Theoretical Ecology: Principles and Applications

By Robert M. May; Angela R. McLean | Go to book overview

CHAPTER 8
Diversity and stability in ecological
communities

Anthony R. Ives

In this lake, where competitions are fierce and continuous beyond any parallel in the worst periods of human history; where they take hold, not on goods of life merely, but always upon life itself; where mercy and charity and sympathy and magnanimity and all the virtues are utterly unknown; where robbery and murder and the deadly tyranny of strength over weakness are the unvarying rule; where what we call wrong-doing is always triumphant, and what we call goodness would be immediately fatal to its possessor–even here, out of these hard conditions, … an equilibrium has been reached and is steadily maintained that actually accomplishes for all the parties involved the greatest good which the circumstances will at all permit.

Stephen A. Forbes, The Lake as a Microcosm (1887)


8.1 Introduction

How the diversity of an ecological community affects its stability is an old and important question (Forbes, 1887; Elton, 1927; Nicholson, 1933). The science of ecology grew out of the study of natural history in the nineteenth century, when nature was viewed as wondrous, mysterious, complex, and largely in balance (even if murderous to experience from an individual’s point of view; Forbes, 1887). Whereas our current scientific view is more textured and guarded, the ‘balance of nature’ still permeates the popular press. Some vestiges also remain in the scientific literature.

Over the last 100 years, conclusions about the relationship between ecological diversity and stability have varied wildly (May, 2001; Ives, 2005). The goal of this chapter is to show that these wildly varying conclusions are due largely to wildly varying definitions of both stability and diversity. To do this, I will take two tacks, one for stability and the other for diversity. For stability, I will give an abbreviated history of the changing definitions of stability, merging both empirical and theoretical studies. I make no pretence of being comprehensive, but will instead pick highlights that show how the definition of stability often changes from one study to the next. For diversity1,

I will present a theoretical model to illustrate how different ‘diversity effects’ on stability can be parsed out. This model shows in a concrete way how any theoretical study (and, for that matter, empirical study) necessarily makes a long list of assumptions to derive any conclusion about diversity and stability. The multiple definitions of stability, and the multiple roles of diversity, argue against any general relationship between stability and diversity.

In the final section of the chapter, I will argue that understanding the relationship between diversity and stability requires the integration of theory and experiment. Theory is needed to define in unambiguous terms the meanings of stability and diversity. Experiments are needed to ground theory in reality. Unfortunately, rarely is this done.


8.2 History of stability

To present an abbreviated history of the changing definitions of stability, I will discuss theoretical and empirical studies side by side. The empirical

1 Throughout this chapter, I use diversity in a broad sense,[lb/]capturing many of the properties that have been used in the[lb/]literature to characterize community complexity. Thus, I am[lb/]using diversity and complexity synonymously.

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