Population fluctuations (zoology)

Every species on Earth is composed of one or more populations of organisms which change over time. The populations of some species change in predictable or cyclical ways, while other species frequently exhibit unpredictable and noncyclic changes. Fluctuations in population size may be caused by changes in the population’s environment. For example, seasonal changes in temperature or moisture produce seasonal fluctuations in population size. Resource limitations may produce density-dependent reductions in the growth rate of a population, which—if the reduction is not instantaneous—can result in oscillations in population size. Interactions with other species also produce population fluctuations. Mathematical models of predator-prey systems typically produce oscillations in the abundance of predators and prey. Finally, natural or anthropogenic disturbances often reduce the size of a population, which either recovers to its former abundance over time or declines further to local (or global) extinction.

The Time Scales of Population Fluctuations

Population fluctuations occur over many different time scales. On a geological time scale (occurring over millions of years), species arise, increase to some level of abundance, and finally become extinct. These long-term patterns of species abundance provide a background for understanding population fluctuations that occur over ecological time (over days, weeks, years, or centuries). Fluctuations at these shorter time scales draw most of the attention of ecologists interested in population dynamics.

Many species of animals, including numerous insects and several small vertebrates, exhibit a more or less annual life cycle, characterized by increasing numbers and higher levels of activity during the summer (or wet season) and by dormancy or decreasing numbers during the winter (or dry season). Even highly mobile animals, such as birds, exhibit a strong seasonal pattern of abundance, if viewed from a local perspective; in North America, for example, most songbirds migrate to more tropical latitudes in the fall and to temperate latitudes in the spring, thereby producing a yearly cycle of abundance in each location. Yearly cycles of abundance are predictable and easily explainable in terms of seasonal patterns of temperature, moisture, and sunlight. Of more interest to ecologists are population fluctuations that appear to be random or unpredictable from year to year or those fluctuations that occur out of synchrony with climatic cycles.

Regular Fluctuations

Nonseasonal fluctuations are of two main types: those that exhibit generally regular cycles of abundance over several years and those that fluctuate irregularly or noncyclically. A three-to-four-year cycle of abundance is characteristic of several species of mice, voles, and other rodents found in far northern latitudes. One of the best-known examples of this type of cycle is observed in lemming species (small rodents) in the northern Arctic tundra of Europe and North America. Lemming populations exhibit very high densities every three to four years, up to a hundredfold increase. Then, in the intervening years, their population reaches such a low density that they are difficult to locate and study. Using time series studies of circumpolar population fluctuations over five years, scientists observed these cycles occur 55 percent of the time and most often followed a 3.7-year pattern. While some sporadic population changes have been observed, climate change was not found to be significantly impacting lemming population cycles.

This boom-or-bust cycle is apparently caused by alternating selection regimes. When lemmings are rare, high reproductive capacity and nonaggressive social behavior are favored, and the population grows rapidly. As the growing population becomes more crowded, aggressive individuals are favored because they can hold territories, secure mates, and protect offspring better than passive individuals. The aggressive interactions, however, inhibit reproductive capacity, increase mortality attributable to fighting and infanticide, and expose more lemmings to predation as subordinate individuals are forced by dominants to occupy more marginal habitats. The behavioral changes that occur in response to crowding apparently persist for some time, even as the density declines, so that aggressive interactions and a depressed birthrate continue until the lemming population reaches very low levels. Finally, passive individuals with high reproductive rates are again favored, and the cycle repeats.

Although the breeding cycles of many predators, including snowy owls, weasels, and foxes, are tied to lemming abundance, it appears that the regular fluctuation of lemming populations is a product of crowding and resource limitation rather than of a classical predator-prey cycle; that is, there is no tight coupling between the population fluctuations of lemmings and those of their predators. There is, however, a tight coupling between the population cycles of the snowshoe hare and the Canadian lynx.

Since about 1800, the Hudson’s Bay Company has kept records of furs produced each year. Both the hare and the lynx show a regular ten-year cycle, with the peaks in lynx abundance occurring about a year behind the hare’s peak abundance. Since the hare is a major food source for the lynx in northern Canada, it is logical to assume that this is a coupled oscillation of population sizes, precisely as predicted by classical predator-prey theory.

Some regular cycles of abundance appear to have evolved as a means of avoiding predation rather than being a direct reduction caused by predation. There is a periodicity in the populations of cicadas and locusts. The hypothesized explanation is that predators cannot reproduce rapidly enough to increase their population sizes quickly in response to the sudden availability of a large food supply. When millions of adult cicadas appear above ground for a few weeks after surviving for seventeen years as nymphs in the soil, predators cannot possibly consume them all: No predator could specialize on adult cicadas unless it also had a seventeen-year cycle.

Several northern bird populations (such as crossbills, grosbeaks, and waxwings) fluctuate dramatically, in some years rising to several times their usual levels. This fluctuation may be a response to changing habitat quality. These bird populations always produce as many eggs as food availability and their natural fecundity allow, even though many offspring will not survive. In a good year, a higher proportion of the offspring survives, and the population experiences an irruption, often leading to intense competition and consequent expansion of the range of the population. In subsequent years, population size returns to preirruption levels. Thus, these fluctuations are entirely consistent with normal density-dependent processes responding to a fluctuating environment.

Irregular Fluctuations

Population fluctuations that occur irregularly or noncyclically often appear to be responses to natural disturbances rather than to density-dependent processes or predator-prey relationships. For example, blue grouse persist at a relatively low level of abundance in coniferous forests until a fire occurs. The species rapidly increases in number following a fire and gradually diminishes again as the forest regenerates over the next several decades.

The population fluctuations of some species are not easily attributed to disturbance or any other single cause. For example, swarming locusts typically remain at low abundance in a restricted area for several years; then, without much warning, they experience an exponential population increase and swarm over large regions, consuming large amounts of vegetation. Locust outbreaks can last for several years before the population declines as rapidly as it initially increased. In the early part of the twentieth century, scientists discovered that locusts exhibit two phases—a solitary phase, corresponding to low abundance, and a gregarious phase, corresponding to an outbreak. These phases are attributed to phenotypic plasticity. According to the environment and weather conditions, this plasticity allows locusts to transform from one phase to another. Their genotype produces different phenotypes according to various environmental stressors, which alters their behavior and causes reproductive, developmental, biochemical, ecological, and physiological changes. Moisture is a common trigger for locust swarms because it influences nymph development and survival, egg development, and predator abundance. However, wind and plant nitrogen levels have also been implicated. Furthermore, it appears that environmental conditions are only effective in inducing a phase transformation if a certain concentration of locusts already exists and if the existing locusts are adequately sensitive to crowding.

Measuring Fluctuations

There are two parts to the study of population fluctuation: detecting and measuring the pattern of the fluctuation and identifying the underlying causes of the fluctuation. In general, any method designed for measuring population size can be used repeatedly over time to detect fluctuations in the population. Reference to a specialized textbook on ecological sampling techniques is strongly recommended when using any of these methods, to assure validity of the sampling for subsequent statistical analysis. The mark-recapture method is commonly used with animal populations. There are many variants of this technique, but they all involve capturing and marking some number of individuals, then releasing them; after some time period appropriate to the study, a second sample is captured and the proportion of marked individuals in the second sample (those that are “recaptured”) is recorded. This proportion is used to estimate the size of the population at the time when the individuals were originally marked.

The quadrat method is used primarily with plants and other sessile organisms. Plots (called quadrats) are laid out, either randomly or in some pattern; all individuals within the plots constitute a sample. Quadrats are usually square, but any regular shape may be used. The appropriate size of each quadrat depends on the sizes of organisms to be sampled and on their spatial distribution. If nondestructive sampling techniques are used, the same quadrats may be sampled repeatedly; otherwise, new quadrats must be established for each sampling episode.

A variety of plotless techniques is available for sessile organisms, in lieu of the quadrat method. These techniques were developed to eliminate some of the uncertainties associated with selecting proper quadrat size and location. Most plotless methods locate points on the ground, then measure distances to nearby organisms; each plotless technique identifies the individuals to be measured in a slightly different way.

None of these techniques is adequate by itself to identify the origin or cause of any fluctuation in population size. Experimental manipulation of a population is necessary to elucidate the underlying mechanisms and determining factors. Populations of small, rapidly reproducing species (such as species of Paramecium or Daphnia) can be manipulated in the laboratory, and hypothesized causes of fluctuation can be tested under controlled conditions. This has been done primarily to develop theoretical predictions regarding environmental conditions (such as temperature, moisture, and humidity), resource limitations and fluctuations, and the effects of predators and competitors.

Identifying Causes of Fluctuation

The most interesting examples of population fluctuation, however, occur over spatial and temporal scales too large to handle in the laboratory. Their underlying mechanisms must be elucidated in the field. Because suites of factors typically produce the complex patterns of population fluctuation observed in nature, an effective field study must include all relevant factors. Generally, the most effective studies have been those that have sought to understand the complete life history of a species. Superb examples include the long-term studies of the wolves of Isle Royal National Park, by David Mech and colleagues, and the equally ambitious studies of the grizzly bears of Yellowstone National Park and surrounding areas by Frank and John Craighead and their many coworkers.

Most equilibrium models of population dynamics are capable of producing regular oscillations that mimic the patterns observed in nature. If the model parameters are properly manipulated, many of these models can produce apparently random fluctuation. More sophisticated models have been constructed that incorporate a mathematical equivalent of random environmental fluctuation, although they usually still assume that a population has a tendency to stabilize and that environmental change simply prevents stabilization. The underlying assumption of almost all these models is that species normally exist at equilibrium. This assumption is consistent with the long-held belief that there is a “balance of nature”—that species exist in harmony with their environments.

If an entire species is considered, perhaps the assumption of equilibrium is warranted, at least for extended periods, but at the level of the population, fluctuation is the rule—indeed, it may be that extreme fluctuation is the rule. As noted earlier, many populations fluctuate so markedly that they often disappear; they are reestablished only by colonization from large populations within dispersal range. If populations become too small or too isolated from one another, this colonization cannot occur. Additionally, because small populations are more subject to extinction associated with fluctuation, there is an additional risk of species extinction if only small populations remain.

The problem of extinction is severe since habitat destruction is occurring at an unprecedented rate on a global scale. The fragments of intact habitat that remain because of inaccessibility or preservation efforts contain populations that are smaller and more isolated than in the past. If an isolated population fluctuates markedly, resulting in its extinction from a habitat fragment, its replacement by recolonization is unlikely. Furthermore, the genetic variation maintained by a complex population structure within a species is reduced. As the genetic variation within a species is lost, the ability of a species to respond to environmental change is reduced, and extinction of the species is more likely.

Ultimately, if populations normally fluctuate severely enough that they can be expected to become extinct at frequent intervals, then effective conservation requires the maintenance of pathways for exchange and dispersal of individuals among populations within a species. It also requires the preservation of the largest population size possible to allow for normal fluctuation without extinction.

Principal Terms

Abundance: In ecology, the number of organisms living in a particular environment

Anthropogenic Disturbance: A change (usually a reduction) in population size caused by human activities

Density-Dependent Population Regulation: The regulation of population size by factors or interactions intrinsic to the population; the strength of regulation increases as population size increases

Irruption: A sudden increase in the size of a population, usually attributed to a particularly favorable set of environmental conditions

Population: All the individuals of the same species in the same place at the same time

Population Density: The number of individuals in a population per unit of area or volume

Quadrat: A sample plot of a specific size and shape used in one method of determining population size or species diversity

Sessile: Not free to move around; mollusks are sessile organisms

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