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.

Abiotic factors in communities

Abiotic factors are the non-living conditions that affect organisms. A change in an abiotic factor can affect how well organisms survive, grow and reproduce. This can change a population’s abundance—how many individuals are present—or its distribution—where they are found.

A community consists of populations of different species living and interacting in the same area. If an abiotic change affects one population, it can also affect populations that depend on it. For example, conditions that reduce plant growth may leave less food for animals that eat the plants. As populations change, the species present and their relative abundances can change, altering the community’s composition.

Abiotic factors and their effects

• Light intensity affects the rate of photosynthesis and therefore plant growth. A plant receiving too little light may grow more slowly.
• Temperature affects biological processes. Species differ in the temperatures they can tolerate, so a temperature change may favour some species while harming others.
• Moisture affects how much water is available to organisms.
• Soil pH and mineral content affect conditions for plant growth and the availability of mineral nutrients.
• Wind intensity affects how exposed organisms are and can increase water loss. Wind direction affects which locations are exposed.
• Carbon dioxide availability can affect the rate of photosynthesis in plants.
• Dissolved oxygen availability affects respiration in aquatic animals.

An increase in a factor is not always beneficial. Its effect depends on the organism and the conditions involved. For example, an increase in temperature may improve conditions for one species but move them beyond the range tolerated by another.

Using data on abiotic factors

Charts, graphs and tables can show how abundance or distribution varies with an abiotic factor. Before comparing values, check the axis labels or table headings and units to identify what is being measured and which organisms or conditions are being compared.

Read the relevant values, then look for a trend or difference. For example, a graph may show that a species is more abundant at some moisture levels than at others. Use that pattern and the context provided to explain how the factor may affect the species and, through its interactions with other populations, the community. A pattern alone does not establish that the abiotic factor caused the difference; other conditions may also have an effect.