Chapter Summary

Population dynamics play an important role in determining how ecosystems work. Biological organisms generally have the ability to produce enough offspring so populations can grow rapidly when resources are available. Given optimum conditions, populations of many organisms can grow exponentially; that is, they grow at a constant rate of increase so that the population size doubles in some regular interval of time. If conditions appropriate for this kind of growth persist and other factors don’t intervene to reduce abundance, exponential growth can produce astronomical numbers of organisms. We describe the rapidly rising curve of an exponentially growing population as a J curve.

Some populations will grow exponentially until they overshoot the carrying capacity of the environment. Mortality rates rise as resources become limited and the population may crash, often dying back as rapidly as it rose. Other species have intrinsic mechanisms that regulate their population growth rate, resulting in an equilibrium at or near the carrying capacity of the environment.

The most important components of population dynamics are natality, fertility, fecundity, life span, longevity, mortality, immigration, and emigration. The sum of all additions to and subtractions from the population determines the net rate of growth. Mortality rates and longevity are often expressed as survivorship rates that reveal much about a species’ place in its ecosystem and the kinds of hazards that eliminate members from the population.

The factors that regulate population dynamics can be either intrinsic or extrinsic to the population. They can be caused by biotic or abiotic forces, and they can act on the population in either a density-dependent or density-independent fashion. The most important abiotic regulatory factors are usually climate and weather. The most important biological factors are usually competition (both interspecific and intraspecific), predation, and disease.

Often, organisms develop specific behavioral patterns that reduce conflict and facilitate resource partitioning. In some cases of extreme crowding, it is thought that physiological responses to excessive intraspecific interaction can result in stress-related disease that can lead to aberrant behavior, reduced reproductive success, increased susceptibility to disease, and high rates of mortality.