The Maintenance and Functional
Consequences of Species Diversity
The core of community ecology is concerned with the question: why are there so many species on Earth? The tremendous diversity of life despite common constraints on the physiology and ecology of organisms is one of the hallmarks of living systems. Community ecology seeks to explain the maintenance of species diversity within ecological systems very much like population genetics seeks to explain the maintenance of genetic diversity within species. A large part of this diversity can be explained by geographical differences in environmental conditions across the globe and by historical circumstances. Many species and genetic variants, however, coexist in any given place and at any given time. Why do so many species and types coexist?
There are two main components to local species diversity, which I shall call vertical and horizontal, respectively. Vertical diversity is the diversity of functionally different types of organisms as defined by their trophic relationships or by other, nontrophic interactions (trophic levels, guilds, functional groups). The term “vertical” comes from the traditional representation of food chains in the form of vertical chains with plants at the bottom and carnivores at the top. By contrast, horizontal diversity is the diversity of species within trophic levels or functional groups. Vertical diversity concerns food webs and interaction networks and will be addressed in chapter 4. In this chapter I shall focus on the maintenance of horizontal diversity within ecological communities.
Explaining the coexistence of species with similar functional roles, or ecological niches, is the subject of competition theory. Competition theory is initially an extension of the theory of density dependence in population dynamics, in which intraspecific competitive interactions among individuals of a single species are extended to include interspecific competition among individuals of different species. This theory, however, has expanded