In theory, populations of many organisms have the potential to grow exponentially. If we let N0 represent the initial number of organisms in a population, t represent the time interval, and r represent the rate of increase, then Nt, the number of individuals present at time t, may be calculated by the formula Nt = N0(er)t = N0(ert). This would be represented by the gold line (J-shaped curve) on the graph to the right.
However, for populations in the real world, exponential growth is not possible. Populations cant continue to grow larger forever because resources are limited and as density increases, so do competition and mortality while natality (fecundity) decreases. Thus, population growth decreases, eventually leveling off at zero population growth. The carrying capacity (symbolized by K) for a given population is the level at which population growth ceases (growth levels off). At this point, the population is theoretically in equilibrium with its environment, and the equation to predict population size must be modified to include this limit. Thus, the equation for logistic growth is , and is represented by the green line (S-shaped or sigmoid curve) on the graph to the right.
Sometimes because of time lags as populations responds to food or other environmental conditions, population numbers may fluctuate/oscillate instead of maintaining a steady level. Classic examples include lynx and snowshoe hare populations, lemmings, and the relationship between mole and songbird populations and Periodical Cicadas. Lynx prey on snowshoe hares, so as the hare population gradually increases, the supply of twigs upon which the hares feed decreases and the lynx population gradually increases, out of sync by several years. As the lynx population increases, the snowshoe hare population decreases due to increased predation (and less twigs per hare). As the hare population decreases, there is less food for the lynx, so their population gradually decreases. Thus levels of these two populations fluctuate in about a nine- to ten-year cycle. Lemming populations typically fluctuate in three- to four-year cycles. Usually, a year or two before Periodical Cicadas emerge, while larger larvae are present underground, the mole population increases in response to the increased food supply, then after the cicadas emerge, drastically decreases due to starvation. However, in the year the cicadas emerge, insectivorous songbirds utilize this ready food supply to feed their young. Thus, in that year, increased numbers of songbirds successfully fledge, resulting in greater competition for food the following year, when the cicadas are not present. Thus, to a certain degree, mole and songbird populations fluctuate in 17-year cycles, influenced by availability of Periodical Cicadas as food.
For any population of organisms, there is an optimum population density. If the density is too high or too low, the rate of growth declines. If there are too many organisms, the competition for food and other resources is too great, and many starve or die of other causes. If the population is too sparse, organsims cant find a mate as easily. The rate of population growth (N/t) is related to the population density by the equation as illustrated in the graph to the right. The green line corresponds to the green line for logistic growth in the example illustrated above. If the growth rate is too low, especially if the rate of increase is less than zero (note the ends of the gold line on this graph), the population will become extinct.
Extinction is a natural process, a normal part of natural selection, thus species (and populations within species) vary in probability of becoming extinct. Extinction may be caused by a single, unusual event, such as the mass extinction of dinosaurs due to sudden environmental changes, or by tremendous ongoing pressure, such as ongoing human habitat destruction (tropical rainforests, wetland areas) or direct killing (bison).
Factors such as natality, mortality, migration, etc. affect population density. The density of a species is the number of organisms of that species per unit area/volume. As discussed in the Statistical Analysis Worksheet, distribution patterns (dispersion) within a population are also important. The dispersion of a species refers to the spatial distribution of the individuals. Generally, these are arranged in one of three ways: In uniform dispersion, individuals are evenly scattered throughout the habitat. In random dispersion, individuals are scattered throughout the habitat at random. In clumped dispersion, individuals are clustered together in some locations, while other locations have few/none.
If there is high natality and/or low mortality, a population can become very dense in a specific area, resulting in increased competition for resources. Various organisms cope with this in different ways, including maintaining territories, dispersal (migration), pheromone-triggered decrease in natality, and/or increased mortality due to greater competition.
Dispersal (migration) goes on at a fairly constant rate, but is most noticeable when the population density is high. There is no general rule/guideline as to which organisms in the population will disperse, but often those which leave are sub-adults which have been driven out by aggressive older adults, and these frequently disperse into submarginal and/or unoccupied areas/habitats where only some survive and reproduce. In other cases of dispersal when the population is not under stress, often stronger, genetically more-fit organisms can/do move to a new habitat. In these cases, the habitat is usually adequate to good and these individuals usually survive well (Refer to the Population Genetics Worksheet, which discusses the effects of dispersal on the gene pool). Especially in reference to human populations, the term immigration refers to moving into a new place and emigration emigration refers to leaving a place.
Copyright © 1999 by J. Stein Carter. All rights reserved.
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