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.

Imagine that you are one of the many invertebrate organisms which existed during the Cambrian or one of their descendents living today. Maybe you live in a tide pool which is washed by waves. A storm appears on the horizon. The waves increase in height. You feel yourself being dashed upon the rocks or into the mouth of a much larger and predatory animal. Finally, you begin to see your brothers and sisters die, one by one, as the forces of nature change your unpredictable environment.

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.

Of course, the animal or plant is not thinking: "How do I change my characteristics?" Natural selection is the force for change, not the individual's conscious decision. But, natural selection has produced a gradation of strategies, with extreme r-selection at one end of the spectrum and extreme K-selection at the other end.

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.

If you are in an unstable environment, you are unlikely to ever have population growth to the point where density dependent factors come into play. The population is still at low values relative to the carrying capacity of the environment and thus is growing exponentially with intrinsic reproductive rate r (when it is not subject to environmental perturbations.), hence the name r-strategist.

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.

One of the interesting differences between r and K strategists is in the shape of the survivorship curve. We can generate a survivorship curve by ploting the log of the fraction of organisms surviving vs. the age of the organism. To compare different species, we normalize the age axis by stretching or shrinking the curve in the horizontal direction so that all curves end at the same point, the maximum life span for individuals of that species. Notice that the vertical axis is on a log scale, dropping from 1.0 (100%) to 0.1 (10%) to 0.01 (1%) to 0.001 (0.1%) in equally spaced intervals.

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.