Ecosystem Concepts and Distribution

Ecosystems are connected systems

An ecosystem includes living organisms and the physical environment with which they interact. It is more than a collection of plants and animals: the connections between its parts are what make it a system. Ecosystems exist at different scales, from a garden pond to an extensive forest. A large-scale ecosystem type is called a biome.

Climate, soil, water, plants and animals are interdependent. Temperature and rainfall influence the conditions in which organisms live. On land, soil provides a growing medium, water and nutrients for plants. Plants provide food and habitats for animals, so animals depend on conditions that support plant growth.

The connections also work in the other direction. Animals can disperse plant seeds, while decomposers such as fungi and bacteria break down dead organisms and waste, returning nutrients to the soil. In a woodland, squirrels bury oak acorns; forgotten acorns may germinate. Fallen leaves decompose and supply nutrients that support further plant growth.

A change in one component can therefore affect several others. A drought reduces available water, which can limit plant growth and leave plant-eating animals with less food. These components do not have equal importance in every ecosystem: soil is important on land, whereas seawater and the availability of light are especially important to shallow coral reefs.

Understanding global distribution

The major ecosystem groups considered here are polar regions, coral reefs, grasslands, temperate forests, tropical forests and hot deserts. To compare them, separate three questions: where are they found, what conditions occur there, and what lives there? These concern distribution, climate, and flora and fauna respectively.

Global distribution is closely linked to climate. Latitude measures distance north or south of the Equator in degrees. Broadly, low latitudes near the Equator are warmer than high latitudes near the poles. Rainfall patterns also matter: warm, wet conditions support tropical forests, whereas very dry conditions support deserts.

Ecosystems do not form perfectly continuous bands. Altitude, distance from the sea and ocean currents also influence local climates. A useful distribution description therefore combines a broad latitude pattern with named regions, rather than suggesting that every place at the same latitude has the same ecosystem.

Polar regions: cold environments at high latitudes

Polar regions occur around the North and South Poles. The Arctic lies around the North Pole, broadly within the Arctic Circle at 66.5° N. It is an ocean surrounded by land, including northern Canada and Siberia. Antarctica is a continent surrounded by ocean, lying mostly south of the Antarctic Circle at 66.5° S.

Both have cold climates and low precipitation. Antarctica is generally colder than the Arctic, especially in its interior. Low temperatures and short growing seasons restrict plant growth, so terrestrial biodiversity is low.

On Arctic tundra, the surface soil thaws during the brief summer, but permafrost remains beneath it. Mosses, lichens, grasses and low-growing shrubs occur here. Arctic fauna include caribou, Arctic foxes, polar bears and seals. These are examples from different habitats within the Arctic, rather than animals all living together in one place.

Antarctica has very little land vegetation, mainly lichens and some mosses. Photosynthetic phytoplankton in the surrounding ocean support marine food chains. Penguins, seals and whales depend on the sea for food. Polar bears are Arctic animals, not Antarctic animals.

Tropical forests: warm and wet near the Equator

Tropical rainforests are concentrated near the Equator, mainly between about 15° N and 15° S, within the wider tropical zone. Major areas include the Amazon Basin in South America, the Congo Basin in central Africa, and South-East Asia, including Malaysia and Indonesia.

Temperatures remain high throughout the year, commonly around 26–28°C, with little seasonal variation. Annual rainfall is often over 2,000 mm. These warm, wet conditions allow a long, continuous growing season.

Dense vegetation includes tall trees, smaller trees and shrubs arranged in layers. Tropical rainforests have very high biodiversity: the range of vegetation provides many habitats and food sources. Fauna include insects such as leaf-cutter ants, tree frogs and snakes. The climate, vegetation structure and variety of animal life distinguish these forests from colder or drier ecosystems.

Grasslands: tropical and temperate forms

Grasslands are dominated by grasses rather than continuous tree cover, but they do not all have the same climate.

Tropical grasslands, often called savannas, occur in tropical regions such as central Africa, parts of South America and northern Australia. They are warm throughout the year but have distinct wet and dry seasons. Grasses dominate, with scattered trees such as acacia and baobab. African savanna fauna include zebras, giraffes, elephants and lions. The seasonal availability of water and vegetation influences the food available to these animals.

Temperate grasslands occur farther from the Equator, including parts of North America, Russia and southern Africa. They have greater seasonal temperature contrasts, with hot summers and cold winters in continental interiors. Rainfall is limited, commonly around 250–750 mm a year. Grasses dominate and trees are scarce. Fauna include bison in North America, as well as deer and snakes in suitable grassland habitats.

The important distinction is between the wet–dry seasonal pattern of tropical grasslands and the stronger summer–winter temperature contrast of temperate grasslands.

Temperate forests: seasonal woodland

Temperate forests occur in the middle latitudes, including Europe and North America. Temperate deciduous woodland is a useful example, with trees such as oak, beech and ash.

These forests experience seasonal temperature changes, with warm summers and cooler winters, and enough precipitation to support tree growth. Deciduous trees shed their leaves as the colder season approaches. Their fallen leaves form litter that decomposes and adds nutrients to the soil.

Flora also include shrubs and woodland-floor plants. In UK woodland, bluebells can flower in spring before the trees develop a full leaf canopy, when more sunlight reaches the ground. Fauna include deer, rabbits, squirrels and woodland birds. Seed dispersal and decomposition connect these organisms to the soil and seasonal climate.

Hot deserts: scarce water and sparse vegetation

Many hot deserts occur around 20–30° north and south of the Equator. They are found in Africa, Australia and parts of North and South America. The Sahara in North Africa is an example of this broad subtropical distribution.

Their defining characteristic is very low precipitation, usually less than 250 mm a year. Daytime temperatures can be extremely high, but nights can be much cooler. Not every desert night falls below freezing.

Water scarcity limits plant growth, leaving sparse vegetation and relatively low biodiversity. Plants include cacti in American deserts and yucca. Fauna include scorpions, spiders and camels in appropriate regions. These examples are not found in every desert: the broad ecosystem type can contain different species on different continents.

Coral reefs: warm, shallow and well lit

Most shallow reef-building stony corals occur in tropical and subtropical seas, broadly between 35° N and 35° S. This describes a broad global range, not an unbroken belt of reefs: suitable local conditions are also necessary. Important concentrations occur around South-East Asia, including Indonesia, and north-east Australia.

Shallow reef-building corals generally need warm, salty water and plenty of light. Many grow best at water temperatures of 23–29°C. Most reef-building species are restricted to the sunlit water above approximately 30 metres depth. Clear water allows sunlight to reach organisms living within coral tissues; suspended material can reduce that light.

These conditions describe shallow tropical reefs, rather than every coral habitat. Deep-sea corals can live in dark water as cold as 4°C.

Coral animals and photosynthetic algae depend on each other

Coral polyps are animals, not plants. Most reef-building corals contain microscopic photosynthetic algae called zooxanthellae. These algae use sunlight, carbon dioxide and water to produce organic material through photosynthesis. They are an important part of the reef's photosynthetic life, while the corals themselves are part of its fauna.

The relationship benefits both organisms. The coral provides the algae with a protected environment and supplies carbon dioxide and water. In return, the algae transfer products of photosynthesis to the coral, supporting its growth and reproduction. This is mutualism, a particularly clear example of interdependence.

This relationship connects living organisms to their physical environment. Clear water allows light to reach the algae; the algae photosynthesise; and the coral receives material needed for growth. It also helps explain how reefs can be highly productive and diverse even in nutrient-poor tropical waters: nutrients are tightly recycled between the coral and its algae.

Ecosystem essentials

• Climate, soil, water, plants and animals are interdependent: changing one component can affect others.
• Plants provide food and habitats; animals can disperse seeds; decomposers return nutrients to soil.
• Distribution follows broad climate patterns, modified by local conditions.

Global ecosystem contrasts

Coral interdependence

• Many shallow reef-building corals grow best at 23–29°C; most occur above approximately 30 m depth.
• Coral supplies protection, carbon dioxide and water; algae supply products of photosynthesis.
• Deep-sea corals are an exception to the warm, sunlit-water pattern.