habitat loss

habitat loss, in ecology, the reduction or elimination of the space in which a species or community of organisms lives and reproduces. Habitat loss, which may be caused by natural disturbances (such as volcanic eruptions, floods, and landslides), is largely the product of human development of natural areas for profit (such as deforestation, strip mining, agriculture, and residential and commercial development) and resultant pollution. After a critical point of lost habitat, ecosystems may no longer be able to provide the environmental resources needed to ensure the survival of the plants, animals, and other forms of life that live there, increasing their chances of becoming extinct. Habitat loss is the leading cause of the global decline in biodiversity (see also biodiversity loss). There are three major types of habitat loss: habitat destruction, habitat fragmentation, and habitat degradation.

Habitat destruction occurs when an ecosystem is changed to such a degree that it can no longer support native species. For example, forests are often cut down to make room for crops or grazing land for livestock. This process often involves the removal of most of the forest’s vegetation (much of which serves as food and shelter for animals and other forms of life). Deprived of living space, organisms in the area must either migrate or risk dying in the transformed environment.

Habitat fragmentation is similar in that it involves the destruction of some amount of habitat, reducing the overall amount available to species in the ecosystem. However, instead of completely transforming the ecosystem, it subdivides larger habitat blocks into smaller disconnected pieces or patches. The effects of fragmentation on different forms of life vary according to species, and the effects depend on the distance between remaining pieces of habitat and the threats posed by the region that separates one patch of habitat and another, or the matrix. For example, habitat fragmentation may have little effect on birds and other animals that can move quickly between one habitat and the next, if the matrix is small (such as a narrow forest cut, road bed, or a power-line corridor). With increased distance between habitat patches, an animal is at a higher risk of encountering predators, being struck by a vehicle, staying too long in less-than-hospitable conditions while moving between patches, or becoming isolated from others. For immobile species, such as plants, or for mobile species in situations where long distances separate habitat patches, in addition to altering important abiotic factors of the nearby environment (such as water availability, temperature, soil nutrients, wind exposure, etc.), habitat fragmentation can hinder reproduction by reducing the chances that males and females of reproductive age meet each other.

Habitat degradation occurs when the ecosystem is disrupted but not destroyed, increasing the difficulty of survival for native plants and animals. Often habitat degradation results from pollution or the invasion of exotic species. The effects of pollution depend largely on the type of pollutant and how intensely various forms of life are exposed to it. In general, pollution increases the stress on ecosystems and its members. For example, nitrogen fertilizers that run off into lakes can supercharge the growth of algae, which, after they die and begin to decompose, reduce the amount of dissolved oxygen in the water available to fishes and other species (see also eutrophication). Invasive species can also disrupt native ecosystems. An invasive predator may be so adept at capturing prey that prey populations decline over time, eventually eliminating prey species from the habitat, whereas other invasive plants and animals may outcompete native species by preventing them from obtaining food, water, living space, or other resources. One example of an invasive competitor in North America is the common buckthorn (Rhamnus cathartica), which grows in a variety of conditions, including full sun and dense shade. Brought from Eurasia, the common buckthorn creates a wall of dense vegetation that blocks sunlight, displacing native plants.