1. Population Dynamics and Carrying Capacity A. Populations change in size, density, and age distribution; most members of populations live together in clumps or groups. 1. Three general patterns of population distribution occur in a habitat: clumping, uniform distribution, and random dispersion. Most species live in clumps or groups. a. Availability of resources varies from place to place. b. Living in groups offers better protection from predators. c. Some predator species live in packs to better have a chance to get a meal. d. Temporary groups may form for mating and caring for young. 2. Uniform pattern distribution may occur where a resource, such as water, is scarce. B. Four variables influence/govern population size: births, deaths, immigration, and emigration. 1. Increase in population occurs by birth and immigration. 2. Decrease in population occurs by death and emigration. 3. Age structure of a population is usually described as the pre-reproductive stage, the reproductive stage and the post-reproductive stage. A population with a large reproductive stage is likely to increase, while a population with a large post-reproductive stage is likely to decrease. C. No population can grow indefinitely due to limited resources such as light, water, and nutrients and also due to competitors and/or predators. 1. The biotic potential is the population’s capacity for growth. 2. The intrinsic rate of increase (r) is the rate of population growth with unlimited resources. 3. Rapidly growing populations have four characteristics. a. Individuals in the population reproduce early in life. b. Individuals have short periods between generations. c. Individuals have long reproductive lives. d. Individuals produce multiple offspring each time they reproduce. D. Environmental resistance consists of factors that limit population growth. 1. Carrying capacity (K) is determined by biotic potential and environmental resistance. This is the number of a species’ individuals that can be sustained indefinitely in a specific space. 2. As a population reaches its carrying capacity, its growth rate will decrease because resources become more scarce. E. A population can grow rapidly with ample resources. 1. With few resource limitations, a population will have exponential growth. This is a fixed rate of growth that will take be a J-shaped growth curve as the base size of population increases. This represents its intrinsic rate of increase (r) or biotic potential. 2. This exponential growth is converted to logistic growth when the populations get larger and face environmental resistance. In logistic growth, the growth rate levels off as population size reaches or nears carrying capacity. 3. The sigmoid (s-shaped) population growth curve shows that the population size is stable, at or near its carrying capacity. F. When population size exceeds its carrying capacity, organisms die unless they move or switch to new resources. 1. Exponential growth leads to logistic growth and may lead to the population overshooting the environment’s carrying capacity. a. Overshooting an environment’s resources often is a result of a reproductive time lag. b. The reproductive time lag can produce a dieback/crash of organisms unless the organisms can find new resources or move to an area with more resources. 2. If the carrying capacity of an area is exceeded, changes in the area itself can reduce future carrying capacity. Reducing grass cover by overgrazing allows sagebrush to move in and reduces the number of cattle that the land can support. 3. Technological, social, and cultural changes have extended the earth’s carrying capacity for human beings, for the time being. G. The density of a population may or may not affect how rapidly it
