An ecosystem is a natural unit consisting of the abiotic (non-living) portion of an environment interacting with the biotic (living) plants and animals that are adapted to it. There are hundreds of ecosystems on the planet, divided into primary categories (i.e., desert) and numerous subcategories (i.e., Western Australia Desert). The major ecosystems on the planet are marine ecosystems, including the photic zone (within 200 m of the surface) and the abyssal plain (the ocean floors). The concept of an ecosystem is very similar to that of a biome.
The major ecosystems on the planet aside from aquatic ecosystems are the taiga (Canadian and Siberian coniferous forests), grasslands (covering most of Asia and North America), and the deep subterranean ecosystem, which extends 2-4 miles beneath the surface of the Earth and is primarily populated by bacteria. The latter is also known as the Subsurface Lithoautotrophic Microbial Ecosystem and is one of the least studied ecosystems on Earth, despite being one of the largest. Much of the planet can also be regarded as human-influenced ecosystems, as humans are so numerous and intervene in nature so frequently that few regions are untouched.
Ecosystems are often defined in terms of the primary producers (vegetation), which serve as the base of the ecosystem and which everything else eats, either directly (consumers) or indirectly (predators). In order of rough proximity to the equator, some major ecosystems defined by vegetation are tropical rainforest, subtropical rainforest, coniferous forest, grasslands, taiga, and tundra. These are intermixed with mountainous and desert ecosystems, each with their own unique flora and fauna. There are also a number of human major ecosystems, such as croplands and cities.
The most important terrestrial major ecosystem is rainforest – rainforests contain more than half of all biodiversity, maybe as much as 80%. Unfortunately, rainforests are also one of the fastest-disappearing ecosystems, destroyed by land clearing for agriculture. Unless this land clearing is aggressively halted, rainforests could disappear by 2100 or earlier.