An ecosystem is a community of living organisms interacting with one another and with the nonliving parts of their environment. Ecosystems can be grouped into several major types based mainly on where they occur and the physical conditions that shape life there. The broadest categories are terrestrial ecosystems, which occur on land, and aquatic ecosystems, which occur in water.
Within these categories are many distinct ecosystems, including forests, grasslands, deserts, freshwater systems, and marine environments. Each has characteristic environmental conditions, organisms, and patterns of energy flow and nutrient cycling.
Terrestrial ecosystems
Terrestrial ecosystems develop on land. Temperature, rainfall, soil, sunlight, and seasonal patterns strongly influence which organisms can survive in them.
The major terrestrial ecosystems include forests, grasslands, deserts, and tundra. These categories contain many more specific ecosystems, and their boundaries are not always sharply defined.
Forest ecosystems
Forest ecosystems are dominated by trees and typically contain several layers of vegetation, from the canopy at the top to smaller plants and organisms on or near the ground.
The amount and seasonal pattern of precipitation and temperature help determine what kind of forest develops. Tropical forests occur in warm regions with generally high rainfall and can support extremely diverse communities. Temperate forests experience more pronounced seasonal changes, while boreal forests, also called taiga, occur in colder northern regions and are dominated largely by coniferous trees.
Forests contain producers such as trees, shrubs, and other plants, along with herbivores, predators, decomposers, fungi, and microorganisms. Fallen leaves, dead wood, and other organic material provide resources for organisms that break down and recycle nutrients.
Grassland ecosystems
Grasslands are ecosystems in which grasses and other herbaceous plants make up much of the vegetation, while trees are relatively limited.
Rainfall is usually sufficient to support substantial plant growth but not consistently high enough to produce dense forests. Fire, grazing, seasonal drought, and other environmental factors can also help maintain grasslands by limiting the establishment of trees.
Grasslands occur in many parts of the world and include ecosystems such as prairies, steppes, and savannas. Savannas differ from many temperate grasslands because they commonly contain scattered trees and shrubs.
Desert ecosystems
Deserts receive very little precipitation relative to the potential water loss from the environment. They are not necessarily hot; deserts can occur in both hot and cold regions.
Because water is scarce, desert organisms have adaptations that help them obtain, conserve, or use water efficiently. Plants may have reduced leaves, water-storage tissues, or extensive root systems, while many animals limit activity during the hottest or driest periods.
Desert ecosystems can support surprisingly varied communities despite their harsh conditions. Temperature, soil, seasonal rainfall, and the availability of water all influence which organisms live in a particular desert.
Tundra ecosystems
Tundra ecosystems occur in regions where low temperatures and short growing seasons restrict plant growth. They are generally characterized by low vegetation such as mosses, lichens, grasses, sedges, and small shrubs rather than large trees.
Arctic tundra occurs at high northern latitudes. Alpine tundra occurs at high elevations where conditions are too cold or otherwise unsuitable for forests.
In much of the Arctic tundra, permanently frozen ground, known as permafrost, influences drainage, soil conditions, and plant growth. During the short growing season, plants and animals can become highly active as environmental conditions temporarily improve.
Aquatic ecosystems
Aquatic ecosystems develop in water and are broadly divided into freshwater ecosystems and marine ecosystems. The concentration of dissolved salts is one of the most important differences between these categories.
Water depth, temperature, light availability, dissolved oxygen, nutrients, water movement, and chemistry all influence aquatic communities.
Freshwater ecosystems
Freshwater contains relatively little dissolved salt compared with seawater. Freshwater ecosystems include rivers, streams, lakes, ponds, wetlands, and groundwater-associated environments.
Rivers and streams are flowing-water ecosystems. Their organisms must cope with currents, changes in water flow, and differences in oxygen and nutrient availability. Conditions can change considerably from the source of a river to its downstream sections.
Lakes and ponds are standing-water ecosystems, although water within them still moves and mixes. Light penetration, temperature, oxygen concentration, and nutrient availability can vary with depth and season.
Wetlands are ecosystems in which water covers or saturates the soil for periods long enough to strongly influence the environment and the organisms that live there. They include marshes, swamps, and other wet habitats. Wetlands can support distinctive plant and animal communities and play important roles in processes such as nutrient cycling and water movement.
Marine ecosystems
Marine ecosystems occur in oceans and other saltwater environments. They cover much of Earth’s surface and include a wide range of habitats with very different physical conditions.
Open-ocean ecosystems contain organisms ranging from microscopic plankton to large marine animals. Near the surface, sunlight allows photosynthetic organisms to capture energy, while deeper waters receive little or no sunlight.
Coral reef ecosystems occur in warm, relatively shallow marine environments and are built around the activities of reef-forming organisms. They support complex communities of fish, invertebrates, algae, microorganisms, and other organisms.
Estuaries occur where freshwater from rivers or streams mixes with seawater. Their salinity can change with tides, rainfall, and freshwater flow, creating conditions that support organisms adapted to changing environments.
Other marine ecosystems include mangrove forests, salt marshes, seagrass ecosystems, rocky shores, sandy beaches, and deep-sea environments. Each is shaped by factors such as depth, light, temperature, tides, salinity, pressure, and availability of nutrients.
How ecosystems differ from one another
The major types of ecosystems are distinguished not simply by the organisms they contain but by the physical conditions under which those organisms live.
Climate is especially important for terrestrial ecosystems. Temperature and precipitation influence plant growth, which in turn affects the animals and other organisms that can survive in an area. In aquatic ecosystems, factors such as salinity, depth, water movement, temperature, light, and dissolved oxygen play major roles.
Ecosystems also differ in how energy and materials move through them. Plants, algae, and some microorganisms capture energy through photosynthesis and form the base of many food webs. Consumers obtain energy by feeding on other organisms, while decomposers break down dead organic matter and help return nutrients to the environment.
Ecosystems can overlap and vary gradually
The major ecosystem categories are useful for understanding Earth’s environments, but nature does not always fit into sharply separated boxes. An ecosystem can contain characteristics of more than one broad type, and conditions often change gradually across landscapes.
For example, a river may flow through a forest, pass into a wetland, and eventually reach an estuary. Along that transition, temperature, salinity, water movement, vegetation, and animal communities change. Likewise, a grassland can gradually become more wooded as rainfall, soil conditions, fire frequency, or grazing patterns change.
For this reason, scientists often use more specific terms to describe particular ecosystems or habitats. The broad categories—forests, grasslands, deserts, tundra, freshwater ecosystems, and marine ecosystems—provide a useful framework, but Earth’s ecosystems form a continuous and interconnected variety of environments rather than a fixed set of separate types.
