Ecosystems and habitats
The thin layer of the Earth's surface where living things are able to survive is called the biosphere. It is about 20km thick, from the bottom of the oceans high up into the atmosphere. Physical conditions outside the biosphere are too extreme to support life. Ecosystems are discrete, recognisable, self-sustaining units within the biosphere, such as woodlands, ponds, salt marshes and rocky shores. Every ecosystem has a living (biotic) component, i.e. the organisms that live there, and a non-living (abiotic) component, i.e. the physical conditions in which an organisms live, such as landscape, climate and type of soil. Specific and readily quantifiable abiotic factors might include temperature, pH, humidity, wind direction, water current, light quality, nutrients, pollutants, salinity, slope, aspect and altitude. An organisms environment refers to the complete range of conditions in the ecosystem which affect its way of life, not only abiotic factors but biological factors such as competition for food, space and shelter. The study of how living organisms interact with each other and with their environment is called ecology.
Although ecosystems are relatively self-contained and perpetuate themselves by the cycling of minerals, they are not completely closed as if surrounded by an invisible box. There is some movement of energy and materials between them. All the energy entering an ecosystem ultimately comes from the sun; sediment is eroded from hills, transported by streams and deposited in ponds; animals may enter and leave, perhaps on a large scale (e.g. migratory birds, butterflies and fish) or on a smaller scale (e.g. a woodpigeon flying from a wood to feed in a meadow). However, this two-way traffic between ecosystems is limited because animals are usually adapted to the particular conditions of only one ecosystem. Sometimes the natural boundaries between ecosystems overlap or are hard to define, such as those between rivers, estuaries and the sea.
The specific place where an organism lives is called its habitat. Habitats may exist on a range of scales: woodland is a habitat for spotted woodpeckers; an individual tree may provide the habitat for certain mosses, bark beetles; individual leaves provide a habitat for leaf miners, although at this small scale we might call it a microhabitat. The body of a host animal or plant provides a habitat for an internal parasite.
ContentsDiversity of organisms