The Biology Book: From the Origin of Life to Epigenetics, 250 Milestones in the History of Biology (2015)

Factors Affecting Population Growth

Thomas Malthus (1766–1834), Harry S. Smith (1883–1957)

1935

Thomas Malthus noted, “Population, when unchecked, goes on doubling itself every twenty-five years or increases in a geometrical ratio.” Under ideal conditions, plant and animal populations continue to grow indefinitely, but this is not the way it is in nature. As resources become limited, birth rates typically decrease and death rates increase, slowing population growth. But does the population density in a given area influence its further decrease or increase?

Density-dependent factors are those that increase the death rate or decrease the birth rate in response to a rise in population. Such high-density pressures are often relieved by outward migrations of animal populations to less populated areas with more plentiful resources. Organisms in close proximity to one another, because of overpopulation, are more likely to be exposed and succumb to highly contagious diseases. Examples include the American chestnut tree blight, caused by a fungus, and smallpox and tuberculosis, resulting from a virus and bacterium, respectively. In 1935, Harry Smith, an entomologist at the University of California-Riverside, described the biological control of pest populations using such biotic weapons as predators, pathogens, and parasites. Predators play a major role in controlling population size. The increasing size of the potential prey population provides an incentive for predators to inhabit a geographic area, as with the four-year cycles of increases and decreases seen in the population of lemmings, which are related to their predator’s activities.

Abiotic, density-independent factors that occur regardless of population size can rapidly and dramatically reduce and even decimate a population by leaving nutrients short in number and inferior in quality. Recent examples include such catastrophic events as forest fires, hurricane Katrina (2005), and the 1989 Exxon Valdez and 2010 Deepwater Horizon (BP) oil spill. Heavy frosts and drought conditions represent some of the climatic factors. Environmental pollutants, such as agricultural pesticides and fertilizers and mining runoffs, have taken their toll on the plant and animal populations, with amphibians, fish, and birds at particular risk.

SEE ALSO: Amphibians (c. 360 Million BCE), Population Growth and Food Supply (1798), Population Ecology (1927), Food Webs (1927), Green Revolution (1945), Silent Spring (1962), Deepwater Horizon (BP) Oil Spill (2010), American Chestnut Tree Blight (2013).

The population of the Adélie penguin colony on Antartica’s Beaufort Island increased 84 percent as ice fields receded from 1958 to 2010. The warmer temperatures increased the ice-free habitat for their breeding.