r and K strategy

R. Mac Arthur and Wilson study on island biogeography coined the term r/k selection.

In an organism the selected combination of traits decides the quality and quantity of new individuals produced and this relates to r/k selection theory.

For successful living in a particular environment the r strategy focus upon the investment of parent to produce large number of offspring i.e. quantity matters while in K strategy few offspring are produced with increase parental care.

Verhulst model of population dynamics illustrated an ecological algebra from where these terms have evolved- r and k.

It was hypothesized that the evolution is drawn to any of the two ways i.e. either r selection or K selection under the influence of selection pressure.

The algebra equation of Verhault model is

dN/dt= rN (1- N/K)

This equation of population dynamics represented, K as the carrying capacity, r as maximum growth rate and N as the population.

a.     r-selection

r-selected or r-strategists are those organisms whose life history is regulated by r-selection.

Various Semelparous cephalopods, bacteria, grasses, diatoms, insects and mammals (especially small rodents) are all subject to r-selection and exhibit r-selection triats.

The r-selection traits are:

a.     Generation time is short

b.     Small body size

c.      Produce high number offsprings i.e. high fecundity rate

d.     Onset of early maturity and

e.      The dispersal is high for offsprings.

Hence, r-selected species are also termed as “opportunistic” and are characterized by high “r” and low “K” as they have higher growth rate (r), they predominate in a low density niche for producing high number of offspring with less survival probability. Dandelion Taraxacum genus is a typical example of r-species.

Figure 25: Mice giving birth to many offspring’s is an example of r selected species.

As the r-selected species has high fecundity rate therefore, it is easier for them to sustain and predominant under unstable and changing environmental conditions. Hence, offer low competition as the environment is ever changing there is no need to waste energy in adapting into it, so does not permit competition with other organisms.

b.     K-selection

K-selection species are occasionally referred to as “equilibrium” exhibiting traits such as:

a.     Longer generation time

b.     Large body size

c.      Low fecundity rate

d.     High parental care requires to reach maturity

They are characterized by low “r” i.e. growth rate while high “K” as the density of organism is very much close to carrying capacity with higher life expectancy. Parents invest in fewer offspring for their survival in crowded niche with strong competition between organisms.

Figure 26: Elephant and Whales are K selected species.

K-strategists are organism characterized with K-selected traits. For example whales, humans, Elephant and Arctic terns (exceptional as small but they are long lived). Unlike r-strategists whose population size is controlled by the unstable environment, the K-strategists populate much stable environment and offer strong competition for limited resources. The population density reaches maximum the niche can tolerate and is very much constant.

c.     r/K dichotomy or continuous organisms

Some organisms share traits common to both r-selected as well as K-selected patterns. For example reptiles like sea turtle and trees. Sea turtle are large body size with high life expectancy (K-selected traits) organisms but produce number of un-nurtured offspring (r-selected traits). Similarly trees are long lived and strong competitors (K-selected) but also produce thousands of offspring supporting s-selected traits.