Species composition tells us who is in the community
A final element of a community's structure is its species composition: the identity of the species present in the community. Species composition is an obvious but important characteristic that is not revealed in species diversity indices.
For example, two communities might have the same species diversity value but have completely different members. For example, a study in Scottish pastures showed that two bacterial communities had diversity indices that were nearly identical but their taxonomic composition differed (McCaig et al. 1999). Five taxonomic groups of bacteria out of the 20 the researchers found werepresent in one or the other pasture, but not in both.
In many ways, community structure is the starting point for more interesting questions: How do species in the community interact with one another? Do some species play greater roles in the community than others? How is species diversity maintained? How does this information shape our view of communities in terms of conservation and the services they provide to humans? Let's move from the rather static view of communities as groups of species occurring together in space and time to a more active view of them as complex networks of species with connections and interactions that vary in strength, direction, and significance.