Biodiversity and species richness

Biodiversity describes the variety of living organisms in an area. The area being considered can be small, such as a local habitat, or as large as the whole Earth. The scale must therefore be clear when describing biodiversity.

Within a community, species richness is the number of different species present. To find it, count each species once, regardless of how many individuals belong to it.

For example, a community with 90 individuals of one species and 5 individuals of each of two other species has a species richness of 3. A community with 34, 33 and 33 individuals of three species also has a species richness of 3.

The two communities have the same species richness, even though their individuals are distributed differently among the species. Species richness tells us how many species are present, not how individuals are shared among them.

Interdependence in communities

A community consists of the populations of different species living in the same place. Species in a community are interdependent: they depend on other species for resources or services.

These dependencies include food and shelter. They also include pollination, when animals transfer pollen between flowers, and seed dispersal, when animals carry seeds away from the parent plant. These services help plants reproduce and spread.

If one species is removed, the populations that depend on it may change. Those changes can then affect other species throughout the community. For example, removing a plant could reduce the food available to insects that feed on it. If those insects become less numerous, birds that eat them may have less food too.

Stability in a community

A stable community is one in which the species and environmental factors are in balance, so population sizes remain fairly constant over time. Stability does not mean that the populations never change: their sizes can fluctuate while the community remains stable.

Biodiversity and ecosystem stability

An ecosystem contains species that depend on one another for resources and suitable conditions. Biodiversity is the variety of living organisms in an area. High biodiversity can make an ecosystem more stable because a species may be less dependent on any one other species.

For example, if an animal can obtain food from several species, the loss of one food source may not leave it without food. Similarly, several species may provide shelter or help maintain the physical environment. When more than one species can contribute to a role, the loss of one is less likely to disrupt the ecosystem severely.

High biodiversity does not mean that losing a species has no effect, or that an ecosystem is guaranteed to remain stable. The effect depends on the species lost and on whether other species can still support the roles it performed.

Biodiversity and human survival

Humans depend on functioning ecosystems for resources and environmental conditions, including food and clean water. If biodiversity is reduced, ecosystems may become less able to continue providing these things.

Many human activities reduce biodiversity, potentially weakening the ecosystems on which humans depend. In recent times, measures have been introduced to try to stop biodiversity loss, helping to maintain the ecosystems needed for human survival.