Every organism and population, or group of organisms of the same kind, has a habitat-a place where they live naturally. A habitat, however, can support only a limited number of organisms.
Imagine inviting some relatives to dinner. You prepare enough food for six people, but two aunts and an uncle hear about your dinner party and drop by unexpectedly. There is a limited amount of food to offer to your guests, and if any more relatives appear, there will be even less food for everyone. There are similarities between your experience and a habitat’s carrying capacity.
A habitat has a limited number of resources that organisms in that habitat must share, meaning it has a carrying capacity. Carrying capacity is the number of organisms within a population that a habitat can support without losing its resources or damaging them severely.
Food, water, shelter, and space are some of the critical resources in every habitat. In fact, each is a limiting factor within a habitat. A limiting factor is any factor that limits the growth, the numbers of, or the distribution of organisms within an ecosystem.
Each species, or kind of organism, has a range of tolerance. In other words, there are lower and upper limits of any factor that a species can tolerate, or allow, and still survive. For example, all organisms require water for survival. Too little water limits the number of organisms that can survive in a habitat, but too much water can do the same.
Relationships in a Habitat
Every living organism competes with other living organisms for limited resources, including water, food, shelter, and space. Competition exists among organisms within the same species. It also occurs between species. Take squirrels and blue jays in a forested habitat as an example.
Squirrels and blue jays eat many of the same kinds of food. Acorns are a particular favorite. Each species competes for the acorns. As the acorn supply decreases, both animals must find other food resources, such as berries or seeds. The species will compete for these resources, too. Ultimately, there may not be enough food to support both populations of animals. Some squirrels and blue jays may leave to find food in a different habitat. Others may die. In either case, populations shrink because the habitat’s carrying capacity has been exceeded. It has gone beyond the natural limits by which all organisms can be sustained.
Other relationships besides competition among species limit a population’s growth. Predator-prey relationships are an example. A predator hunts for, captures, and eats all or part of its prey. The red fox, for example, lives in a variety of habitats, including forests, grasslands, mountains, deserts, and even human neighborhoods. Among other things, red foxes prey upon rodents such as squirrels, rabbits, and mice.
When there is an abundance of rodents, meaning the population of rodents is high, food is not a limiting factor, and the fox population grows quickly. The increased pressure on rodents, however, causes their populations to fall. Food supply becomes a limiting factor for the fox population. Foxes must compete for fewer food resources. Some foxes may move elsewhere in search of food. Others may suffer from disease or starvation. Consequently, the fox population declines. It moves toward equilibrium, or a balancing point. The fox and rodent populations may increase and decrease a number of times before equilibrium is established.
Reindeer on the Pribilof Islands
The reindeer of the Pribilof Islands offer a good example of what happens when a population exceeds a habitat’s carrying capacity. There are four volcanic islands within the group called the Pribilof Islands. Two of the largest islands are St. Paul and St. George. These rocky islands off the coast of Alaska are covered with tundra, which is flat, treeless, Arctic land. Tundra is the coldest and driest of all biomes. Permafrost, or permanently frozen ground, exists most of the year, melting only in summer. So little liquid water and poor soil limit what grows in the tundra.
Lichens are among the few organisms that can survive in the tundra. While they are able to colonize bare rock, they grow slowly because they get their nutrients from air, rain, and melted water.
In 1911, government officials introduced 25 reindeer to St. Paul Island. Ten years later, the population had grown to 250 reindeer. The number continued to grow until there were about 2,000 reindeer on the island in 1938. Then a number of factors combined to reduce the population drastically. Illegal hunting and harsh winter weather affected the population, but lichens proved to be the most important limiting factor.
Lichens are a major source of food for reindeer. As the reindeer population grew, so did the demand for lichens. Lichens did not grow fast enough to continue feeding the reindeer. The reindeer exceeded the island’s carrying capacity, and their numbers dropped dramatically. By 1950, only eight reindeer remained alive on St. Paul Island.
Migration and starvation are possible responses to scarce food resources, but so is dietary diversity, or variety. Scientists have observed that when a preferred food becomes hard to find, animals usually diversify, or broaden their diets to include different kinds of foods. Sea otters living in California’s coastal waters are no exception.
In their research, scientists learned that red sea urchins, a favorite food among sea otters, are plentiful in the waters around San Nicolas Island. The sea otters living there feed on these spiny sea animals, as well as marine snails and crabs. Within the population, there is little diversity in the eating habits of individual sea otters.
Fewer red urchins live in the waters off the Central Coast. So it was no surprise to scientists that the diets of sea otters living along the Central Coast were more diversified than those of the San Nicolas sea otters. However, studies of individuals in the population show more specialized diets, and that was surprising to scientists. The finding suggests that food webs are more complex than scientists think. Wildlife managers who monitor animal populations may need to change their focus from the dietary habits of the group to the dietary habits of the individual, making their studies of food limitations even more challenging.