RSelected versus KSelected Organisms

If the intrinsic rate of increase (r) is large, as is true for many small organisms like many species of insects, populations can increase rapidly in an exponential fashion for some period of time. Likewise, the greater duration of time (t) would predict greater numbers of individuals in the future. Of course, no organism can increase exponentially forever; various density-dependent factors cause a slowing of population growth and eventual possible stabilization and variability around a carrying capacity. As the population reaches maximum numbers, various factors act to decrease reproductive rates and increase mortality rates, hence the per capita growth rate diminishes. Species with a greater intrinsic rate of increase generally have lesser generation times (average time between initiation of parent and initiation of their offspring) and greater reproductive potential, such as increased fecundity and reproduction starting at earlier ages. Abundance of these organisms can change rapidly, either increasing or decreasing. They have been called r-selected species because they have evolved traits that enhance their intrinsic rate of increase, and allow them to exploit times or areas of optimum conditions. r-selected species also are short-lived because the traits that increase reproductive potential and decrease generation time have a negative influence on survival. Therefore, there is an assumed tradeoff between increased reproductive capacity and survival and longevity. Organisms that put a large amount of energy into reproduction, via decreased age at first reproduction and increased fecundity will have less energy placed in growth and survival. These species have greater values of r and perhaps greater lag times so their oscillations around K may be great. At the other end of the continuum are so called K-selected species that have a greater body size, greater generation time, lesser reproductive potential, and greater longevity. These species presumably are adapted to maintain abundance in regions with minimal environmental variability, so their abundance is less variable around the carrying capacity than r-selected species. Life-history traits might also be affected by physiological, physical, or behavioral limitations. For instance, many large vertebrates must reach a certain size, maturity, and understanding of the reproductive system before they can defend territories or gain access to females for mating. Because the intrinsic rate of increase is less for K-selected species and lag time may be less, the oscillations around the K may be more dampened than r-selected species. These are all generalities, for instance, not all species that are K-selected have relatively invariant abundances, and their populations can change dramatically with changes in intrinsic and extrinsic forces. Recognize also that there is a continuum of r-selected and K-selected species, and that many species rx lxmx have combinations of r-selected and K-selected traits that affect their population growth rate. The idea of r—Kselection has not really proven useful for predicting life-history traits; hence some researchers have abandoned the concept. The idea that environmental conditions affect life-history traits, however, is useful.

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