A population is a group of individuals (all members of a single species) who live together in the same habitat and are likely to interbreed. Each population has a unique physical distribution in time and space. It may contain individuals of different ages and its size (density) is likely to change over time, growing or shrinking according to the reproductive success of its members. The study of population dynamics focuses on these changes — how, when, and why they occur. In entomology, a good understanding of population dynamics is useful for interpreting survey data, predicting pest outbreaks, and evaluating the effectiveness of control tactics.
Birth (natality), death (mortality), immigration, and emigration are the four primary ecological events that influence the size (density) of a population. This relationship can be expressed in a simple equation:
All other factors (both biotic and abiotic) exert their impact on population density by influencing one (or more) of the variables on the right-hand side of the above equation. Such factors, known as secondary ecological events, may affect the frequency, extent, magnitude, or duration of a primary ecological event. Cold winter temperatures, for example, could increase mortality and reduce population density. On the other hand, low predation rates in the summer might increase natality and allow the population to grow. Most secondary ecological events act as “population regulating factors”. Whenever they limit a population from reaching its maximum reproductive potential, they are regarded as “environmental resistance”.
Secondary ecological events can be divided into two broad categories: density-independent factors and density-dependent factors.
- Density-Independent Factors include events or conditions, often weather- or climate-related, that affect all individuals equally, regardless of the overall population density. A hard freeze, for example, will kill the same high percentage of the potato leafhoppers in a farmer’s peanut field — no matter if the population contains a few hundred or a few million individuals. In another species, high temperatures and/or low humidity might have a similar, non-selective impact on mortality. Favorable climatic conditions can have a positive effect on population density just as much as unfavorable conditions can have a negative effect. Larvae of Japanese beetles, for example, thrive in years when ample summer rainfall keeps soil conditions moist. Other density-independent events might include wildfires, hurricanes, or hail storms. For an aquatic species, a low concentration of dissolved oxygen or a flash flood after heavy rainfall would qualify as density-independent events because a small population would suffer the same percent mortality as a large population.
- Density-Dependent Factors include events or conditions that change in severity as a population’s size increases or decreases. Common examples of density-dependent factors include predation, parasitism, and disease (one species exploiting another). A large, dense population, for example, is usually more susceptible to the spread of parasites or contagious disease than a small, sparse population. Predators often adapt to changes in the density of their prey populations by migrating into areas of high prey density (numerical response) or by focusing their attention primarily on the most abundant prey species (behavioral response). As a result, large and small populations tend to suffer different rates of predation. Competition for limited resources is also density-dependent — each individual’s share of the “pie” decreases as a population grows numerically. In a small population, members may face competition mostly from individuals of other species who use the same resources (interspecific competition). In large populations, however, competition may also come from other members of the same species (intraspecific competition). In either case, competition undermines survival and reproduction. Any physical trait or behavioral adaptation that reduces or eliminates competition is likely to be favored by natural selection.