The term biodiversity refers to the variety of forms of life in an area. Biologists study and describe biodiversity in several ways. One measure of biodiversity is species diversity. Species diversity, in turn, is reflected by both species richness and species evenness. Species richness refers to the number of unique species within an area. This kind of diversity is easiest to notice and is most often what is meant when referring to biodiversity. Species evenness refers to the relative number of individuals of each species in an area.

Species evenness is harder to study than species richness but may be more important to understanding ecosystems. Two locations may have the same species richness yet differ in species evenness. For example, suppose that five tree species grow in each of two forests. In one forest, the numbers of each kind of tree are about the same, but in the other forest, one kind of tree far outnumbers the other kinds. A disruption such as a drought would affect the two forests quite differently.

Another measure of biodiversity is genetic diversity, or the amount of variation in the genetic material within all members of a population. Genetic diversity is important because it affects a population's ability to adapt in the face of environmental change. Recall that evolution by natural selection acts upon genetic variation within populations. When populations are reduced to small numbers, the amount of genetic variation is reduced for many generations to come. A reduction in a species' genetic variation reduces the likelihood that the species will survive natural selection.

Measuring Biodiversity

Often, when people think of the importance of other species, they focus on mammals such as pandas, lions, or dolphins. But mammals are a small fraction of biodiversity. Of all of the known species, a huge proportion is made up of insects, as shown in Figure 22-4. We know very little about most of these species.


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