An ecosystem is comprised of every living organism and every nonliving element in a given area, plus the interactions between them. That area can be as vast as an ocean basin or as small as a rotting log. What makes it an ecosystem, rather than just a collection of things, is the constant exchange of energy and matter flowing among all the parts.
Living Components
The living, or biotic, side of an ecosystem includes every organism present: large plants and trees, animals of all sizes, fungi, and the enormous populations of bacteria and other microorganisms in the soil, water, and air. These organisms are typically sorted by what they eat and how they get energy.
Producers sit at the base. On land, these are mostly green plants and algae that capture sunlight and convert it into chemical energy through photosynthesis. In deep-sea ecosystems, certain bacteria fill this role by drawing energy from chemical reactions instead of light. Everything else in the ecosystem depends, directly or indirectly, on the energy these producers create.
Primary consumers eat the producers. Think of deer grazing on grass, caterpillars chewing leaves, or zooplankton filtering algae from lake water. Secondary consumers are carnivores that eat those plant-eaters, and tertiary consumers eat other carnivores. Some ecosystems support a fourth level, with apex predators like wolves, sharks, or eagles sitting at the top of the chain with no natural predators of their own.
Decomposers complete the cycle. Fungi, earthworms, bacteria, and other soil organisms break down dead plants and animals, returning their nutrients to the soil or water where producers can use them again. Without decomposers, dead matter would pile up and nutrients would stay locked away, eventually starving the rest of the system.
Nonliving Components
The nonliving, or abiotic, elements form the physical stage on which the living community operates. The most fundamental ones are sunlight, water, air, and soil. Sunlight provides both warmth and the energy that drives photosynthesis. Water sustains every organism and carries nutrients through the system. Oxygen allows most organisms to breathe. Soil gives plants a place to root and, after those plants die, helps decompose them.
Beyond those basics, several other physical factors shape what an ecosystem looks like and what can survive there. Climate and temperature set broad limits on which species can thrive. Topography (the shape of the land, whether flat, hilly, or mountainous) influences water drainage, sun exposure, and wind patterns. Altitude matters because higher elevations bring cooler temperatures, thinner air, and more intense sunlight. Even the type of rock underlying an area affects which minerals are available in the soil, which in turn determines what plants can grow.
How Energy Moves Through the System
Energy flows through an ecosystem in one direction: from the sun to producers, from producers to consumers, and from consumers to decomposers. At each step, a large share of that energy is lost as heat. A widely used rule of thumb is that only about 10% of the energy at one level passes to the next. A field of grass might capture a certain amount of solar energy, but the rabbits eating that grass absorb only a tenth of it, and the fox eating the rabbits absorbs only a tenth of what the rabbits held.
This steep drop-off is why food chains rarely exceed four or five levels. By the time energy reaches a top predator, there simply isn’t enough left to support another layer of consumers above it. It also explains why large predators need vast territories and why their populations are always far smaller than the populations of the herbivores they hunt.
How Matter Cycles Through the System
Unlike energy, matter doesn’t leave an ecosystem. It cycles. The same atoms of carbon, nitrogen, phosphorus, and sulfur move between living organisms and the nonliving environment in loops called biogeochemical cycles. These cycles are a core part of what holds an ecosystem together.
The carbon cycle is probably the most familiar. Plants pull carbon dioxide from the air during photosynthesis and store the carbon in their tissues. When those plants die, microorganisms decompose them and integrate the carbon into soil. Some carbon returns to the atmosphere when organisms breathe or when organic matter decays. Over much longer timescales (millions of years), carbon can be locked into rock or fossil fuels and released through volcanic activity or weathering.
The nitrogen cycle depends heavily on bacteria. Atmospheric nitrogen gas is unusable by most plants, so specialized bacteria convert it into ammonia, a form plants can absorb. Some of these bacteria live in the roots of legumes like beans and clover, while others are free-living in the soil. Lightning also contributes by producing nitrogen oxides that rain carries into the soil as nitrate. On the return trip, other bacteria convert nitrate back into nitrogen gas, releasing it to the atmosphere.
Phosphorus works differently because it has no significant gas phase. It enters ecosystems almost entirely through the slow weathering of rocks. Once released into soil, phosphorus is taken up by plants, passed through the food chain, and eventually returned to the soil through decomposition. This makes phosphorus-rich soils especially valuable and phosphorus loss especially hard to replace on human timescales.
Relationships Between Species
The organisms within an ecosystem don’t just eat each other. They interact in a range of ways that ecologists group under the term symbiosis, meaning two unrelated species living in close association. Three main types shape ecosystem dynamics.
In mutualism, both species benefit. Honeybees drink nectar from flowers and, in the process, carry pollen between plants, helping them reproduce. Clownfish live among sea anemones, gaining protection from predators while their waste provides the anemone with nutrients. These partnerships can be so tightly woven that neither species thrives without the other.
In commensalism, one species benefits while the other is unaffected. Remora fish hitch rides on sharks, gaining transportation, protection, and food scraps without helping or harming the shark. Cattle egrets follow grazing cattle, snapping up insects disturbed by hooves, while the cattle carry on unaffected.
In parasitism, one organism benefits at the other’s expense. Ticks feed on the blood of mammals, sometimes transmitting diseases like Lyme disease. Common cuckoo birds lay eggs in other birds’ nests, and the cuckoo chicks often push the host’s eggs out, commandeering all the parental care. Parasites rarely kill their hosts outright, since a dead host means a lost home, but they can weaken them significantly.
Where One Ecosystem Ends and Another Begins
Ecosystems don’t have hard borders. Where two ecosystems meet, you find an ecotone: a transition zone with characteristics of both neighbors and sometimes unique properties found in neither. The word itself comes from Greek roots meaning “home” and “tension,” capturing the idea of competing influences. A marsh between open water and dry land is a classic example. Mountain treelines, where forest gives way to alpine meadow, are another.
Ecotones exist at every scale, from the edge where a forest meets a grassland spanning hundreds of kilometers to a tiny boundary between a sunny clearing and the shaded forest floor. Some are natural, shaped by altitude, latitude, or moisture gradients. Others are human-generated, like the sharp edge between a clear-cut area and intact forest or the border of an urban park against surrounding pavement. These transition zones often support unusually high biodiversity because species from both neighboring ecosystems overlap there.
What Ecosystems Do for People
Ecologists recognize four broad categories of services that ecosystems provide to humans. Provisioning services are the tangible products: food, fresh water, timber, fiber, and fuel. Regulating services are the behind-the-scenes controls that ecosystems exert over floods, disease outbreaks, water purification, and climate stability. Cultural services include recreation, spiritual significance, and the aesthetic value people draw from natural places. Supporting services are the foundational processes, like nutrient cycling and soil formation, that make all the other services possible.
The scale of these services is enormous and largely invisible until they fail. Wetlands that once absorbed floodwaters get drained, and downstream communities flood. Pollinator populations decline, and crop yields follow. Understanding what an ecosystem is comprised of helps explain why losing any piece, whether a single species, a nutrient cycle, or a physical feature, can ripple outward in ways that affect everything else in the system, including us.
Scale and Current Threats
An ecosystem can be defined at virtually any scale. A puddle of water teeming with microorganisms qualifies, and so does the entire biosphere. The boundaries are set by whoever is studying or describing the system. What matters is that within those boundaries, living organisms and their physical environment form an interconnected network of energy, matter, and information transfer.
Globally, protection of key biodiversity areas has roughly doubled over the past 25 years. On land, coverage of these critical sites increased from about 27% in 2000 to nearly 45% by 2024. Marine coverage rose from about 26% to 46% over the same period. Still, more than half of each key biodiversity site remains unprotected, and roughly 3.5% of the world’s mountain area experienced harmful land cover changes between 2000 and 2021. The composition of ecosystems everywhere is under pressure from habitat loss, climate shifts, and pollution, all of which disrupt the living and nonliving connections that keep these systems functioning.