Ecosystem organisation

Living things in an area can be considered at different levels, from a group of one species to the living and non-living parts of the whole environment.

A population consists of organisms of the same species living in a particular area. For example, all the water lilies of one species in a pond form a population.

A community consists of the populations of different species living together in an area. In a pond, the populations of plants, animals and microorganisms together form the community.

An ecosystem consists of a community and the non-living, or abiotic, components of its environment, together with the interactions between them. In a pond ecosystem, organisms interact with one another and with abiotic components such as water, light and temperature. For example, water lilies use light to grow and provide habitats for other organisms.

Ecosystems can be very small or very large: a pond can be an ecosystem, as can a large forest.

Investigating environmental effects on species distribution

To investigate whether an environmental factor is associated with the distribution of a species, measure the factor and record the species at the same sampling locations. Each location then provides a paired observation: a value for the factor and a record of the species.

For example, an investigation could compare light intensity with the abundance of a named plant species. At each location, measure light intensity and count the plants of that species in a quadrat: a frame marking a fixed sampling area. A count of zero is also useful because it records that the species was not found in that sample.

Sampling across an environmental gradient

A gradient is a gradual change in an environmental factor across an area. Sampling at intervals along a gradient can provide observations at a range of factor values. In the light investigation, sampling might extend from a shaded area towards a more exposed area.

Use consistent methods at every location. For example, use the same size of quadrat, count the species in the same way, and measure light using the same method under comparable conditions. This makes differences between observations easier to interpret.

Take replicate samples across the range of conditions rather than relying on one observation at each level of the factor. Choose sampling locations appropriately so that they cover the area and range being investigated. Replication reveals variation between locations and reduces the influence of an unrepresentative sample.

Comparing the factor with species distribution

Compare the paired measurements to look for an association. For example, the plant may tend to be more abundant where measured light intensity is higher. The data may instead show the opposite pattern or no clear pattern.

An association alone does not establish that the measured factor caused the distribution. Locations that differ in light may also differ in other environmental variables, which could help explain the pattern. The investigation identifies a relationship in the observations, not necessarily its cause.