Ecosystems: energy, matter and why food chains are short

This single sentence carries most of ecology. Energy enters an ecosystem from the sun, passes through organisms, and leaves as heat. It makes a one-way trip and must be continually resupplied. Matter, by contrast, is finite and recycled: the same carbon and nitrogen atoms are used repeatedly.

Producers, mostly plants and algae, capture light energy through photosynthesis and store it in chemical bonds. Consumers obtain energy by eating. Decomposers break down dead material, releasing the locked-up matter back into circulation, which is why they are indispensable rather than merely tidy.

Each feeding step is a trophic level: producers, primary consumers, secondary consumers and so on. Only a small fraction of the energy at one level, often around ten percent, becomes biomass at the next. The rest is lost as heat from respiration, spent on movement, or never eaten and never digested.

This loss explains several patterns at once. Food chains rarely exceed four or five levels, because there is not enough energy left to support another. Top predators are rare and need large territories. And a given area of land can feed many more people growing crops than raising livestock, since eating at a lower trophic level skips a lossy step.

• Carbon: fixed by photosynthesis, released by respiration, combustion and decomposition, stored in oceans, rocks and fossil fuels.
• Nitrogen: abundant in air but unusable by most life until nitrogen-fixing bacteria convert it, then passed along food chains and eventually returned by denitrifying bacteria.
• Water: evaporation, transpiration from plants, condensation, precipitation, and flow through ground and rivers.
• Phosphorus: has no significant gas phase, moving slowly through rock weathering, soil, organisms and sediment, which makes it a common limiting nutrient.

A population growing without constraint follows an exponential curve, but no environment permits that for long. Food, space, water, disease and predation impose a carrying capacity, the number an environment can sustain, and growth levels off into an S-shaped logistic curve.

Limiting factors are described as density dependent when their effect strengthens as crowding increases, like disease and competition, and density independent when they hit regardless of numbers, like a flood or a hard frost.

Succession is the predictable change in a community over time. Primary succession begins on bare rock with pioneer species such as lichens that create the first soil. Secondary succession begins where soil already exists, after a fire or an abandoned field, and moves much faster.

Each stage alters conditions in ways that favour the next, until a relatively stable community forms. Biodiversity generally rises through succession, and diverse ecosystems tend to be more resilient because the loss of one species is more easily absorbed.