Populations and ecosystems are studied by sampling organisms, measuring environmental conditions and looking at how both change over time. A population is a group of organisms of the same species living in the same place at the same time. An ecosystem includes a community of organisms and the non-living conditions around them.

Studying populations

For plants and other organisms that do not move around, place quadrats at random coordinates to estimate abundance without choosing only convenient areas. A belt transect uses quadrats along a line to investigate how a species’ distribution changes across an environmental gradient, such as increasing distance from a pond. Measure a relevant factor, such as soil moisture, alongside the organisms.

For mobile organisms, mark-release-recapture estimates population size. Capture and mark a sample, release it, then take a second sample and count how many marked individuals return. For example, if $40$ are marked, $50$ are caught later and $10$ of those are marked, the estimate is

\frac{40 \times 50}{10}=200.

This assumes marks remain visible, marked organisms mix with the population, and the population does not change substantially between samples.

Carrying capacity and succession

Carrying capacity is the population size an environment can support under its current conditions. It can change as resources, competition, predation or abiotic factors such as temperature change. It is not a fixed maximum for a species everywhere.

Succession is the change in a community over time. Pioneer species first colonise an area and alter its conditions, allowing other species to establish. Further stages may lead to a relatively stable climax community. Comparing species and environmental measurements at different stages helps investigate this process; conservation may involve managing succession.

Exam mistake: a transect shows where organisms occur, but a pattern alone does not prove which environmental factor caused it.