Two different environments or regimes of selection. An r-selection regime consists of environments with abundant resources, but which are short-lived and unstable, and therefore liable to produce high random rates of mortality. Consequently, the optimal parenting strategy is the production of numerous offspring and a low degree of investment in their nurture and protection. With upper limits to fecundity, having offspring whose maturation is accelerated facilitates this strategy. Examples of r-species are nidifugous birds, mice and rabbits. In contrast, a k-selection regime involves environments that are relatively stable, but in which the vital resources are widely distributed. Populations existing under such a regime are near to the carrying capacity (k) of the environment and their mortality rates are more dependent on population density than on marked fluctuations in resources and climate. It is a regime favoring quality over quantity, an investment of reproductive effort into a small number of offspring who will become efficient competitors for the limited resources. The appropriate parenting strategy is then one of extensive and extended nurturing and protection of the offspring. Examples of k-species are elephants, humans and tortoises. The table below summarizes the main differences in the attributes of r- and k-species.
Feature |
r-selection | k-selection |
| Climate | Variable and unpredictable | Consistent and more certain |
| Mortality | Catastrophic and density-independent | More constant, predictable and density-dependent |
| Population size | Variable and below carrying capacity, with the need to re-colonize each year | Constant and close to carrying capacity |
| Intraspecific and interspecific competition | Variable and often weak | Usually strong |
| Selection favors | • Rapid development | Slow development |
| • Early reproduction | Delayed reproduction | |
| • Single reproduction | Repeated reproduction | |
| • Small body size | Larger body size | |
| • Many small offspring | Fewer, larger offspring | |
| • High population growth rate | Greater competitive ability | |
| Life span | Usually shorter | Usually longer |
| Leads to | High productivity | High efficiency |
See Altrical, Parenting, Precocial, Reproductive success, Species, Theory of natural selection, Theory of population pressure