A habitat's carrying capacity, or the maximum number of individuals in a population that can be supported, is determined by limiting factors. For example, a beaver dam turns a freshwater river into a small lake in which trout populations can increase. The trout population increases because there are few predators present that normally feed on trout and trout eggs. The large trout population reduces the oxygen level in the lake, which rapidly depletes food supplies. Predators and disease can also increase the trout's mortality rate. At this point, the trout population has reached carrying capacity—the population size remains stable with births and deaths relatively equal, resulting in a growth rate of zero. When external factors do not control growth, populations exceed the carrying capacity of their environment, and death rates increase.
Initially, a newly introduced organism may arrive in an ecosystem and find extremely positive conditions. Food and water are plentiful, and there is minimal competition for resources. As populations approach their carrying capacity, both living and nonliving factors begin to control population numbers. Density-dependent factors are limitations to population growth that vary with population density, such as availability of food and water, competition for food, disease, and predation. Living factors may be density-dependent in that the size of the population itself influences its numbers. Density-independent factors are limitations to population growth that are not dependent on population density, such as pollution, weather extremes, seasonal changes, or catastrophic events. Wildfires, hurricanes, earthquakes, volcanic eruptions, and floods create catastrophic events that may affect few or all populations in an ecosystem.
Whether density-dependent or density-independent, major changes to any population create an imbalance that can allow a species to have more individuals than the environment can support. When wolves were eliminated from Yellowstone National Park, deer and elk populations soared with no predator to reduce them. Competition for food increased, and both populations spread to new areas, totally changing the food supply available to other species in meadow and riverine areas. The large herbivores out-competed smaller herbivores for food. As bird and rodent populations decreased, predators (owls, hawks, weasels, etc.) lost their food supplies. The imbalance was corrected by reintroducing wolves into Yellowstone. The wolves preyed on elderly and sick deer and elk, and they slowly reduced herd sizes to their natural carrying capacity. Meadow and riverine ecosystem flora recovered, encouraging the growth of bird and rodent populations and a resurgence of small-animal predators.
Factors Affecting Carrying Capacity
|Factors that Increase||Factors that Decrease|
|Changing climate or other environmental factors||Changing climate or other environmental factors|
|Technologic efficiency advances||Environmental degradation|
|Decrease in body size and resource demand||Depletion of nonrenewable resources|
|—||Introduction of a new competitor|
Abiotic and Biotic Factors
A limiting factor is an environmental factor that restricts the growth of populations. Limiting factors can be categorized as abiotic (non-living) and biotic (living). Organisms depend on food, shelter, water, and the opportunity to reproduce. Without the basic elements of life, organisms do not reproduce, and populations become severely limited, or become extinct.
An abiotic factor is any nonliving thing that affects an ecosystem: water, sunlight, oxygen, soil, and climate are all abiotic factors. Animals either live in water or obtain water through drinking or eating foods that contain water. In desert environments, plants have adapted to retain water, limit the loss of water, or reach water through extensive root systems. Animals have adapted to eating water-bearing foods, estivating (slowing their movements, similar to hibernation) during the hottest and driest periods, and limiting reproduction to times when water and food are more plentiful. Sunlight is essential for green plants to function, and competition for sunlight in forests is fierce. Without sunlight, plants cannot flower and produce seeds, and animal populations that feed on plant matter decrease. Soil, composed of rock particles and organic matter, provides nutrients and water for flora and shelter for burrowing animals. Climate strongly influences plant and animal populations, as does space, which affects shelter availability and resources.A biotic factor is a living thing that influences an ecosystem, such as prey, predators, and pathogens. Food may be abundant or scarce, and the availability of food may vary among species within an ecosystem. For example, there may be plenty of grasses for rabbits to eat but a dearth of flowers to feed bees. Some species are food for other species, which is known as predation. Predation occurs to both animals and plants; for example, rabbits prey on the grasses, and hawks may prey on the rabbits. When a resource is scarce, competition occurs. Competition can be direct or indirect. In direct competition, individuals fight each other for a single resource, such as when deer fight for a mate. In indirect competition, one individual's use of a resource means the other does not have it, such as when deer eat all the grasses, leaving none for rabbits. Competition can lead to a decrease in the size of one population, an increase in the size of another population, or a shift within a population. For example, if the deer eat all the grasses, the number of deer may increase while the number of rabbits decreases. When deer fight for a mate, the larger, stronger one may win, passing on genes for size and strength to its offspring, leading to a general increase in size and strength of deer over time. Populations can also be affected by pathogens, which are organisms that cause disease. The more dense the population, the greater the chances the disease will spread.