Population Dynamics

A population is a collection of individual organisms of the same species that occupy some specific area. The term “population dynamics” refers to how the number of individuals in a population changes over time. Biologists study the factors that affect population dynamics because they are interested in topics such as conservation of endangered species (for example, the Florida panther) and management of fish and wildlife. In addition, basic knowledge about the processes that affect population dynamics can be used to predict future patterns of human population growth.

How Do Biologists Characterize Populations?

Biologists distinguish between two main types of populations: unstructured and structured. In an unstructured population, all individuals are subject to the same general ecological pressures. That is, the rates of growth, repro­duction, and mortality are roughly the same for all individuals in the pop­ulation. A bacterial colony is a good example of an unstructured population. Conversely, in structured populations, individuals can differ from one an­other in ways that make some individuals more susceptible to mortality or more likely to reproduce than others. Examples of structured populations include many insects, sea turtles, trees, and fish. In these cases, mortality is often much higher for younger (and/or smaller) individuals. In addition, re­production is often delayed until individuals are older (and/or larger).

How Does Resource Abundance Affect Population Dynamics?

The abundance of environmental resources such as food, water, and space determines how population abundance changes over time. In the presence of unlimited resources, populations grow exponentially. If one plots the number of individuals in an exponentially growing population over time one finds a J-shaped curve where the slope gets ever steeper. This curve is described by the following equation:

N_t = N₀e^rt

Where N₀ is the initial number of individuals, N_t is the number of individ­uals at a future time, r is the rate of increase, t is time, and e is the base of the natural logarithm . The rate of increase (r) is determined by the difference between birth and death rates of the population. In 1999 the U.S. Bureau of the Census estimated the rate of population increase (r) for the world human population to be 0.0129 (or 1.29 percent) per year. Few natural populations grow at exponential rates for extended periods of time because resources typically become limiting when population abun­dance is very high.

The abundance of environmental resources determines the rate of population growth over time.

In an environment where resources become limited, populations exhibit a pattern of growth called logistic growth. In this case, if one plots the num­ber of individuals in the population over time, one finds a sigmoidal, or S- shaped curve. When population abundance is low, the population grows exponentially. However, as population size increases, resources become lim­ited, the population growth rate slows, and the population abundance curve flattens. The number of individuals present in the population when the growth rate slows to zero is referred to as K, the carrying capacity. The car­rying capacity is the theoretical maximum number of individuals that the environment can support. Although estimates of K for humans are contro­versial, most are around 12 billion.

Using concepts from basic population biology, biologists have distin­guished two strategies for population growth. Some species have character­istics that allow them to grow rapidly when an environment with abundant resources is newly created (for example, a new clearing in a forest). These species are referred to as r-selected species and typically reproduce at a young age and produce many offspring. Other species, called K-selected species, have characteristics that make them well suited for life in environments where there is intense competition for limited resources. These species are often strong competitors, reproduce later in life, and produce fewer off­spring than r-selected species.

How Does Variability in Environmental Conditions Affect Population Dynamics?

A key assumption of the logistic population growth model for environ­ments where resources are limiting is that environmental conditions are constant. In nature, environmental conditions may vary substantially over time. In such variable environments, the abundance of individuals in a population may also fluctuate over time. Some populations cycle in a pre­dictable manner. Populations that fluctuate widely or have low abundance are especially vulnerable to extinction, an event in which population abun­dance declines to zero. Extinctions may be local (a population in a par­ticular area is lost) or global (all populations of a species decline to zero and there are no living individuals of the species left on the planet). For example, the passenger pigeon, which was once one of the most numer­ous birds on Earth, went globally extinct in 1914 due to overhunting and habitat loss.

How Do Physical and Biological Factors Regulate Population Dynamics?

Patterns of population abundance are affected by a variety of biological and physical factors. For example, the abundance of a given species (for exam­ple, snails) might be controlled by the abundance of organisms that have a negative effect on the species of interest, such as competitors, predators, and diseases. Similarly, population abundance could be limited by the abundance of organisms that benefit the species of interest (for example, algae con­sumed by the snails).

In fact, some organisms require the presence of other species called sym­bionts with whom they live in direct contact. For example, corals use food molecules synthesized by symbiotic zooxanthellae (a type of algae), and zooxanthellae receive nutrients and protection from corals. However, not all pop­ulations are regulated by biological factors involving interactions with other species. Physical factors like water availability and temperature can control population abundance of some species.

Which type of factor (biological or physical) has a stronger effect on population dynamics? As one might suspect, the answer depends largely on the population that is studied. Some populations are regulated mostly by bi­ological factors, others are controlled by physical factors, and most popula­tions are affected by both biological and physical factors.

References

Cohen, Joel E. “Population Growth and Earth’s Human Carrying Capacity.” Science 269, no. 5222 (1995): 341-346.

Ehrlich, Paul R. The Population Bomb. San Francisco, CA: Sierra Club, 1969.

Molles, Manuel C. Ecology: Concepts and Applications. Boston: McGraw-Hill, 1999.

U.S. Bureau of the Census, International Data Base. http://www.census.gov/pub/ipc/ www/worldpop.html.