0.4 What is biodiversity? a comparison of spider communities  (Page 3/3)

Again, tally how many individuals belonging to each species occur in each site's spider collection (use yourclassification of spiders completed for Site 1 during Level 1 of the exercise). Specifically, construct a table of species (rows)by site (columns). In the table's cells put the number of individuals of each species you found in the collection fromthe island. You can then analyze these data to generate different measures of community characteristics to help youto decide how to prioritize protection of the forest patches. Recall that you need to rank the patches in termsof where protection efforts should be applied, and you need to provide a rationale for your ranking.

You will find it most useful to base your decisions on three community characteristics: species richness and speciesdiversity within each forest patch, and the similarity of spider communities between patches. Species richness issimply the tally of different spider species that were collected in a forest patch. Species diversity is a morecomplex concept. We will use a standard index calledSimpson Reciprocal Index, $\frac{1}{D}$ where $D$ is calculated as follows: $$D=\sum p_i^2$$ where $p_i$ = the fractional abundance of the $i^{\mathrm{th}}$ species on an island. For example, if you had a sample of two species with five individuals each, $D=\frac{1}{0.5^2+0.5^2}=2$ . The higher the value, the greater the diversity. The maximum value is the number of species in thesample, which occurs when all species contain an equal number of individuals. Because this index not only reflectsthe number of species present but also the relative distribution of individuals among species within a communityit can reflect how balanced communities are in terms of how individuals are distributed across species. As a result, twocommunities may have precisely the same number of species, and hence species richness, but substantially differentdiversity measures if individuals in one community are skewed toward a few of the species whereas individuals aredistributed more evenly in the other community.

Diversity is one thing, distinctiveness is quite another. Thus another important perspective in ranking sitesis how different the communities are from one another. We will use the simplest available measure of communitysimilarity, that is, the Jaccard coefficient of community similarity, to contrast community distinctiveness betweenall possible pairs of sites: $${\mathrm{CC}}_j=\frac{c}{S}$$ where $c$ is the number of species common to both communities and $S$ is the total number of species present in the two communities. For example, if onesite contains only 2 species and the other site 2 species, one of which is held in common by both sites, the totalnumber of species present is 3 and the number shared is 1, so $1/3=33$ %. This index ranges from 0 (when no species are found in common between communities) to 1 (when all speciesare found in both communities). Calculate this index to compare each pair of sites separately, that is, compare Site 1 with Site 2 , Site 1 with Site 3 ,, Site 4 with Site 5 for 10 total comparisons. You might find it useful to determine the average similarity ofone community to all the others, by averaging the ${\mathrm{CC}}_j$ values across each comparison a particular site is included.

Once you have made these calculations of diversity (species richness and Simpson's Reciprocal Index) you cantackle the primary question of the exercise: How should you rank these sites for protection and why? Making an informeddecision requires reconciling your analysis with concepts of biological diversity as it pertains to diversity anddistinctiveness. What do you recommend?

Level 3: considering evolutionary distinctiveness

When contrasting patterns of species diversity and community distinctiveness, we typically treat each species as equallyimportant, yet are they? What if a species-poor area actually is quite evolutionarily distinct from others?Similarly, what if your most species-rich site is comprised of a swarm of species that have only recently diverged fromone another and are quite similar to species present at another site? These questions allude to issues of biologicaldiversity at higher taxonomic levels. Only by looking at the underlying evolutionary relationships among species can wegain this additional perspective. We have provided in a phylogeny of the spider families that occur in your collections (a genuine phylogeny for thesefamilies based in large part on Coddington and Levi 1991 ). In brief, the more closely related families (and species therein) arelocated on more proximal branches within the phylogeny. Based on the evolutionary relationships amongthese families, will you modify any of the conclusions you made on prioritizing forest patches for protection based onpatterns of species diversity alone? If so, why?