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5-2 Limits to Growth
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Limiting Factors
What factors limit population growth?
Limiting Factors
The primary productivity of an ecosystem can be reduced when there is an insufficient supply of a particular nutrient.
Ecologists call such substances limiting nutrients.
A limiting nutrient is an example of a more general ecological concept: a limiting factor.
In the context of populations, a limiting factor is a factor that causes population growth to decrease.
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4. Density-Dependent Factors
A limiting factor that depends on population size is called a density- dependent limiting factor.
Density-dependent limiting factors include:
competition
predation
parasitism
disease
Density-dependent factors operate only when the population density reaches a certain level. These factors operate most strongly when a population is large and dense.
They do not affect small, scattered populations as greatly.
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5. Density-Dependent Factors
Competition
When populations become crowded, organisms compete for food, water space, sunlight and other essentials.
Competition among members of the same species is a density-dependent limiting factor.
Competition can also occur between members of different species.
This type of competition can lead to evolutionary change.
Over time, the species may evolve to occupy different niches.
Predation
Populations in nature are often controlled by predation.
The regulation of a population by predation takes place within a predator-prey relationship, one of the best-known mechanisms of population control.

6. Density-Dependent Factors
Wolf and Moose Populations on Isle Royale
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.
Moose Wolves
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7. Density-Dependent Factors
Parasitism and Disease
Parasites can limit the growth of a population.
A parasite lives in or on another organism (the host) and consequently harms it.
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8. Density-Independent Factors
Density-independent limiting factors affect all populations in similar ways, regardless of the population size.
Examples of density-independent limiting factors include:
unusual weather
natural disasters
seasonal cycles
certain human activities—such as damming rivers and clear-cutting forests
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5-2
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5-2
A limiting factor that affects all populations in similar ways regardless of their size might be
drought.
disease.
predation.
crowding.
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5-2
Which of the following would be a limiting factor affecting the panda population of China?
programs that educate people about endangered species
capture of some pandas for placement in zoos
laws protecting habitat destruction
a disease that kills bamboo plants
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5-2
Density-dependent factors operate most strongly when a population is
large and dense.
large but sparse.
small and sparse.
small, but growing.
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5-2
Within a limited area, if the population of a predator increases, the population of its prey is likely to
increase.
decrease.
remain about the same.
become extinct.
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5-2
Which of the following is a density-independent factor affecting populations?
predation
disease
a destructive hurricane
parasites
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15. END OF SECTION