Predation

Predators and Their Prey. These lionesses feed on the carcass of a zebra.

Predation is a relationship in which members of one species (the predator) consume members of another species (the prey). The lionesses and zebra in Figure above are classic examples of predators and prey. In addition to the lionesses, there is another predator in this figure. Can you spot it? The other predator is the zebra. Like the lionesses, it consumes prey species, in this case species of grass. However, unlike the lionesses, the zebra does not kill its prey. Predator-prey relationships such as these account for most energy transfers in food chains and food webs.



Predation and Population

A predator-prey relationship tends to keep the populations of both species in balance. An idealized model of this relationship is shown by the graph in Figure below. As the prey population increases, there is more food for predators. So, after a slight lag, the predator population increases as well. As the number of predators increases, more prey are captured. As a result, the prey population starts to decrease. What happens to the predator population then?

Below is an example of predator-prey relationship that exists between lynx and the Snowshoe hare. In this predator-prey example, one factor limits the growth of the other factor. As the prey (hare) population deceases, the predator (lynx) population begins to decrease as well. The prey population is a limiting factor. A limiting factor limits the growth or development of an organism, population, or process.

Snowshoe hare is the primary food of the lynx. The population cycles of these two species are closely linked.



Logistic Growth Patterns

Most populations do not live under ideal conditions. Therefore, most do not grow exponentially. Certainly, no population can keep growing exponentially for very long. Many factors may limit growth. Often, the factors are density dependent (known as density-dependent factors). These are factors that are influential when the population becomes too large and crowded. For example, the population may start to run out of food or be poisoned by its own wastes. As a result, population growth slows and population size levels off. Curve B in Figure above represents this pattern of growth, which is called logistic growth.

At what population size does growth start to slow in the logistic model of growth? That depends on the population’s carrying capacity. The carrying capacity (K) is the largest population size that can be supported in an area without harming the environment. Population growth hits a ceiling at that size in the logistic growth model.

Eastern Cottontail

The story of the Eastern Cotttontail offers a great example of what happens when a habitat’s carrying capacity is exceeded. Before North America was colonized by Europeans, the North American rabbit population was kept in check by foxes. Once settlers arrived they began to recognize foxes as a nuisance and danger to their communities so hunted the animals, and fox populations were severely decreased. As a result of the loss of their main predator, the rabbit population began to boom. Rabbits feed on all sorts of plants, shrubs, flowers, and grass, and require large quantities of such vegetation to sustain themselves. Eventually, rabbits began to starve because their large numbers depleted their main source of food; they had reached their carrying capacity. The figure below depicts the logistic growth pattern of Eastern Cottontail in New York State as effected their carrying capacity.

Summary

  • Predation is a relationship in which members of one species (the predator) consume members of another species (the prey).
  • A predator-prey relationship keeps the populations of both species in balance.
  • Under ideal conditions, populations can grow exponentially.
  • The growth rate increases as the population gets larger.
  • Most populations do not live under ideal conditions and grow logistically instead.
  • Density-dependent factors slow population growth as population size nears the carrying capacity.



References:

CK12-Foundation. (n.d.). Flexbooks.ck12.org. https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/6.13/primary/lesson/predation-bio/

Four Examples of Carrying Capacity: When a Population Hits Its Limit - Population Education. (2019, January 17). Population Education. https://populationeducation.org/4-examples-of-carrying-capacity-when-a-population-hits-its-limit/