Population Ecology

My purpose in this section is not to give an overview of population ecology in general, but rather to highlight one key distinction that is often borrowed from this field into other disciplines: the r/K selection distinction.

The symbols “r” and “K” are said to be taken from mathematical models in ecology known as logistics equations. The logistics in question are the logistics of reproduction in continuously reproducing (as opposed to seasonally reproducing) populations, given such variables as the population density and the availability of resources. The core idea is that when resources are abundant, organisms that reproduce the quickest are favored. When resources get depleted and the habitat becomes crowded, slower-growing, slower-reproducing organisms do better. Thus, the r/K-selection distinction identifies two broad regimes of natural selection that give rise to qualitatively different kinds of biological order.

The letters “r” and “K” refer to variables that describe rate of reproduction and carrying capacity of the environment supporting the population, respectively. When resources are so abundant that they are effectively unlimited, the only limit on reproduction is the time it takes to reproduce, i.e. the maximum rate of reproduction (rmax). This maximum rate becomes the selection factor, such that the fastest, most prolific replicators make the largest contributions to future generations. These rate-adapted organisms are called r-selected.

Of course, this growing population will consume resources, changing the logistics of reproduction. Eventually the diminishing availability of key resources will come to be the factor limiting reproduction. There will come a point when the population is as large as the environment can sustain. This upper limit of population density is called carrying capacity and it is represented by the variable K. At K, the net rate of reproduction R0 is constant (at unity), and the intrinsic rate of increase r is zero. At that point, replicators which contribute the highest proportion of surviving offspring across generations exhibit the greatest fitness. These organisms must be adapted to survive in densely populated environments with limited resources and intense competition[1,2].

Organisms generally get larger under K-selection. Various kinds of internal and external specializations become necessary, boosting complexity and the need for organization and coordination. Since the environment is crowded and free resources are not easily available, scattering many offspring far and wide will not accomplish much. Fewer, larger offspring need to be created, packaged with on-board food resources (e.g. eggs that contain yolk sacks). This kind of adaptation is kind of like a plan to have resources available at a steady rate when needed. By developing this internal complexity and differentiation, the organism is able to buffer itself from
some of the instabilities of the outside world.

In organizational lifecycle terms, the transition from an r-selection regime to a K-selection regime is like the transition from the P-heavy early stages of Infancy and Go-Go to the more A-heavy stage of Adolescence. In organizations in the earlier stages, the rapid completion of tasks by any means possible is both normal and needed. However, with growing success comes the need to get organized for greater efficiency. Further growth depends on getting some internal process stability and doing some planning for longer business cycles, buffering the organization a bit from instability
and volatility in the external environment.

The r/K selection shift is thus a good model for the PA distinction. It does not help us illuminate transitions to E and I-type selection pressures. Holling’s adaptive renewal cycle sheds more light in that area.

1. Pianka, E. R. (1970). “On r- and K-selection.” American Naturalist, 104, 592-597.
2. Pianka, E. R. (1988). Evolutionary Ecology (4 ed.). New York: Harper and Row.
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