2.1 Individuals, populations, communities and ecosystemsIB Environmental Systems and Societies SL: Revision notes

The biosphere is organised into individuals, populations, communities and ecosystems. An individual is a member of a species. A population is a group of the same species living in the same area at the same time and able to interbreed. A community is all the interacting populations in an ecosystem. Habitat is the location where an organism, population or community lives.

Classification lets us identify organisms efficiently and predict their characteristics from the group they belong to. Taxonomists use tools such as dichotomous keys, field guides and DNA analysis to identify organisms.

The distribution of a population is limited by abiotic factors (temperature, sunlight, pH, salinity, dissolved oxygen, soil texture) and biotic factors (food, competitors, predators, disease).

A niche is the particular set of abiotic and biotic conditions and resources on which an organism or population depends. Populations interact by herbivory, predation, parasitism, mutualism, disease and competition. These interactions have ecological, behavioural and evolutionary consequences: for example, competition between red and grey squirrels in Britain has reduced the range of red squirrels.

Carrying capacity is the maximum population size that the resources of an area can support, set by competition for limited resources. Population size is regulated by density-dependent factors (competition, predation, disease) that act more strongly as density rises, and by negative feedback, which returns the population towards carrying capacity.

Population growth can be exponential (when resources are plentiful) or limited by carrying capacity (levelling off). Limiting factors on human populations have increasingly been removed by agriculture, medicine and technology, with consequences for the sustainability of ecosystems. Carrying capacity is hard to assess for humans because technology, consumption and trade change what resources are available.

Abundance can be estimated by random sampling, systematic sampling or transect sampling. Random quadrat sampling estimates the population of non-mobile organisms: count in randomly placed quadrats, find the mean and scale up to the whole area.

For mobile organisms use capture–mark–release–recapture and the Lincoln index:

N = (n₁ × n₂) ÷ m₂

where n₁ is the number first caught and marked, n₂ the number caught in the second sample and m₂ the number of marked animals recaptured. The method assumes a closed population, harmless marks that persist, and random mixing.

Ecosystems are open systems: energy and matter can enter and leave. Sustainability is a natural property of ecosystems, but human activity can push them past tipping points into a different state. Keystone species, such as sea otters in kelp forests, have a role in sustainability that is much greater than their abundance suggests.

The planetary boundaries model indicates that changes to biosphere integrity have passed a critical threshold. To avoid critical tipping points, loss of biosphere integrity needs to be reversed.