Natural habitat

In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical manifestation of its ecological niche. Thus "habitat" is a species-specific term, fundamentally different from concepts such as environment or vegetation assemblages, for which the term "habitat-type" is more appropriate.

The physical factors may include (for example): soil, moisture, range of temperature, and light intensity. Biotic factors will include the availability of food and the presence or absence of predators. Every species has particular habitat requirements, with habitat generalist species able to thrive in a wide array of environmental conditions while habitat specialist species requiring a very limited set of factors to survive. The habitat of a species is not necessarily found in a geographical area, it can be the interior of a stem, a rotten log, a rock or a clump of moss; a parasitic organism has as its habitat the body of its host, part of the host's body (such as the digestive tract), or a single cell within the host's body.For example:

Habitat types are environmental categorizations of different environments based on the characteristics of a given geographical area, particularly vegetation and climate. Thus habitat types do not refer to a single species but to multiple species living in the same area. For example, terrestrial habitat types include forest, steppe, grassland, semi-arid or desert. Fresh-water habitat types include marshes, streams, rivers, lakes, and ponds; marine habitat types include salt marshes, the coast, the intertidal zone, estuaries, reefs, bays, the open sea, the sea bed, deep water and submarine vents. Habitat types may change over time. Causes of change may include a violent event (such as the eruption of a volcano, an earthquake, a tsunami, a wildfire or a change in oceanic currents); or change may occur more gradually over millennia with alterations in the climate, as ice sheets and glaciers advance and retreat, and as different weather patterns bring changes of precipitation and solar radiation. Other changes come as a direct result of human activities, such as deforestation, the plowing of ancient grasslands, the diversion and damming of rivers, the draining of marshland and the dredging of the seabed. The introduction of alien species can have a devastating effect on native wildlife – through increased predation, through competition for resources or through the introduction of pests and diseases to which the indigenous species have no immunity.

Definition and etymology

The word "habitat" has been in use since about 1755 and derives from the Latin ''habitāre'', to inhabit, from ''habēre'', to have or to hold. Habitat can be defined as the natural environment of an organism, the type of place in which it is natural for it to live and grow. It is similar in meaning to a biotope; an area of uniform environmental conditions associated with a particular community of plants and animals.

Environmental factors

The chief environmental factors affecting the distribution of living organisms are temperature, humidity, climate, soil and light intensity, and the presence or absence of all the requirements that the organism needs to sustain it. Generally speaking, animal communities are reliant on specific types of plant communities.

Some plants and animals have habitat requirements which are met in a wide range of locations. The small white butterfly ''Pieris rapae'' for example is found on all the continents of the world apart from Antarctica. Its larvae feed on a wide range of ''Brassicas'' and various other plant species, and it thrives in any open location with diverse plant associations. The large blue butterfly ''Phengaris arion'' is much more specific in its requirements; it is found only in chalk grassland areas, its larvae feed on ''Thymus'' species and because of complex lifecycle requirements it inhabits only areas in which ''Myrmica'' ants live.

Disturbance is important in the creation of biodiverse habitat types. In the absence of disturbance, a climax vegetation cover develops that prevents the establishment of other species. Wildflower meadows are sometimes created by conservationists but most of the flowering plants used are either annuals or biennials and disappear after a few years in the absence of patches of bare ground on which their seedlings can grow. Lightning strikes and toppled trees in tropical forests allow species richness to be maintained as pioneering species move in to fill the gaps created. Similarly coastal habitat types can become dominated by kelp until the seabed is disturbed by a storm and the algae swept away, or shifting sediment exposes new areas for colonisation. Another cause of disturbance is when an area may be overwhelmed by an invasive introduced species which is not kept under control by natural enemies in its new habitat.

Types

Terrestrial

Terrestrial habitat types include forests, grasslands, wetlands and deserts. Within these broad biomes are more specific habitat types with varying climate types, temperature regimes, soils, altitudes and vegetation. Many of these habitat types grade into each other and each one has its own typical communities of plants and animals. A habitat-type may suit a particular species well, but its presence or absence at any particular location depends to some extent on chance, on its dispersal abilities and its efficiency as a colonizer.

Freshwater

Freshwater habitat types include rivers, streams, lakes, ponds, marshes and bogs. Although some organisms are found across most of these habitat types, the majority have more specific requirements. The water velocity, its temperature and oxygen saturation are important factors, but in river systems, there are fast and slow sections, pools, bayous and backwaters which provide a range of habitat types. Similarly, aquatic plants can be floating, semi-submerged, submerged or grow in permanently or temporarily saturated soils besides bodies of water. Marginal plants provide important habitat for both invertebrates and vertebrates, and submerged plants provide oxygenation of the water, absorb nutrients and play a part in the reduction of pollution.

Marine

Marine habitats include brackish water, estuaries, bays, the open sea, the intertidal zone, the sea bed, reefs and deep / shallow water zones. Further variations include rock pools, sand banks, mudflats, brackish lagoons, sandy and pebbly beaches, and seagrass beds, all supporting their own flora and fauna. The benthic zone or seabed provides a home for both static organisms, anchored to the substrate, and for a large range of organisms crawling on or burrowing into the surface. Some creatures float among the waves on the surface of the water, or raft on floating debris, others swim at a range of depths, including organisms in the demersal zone close to the seabed, and myriads of organisms drift with the currents and form the plankton.

Desert

A desert is not the kind of habitat that favours the presence of amphibians, with their requirement for water to keep their skins moist and for the development of their young. Nevertheless, some frogs live in deserts, creating moist habitat types underground and hibernating while conditions are adverse. Couch's spadefoot toad (''Scaphiopus couchii'') emerges from its burrow when a downpour occurs and lays its eggs in the transient pools that form; the tadpoles develop with great rapidity, sometimes in as little as nine days, undergo metamorphosis, and feed voraciously before digging a burrow of their own.

Wetlands

Other organisms cope with the drying up of their aqueous habitat in other ways. Vernal pools are ephemeral ponds that form in the rainy season and dry up afterwards. They have their specially-adapted characteristic flora, mainly consisting of annuals, the seeds of which survive the drought, but also some uniquely adapted perennials. Animals adapted to these extreme habitat types also exist; fairy shrimps can lay "winter eggs" which are resistant to desiccation, sometimes being blown about with the dust, ending up in new depressions in the ground. These can survive in a dormant state for as long as fifteen years. Some killifish behave in a similar way; their eggs hatch and the juvenile fish grow with great rapidity when the conditions are right, but the whole population of fish may end up as eggs in diapause in the dried up mud that was once a pond.

Urban

Many animals and plants have taken up residence in urban environments. They tend to be adaptable generalists and use the town's features to make their homes. Rats and mice have followed man around the globe, pigeons, peregrines, sparrows, swallows and house martins use the buildings for nesting, bats use roof space for roosting, foxes visit the garbage bins and squirrels, coyotes, raccoons and skunks roam the streets. About 2,000 coyotes are thought to live in and around Chicago. A survey of dwelling houses in northern European cities in the twentieth century found about 175 species of invertebrate inside them, including 53 species of beetle, 21 flies, 13 butterflies and moths, 13 mites, 9 lice, 7 bees, 5 wasps, 5 cockroaches, 5 spiders, 4 ants and a number of other groups. In warmer climates, termites are serious pests in the urban habitat; 183 species are known to affect buildings and 83 species cause serious structural damage.

Microhabitat types

A microhabitat is the small-scale physical requirements of a particular organism or population. Every habitat includes large numbers of microhabitat types with subtly different exposure to light, humidity, temperature, air movement, and other factors. The lichen

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