Population Ecology Worksheet (KEY)

1. Characteristics of PopulationsA. 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—the habitat.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 for these reasons.

a. Suitable physical, chemical, and biological conditions are patchy, not uniform.b. Many animals form social groups.c. Many offspring are not highly mobile and are forced to live “where they landed.”

2. Uniform dispersion is rare in nature; when it does occur, it is usually the result of fierce competition for limited resources.3. Random dispersion occurs in nature if environmental conditions are rather uniform in the habitat and members are neither attracting nor repelling each other.

2. Elusive Heads to CountA. To determine the number of animals in a particular area,

you could try a full count to measure absolute density; this may be difficult especially with elusive animals such as deer.

B. You could divide up the area into smaller quadrats; count the number of deer in one quadrat and extrapolate the number for the whole area.

C. Because many animals migrate a lot, it may be better to use the capture–recapture method.

3. Population Size and Exponential GrowthA. Gains and Losses in Population Size

1. Population size is dependent on births, immigration, deaths, and emigration.2. Population size may also change on a predictable basis as a result of daily or seasonal events called migrations.

B. From Zero to Exponential Growth1. 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 Biotic Potential?1. The biotic potential of a population is its maximum rate of increase under ideal—nonlimiting—conditions.2. The biotic potential varies from species to species because of three parameters: a. at what age each generation starts reproducing,

b. how often reproduction occurs, andc. how many offspring are born each time.

4. Limits on the Growth of PopulationsA. 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.

a. The carrying capacity can vary over time and is expressed graphically in the S-shaped curve pattern called logistic growth.b. The formula for logistic growth is: G = rmaxN[(K-N)/K].

2. Logistic growth deals with density-dependent controls.a. The main density-dependent factors are competition for resources, predation, parasitism, and disease.b. These factors exert their effects in proportion to the number of individuals present.

C. Density-Independent Factors1 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.

5. Life History PatternsA. Each species has a life history pattern that influences

survival, fertility, and the age of first reproduction.B. Life Tables

1. Life tables follow the fate of a group of newborn individuals (cohort) through their lives to calculate the survivorship schedule.2. The number of offspring born to individuals in each age interval is also recorded.

C. Patterns of Survival and Reproduction1. Survivorship 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. 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. A Type II curve is typical of many animals where the chances of survival or death are about the same at any age.c. A Type III curve indicates low survivorship, or conversely, high mortality in early life.

6. Natural Selection and Life Histories A. David Reznick and John Endler studied the differences in

size and survival of guppies in Trinidad.1. They were interested in the effects of predation on guppy populations in two streams with different predators.2. Their hypothesis was that predation by killifish and pike-cichlids is a selective agent that acted to shape guppy life history patterns.

B. They discovered that differences in growth rate and reproductive maturity have a genetic basis because the effects were the same in lab-reared guppies and the natural populations.

C. The experimenters performed other field experiments that stretched over eleven years.

1. Guppies that had been exposed to one predator, and then moved to another, displayed changes.2. Reznick and Endler showed that life history traits can be inherited, but these traits can also evolve.

7. Human Population GrowthA. Notice these startling statistics:

1. The world population reached 6 billion in 1999.2. The annual rate of increase averaged 1.3 percent.

B. How did the human population grow to such large numbers?

1. Humans expanded into new habitats and climatic zones.2. Agriculture increased the carrying capacity of the land to support humans and their animals.

3. Medical practice and improved sanitation conditions removed many population-limiting factors.4. New forms of energy from fossil fuels ushered in the industrial revolution.

C. Consider this: It took 2.5 million years for the world’s human population to reach 1 billion; it took only 12 years to reach the sixth billion.

8. Fertility Rates and Age StructureA. At the present rate of increase, the world human

population may reach 8.9 billion by 2050.1. The total fertility rate (TFR) is the average number of children born to women during their reproductive years, and is currently 2.8 children per female.2. Even if the replacement level of fertility is achieved (about two children per woman), the human population will continue to grow for another sixty years.

B. Any population with a broadly based age structure (many women in reproductive years) will continue explosive population growth.

1. Effective family planning programs can achieve a faster decline in birth rate than economic development alone.2. China has the most far-reaching family planning program.

a. Couples are limited to having one, maybe two, children.b. There are many incentives, and the program has lowered the TFR from 5.7 to 1.8.

9. Population Growth and Economic EffectsA. Demographic Transitions

1. In the demographic transition model, changes in population growth are linked to four stages of economic development.

a. In the preindustrial stage, living conditions are harsh; birth and death rates are high; and there is little increase in population size.

b. In the transitional stage, living conditions improve; death rate drops; and birth rate remains high.c. In the industrial stage, growth slows.d. In the postindustrial stage, zero population growth is reached; birth rate falls below death rate.

2. Some developed countries are in the industrial stage (for example, the United States, Canada, and Japan); some countries (for example, Mexico) are in the transitional stage.

B. A Question of Immigration Policies1. The greatest increase in immigration came in 1986 when legislation gave legal status to undocumented immigrants who proved they had lived in the country for years.2. Economic downturns in the past twenty years have fanned resentment against newcomers and increased calls for limits on new immigrants.

C. A Question of Resource Consumption1. The United States uses about a quarter of the world’s goods, services, and natural resources but constitutes only 4.6 percent of the world’s population.2. Other countries such as India and China are asking for a greater share of the economic pie.

D. Impacts of No Growth1. It takes a large workforce of younger individuals to support individuals in the higher age brackets.2. Can humans defy the laws of nature that dictate the number of individuals, which can be supported per unit of space, or are we just postponing the inevitable?