Interactions Among Populations

Factors That Affect Population Size

Populations of organisms must obtain food and other resources, or materials for living, within their ecosystem to survive. Factors that affect survival also affect population size.

Limiting Factors

Population density is a measure of the number of organisms per unit area. The size of any population in an ecosystem can be controlled by a number of limiting factors, or factors that set an upper limit on the growth rate of a population. Such factors include reproduction, natural resources, space and nesting sites, competition, and waste and disease.


The speed at which a population can grow is set by the species’ reproductive cycle. Some species, such as whales, sharks, and humans, produce a single offspring after a long period of growth. These animals produce few young over their lifetimes. Other species, such as insects and small rodents, mature quickly and produce many young. This is because after only a short time, there will be many breeding members of the population.

A species’ ability to reproduce also largely depends on the availability of resources. During periods when few resources are available, such as during a drought, many animals and plants produce fewer offspring. During periods of abundance, animals and plants produce more offspring.

Natural Resources

Natural resources, such as food and water, can be limiting factors. For example, the population size of an animal that eats only fruit will be limited by the amount of fruit available in the ecosystem. The population size of a predator will be limited by how many animals can be preyed on in the area.

Space and Nesting Sites

Living space and nesting sites can also be limiting factors. Some organisms live closer together than others. For example, millions of herb plants such as garlic mustard can exist in a square kilometer. Wolverines live very far apart with a population density of one individual per 100 square kilometers. Certain animals must have places in which to construct their nests. For instance, many species of birds have plenty of space in which to live but are limited by the number of suitable nesting sites, such as holes in dead trees.


Competition will occur when resources are limited. Competition is the interaction among living things for the resources they need to survive and to produce offspring. Competition can occur within a population or between individuals in different populations.

Wastes and Disease

As a population’s density increases, its waste products can build up to unhealthy levels. Waste can poison an organism or spread diseases that can affect populations of animals that live close together in colonies, such as prairie dogs. It also can affect human population growth.

Carrying Capacity

The number of organisms of one species that an environment can support for a long time is called the environment’s carrying capacity. In most cases, populations start small and grow until they are larger than the carrying capacity of their habitat. Limiting factors cause the population to drop below the carrying capacity for a period of time before growing once again. This graph shows how a population of sheep on the island of St. Kilda in Scotland has changed in size since 1985. The solid line shows how the population fluctuates dramatically each year.

St. Kilda Island, Scotland 1985-2010

Natural Events and Carrying Capacity

Researchers studying the sheep population found that the timing of storms was an important factor influencing population fluctuations. The time it takes a population to return to its carrying capacity depends on the organisms’ reproductive abilities. Aphids, tiny insects that are born pregnant, can repopulate an area in a matter of weeks. Sharks, however, produce only a few young each year, starting when they are several years old. It may take decades for a shark population to recover.


A close relationship between individuals of two or more species is called symbiosis. The partners in symbiosis typically have effects on each other’s lives such as their ability to survive and reproduce. Symbioses are classified according to which species benefits from the relationship. There are three kinds of symbiosis: mutualism, commensalism, and parasitism.


A relationship in which both species benefit is called mutualism. There are many examples involving species from every kingdom of life. Most plants form a type of mutualistic relationship with fungi that is called a mycorrhizal. Fungi have hairlike structures that take up water and nutrients. The fungi live on sugars and other food they collect from plant roots. In tum, the fungi absorb water and minerals that are shared with the plant.

Mutualism results in many dramatic adaptations. Pollinators, such as birds and insects, and the flowers they pollinate are an example. Plants produce flowers with the shape, color, and smell preferred by their pollinators. Some flowers even look much like females of the insect pollinator’s species.


Parasitism is a symbiotic relationship in which one organism, the parasite, feeds on the living body of the other organism. The parasite benefits, while the host, or the organism on which the parasite lives, is harmed. Some parasites, including fleas, mosquitoes, and ticks, live on the outside of the host. Other parasites, such as tapeworms, can live only inside the host’s body. Parasites have evolved to where they may hurt, but not kill, the host. If the host dies, the parasite can also die. Host species adapt by poisoning parasites or attacking them with their immune systems.


A symbiotic relationship in which one organism benefits and the other is not affected is called commensalism. One common type of commensalism occurs between jellyfish and young fish. A young fish benefits from the relationship because it is protected from predators by the jellyfish’s stinging tentacles. The jellyfish is neither helped nor hurt by the presence of the fish. Commensal species adapt by seeking partners that will provide the full benefit of the relationship. If small fish are more likely to survive when being protected by a jellyfish’s tentacles, the fish species will likely adapt to recognize and attach themselves to jellyfish.

Predator-Prey Relationships

Predation is another type of mutualism in which one organism kills and eats another organism. Predation involves two organisms or species-a predator and its prey. The predator benefits by gaining food, while the prey that is eaten can no longer survive and reproduce. Unlike parasitism, which lasts a long time without killing the host, predation is a one-time relationship. Predator and prey species can affect each other’s carrying capacity in a community.

Predation and Carrying Capacity

On Isle Royale in Lake Superior, moose colonized the island, and their population grew rapidly in the absence of predators. Food was one of the limiting factors that influenced the carrying capacity of the moose. When wolves colonized the island, predation became another factor that reduced the carrying capacity of the moose but increased the carrying capacity of the wolf.

How Changes Affect Food Webs

If the top predator in a food web is removed, all the other species in the web will be affected, for better or worse. For example, humans have killed off wolves throughout most of the United States to prevent wolves from eating livestock and to reduce competition between human hunters and wolves. One unexpected result was the significant increase in the number and range of coyotes. Coyotes were competing with wolves for small-animal prey. Also, wolves killed and ate many young coyotes. So, as the population of wolves decreased, the population of coyotes increased. Elk, rabbit, mouse, and deer populations also grew rapidly after the removal of wolves. These increases led to overgrazing and destruction of crops and plant communities.

Adaptation to Predation

Predators and prey both adapt to their relationships. Prey species adapt by becoming harder to find and less tasty to predators. They may also adopt a reproductive strategy of having many offspring at once. For example, all the maple trees in a population produce a heavy crop of seeds at the same time every few years. There are too many seeds for predators to eat them all. By the time the predator populations have grown larger, the young trees are too big to be eaten. Predators also adapt to increase their fitness. They can become better at catching and killing their prey. Behaviors such as speed, teamwork, and quiet movement help lions kill large animals. In addition, many predators can survive for a long time without catching any prey. Spiders can wait months between meals, and wolverines store meat under the snow.