Ecosystems are integrated systems of organisms (biotic components) and their physical environment (abiotic components) that interact to cycle energy and nutrients. Ecosystem structure includes vertical stratification, horizontal patchiness, and species composition at different trophic levels. Function describes how energy flows through trophic levels and how nutrients cycle between living organisms and the abiotic environment.
Observe a real or digital ecosystem and map its biotic and abiotic components. Use cross-section diagrams to understand stratification in forests, aquatic systems, and grasslands.
Ecosystems are not just collections of organisms but require both living and non-living components. Decomposers are equally important as producers and consumers in ecosystem function.
An ecosystem is more than a collection of species living in the same place. It is an integrated system in which living organisms and their physical environment interact continuously through the exchange of energy and matter. The biotic components — all the living organisms — cannot be understood in isolation from the abiotic components: temperature, light, water, soil nutrients, oxygen levels, and other physical and chemical factors that shape what lives where and how productively.
Ecosystems have structure at multiple scales. Vertically, they are stratified: a forest has canopy trees, understory shrubs, ground cover plants, and a soil layer, each occupied by different species with different light, moisture, and temperature tolerances. Horizontally, ecosystems are patchy — different soil types, drainage patterns, and disturbance histories create mosaics of microhabitats within a single landscape. Species are distributed across these structural layers according to their ecological requirements, creating the community composition we observe.
Organisms are organized into trophic levels based on how they acquire energy. Producers (plants, algae, cyanobacteria) capture energy from sunlight through photosynthesis and convert it into organic matter. Primary consumers (herbivores) eat producers. Secondary consumers eat herbivores, and so on up to apex predators. A critical principle is that energy is lost at every trophic transfer — roughly 90% dissipates as heat through metabolism, movement, and excretion. Only about 10% is incorporated into the consumer's own biomass. This is why energy pyramids taper sharply: a grassland can support vast numbers of insects but only a handful of hawks.
Nutrients cycle differently from energy — while energy flows in one direction (in through sunlight, out as heat), elements like carbon, nitrogen, and phosphorus cycle repeatedly between living organisms and the abiotic environment. Plants absorb inorganic nitrogen from soil; herbivores incorporate it into their tissues; when they die, decomposers (bacteria and fungi) break down the organic matter and release inorganic nitrogen back into the soil for plants to absorb again. This nutrient cycling is what makes ecosystems sustainable over long timescales.
Decomposers occupy a frequently underappreciated role. They are not merely the "clean-up crew" — they are the mechanism by which nutrients re-enter the living system. Without decomposers, dead organic matter would accumulate, nutrients would be permanently locked in unusable form, and primary production would collapse within a short time. A forest ecosystem depends just as much on the fungi and bacteria in its soil as on the trees in its canopy. Recognizing the full biotic community — producers, consumers, and decomposers — alongside the abiotic environment is the foundational lens for everything that follows in ecosystem ecology.