The ecological consequences of changes in biodiversity: a search for general principles

Ecology, July, 1999 by David Tilman

INTRODUCTION

The biological diversity of the earth and its origins have long been a source of amazement and curiosity, and an area of formal inquiry ever since Wallace and Darwin. Current interest in diversity centers both on why there are so many species and on how diversity impacts population and ecosystem processes, which is the focus of this paper. In 1961, Hutchinson noted that theory predicted that the number of coexisting species should not exceed the number of limiting resources, but that most lakes contained many times more algal species than limiting nutrients. This paradox of diversity helped to attract me from physics to ecology during my undergraduate education. Ecology offered all that I had been seeking in a career: a chance to combine theory and experiments, to address the "big unknowns" of a discipline, and to have the results be of importance to the long-term welfare of society.

With the guidance of first Stephen Hubbell and then Peter Kilham, my thesis work led to a mechanistic theory of resource competition (Tilman 1977, 1980) and to the first experimental demonstration of the ability of theory to predict the outcome of interspecific interactions (Tilman 1976, 1977). The theory offered a solution to the paradox of diversity, predicting that an unlimited number of competing species could coexist at equilibrium if a habitat had spatial heterogeneity in the relative supply rates (ratios) of two or more limiting resources (Tilman 1980, 1982). Many alternative theoretical explanations for high diversity also have been discovered (e.g., Levin 1970, 1981, May 1975, 1986, Levins 1979, Armstrong and McGehee 1980, Chesson 1986, Chesson and Huntly 1997; reviewed in Tilman and Pacala 1993). These have solved the paradox of diversity, but the mystery remains. We still do not know, for example, how hundreds of plant species and thousands of insect species coexist on a hectare of rainforest or prairie, or how millions of species coexist on earth. Because these mechanisms are poorly understood, we have but a blurry vision of the long-term impacts of habitat conversion and destruction, invasion by exotic species, nutrient enrichment, and other anthropogenic changes on species extinctions.

The ecological consequences of changes in biodiversity, the theme of this paper, are at least as poorly understood. Darwin (1872) suggested that greater plant diversity would lead to greater primary productivity, but his thoughts lay dormant for over a century (McNaughton 1993). Elton (1958) proposed that greater diversity and trophic complexity would increase population and ecosystem stability, but interest in the consequences of diversity declined after May (1973) showed that the stability of model competitive ecosystems decreased as diversity increased. However, the accelerating effects of human activities on biodiversity and the possibility that the loss of biodiversity might impact ecosystem functioning (e.g., Ehrlich and Ehrlich 1981, Wilson 1992) renewed interest in the effects of diversity on ecosystem processes (Schulze and Mooney 1993) and on ecosystem services essential to society (Daily 1997). Moreover, the disciplines of population, community, and ecosystem ecology, which diverged markedly in the 1970s and 1980s, were undergoing a synthesis and reunification (e.g., Vitousek and Hooper 1993, Jones and Lawton 1995). A seemingly outdated idea, originally expressed in the superorganismal perspective of Clements (see Goodman 1975), again became challenging when viewed through the ongoing synthesis of evolutionary, population, and ecosystem ecology.

Recent work on the consequences of changes in biodiversity (e.g., Frank and McNaughton 1991, McNaughton 1993, Vitousek and Hooper 1993, Naeem et al. 1994, 1996, in press, Tilman and Downing 1994, Tilman 1996, Tilman et al. 1996, 1997a, 1998, Hooper and Vitousek 1997, Huston 1997, McGrady-Steed et al. 1997, Naeem and Li 1997, Doak et al. 1998) has led to both insights and debate, as often occurs when ideas are young and paradigms are challenged. A major debate concerns whether plant community diversity depends on productivity (e.g., Grime 1979, Huston 1979, 1997, Tilman 1982, 1988), or productivity depends on diversity (e.g., McNaughton 1993, Vitousek and Hooper 1993, Naeem et al. 1994, Tilman et al. 1996, 1997a, b), or whether causation goes in both directions (e.g., Tilman et al. 1996). Other discussions focus on the distinction between the effects of composition vs. diversity and the proper ways to design and interpret diversity experiments or field studies (e.g., Givnish 1994, Aarssen 1997, Huston 1997, Tilman 1997a, Tilman et al. 1997a, b, Wardle et al. 1997, Lawton et al. 1998, Naeem and Li 1998). The relative importance of species vs. functional diversity is also uncertain (e.g., Vitousek and Hooper 1993, Grime 1997, Hooper and Vitousek 1997, Tilman et al. 1997a).

Many variables, including disturbance, species composition, and climate, are known to influence ecosystem processes. Here, I highlight the search for general principles governing how another variable, the biodiversity of a trophic level or guild, impacts the dynamics and functioning of populations, communities, and ecosystems. Such principles are needed to increase scientific understanding of the ecological consequences of changes in biodiversity, and to guide public policy related to biodiversity, especially to the loss of biodiversity that occurs in simplified ecosystems managed for human benefit.