r and K selection

r and K selection

Organisms that live in stable environments tend to make few, "expensive" offspring. Organisms that live in unstable environments tend to make many, "cheap" offspring.

If you could design a "strategy" to overcome the problems created by an unpredictable environment, you would have two choices - go with the flow or cut and run to a more stable environment.

Suppose you stayed. Then, one thing you could do would be to increase the number of offspring. Make lots of cheap (requiring little energy investment) offspring instead of a few expensive, complicated ones (requiring a lot of energy investment). If you lose a lot of offspring to the unpredictable forces of nature, you still have some left to live to reproductive age and pass on your genes to future generations. Many invertebrates follow this strategy - lots of eggs are produced and larvae are formed but only a few survive to produce mature, reproductive adults. Many insects and spiders also follow this strategy.

Alternatively, you could adapt to a more stable environment. If you could do that, you would find that it would be worthwhile to make fewer, more expensive offspring. These offspring would have all the bells and whistles necessary to ensure a comfortable, maximally productive life. Since the environment is relatively stable, your risk of losing offspring to random environmental factors is small. Large animals, such as ourselves, follow this strategy.

Plants are also subject to the same sorts of forces as animals. Some live in unstable environments such as a floodplain near a river or a gap in the forest caused by falling trees. Others live in a quite stable environment, such as a climax forest.

The two evolutionary "strategies" are termed r-selection, for those species that produce many "cheap" offspring and live in unstable environments and K-selection for those species that produce few "expensive" offspring and live in stable environments.

The following table compares some characteristics of organisms which are extreme r or K strategists:

r Unstable environment, density independent K Stable environment, density dependent interactions small size of organism large size of organism energy used to make each individual is low energy used to make each individual is high many offspring are produced few offspring are produced early maturity late maturity, often after a prolonged period of parental care short life expectancy long life expectancy each individual reproduces only once individuals can reproduce more than once in their lifetime type III survivorship pattern

in which most of the individuals die within a short time

but a few live much longer type I or II survivorship pattern

in which most individuals live to near the maximum life span

The terms "r-selected" and "K-selected" come from a description of the population growth regimes of the two types of organisms.

An extreme K-strategist lives in a stable environment which is not seriously affected by sudden, unpredictable effects. Thus the population of a K-strategist is near the carrying capacity K.

Surviorship curves give us additional insight into r and K-selected strategies. Notice that the vertical axis of the survivorship plots is on a log scale and that horizontal axis is scaled to the maximum lifetime for each species.

Extreme r-strategists, such as the oyster, lose most of the individuals very quickly, relative to the maximum life span for the species. But, a very few individuals do survive much longer than the rest. But, for extreme K-strategists, such as man, most individuals live to old age (again relative to the maximum life span for the species).

These survivorship data are very valuable when studying the ecology of various organisms. Two components are involved in reproduction: 1) How many females survive to each age and 2) the average number of female offspring produced by females at each age. By using these data, we can compute the intrinsic rate of reproduction, r, a key parameter in models of population growth.