Surrogate Measures of Overall Biodiversity

The discussion above illustrates the many different ways of defining biodiversity, and each way depends on how we want to characterize biodiversity. For example, we may want to show the genetic diversity between populations from different regions, or we may want to show the diversity of trophic levels represented by the species in different ecosystems. But how do we provide an account of the overall biodiversity of an area in terms of the diversity of the organisms, communities, ecosystems, and interactions present? It is usually difficult, if not impossible, to measure all these aspects of the biodiversity of a region, so we must select some representative or surrogate measure of the overall diversity.

What do we mean by surrogate? Essentially we need to measure an aspect of biodiversity that is feasible to quantify, and we need to choose something that best represents the nonmeasured aspects of biodiversity. We take baseline information on these surrogates and monitor them over time to determine changes in the status of biodiversity based on a management strategy.

The number of species present in an area, or the species richness of an area, is one of the most common surrogates for estimating overall biodiversity. A greater number of species implies a greater level of genetic, organismal, and ecosystem diversity. However, species richness can oversimplify the extent of diversity, because it does not account for possible variation in the types of species present—that is, the taxo-nomic or phylogenetic diversity of the species present. Table 4 compares three different regions with three communities of species.

Table 4

Comparison of Species in Three Regions

Table 4

Comparison of Species in Three Regions

Region A

Region B

Region C

Plant 1

Plant 1

Plant 1

Plant 2

Snail 1

Plant 2

Plant 3

Fish 1

Plant 3

Plant 4

Lizard 1

Plant 4

Plant 5

Bird 1

Plant 5

Bird 1

Snail 1

Bird 2

Fish 1

Bird 3

Lizard 1

Bird 4

Bird 1

Bird 5

Region A is clearly more diverse than region B in terms of species richness, because it has twice as many species. However, region B is more taxonomically diverse, having representatives from five different taxonomic groups (plants, mollusks, fishes, lizards, and birds) compared with only two groups (plants and birds) in region A. This greater level of taxo-nomic diversity for region B implies that it is genetically and ecologically richer, despite the fact that it has fewer species.

Let us consider the relative contribution that each of the different taxonomic groups makes to the overall species diversity for regions A and B. In region A, plants and birds both contribute 50 percent of the total number of species present, and 50 percent to the taxonomic diversity. In region B, each of the taxonomic groups contributes 20 percent to the total number of species present, and 20 percent to the taxonomic diversity. Now let us compare region B with region C. Region C has the same number of taxonomic groups as region B, but it differs by having multiple species of plants. So each taxonomic group still contributes 20 percent to the taxonomic diversity (as in region B), but plants contribute 56 percent to the total number of

Table 5

Abundance of Species* in Three Ecosystems, with Measures of Richness and Evenness

Table 5

Species

Ecosystem A

Ecosystem B

Ecosystem C

220

80

120

2

170

65

65

120

50

10

4

70

0

0

Richness (S)1

4

3

3

Shannon's Diversity Index (H)2

1.3086

1.0807

1.0323

Evenness (E)3

0.94

0.98

0.94

* Number of specimens per hectare

Sources: Gibbs, J. P., M. L. Hunter Jr., and E. J. Sterling. 1998. "Problem-solving in Conservation Biology and Wildlife Management. Exercises for Class, Field, and Laboratory." Boston: Blackwell Science; Gross, L. J., et al., eds. "Alternative Routes to Quantitative Literacy for the Life Sciences," a project supported by the National Science Foundation through award DUE-9752339 to the University of Tennessee, Knoxville, August 1, 1998-July 31, 2000. The Institute for Environmental Modelling, University of Tennessee, Knoxville. http://www.tiem.utk.edu (cited June 21, 2002) for discussion and examples; Magurran, Anne E. 1988. Ecological Diversity and Its Measurement. Princeton: Princeton University Press also provides discussion of the methods of quantifying diversity.

* Number of specimens per hectare

Sources: Gibbs, J. P., M. L. Hunter Jr., and E. J. Sterling. 1998. "Problem-solving in Conservation Biology and Wildlife Management. Exercises for Class, Field, and Laboratory." Boston: Blackwell Science; Gross, L. J., et al., eds. "Alternative Routes to Quantitative Literacy for the Life Sciences," a project supported by the National Science Foundation through award DUE-9752339 to the University of Tennessee, Knoxville, August 1, 1998-July 31, 2000. The Institute for Environmental Modelling, University of Tennessee, Knoxville. http://www.tiem.utk.edu (cited June 21, 2002) for discussion and examples; Magurran, Anne E. 1988. Ecological Diversity and Its Measurement. Princeton: Princeton University Press also provides discussion of the methods of quantifying diversity.

1 The total number of species in an area.

2 Shannon's Diversity Index (H) = -Xpi In pi, where pi is the proportion of the total number of specimens of species i expressed as a proportion of the total number of specimens for all species in the ecosystem. The product of (pi In pi) for each species in the ecosystem is summed and multiplied by -1 to give H.

3 The species evenness index (E) is calculated as H/Hmax, where Hmax is the maximum possible value of H and is equivalent to In(S). Thus E = H/In(S)

species, and all other taxonomic groups contribute only 11 percent.

Another factor to compare against species richness (that is, the total number of species present in an area) is the evenness with which species are represented. Table 5 shows abundance of species (number of individuals per hectare) in three ecosystems and gives the measures of species richness and evenness and the Shannon diversity index.

Ecosystem A shows the greatest diversity in terms of species richness, but ecosystem B could be described as being richer, insofar as all the species present are more evenly represented (The E value is larger). This example also illustrates a condition that is often seen in tropical ecosystems, where disturbance of the ecosystem causes uncommon species to become even less common, and common species to become even more common. Disturbance of ecosystem B may produce ecosystem C, where the uncommon species 3

becomes less common, and the relatively common species 1 has become more common. There may even be an increase in the number of species in some disturbed ecosystems, but, as noted above, this may occur with a concomitant reduction in the abundance of individuals or local extinction of the rarer species.

Also, individuals of any one species might be abundant in one part of the region under consideration but absent in all other parts. Another species might have the same number of individuals, but they are more widespread over the entire area. For example, if we can consider an ecosystem with a total area of 1 hectare, containing 60 specimens of two species (species X and species O shown in Figure 3). If we divide the ecosystem area into a grid of 100 smaller units, each 0.01 hectares in size, we might see a distribution of the two species similar to that in Figure 3.

There are 60 specimens of both species in the 1-hectare grid, but species X shows all the

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