A. Human populations, such as those in India, are outstripping the earth’s resources.
B. Efforts to limit human populations have not been widely adopted nor successful.
C. Ecology is the study of the interactions of organisms with one another and with the physical and chemical environment.
I. Characteristics of Populations
A. Each population—a group of individuals of the same species living in the same area (habitat)—has certain characteristics:
1. The population size is the number of individuals making up its gene pool.
2. Age structure defines the relative proportions of individuals of each age—especially with respect to reproductive years.
3. Population density is the number of individuals per unit of area or volume.
4. Population distribution refers to the general pattern in which the population members are dispersed through its habitat.
B. Populations can be dispersed in three patterns:
1. Members of a population living in clumps is very common.
2. Uniform dispersion is rare in nature, and is usually the result of fierce competition for limited resources.
3. If environmental conditions are rather uniform in the habitat and members are neither attracting nor repelling each other, the dispersion tends to be random.
II. Focus
on Science: Elusive Heads to Count
III. Population Size and Exponential Growth
A. Gains and Losses in Population Size
1. Population size is dependent on births, immigration, deaths, and emigration.
2. Migrations are not considered in population demographics because they are usually round trip events.
B. From Ground Zero to Exponential Growth
1. Zero population growth designates a near balance of births and deaths.
2. Rate of increase (r) = net reproduction per individual per unit time.
3. The growth rate formula is: G = rN.
a. A graphic plot of exponential growth results in a J-shaped curve that becomes steeper with advancing time.
b. As long as r is positive, the population will continue to increase at ever-increasing rates—easily measured by noting the “doubling time.”
C. What Is the Biotic Potential?
1. The biotic potential of a population is its maximum rate of increase under ideal—nonlimiting—conditions.
2. The actual rate depends on the age at which each generation starts to reproduce, how often each individual reproduces, and how many offspring are produced.
A. What Are the Limiting Factors?
1. The actual rate of increase of a population is influenced by environmental conditions.
2. Limiting factors (nutrient supply, predation, competition for space, pollution, and metabolic wastes) provide environmental resistance to population growth.
B. Carrying Capacity and Logistic Growth
1. The sustainable supply of resources defines the carrying capacity for a particular population in a given environment.
2. The carrying capacity can vary over time and is expressed graphically in the S-shaped curve pattern called logistic growth.
3. The formula for logistic growth is: G = rmaxN[(K-N)/K].
C. Density-Dependent Controls
1. The main density-dependent factors are competition for resources, predation, parasitism, and disease.
2. These factors exert their effects in proportion to the number of individuals present.
D. Density-Independent Factors
1 Some events, such as weather, tend to increase the death rate without respect to the number of individuals present.
2. Lightning, floods, snowstorms, and the like affect large populations as well as small groups.
A. Life Tables
1. Age-specific patterns of particular populations can be studied to determine changes.
2. Life tables follow the fate of a group of newborn individuals (cohort) through their lives to calculate survivorship schedules.
B. Patterns of Survival and Reproduction
1. These curves are plots of the age-specific patterns of death for a given population in a given environment.
2. Most animals are characterized by one of these types of curves:
a. Type I curve is typical of large mammals where few offspring are produced and cared for so that infant mortality is low; death usually comes after an extended life.
b. Type II curve is typical of many animals where the chances of survival or death are about the same at any age.
c. Type III curve indicates low survivorship, or conversely, high mortality in early life.