Do Communities Evolve? A Major Question in Evolutionary Paleoecology

Chapter 7 of Jim Valentine's classic book, Evolutionary Paleoecology of the Marine Biosphere, is titled "Community Ecology and Evolution" (Valentine 1973, 269-336). The first sixty-two pages of the chapter are about various general properties of community ecology and paleoecology, the next three and one-half pages concern the evolution of species within communities, and only the last two and one-half pages focus on communities through geological time. Valentine headed this short concluding section, "Many More Data Are Needed On Fossil Communities." Two decades later the data exist to deal more explicitly with the question of community evolution (103 of the 122 references used in this chapter postdate the writing of Valentine's book). We hope to use these data to answer the question, do communities evolve? The focus in this chapter is primarily on marine paleoecology, in parallel with Valentine's original subject. The first questions asked in discussions of theory about communities are (1) what are communities, and (2) do they exist as identifiable entities? Numerous concepts and definitions of communities exist, ranging from the idea of the community as a superorganism to the view of the community as just a random aggregation of populations in a local habitat community definitions are ultimately rooted in the fact that populations of individuals of a variety of species live together in local geographic areas. Whether these populations are linked together tightly in a coevolved structure or not, a variety of interactions (ranging from trophic activity to habitat modification) take place among these organisms, either contemporaneously or over time. The reality of coexistence and interaction justifies the recognition of community as a level in the ecological hierarchy for associations of populations of noncon-specific organisms (Eldredge 1985, 162-73; Salthe 1985; Eldredge and ~ i e n e 1992). One reason why agreement on what is meant by terms such as community has never been achieved is that both ecological interactions and species occurrence patterns take place at a variety of scales. One worker may focus on a local patch, another on a widespread environment. Each may consider that the fauna and flora in the geographic region studied comprises a community, yet they may not be dealing with comparable entities. However, by using the concept of an eco