Ecosystems and biomes

##Key points

• The term symbiosis describes close, long-term interactions between individuals of two different species. Types of symbiosis include mutualism, commensalism, and parasitism.

• Mutualism occurs when two organisms of different species provide each other with resources or services, and both organisms benefit. For example, sea anemones and clownfish share a mutualistic relationship in which anemones provide shelter and protection, and clownfish provide nutrients in the form of waste.

• Commensalism is an interaction between species where one organism benefits, and the other organism is not significantly affected. A barnacle and a whale exhibit commensalism when the barnacle attaches to the whale. The whale is unaffected, while the barnacle is able to gather food from the water as the whale swims.

• Parasitism is an interaction between species in which one organism—the parasite—benefits by living and feeding on another organism—the host—who is harmed but not immediately killed. For example, a sea lamprey is a parasite that attaches itself to the body of a host fish and consumes the fish's body fluids. The lamprey draws nourishment from the fish, and the fish gradually weakens over time.

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##Key points

• A biome is a community of plants and animals living in, and adapted-to, a certain climate.

• Many of Earth’s biomes are terrestrial, or land-based. The characteristics of terrestrial biomes are heavily influenced by temperature and rainfall. The major terrestrial biomes are listed below.

• Tropical rainforests are found at latitudes near the equator. In these areas, average temperatures and rainfall are high throughout the year. This warm, wet climate allows tropical rainforests to have high biodiversity.

• Temperate forests are generally found at latitudes between 30° and 60° north and south of the equator. These forests experience well-defined seasons, with a distinct period of winter.

• Temperate deciduous (seasonal) forests have cold, dry winters, and hot, humid summers. The deciduous trees of these forests shed their leaves each winter to save energy.

• Temperate rainforests have mild, frost-free winters and rainfall that is evenly distributed throughout the year.

• Coniferous forests (also known as boreal forests or taigas) are found at latitudes between 50° and 60° north of the equator. These forests experience short, warm, moist summers and long, cold, dry winters. The coniferous trees of these forests have thin, needle-like leaves that do not shed in winter. They also produce seeds in cones, which forms the root term of the name coniferous.

• Deserts exist at many latitudes, but most form at latitudes around 30° north and south of the equator. Rainfall in deserts is low and highly variable. Deserts closer to the equator have warmer temperatures than deserts further from the equator. Different types of plants live in warm or cold deserts, but they all have special ways to conserve water in these arid environments.

• Grasslands can occur at any latitude. Grasslands experience seasonal drought, occasional fire, and grazing by herbivores. These features stop trees and shrubs from overgrowing grasses. Warmer grasslands with scattered trees are called savannas. Milder grasslands with very few trees are called temperate grasslands, which include prairies and steppes.

• Scrublands (also known as shrublands, chaparrals and woodlands) are located in western coastal regions between 30° and 40° north and south of the equator. Scrublands usually get more rain than deserts and grasslands, but less than forested areas. As a result, scrublands are dominated by shrubs and short trees.

• The tundra biome is located primarily in the arctic, and is the most inhospitable of the biomes. It is cold and dry, with a short growing season and a layer of permanently frozen soil called permafrost. The tundra is relatively low in biodiversity, but is home to various species of plants and animals that are adapted to the tundra's cold, dry conditions.

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##Key points

• A biome is a community of plants and animals living in, and adapted to, a certain climate.

• Many of Earth’s biomes are aquatic, or water-based. The characteristics of aquatic biomes are heavily influenced by the concentration of dissolved oxygen and nutrients in the water.

• Some aquatic biomes are freshwater biomes, where the water contains little or no salt. Freshwater biomes include ponds, lakes, streams, and rivers, and are a vital source of drinking water.

• Ponds and lakes are natural bodies of standing water. Ponds are smaller and may be seasonal, whereas lakes are larger and more permanent. Large lakes have four main zones.

• The littoral zone is the top, near-shore layer of a lake. This zone is shallow with plentiful sunlight and nutrient inputs from the surrounding land. As a result, this zone supports a wide variety of plants and animals.

• The limnetic zone (or open-water zone) is a lake’s sunlit surface layer where most photosynthesis takes place. It starts away from shore, just past the littoral zone, and extends to the depth penetrated by sunlight. This zone supports phytoplankton and zooplankton, along with freshwater fish.

• The profundal zone (or deep-water zone) is a lake’s deep, open-water layer, which is too dark for photosynthesis. The water in this zone is cooler and contains less dissolved oxygen than the shallower zones. This zone supports fish adapted to these cooler waters.

• The benthic zone includes the soil and soil organisms that live at the bottom of a lake. This zone is inhabited mostly by decomposers and other organisms that feed on dead and decaying material, and can withstand a low-oxygen environment.

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• Streams and rivers are flowing bodies of water that drain the landscape, and are important agents of erosion. The flow of a typical stream or river can be characterized by three zones.

• Streams and rivers start at the source zone, or headwaters, where water at high elevations collects from precipitation and snowmelt. This water is cold, fast moving, has lots of dissolved oxygen, and a low nutrient content. There are usually only a few plant species that grow near a river's source.

• Headwaters merge together to form the transition zone of a river. Here, the river is wider, slower moving, has less dissolved oxygen, and carries more sediment than the source zone. The water is warmer and more nutrient-rich, so this zone tends to support a larger variety of plants and animals.

• From the transition zone, a river meanders toward its mouth through a flat, wide floodplain zone. This zone forms because, occasionally, the bulk of water in the river becomes greater than the river channel itself, and water spills out onto the land forming wetlands and temporary lakes. The water in this zone usually contains more sediment and less dissolved oxygen than the transition zone. This zone's warm, nutrient-rich waters support the greatest variety of plants and animals.

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