The third principle of population dynamics recognizes that individuals living in dense populations may have difficulty acquiring the resources they need to survive and reproduce. This principle is similar, in some ways, to the economic law of diminishing returns, in which firms and bureaucracies tent to become less efficient as they get larger because of competition between individuals, departments, and such. It is also similar to the evolutionary principle of struggle for existence which was first used by Malthus, and later by Darwin as a pillar of his theory of evolution.
Intraspecific competition
The struggle between members of a population for scarce resources is usually called intra-specific competition. A major consequence of intra-specific competition is that the survival and/or reproduction of individual organisms tend to decline as the density of the population rises. This is commonly called "density dependence" in the literature. In other words, the action of the 3rd principle causes the birth rate of individuals to fall and/or the death rate to rise as population density increases, the net result being that the realized per-capita rate of change R declines with population density (see Figure).
Regulation or Stabilization
Notice that the R-function now has a negative slope (see Figure). What this means is that the 3rd principle acts as a –feedback on population density and, therefore, tends to stabilize population dynamics - the population is also said to be regulated by the –feedback process. Hence, populations under the influence of the 3rd principle will tend to fluctuate around a stabilizing equilibrium in a typical "saw-toothed" pattern, much like the sycamore aphid. If the environment become more favorable for the species, then the equilibrium density is expected to increase. In fact K can be considered a measure of the favorability of the environment for the species in question, in terms of the abundance food or other essential resources, and the presence or absence of enemies. It is also called the "carrying capacity of the environment".
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© 1998 Alan A. Berryman