The study of populations and population ecology is a growing field of biology. Plants and animals are studied both singly and in relationship to one another. Factors that affect population growth and overall health are constantly being sought and analyzed.
Animal populations are a bit easier to discuss since the genetic basis from which animal populations arise is not as complicated as the genetic basis of plants. Animal populations are more constrained by genetic variation than are plants. No haploid or polyploid animal exists or reproduces. Population dynamics (growth, death, and reproduction rate) are more easily explained in animals and many models, or predictions of population success, can be used to examine and learn from animal populations. Insects, in particular, provide a wealth of interesting population models since they tend to reproduce rapidly and in high numbers and their life cycles are fairly short.
A great deal of healthy debate exists regarding the definition of a population. Not all species fit neatly into any one definition. In general, a population is described as a group of individuals of a species that lives in a particular geographic area. It is a sexually reproducing species in which individuals add to the continued growth or sustenance of the population. The sexually reproducing component of the definition is critical in that many endangered populations are at risk simply because they are not reproducing effectively enough to sustain their populations.
An example of this was found in the remnant population of the California condor. Its numbers in the wild had dwindled to approximately twenty individuals. The condors were not successfully reproducing in their natural habitat. Each year the eggs were infertile or crushed and no offspring were being reared. The population continued to dwindle as the older birds died and no young birds were born to replace them. Eventually, they were all captured and artificially bred in hopes of preserving the species. The population has increased in captivity, but California condors still face the challenge of becoming a viable biological population in the wild.
A population can exist over a broad geographic expanse, such as the North American continent or even the Earth, but it can also exist in one small pond. Many different populations of desert pupfish live only in their own small pond throughout the Mojave Desert. Populations of mites or parasites may live on one specific host or they may prefer one area of the host organism. For instance, a population of tapeworms may live only in the intestinal tract of the host while a louse population exists in the external environment. Most often a population for biological study is one in which a distinct geographical range can be assessed, such as a valley, plain, or forest, and a definite genotype can be identified.
A great deal of genetic variation exists within a population and the predictions of how this variation may be expressed is the fascinating work of population geneticists. Mathematics is used to help predict how a trait will move through a population or how a population will respond to an environmental pressure. Mathematical models that help scientists study population response to internal (genetic) and external (environmental) pressures are predictive only and are never entirely correct.
It is the very nature of scientific models to be incorrect. No human or computer can ever account for all the existing variables or potential variables that may affect a population. How would one anticipate an intensely cold and violent storm three years from now? How would it be possible to predict a specific genetic mutation? None of this can be done, but all natural variables eventually affect populations in some way or another.
All populations are ultimately controlled by the carrying capacity of their environment. The carrying capacity is the sum of all resources needed by a specific species in order to survive. The abundance of food is a major control. When food is short, young, old, and unfit members will die.
If two or more species are competing for one food source, additional pressure is placed on both populations. If two species of birds rely heavily on a certain insect for protein and nutrition, the availability and abundance of that insect is crucial. Any reduction in the population of insects will result in a loss of species from either bird population. There may not be enough food to feed and rear the young. Older birds unable to fly long distances for alternate food will also perish.
Food is not the only limiting factor that affects carrying capacity. Shelter and places to safely rear young are also part of the limits for many populations. Ground squirrels often rear their young underground or in rock shelters. This protects the young from predators, such as foxes or birds of prey. If there is a lack of adequate shelter the young are in peril. The rock squirrel lives in the crevices of rocks. If there are no rocks in a particular feeding area the squirrel may be able to eat out in the open or under a tree, but hiding from snakes and birds would be difficult under such circumstances. Rock squirrels in an area without the necessary physical habitat would be unable to successfully raise young to replenish the next generation.
Climate variables can also affect carrying capacity. If normal weather conditions change significantly over a period of time, certain plant species may not survive. Animals living on those plants will also perish. Pollution may also prevent populations from surviving; water and food may become toxic and make the environment unsuitable for existing populations.
Ultimately, the carrying capacity of an area affects the growth of a population since the numbers of species will not grow if there are not enough resources for the individuals to survive. The population will be regulated by the availability of food and other resources. Numbers of individuals will not increase, but in all likelihood remain the same.
Population regulation is achieved through several mechanisms. The environment is always at the top of the list of population regulation factors for reasons just mentioned. Another factor is the number of predators in any given area. If an animal that feeds on other animals is maintaining a stable population, it will only eat so many prey species every year. If, however, the prey species begins to flourish and increase in numbers, there will be more food for the predator. The predator population will also become more successful and increase in numbers. Eventually the predators will eat all the available prey and they will have reached the limits or carrying capacity of their environment.
The classic example of this type of population regulation is the Canadian lynx and the snowshoe hare. With regular cyclicity, hare populations increase in their habitats. As they rear more and more young, the available food for the Canadian lynx increases. With readily available food, the lynx is also more successful at raising young, so its population grows in response. Eventually the limit of how many snowshoe hares can exist in a particular region is reached. The lynx eat more and more hares, thus regulating and even reducing the population of hares. As the snowshoe hare population declines so does the population of Canadian lynx. Each population is regulating the growth of the other.
Density Dependence and Independence
Density-dependent and density-independent populations are the focus of many scientific models. A density-dependent population is one in which the number of individuals in the population is dependent on a variety of factors including genetic variability and the carrying capacity of its environment. In a density-dependent population the ability to find a mate is critical. When population numbers are low this may be a critical factor in the survival of the species. If no mates are found during a season there will be no offspring.
Another effect of density-dependency is intraspecific (within the species) competition. When members of a population are all competing for food resources or adequate habitat, the less fit (unhealthy, young, or old) members will lose out and perish. As a population grows, this type of intraspecific competition serves as a self-regulating mechanism, eliminating many members of the population.
Density-independent populations are those that are regulated by catastrophic or unusual events. Hurricanes constantly provide population controls on the coasts of North America or neighboring islands. Winds bring down trees where birds and other animals find shelter. Water drowns populations of squirrels or other mammals. Freshwater fish are inundated by saltwater.
The list of catastrophic effects is long, but the end result is that the local populations suffer and must rebuild until the next hurricane comes along. Other catastrophic events may include climatic changes or the accidental dumping of toxic waste. Oil spills have become an increasing threat to coastal populations of all types. In a truly severe event the populations may never return and the members that survive may not be able to live in the wasted environment.
Human populations defy the ecological rules imposed on other animal populations. Because we can modify our environment, humans can live beyond the carrying capacity of our environment by growing extra food and building shelter. Medicine has helped the survivorship of our young and elderly. It is hard to describe human populations in terms of density dependence or independence. We are a highly successful species that is increasingly intelligent about survival in formerly inhospitable habitats.
Most scientists agree, however, that there is a limit to which the human population can grow. Eventually all the food that can be produced may still be inadequate to support the population. Resources like water and energy may reach their limits. There is no reason to believe that human populations will not also be regulated by environment. Mass starvation has already occurred in regions where natural catastrophes destroy food resources and hinder the attempts of other populations to help. There are scientists and governments that advocate population regulation on a voluntary basis to keep the human population from exceeding the carrying capacity of the Earth.
Extinction is a part of population biology. It is a natural process that has been or will be experienced by every population. The fossil record is full of animals that flourished for millions of years and then vanished.
Extinction and the mechanisms that compose it are not always well understood. The Earth is still experiencing extinctions at an amazing rate. The mammoths and saber-toothed cats are among the better known populations to have vanished in the last ten to twenty thousand years. Animal populations still disappear every day. However, when one population disappears, quite often another grows. Humans are blamed for the loss of many species. As human populations stabilize, additional populations will begin to flourish. This is the nature of population ecology cycles.
see also Human Populations; Population Dynamics.
Brook Ellen Hall
Gutierrez, A. P. Applied Population Ecology: A Supply-Demand Approach. New York: John Wiley & Sons, 1996.
MacArthur, Robert H., and Joseph H. Connell. The Biology of Populations. New York: Wiley, 1996.
Raising captive baby condors requires all sorts of skills. After feeding the young birds had become an issue, their caretakers started to wear hand puppets designed in the image of the head of a female California condor. The babies quickly adjusted to taking their food from the mouth of their fabricated "mother." This gimmick made a difference in the number of babies surviving in captivity.