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:
= (Births + Immigration) - (Deaths + Emigration)
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.
Contest Competition: In situations where resources (food, space, etc.) are fairly stable over time, intraspecific competition may take the form of "contests" in which individuals lay claim to a "territory" and defend it from all intruders. Each territory generally provides enough resources for the owner's survival and reproduction; failure to "win" a territory can be a competitive disadvantage. Since only the strongest (most "fit") individuals are likely to hold a territory, they have the best chance to pass on their genes to the next generation.
Scramble Competition: In situations where resources are temporary or transient, there is little or no advantage to defending a territory. Insects that compete for these types of resources (blow flies on a corpse, for example) "scramble" for access. The first arrivals encounter the best conditions for survival and reproduction (first come, first served). Latecomers encounter a depleted resource that may no longer support growth and development.
Each species occupies a unique ecological niche within its community. The niche is a Gestalt-like concept encompassing all of the biotic and abiotic parameters that determine where a population lives (its "habitat") as well as the role it plays within the food web (its "profession"). Interspecific competition occurs whenever the niche parameters of two (or more) different species overlap. The more the overlap, the greater the competition.
Interspecific competition usually leads to one of three possible evolutionary outcomes:
Density-dependent emigration (movement away from crowded conditions) is another important regulator of population size. It not only reduces overcrowding in the home range, but it also increases the likelihood of establishing new populations elsewhere. In the long term, emigration benefits the individuals who remain behind as well as the pioneers who find new places to live.
Cooperative interactions may also give populations a competitive advantage, allowing them to reduce mortality, use resources more efficiently, or accomplish tasks that could not be performed by solitary individuals. Intraspecific cooperation has certainly contributed to the evolutionary success of all social insects (ants, bees, wasps, and termites). These species outnumber all other animals in many terrestrial habitats and, despite their small size, they usually play dominant roles in community ecology, both as consumers and as decomposers. Cooperative interactions between different species (i.e. mutualism and commensalism) are also common in the insect world. These symbiotic relationships occur not only between different insect species (e.g. ants and aphids) but also between insects and microorganisms, between insects and vertebrates, and between insects and plants.
When food is abundant and growing conditions are favorable, a population has the potential to increase in number from generation to generation -- just as money in a bank savings account accrues interest over time. The population's intrinsic growth rate ("r") is similar to the bank account's interest rate -- it is a measure of how quickly the increase occurs. Growth is said to be geometric when each generation's increase is a constant percentage of the total population size. Geometric growth is also known as exponential growth because the larger the population gets, the faster it grows. With a 5% growth rate, for example, a population of 50 beetles would grow by only 31 individuals in 10 generations whereas a population of 10,000 would grow by 6,289 during the same amount of time. The "J-shaped" curve in Figure 1 represents the typical form of an exponential growth curve.
Obviously, exponential growth cannot continue indefinitely in a resource-limited environment. Eventually a population becomes so large that it runs out of free space, outgrows its food supply, or exhausts other assets. The upper limit on population density is called the environmental carrying capacity (usually represented by the symbol "K"). As population density approaches the carrying capacity, competition becomes more intense, mortality increases, the birth rate drops, and any one of the following alternatives is possible:
- The population may level out and stabilize below the carrying capacity. This pattern is known as a logistic or sigmoid (S-shape) growth curve.
- The population may briefly overshoot the carrying capacity and then crash, resulting in repeated cycles of "boom" and "bust".
- The population may oscillate around (or below) the carrying capacity.
In reality, all of these models are gross over-simplifications. Natural populations respond to a wide range of environmental conditions that are rarely constant over time. Some species have complex life cycles requiring different resources or conditions at each stage of development. Others show distinct temporal or seasonal variability in their response to environmental conditions.
r- and K-selection. Some insects are ecological opportunists. They exploit disturbed or unstable environments, take full advantage of transient resources, produce large numbers of offspring in short periods of time, and rapidly disperse into new habitats when conditions turn unfavorable. This life history strategy, often called "r-selection," is typically found in species that have a short life cycle, small body size, and high mobility (ex. house flies). Most individuals in "r-selected" populations die before reaching sexual maturity so a high reproductive potential is essential for the species to avoid extinction. These insects often play a major role as colonizers in the early stages of ecological succession -- they are also likely to be regarded as pests if their colonial empire spreads into farms, ranches, or human habitations!
On the other hand, life expectancy is usually longer for species that live in stable habitats (like mature grasslands or climax forests). More of these individuals reach sexual maturity and populations tend to stabilize near the environmental carrying capacity. Under these conditions, often called "K-selection," there is no particular advantage to having large numbers of offpring. Selection pressures focus on intraspecific competition and efficient use of resources. "K-selected" species often have longer life cycles, larger body size, and relatively low growth rates.
Ecologists recognize that r- and K-selection are opposite ends in a broad spectrum of life history strategies. Most species fall somewhere in the middle of the range with a blend of "r-selected" and "K-selected" characteristics. Ants and termites, for example, produce large numbers of small, expendable offspring but they have long-lived colonies that are highly competitive in stable environments.