Habitat

In ecology, a habitat is the specific collection of resources and biotic factors that support a species' survival and reproduction. It is the physical manifestation of a species' ecological niche. While the terms environment or vegetation assemblage describe general areas, habitat is a species-specific term. It includes both physical factors, such as soil, moisture, temperature range, and light intensity, and biotic factors, like food availability and the presence of predators.

Every species has unique habitat requirements that dictate where it can live. Some organisms are habitat generalists, meaning they can thrive across a wide array of environmental conditions. An example is the small white butterfly, Pieris rapae, which lives on every continent except Antarctica because its larvae feed on many different plant species. In contrast, habitat specialists require a very limited set of factors. The large blue butterfly, Phengaris arion, is a specialist that only lives in chalk grassland areas. It requires specific Thymus species for its larvae and depends on the presence of Myrmica ants to complete its complex life cycle.

Habitats exist at many different scales, including microhabitats. A microhabitat is a small-scale area with unique physical requirements like light, humidity, or air movement. For instance, the lichens on the north face of a boulder differ from those on the south face or the top. Within a single wood, microhabitats include tree trunks, branches, leaves, and even the soil. Even a single leaf, such as those of the Alnus nepalensis tree, can serve as a microhabitat for a leaf beetle.

Scientists categorize habitats into several broad types. Terrestrial habitat types include forests, grasslands, wetlands, and deserts. Arid habitats, such as the deserts in Egypt, are defined by very little available water.

Freshwater habitats include running waters like rivers and streams, or standing waters like lakes and bogs. Marine habitats cover the oceans, ranging from coral reefs and estuaries to the deep benthic zone of the seabed.

Some organisms even find habitats within other living things, such as a parasite living in the digestive tract or cells of a host.

Life in extreme habitats requires specialized biological adaptations. In arid environments, some frogs live underground to create moist habitats and hibernate during dry periods. The Couch's spadefoot toad emerges from its burrow only during downpours to lay eggs in transient pools. Its tadpoles can undergo metamorphosis in as little as nine days. Similarly, fairy shrimps can lay "winter eggs" that resist drying out. These eggs can remain dormant in the dust for as long as fifteen years until new water arrives.

Habitats are not permanent and can change through various processes. Change can be caused by violent events like volcanic eruptions, earthquakes, tsunamis, or wildfires.

Changes can also occur gradually over millennia through climate shifts, such as the advance or retreat of glaciers. Human activities also cause significant changes, including deforestation, the damming of rivers, and the dredging of the seabed. Furthermore, the introduction of alien species can devastate native wildlife through competition, predation, or the spread of new diseases.

Even human-made urban environments function as habitats for adaptable generalists. In cities, pigeons and sparrows use buildings for nesting, while bats use roof spaces for roosting. Many animals, such as rats, mice, and foxes, have followed humans globally. In Chicago, it is estimated that about 2,000 coyotes live in and around the city. In warmer climates, urban habitats also host many species of termites, with 183 species known to affect buildings.