Levels of organisation in an ecosystem

Ecology • Adaptations, interdependence and competition

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What is the difference between abundance and distribution?

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Abundance is the number of individuals; distribution is the spatial pattern of where they occur.

Key concepts

What you'll likely be quizzed about

Individual organism

An individual organism is a single living entity capable of carrying out life processes such as respiration, growth and reproduction. Individual traits determine survival and reproduction; trait differences cause variation within a population and influence competition for resources. Limiting factors for individuals include availability of food, water and suitable microhabitat; if any of these fall below a critical level then survival and reproduction decline, causing population change.

Population

A population is all organisms of the same species living in a defined area at a given time. Population size changes when births, deaths, immigration and emigration do not balance; higher birth rates or immigration increase population size while higher death rates or emigration decrease it. Density-dependent limiting factors such as food supply and disease become stronger as population size rises, causing competition to increase and growth rate to fall.

Community

A community is the set of populations of different species living and interacting in the same area. Interactions such as predation, competition and mutualism shape community structure and determine which species persist. Removal or addition of one species causes cascading effects; for example, predator removal allows prey populations to increase, which then changes resource availability for other species and alters community balance.

Ecosystem

An ecosystem consists of a biological community plus the non-living (abiotic) environment in which it exists, including factors such as light, temperature, soil pH and water availability. Energy flow and nutrient cycling link producers, consumers and decomposers and determine ecosystem function. Abiotic factors act as limiting factors; for example, reduced light limits photosynthesis, which reduces producer biomass and therefore the energy available to higher trophic levels.

Sampling, quadrats and measurement

Sampling uses representative parts of a habitat to estimate abundance and distribution across a larger area because counting every individual is impractical. Use of random sampling, quadrats and transects reduces bias and allows calculation of mean abundance and percentage cover for plants and slow-moving organisms . If sampling size is too small or non-random then estimates are biased; increasing sample number and using random selection improves accuracy and precision of ecosystem-level conclusions.

Key notes

Important points to keep in mind

Individual → population → community → ecosystem describes scale from single organism to environment.

Population change results from births, deaths, immigration and emigration.

Higher population density increases competition and strengthens density-dependent limiting factors.

Abiotic factors such as light, temperature and pH limit biological processes and determine species distribution.

Biotic interactions (predation, competition, mutualism) shape community composition and stability.

Sampling with random quadrats and adequate sample size reduces bias and improves estimates of abundance.

Percentage cover, count per quadrat and mean density are standard measures for plant abundance.

Removal or introduction of a species causes cascading effects through food webs and ecosystem function.

Producers form the base of energy flow; reduced producer biomass decreases energy available to higher trophic levels.

Correlation between two ecological variables does not prove causation; identify the causal mechanism.