Life History Strategies
Life History Strategies
Among biological organisms, there is a continuum in life history strategies between what are referred to as r -selected species and k -selected species. Life history strategy is correlated with many aspects of an organism's reproductive strategy and life history, as well as with demographic variables such as generation time and life span, and population parameters such as population density and population dynamics . Where individual species fall on the r-k continuum is largely determined by the environment in which they live.
The variables r and k in r -and k -selection come from the logistic equation for population growth. This equation describes how population density changes over time. In the initial phase, population growth is very rapid and largely dependent on the variable r, which represents the intrinsic rate of natural increase of a species. Mathematically, r is the birth rate plus the immigration rate, minus the death rate and the emigration rate. A population grows rapidly in its initial phase because there are abundant resources, and consequently little or no competition between individuals. Thus, r -selected species exist in circumstances where they are often at this stage of rapid growth.
The k refers to the maximum density at which a population is able to exist in a given environment, and is called the carrying capacity of that environment. The value of k depends on the resources available. Once population densities are close to the carrying capacity , growth slows and population density levels off at or around k. Because resources are no longer in abundant supply in this saturated environment, there is significant competition between the individuals of a population. Species that are k -selected describe those for which the population density is usually close to the carrying capacity. Differences between r -and k -selected species exist over a wide variety of traits.
Those species that are r -selected exist well below the carrying capacity of their environment. This may be a consequence of either biotic or abiotic qualities of the environment. For example, many r -selected species are associated with unstable environments that alternate between periods of abundance and periods of high mortality. So long as resources are abundant, populations grow exponentially. Then, the population is decimated and the cycle begins again. On the other hand, population densities may remain well below the carrying capacity of an environment as a consequence of biological factors such as predation.
Because r -selected species exist well below the carrying capacity of the environment, there is generally little competition between individuals. In addition, mortality may depend largely on chance. Under these circumstances, the production of high-quality offspring may not necessarily pay off. It is more important to produce a large number of offspring as quickly as possible, thus increasing the probability that at least a few of them will survive long enough to contribute to the next generation. The emphasis is on the quantity of offspring produced rather than on their quality.
Features associated with r -selected species include small size, a short generation time, reproduction early in life, and the production of large numbers of offspring in which comparatively little investment is made. Some r -selected species are semelparous , meaning that individuals reproduce in one big reproductive bout and then die. This is sometimes referred to as big-bang reproduction.
In terms of life history, r -selected species exhibit what is called a Type III survivorship pattern, with very high mortality in the early stages of life, and only a very small proportion of individuals surviving into adulthood. The population patterns of r -selected species often show periods of rapid, exponential growth , followed by sudden crashes. Often, r -selected species are the first colonizers of a new habitat.
K -selected species, on the other hand, generally occupy comparatively stable environments. Because there are long periods of environmental stability, populations are able to increase in size until population densities are close to the carrying capacity k of the environment.
In these saturated environments, crucial resources are in short supply, and there is intense competition between individuals of the population. As a result, competitive ability becomes very important. The number of offspring produced becomes less important, while the quality matters more. (There is a necessary trade-off between the quality and quantity of offspring produced, because of the limited resources that a parent is able to acquire and process.)
In k -selected species, individuals produce fewer, high-quality offspring that will perform and survive better in a competitive world. This is associated with such traits as larger body size, longer generation time, slower development, and reproduction later in life. K -selected species also often exhibit parental care.
K -selected species are more likely to be iteroparous , that is, to reproduce in numerous bouts. They also tend to invest in survival more than r -selected species, and may generally be more intelligent. Unlike r -selected species, k -selected species have a Type I survivorship curve, where survival early in life is relatively high and most individuals live to a comparatively late age. Population sizes in k -selected species are relatively stable, at or near the carrying capacity of the environment. Unlike r -selected species, k -selected species are not effective colonizers. Instead, they tend to be found in climax communities (stable, long-established ecological communities).
It is important to remember that there is a continuum between r and k strategies among biological organisms, and that it sometimes does not make sense to try to pigeonhole species as one or the other. Mice, for example, seem to be k -strategists compared to clams, which are perpetually emitting tiny eggs. Mice are also characterized by parental care.
However, when compared to other species of mammals, mice are closer to the r -strategist extreme. Their generation time is shorter than that of most other mammals, as is their life span and time to reach reproductive maturity. In addition, mice have much larger litters than most mammals.
Small mammals such as rodents and rabbits are often closer to the r -selected extreme, while larger mammals such as humans and elephants are more k -selected. Among plants, weedy species are r -selected, while larger species with longer life spans, such as trees, are k -selected.
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