An ecosystem is a collection of plants and animals in a particular geographic area, where climate and landscape directly affect the habitats and interactions of species. There are three main types of ecosystems: freshwater, ocean, and terrestrial. Each type of ecosystem can house a wide variety of habitats and thus accounts for the diversity of plants and animals on planet Earth.
Freshwater ecosystems include lakes and rivers, ponds and wetlands, reservoirs and groundwater. As a resource, freshwater is used for drinking, agriculture, industry, sanitation, recreation, and transportation. The various freshwater ecosystems serve as home to a wide variety of organisms, such as fish, amphibians, reptiles, birds, and mammals, as well as protozoans, worms, and mollusks. Plants, algae, and phytoplankton are abundant as well and form the basis of the freshwater food web.
Ocean, or marine, ecosystems cover approximately 70 percent of the Earth's surface. Marine ecosystems include the oceans, estuaries, coral reefs, and coastal areas. Marine ecosystems differ from freshwater ecosystems in that the water contains salt, so the plants and animals living there must be at least somewhat salt-tolerant, depending on the specific location in which they dwell. Fish such as flounder and sea bass as well as larger animal like whales, dolphins, and seals are just a sampling of the very diverse animal life found in the ocean ecosystem. Seaweeds, phytoplankton, and algae adapted to survival in saltwater are also abundant. The diverse inhabitants are important to human survival as well, as many of them are utilized as food sources.
A terrestrial ecosystem is a community of plants and animals and other organisms that inhabit a specific land area. There is much less water available for survival than in a freshwater or marine ecosystem; therefore, water acts as a limiting factor for survival. These ecosystems experience greater fluctuations in temperature. Gases are necessary for life: oxygen for animals and carbon dioxide for plants. Terrestrial environments include forests and grasslands and are the source of many items integral to human survival, such as food and materials for shelter.