CHAPTER 1

Communities

"Ecology is the science of communities. A study of the relations of a single species tothe environment conceived without reference to communities and, in the end, unrelatedto the natural phenomena of its habitat and community associations is not properlyincluded in the field of ecology." Victor Shelford (1913)

1.1 Overview

This chapter briefly describes how ecological communities are defined and classified,and introduces some of the properties and interactions that community ecologistsstudy. The major interspecific interactions, or elementary processes, among pairs ofspecies include competition, predation, and mutualism. Complex indirect interactionscan arise among chains of three or more interacting species. Important communityproperties include the number of species present, measures of diversity, which reflectboth the number and relative abundances of species, and statistical distributions thatdescribe how different species differ in abundance.

Observations of natural patterns and explorations of mathematical models haveinspired generalizations about the underlying causes of community organization. Onepattern important in the historical development of community ecology concerns anapparent limit to the similarity of coexisting species. The case of limiting similarityprovides a cautionary example of the way in which community patterns are initiallyrecognized, explained in terms of causal mechanisms, and eventually evaluated.Community patterns are the consequence of a hierarchy of interacting processes thatinteract in complex ways to mold the diversity of life on Earth.

1.2 Communities

Our best estimates suggest that somewhere between 1.5 million and 30 million differentspecies of organisms live on Earth today (Erwin 1982; May 1990). The smallfraction of this enormous global collection of species that can be found at any particularplace is an ecological community. One important goal of community ecology isto understand the origin, maintenance, and consequences of biological diversitywithin local communities. Different processes, operating on very different time scales,can influence the number and identity of species in communities. Long-term evolutionaryprocesses operating over time scales spanning millions of years can producedifferent numbers of species in different locations. Short-term ecological interactionscan either exclude or facilitate species over shorter time scales ranging from a fewhours to many years. This book provides an overview of community patterns and theprocesses that create them.

Like many fields of modern biology, community ecology began as a descriptivescience. Early community ecology was preoccupied with identifying and listing thespecies found in particular localities (Clements 1916; Elton 1966). These surveysrevealed some of the basic community patterns that continue to fascinate ecologists.In many temperate zone communities, a few species are much more common thanothers. The dominant species often play an important role in schemes used to identifyand categorize different communities. But why should some species be much morecommon than others? Communities also change over time, often in ways that arequite repeatable. But what processes drive temporal patterns of community change,and why are those patterns so regular within a given area? Different communities canalso contain very different numbers of species. A hectare of temperate forest in NewJersey in northeastern North America might hold up to 30 tree species (Robichaudand Buell 1973), while a similar sized plot of rainforest in Panama can yield over 200tree species (Hubbell and Foster 1983). More than 10 different ideas have been proposedto explain the striking latitudinal gradient in biodiversity that contributes tothe differences between temperate and tropical communities (Pianka 1988)! Whilethere are many reasonable competing explanations for the commonness and rarity ofspecies, and for latitudinal differences in biodiversity, the exact causes of these verybasic patterns remain speculative. Related questions address the consequences ofbiodiversity for community processes. Do communities with many species functiondifferently from those with fewer species? How do similar species manage to coexistin diverse communities?

The central questions in community ecology are disarmingly simple. Our ability toanswer these questions says something important about our understanding of thesources of biological diversity and the processes that maintain biodiversity in anincreasingly stressed and fragmented natural ecosystem. Answering these questionsallows us to wisely manage the human-dominated artificial communities that includethe major agricultural systems that we depend on for food and biologically producedmaterials, and to restore the natural communities that we have damaged eitherthrough habitat destruction or overexploitation.

Ecologists use a variety of approaches to explore the sources of community patterns.Modern community ecology has progressed far beyond basic description of patterns,and often experiments can identify which processes create particular patterns (Hairston1989). However, some patterns and their underlying processes are experimentallyintractable, owing to the fact that the organisms driving those processes are so large,long-lived, or wide-ranging that experimental manipulations are impossible.Consequently, community ecologists must rely on information from many sources,including mathematical models, statistical comparisons, and experiments to understandwhat maintains patterns in the diversity of life. The interplay among description,experiments, and mathematical models is a hallmark of modern community ecology.

Before describing how ecologists identify and classify communities, it is importantto recognize that the term "community" means different things to different ecologists.Most definitions of ecological communities include the idea of a collection of speciesfound in a particular place. The definitions part company over whether those speciesmust interact in some significant way to be considered community members. Forinstance, Robert Whittaker's (1975) definition

"... an assemblage of populations of plants, animals, bacteria and fungi that livein an environment and interact with one another, forming together a distinctiveliving system with its own composition, structure, environmental relations,development, and function."

clearly emphasizes both physical proximity of community members and their variousinteractions. In contrast, Robert Ricklefs's (1990) definition

"... the term has often been tacked on to associations of plants and animals thatare spatially delimited and that are dominated by one or more prominent speciesor by a physical characteristic."

doesn't stress interactions, but does emphasize that communities are often identifiedby prominent features of the biota (dominant species) or physical habitat. Other succinctdefinitions include those by Peter Price (1984)

"... the organisms that interact in a given area."

and by John Emlen (1977)

"A biological community is a collection of organisms in their environment."

that emphasize the somewhat arbitrary nature of communities as sets of organismsfound in a particular place. Charles Elton's (1927) definition, while focused onanimals, differs from the previous ones in drawing an analogy between the roles thatvarious individuals play in human communities and the...