Ecosystems, food webs and populations

Define the system and its components

An ecosystem includes living organisms, non-living environmental components and their interactions. Light, temperature, water and mineral availability can influence which organisms survive and reproduce. A habitat is the place an organism lives; an ecological niche includes how it uses resources and interacts with its environment. Two species can share a habitat while using different food sources or being active at different times. A shared location therefore does not prove that their ecological roles are identical.

Within an ecosystem, producers build organic compounds from inorganic carbon. Many use light through photosynthesis; some use chemical energy. Consumers obtain organic matter by feeding on other organisms or their products. Decomposers such as many fungi and bacteria break down dead organic matter, making nutrients available for further cycling. These are functional roles rather than a complete classification of evolutionary relationships. An organism's role depends on the process under discussion and the food web being considered.

Read the arrow before following the chain

In a food chain, an arrow conventionally points from a food resource to the organism receiving matter and energy from it. Grass → grasshopper → frog means that the grasshopper eats grass and the frog eats grasshoppers. A food web combines many feeding relationships. An omnivore may feed at more than one trophic level, so it need not have a single fixed position in every chain. Always check the diagram's legend because a specially labelled diagram can use a different arrow convention.

Only part of the energy available at one trophic level becomes new biomass available to the next. Organisms use energy for metabolism, some material is not eaten, and some eaten material is not assimilated. Energy is eventually dispersed as heat. For a numerical problem, use the transfer efficiency supplied; do not assume that a single percentage is a universal constant. If two transfers occur, apply both efficiencies successively rather than subtracting the same percentage of the original amount twice.

Nutrients can cycle through living organisms and environmental reservoirs. Carbon enters many food webs when producers fix carbon dioxide and returns through respiration, decomposition and other processes. Water and mineral nutrients also circulate. Energy flows through the system rather than being recycled in the same way. Decomposers return matter to available forms, but they do not restore all dispersed energy to sunlight. This distinction explains why ecosystems need continuing energy input even though many atoms can be reused.

Classify interactions by consequences

Predation benefits a predator while harming the prey. Herbivory similarly benefits a consumer at a plant's expense. Parasitism benefits a parasite while harming its host, often through a prolonged association. Competition occurs when organisms use a limiting resource in ways that reduce its availability to one another; it can occur within a species or between species. Mutualism benefits both partners, whereas commensalism benefits one without a substantial effect on the other under the stated conditions.

Descriptions matter more than familiar labels. If one organism obtains shelter and the other gains protection, both benefit, supporting mutualism. If both lose access to a scarce nutrient, the interaction is competition. Effects can depend on conditions, so avoid assuming that every association is permanently beneficial or harmful. Likewise, removing one species from a food web can have indirect effects on others; the immediate feeding link is only the beginning of the explanation.

Account for changes in population size

Population size changes through births, deaths, immigration and emigration. Births and immigration add individuals; deaths and emigration remove them. In a closed population, immigration and emigration are absent. When resources are abundant, a population may grow rapidly, but limited food, space, disease or predation can slow growth. Carrying capacity describes the population size an environment can sustain under particular conditions, not an unchangeable property of a species.

Density-dependent effects, such as competition for food, often intensify as individuals become more crowded. Other disturbances can affect populations without depending primarily on density. A graph that flattens does not prove that births have stopped: births and immigration may be balanced by deaths and emigration. Interpret a population graph as a net outcome and ask which processes could produce it. This prevents replacing a measured change with an unsupported explanation of its cause.