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Transcript of Lesson 25
A population is a group of organisms of a single species that lives in a given area.
The area inhabited by a population is called its geographic range.
Population density refers to the number of individuals per unit area.
A population of ducks in a pond may have a low density, while fish and other animals in the same pond community may have higher densities.
Distribution refers to how individuals in a population are spaced out across the range of the population—randomly, uniformly, or mostly concentrated in clumps.
A population’s growth rate determines whether the population size increases, decreases, or stays the same.
Age structure—the number of males and females of each age a population contains.
The factors that can affect population size are the birthrate, death rate, and the rate at which individuals enter or leave the population.
A population may grow if individuals move into its range from elsewhere, a process called immigration.
A population may decrease in size if individuals move out of the population’s range, a process called emigration.
Under ideal conditions with unlimited resources, a population will grow exponentially.
In exponential growth, the larger a population gets, the faster it grows.
Logistic growth occurs when a population’s growth slows and then stops, following a period of exponential growth.
Carrying capacity is the maximum number of individuals of a particular species that a particular environment can support.
How Populations Grow
Limits to Growth
A limiting factor is a factor that controls the growth of a population.
Density-dependent limiting factors include competition, predation, herbivory, parasitism, disease, and stress from overcrowding.
When populations become crowded, individuals compete for food, water, space, sunlight, and other essentials.
Competition is a density-dependent limiting factor.
The more individuals living in an area, the sooner they use up the available resources.
Unusual weather such as hurricanes, droughts, or floods, and natural disasters such as wildfires, can act as density-independent limiting factors.
Once a population reaches the carrying capacity of its environment, a variety of factors act to stabilize it at that size.