Community Ecology

Peter J. Morin

• Publisher: Wiley-Blackwell
• Year: 2011
• Print ISBN: 9781405124119, 9781444338218
• eBook ISBN: 9781444341942
• Edition: 2
• Topic: Ciencias biológicas
• Subtopic: Ecología

All life on earth occurs in natural assemblages called communities. Community ecology is the study of patterns and processes involving these collections of two or more species. Communities are typically studied using a diversity of techniques, including observations of natural history, statistical descriptions of natural patterns, laboratory and field experiments, and mathematical modelling. Community patterns arise from a complex assortment of processes including competition, predation, mutualism, indirect effects, habitat selection, which result in the most complex biological entities on earth – including iconic systems such as rain forests and coral reefs.

This book introduces the reader to a balanced coverage of concepts and theories central to community ecology, using examples drawn from terrestrial, freshwater, and marine systems, and focusing on  animal, plant, and microbial species. The historical development of key concepts is described using descriptions of classic studies, while examples of exciting new developments in recent studies are used to point toward future advances in our understanding of community organization. Throughout, there is an emphasis on the crucial interplay between observations, experiments, and mathematical models.

This second updated edition is a valuable resource for advanced undergraduates, graduate students, and  established scientists  who seek a broad overview of community ecology. The book has developed from a course in community ecology that has been taught by the author since 1983.

Figures and tables can be downloaded for free from www.wiley.com/go/morin/communityecology

Part 1: Communities: Basic Patterns and Elementary Processes

1

Communities

“Ecology is the science of communities. A study of the relations of a single species to the environment conceived without reference to communities and, in the end, unrelated to the natural phenomena of its habitat and community associations is not properly included 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 ecologists study. The major interspecific interactions, or elementary processes, among pairs of species include competition, predation, and mutualism. Complex indirect interactions can arise among chains of three or more interacting species. Important community properties include the number of species present, measures of diversity, which reflect both the number and relative abundances of species, and statistical distributions that describe how different species differ in abundance.

Observations of natural patterns and explorations of mathematical models have inspired generalizations about the underlying causes of community organization. One pattern important in the historical development of community ecology concerns an apparent limit to the similarity of coexisting species. The case of limiting similarity provides a cautionary example of the way in which community patterns are initially recognized, explained in terms of causal mechanisms, and eventually evaluated. Community patterns are the consequence of a hierarchy of interacting processes that interact 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 different species of organisms live on Earth today (Erwin 1982; May 1990). The small fraction of this enormous global collection of species that can be found at any particular place is an ecological community. One important goal of community ecology is to understand the origin, maintenance, and consequences of biological diversity within local communities. Different processes, operating on very different time scales, can influence the number and identity of species in communities. Long-term evolutionary processes operating over time scales spanning millions of years can produce different numbers of species in different locations. Short-term ecological interactions can either exclude or facilitate species over shorter time scales ranging from a few hours to many years. This book provides an overview of community patterns and the processes that create them.

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

The central questions in community ecology are disarmingly simple. Our ability to answer these questions says something important about our understanding of the sources of biological diversity and the processes that maintain biodiversity in an increasingly stressed and fragmented natural ecosystem. Answering these questions allows us to wisely manage the human-dominated artificial communities that include the major agricultural systems that we depend on for food and biologically produced materials, and to restore the natural communities that we have damaged either through 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 (Hairston 1989). However, some patterns and their underlying processes are experimentally intractable, 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 understand what 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 important to recognize that the term “community” means different things to different ecologists. Most definitions of ecological communities include the idea of a collection of species found in a particular place. The definitions part company over whether those species must interact in some significant way to be considered community members. For instance, Robert Whittaker’s (1975) definition

“… an assemblage of populations of plants, animals, bacteria and fungi that live in an environment and interact with one another, forming together a distinctive living system with its own composition, structure, environmental relations, development, and function.”

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

“… the term has often been tacked on to associations of plants and animals that are spatially delimited and that are dominated by one or more prominent species or by a physical characteristic.”

doesn’t stress interactions, but does emphasize that communities are often identified by prominent features of the biota (dominant species) or physical habitat. Other succinct definitions 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 organisms found in a particular place. Charles Elton’s (1927) definition, while focused on animals, differs from the previous ones in drawing an analogy between the roles that various individuals play in human communities and the functional roles of organisms in ecological communities.

“One of the first things with which an ecologist has to deal is the fact that each different kind of habitat contains a characteristic set of animals. We call these animal associations, or better, animal communities, for we shall see later on that they are not mere assemblages of species living together, but form closely-knit communities or societies comparable to our own.”

(Elton, 1927).

Elton’s emphasis on the functional roles of species remains crucial to our understanding of how functions and processes within communities change in response to natural or anthropogenic changes in community composition.

For our purposes, community ecology will include the study of patterns and processes involving at least two species at a particular location. This broad definition embraces topics such as predator-–prey interactions and interspecific competition that are traditionally considered part of population ecology. Population ecology focuses primarily on patterns and processes involving single-species groups of individuals. Of course, any separation of the ecology of populations and communities must be highly artificial, since natural populations always occur in association with other species in communities of varying complexity, and since populations often interact with many other species as competitors, consumers, prey, or mutually beneficial associates.

Most communities are extraordinarily complex. That complexity makes it difficult even to assemble a complete species list for a particular locale (e.g., Elton 1966; Martinez 1991). The problem is compounded by the fact that the taxonomy of smaller organisms, especially bacteria, protists, and many invertebrates, remains poorly known (Wilson 1992; Foissner 1999; Hughes et al. 2001). Consequently, community ecologists often focus their attention on conspicuous readily-identified sets of species that are ecologically or taxonomically similar. One important subset of the community is the guild, a collection of species that use similar resources in similar ways (Root 1967; Fauth et al. 1996). There are no taxonomic restrictions on guild membership, which depends only the similarity of resource use. For example, the granivore guild in deserts of the southwestern USA consists of a taxonomically disparate group of birds, rodents, and insects that all consume seeds as their primary source of food (Brown and Davidson 1977). Another term, taxocene (Hutchinson 1978), refers to a set of taxonomically related species within a community. Ecologists often refer to lizard, bird, fish, and plant communities, but these assemblages are really various sorts of taxocenes. Unlike the guild, membership in a taxocene is restricted to taxonomically similar organisms. Although ecologists often study taxocenes rather than guilds, the use of the term taxocene to describe such associations has been slow to catch on.

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Table of contents

• Cover
• Title page
• Copyright page
• Preface to the Second Edition
• Preface to the First Edition
• Part 1: Communities: Basic Patterns and Elementary Processes
• Part 2: Factors Influencing Interactions Among Species
• Part 3: Large-Scale, Integrative Community Phenomena
• Appendix: Stability Analysis
• References
• Index