Ecosystems and nutrient cycles: active recall

A habitat is where an organism lives. A population comprises members of one species in a defined area, while a community includes interacting populations of different species. An ecosystem includes the community and its abiotic environment, linked by processes such as energy transfer and nutrient cycling.

It describes how an organism uses resources and interacts with biotic and abiotic conditions, including its role within the community. It is broader than its physical location. Two species can occupy the same habitat while differing in feeding, timing, tolerances or other niche dimensions.

A justified random procedure reduces deliberate placement bias and gives locations a defined chance of selection. Sufficient independent samples help represent spatial variation. Randomness does not guarantee perfect representation, and a biased sampling frame can still exclude important parts of the habitat.

It is useful for recording changes along a spatial gradient, often with systematic observations at defined positions. A relationship between species abundance and an environmental measurement can support a pattern. Other changing factors can contribute, so the association alone does not establish causation.

Population change is births plus immigration minus deaths minus emigration over a stated interval. Use a consistent area and time period. A rising population count does not establish an increased birth rate if migration or survival could also explain the change.

Available resources, predation, disease and physical conditions can change, altering the population size that an environment can sustain. Density-dependent effects often intensify as population density increases. A simple model is useful, but changing conditions and time delays can produce fluctuations around an estimate.

Establishing organisms can alter soil, organic matter, shading and other conditions, affecting which later species can persist. Communities therefore influence the environment as well as responding to it. Disturbance and local conditions can alter the trajectory; succession is not always one inevitable fixed sequence.

Not all biomass is eaten or assimilated, and assimilated energy supports respiration and other processes. Energy dissipates as heat rather than being recycled indefinitely through the food chain. Use the exact budget and reference quantities supplied when calculating a transfer efficiency.

Gross primary production is the rate at which producers capture energy or form organic material, according to the stated units. Subtract the producers’ respiratory loss to obtain net primary production. The net quantity can support growth and subsequent consumers; keep rates, totals and biomass measurements distinct.

Nitrogen fixation converts atmospheric nitrogen into biologically usable nitrogen compounds, carried out by certain microorganisms. Nitrification oxidises ammonium compounds through nitrite to nitrate. These are different transformations involving different starting materials; plants do not generally absorb atmospheric nitrogen gas directly.

Decomposers break down organic material and can release mineral nutrients, including ammonium through nitrogen-containing material breakdown. Denitrifying microorganisms can convert nitrate to gaseous nitrogen under suitable conditions. Matter moves between reservoirs, while energy transfer through these processes is not a closed recycling loop.

Compare population trends, habitat condition and likely causes of change, then assess the expected benefits and costs of an intervention. Use representative monitoring and recognise uncertainty. An increase in one target species can be encouraging without proving that every part of the ecosystem has recovered.