic area, and population ecology examines factors that influence populations and how these populations interact with their environment. In his 1925 book, Elements of Physical Biology, Lotka noted that four variables affected population—death, birth, immigration, and emigration—and that a dynamic equilibrium occurred when losses and gains were equal. The population size is affected by interactions with abiotic and biotic environmental factors: among the abiotic factors are climate and food supply, while biotic factors include predation and intraspecies and interspecies competition. A number of dynamic processes influence population dispersion, population density, and demographic trends, which describe how populations change over time.

The zebra mussel (Dreissena polymorpha), native to the Black Sea and Caspian Sea, were first detected in the Great Lakes in 1988. They feed aggressively on phytoplankton (microscopic plants) but also zooplankton (animal life), which larval fish and native mussels depend upon to survive.

Scientists estimate that 99 percent of all species that have ever lived are now extinct. In addition to negative abiotic and biotic factors, humans have contributed to extinction and marked population declines. Human contributions include: pollution, such as runoffs of industrial wastes and agricultural fertilizers; global warming; the introduction of invasive species, such as zebra mussels in Lake Erie and kudzu in the southern United States; and the removal of competing species or predators.