 |  Environmental Science: A Global Concern, 7/e William P. Cunningham,
University of Minnesota
Mary Ann Cunningham,
Vassar College
Barbara Woodworth Saigo,
St. Cloud State University
Population Dynamics
BE ALERT FOR: The Nature of Exponential Growth
The concept of growth pops up again in later chapters. Be sure to understand the distinction between exponential and other types of growth. Think of exponential growth as growth by some percentage increase per unit of time. A savings account that earns 4 percent annual interest is growing exponentially.
Another form of growth occurs when an amount is increasing by some fixed amount. A tree adding 10 centimeters to its height each year is growing nonexponentially.
As you will see, exponential growth produces explosive growth compared to other ways of increase.
BE ALERT FOR: How Life Span Differs from Life Expectancy
Don’t miss the distinction between life span and life expectancy. Life span refers to the maximum age a member of a species can attain. Life expectancy, on the other hand, is calculated as the average age of death of population members.
Compare population A, with five individuals that die at ages 1, 1, 5, 8, and 10 years, with population B, with five individuals that die at ages 4, 6, 7, 9, and 9. The apparent life span for the species is 10 years. The life expectancy in population A is 5 years [1+1+5+8+10)/5], whereas that of population B is 7 years.
BE ALERT FOR: Density-Dependent and -Independent Factors
Any factor that causes death or otherwise lowers reproductive gains in a population is part of environmental resistance. The severity with which some factors operate depends on the density of the population. Other factors are not linked to density and are called density-independent. Density is an expression of the number of organisms per unit of space and, therefore, is an index of the degree of crowding.
Assume a population of carp gets stranded in a floodplain pool that eventually dries up. The density of fish in this population has absolutely no impact on the outcome; all the fish will die. The drying-up event, then, is operating as a density-independent factor. Abiotic factors such as temperature and others related to weather typically act in a density-independent manner.
Density-dependent factors, on the other hand, regulate growth such that population density does affect severity of impact. A contagious disease might have very little impact on a population with low density because of the difficulty in transmission. The very same disease might decimate a dense population, however, because of the greater ease of transmission from organism to organism.
A number of animals seem to produce smaller litter sizes under conditions of higher density. It appears the circumstance of crowding affects reproductive physiology. Such a reduction in fertility is another type of density-dependent factor.
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2003 McGraw-Hill Higher Education