R and k selection

1. Rounak Choudhary
M.Sc. (Gold Medalist), UGC-NET & ICAR-ASRB NET Environmental Science, DCB Ornithology, PGD Industrial Safety, Health and Environment
r - and K -selection

2. • One way of classifying environments (or species
habitats) relates to their variability in time. We can
envision two contrasting types of habitats: (1) those
that are variable in time or short-lived, and (2) those
that are relatively stable (long-lived and constant), with
few random environmental fluctuations.
• The ecologists Robert MacArthur of Princeton
University, E. O. Wilson of Harvard University, and later
E. Pianka of the University of Texas used this
dichotomy to develop the concept of r - and K -
selection.

3. • The theory of r - and K -selection predicts that
species adapted to these two different
environments will differ in life history traits
such as size, productiveness, age at first
reproduction, number of reproductive events
during a lifetime, and total life span.

4. • Species popularly known as r -strategists are
typically short-lived.
• They have high reproductive rates, rapid
development, small body size, large number of
offspring (with low survival), and minimal
parental care.
• They make use of temporary habitats. Many
inhabit unstable or unpredictable environments
that can cause catastrophic mortality
independent of population density.
• Environmental resources are rarely limiting. They
exploit noncompetitive situations. Some r -
strategists, such as weedy species, have means of
wide dispersal, are good colonizers, and respond
rapidly to disturbance.

5. • K -strategists are competitive species with stable
populations of long-lived individuals.
• They have a slower growth rate.
• K -strategists can cope with physical and biotic pressures.
• They possess both delayed and repeated reproduction and
have a larger body size and slower development.
• They produce few seeds, eggs, or young. Among animals,
parents care for the young; among plants, seeds possess
stored food that gives the seedlings a strong start.
• Mortality relates more to density than to unpredictable
environmental conditions. They are specialists, efficient
users of a particular environment, but their populations are
at or near carrying capacity (maximum sustainable
population size) and are resource limited.
• These qualities, combined with their lack of means for wide
dispersal, make K -strategists poor colonizers of new and
empty habitats.

6. • The terms r and K used to characterize these
two contrasting strategies relate to the
parameters of the logistic model of population
growth.
• r is the per capita rate of growth, and K is the
carrying capacity (maximum sustainable
population size).

11. MCQs

14. Which is not the characteristic of ‘r’
selected species?
1. Reproduce quickly
2. Parental care
3. A low survival rate of progenies
4. Produce a large number of progenies