Populations - Application of Biology - Lecture Slides, Slides of Biology

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Populations

Chapter 8 section 1

Population

• Population: All the

same members of a

species living in the

same place at the same

time

Properties of Populations

• Density: The number of individuals per unit

area

• Dispersion: The relative arrangement of a

population’s individuals

– Even

– Clumped

– Random

Density

Three factors can affect population size: the number of births, the number of deaths, and the number of individuals that enter or leave the population.

The J-shaped curve indicates that the population is undergoing exponential (eks-poh-NEN-shul) growth. Exponential growth occurs when the individuals in a population reproduce at a constant rate.

Exponential Growth In the presence of unlimited resources and in the absence of predation and disease, a population will grow exponentially. Both of these hypothetical graphs show the characteristic J-shape of exponential population growth.

• In the context of populations, a limiting factor is a factor that causes population growth to decrease.
• A limiting factor that depends on population size is called a density- dependent limiting factor. Density- dependent factors become limiting only when the population density—the number of organisms per unit area—reaches a certain level.

Density-dependent limiting factors include competition, predation, parasitism, and disease.

Density-independent limiting factors affect all populations in similar ways, regardless of the population size. Unusual weather, natural disasters, seasonal cycles, and certain human activities—such as damming rivers and clear-cutting forests—are all examples of density-independent limiting factors. In response to such factors, many species show a characteristic crash in population size. After the crash, the population may soon build up again, or it may stay low for some time.

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Wolf and Moose Population The relationship between moose and wolves on Isle Royale illustrates how predation can affect population growth. In this example, the moose population was also affected by changes in food supply, and the wolf population was also affected by disease. Interpreting Graphics How are the increases and decreases in the moose population related to the changes in the wolf population?