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For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial From deLaplante Kevin, and Picasso Valentin, The Biodiversity-Ecosystem Function Debate in Ecology. In: Dov M. Gabbay, Paul Thagard and John Woods, editors, Handbook of The Philosophy of Science: Philosophy of Ecology. San Diego: North Holland, 2011, pp. 169-200. ISBN: 978-0-444-51673-2 © Copyright 2011 Elsevier B. V. North Holland. Author's personal copy THE BIODIVERSITY–ECOSYSTEM FUNCTION DEBATE IN ECOLOGY Kevin deLaplante and Valentin Picasso 1 INTRODUCTION Population/community ecology and ecosystem ecology present very different per- spectives on ecological phenomena. Over the course of the history of ecology there has been relatively little interaction between the two fields at a theoretical level, despite general acknowledgment that many ecosystem processes are both influ- enced by and constrain population- and community-level phenomena. However, recent years have seen a growing interest in theoretical models and experimental studies aimed at investigating the relationship between biological diversity and higher-level community and ecosystem properties, such as invasibility and pro- ductivity. This research on the relationship between biodiversity and ecosystem functioning has spawned a large and growing literature that holds great promise for productive engagement between community ecology and ecosystem ecology. Indeed, some have argued that the synthetic viewpoints developing out of this research represent a genuine “paradigm shift” in ecology [Naeem, 2002]. However, this research has also generated heated debate among ecologists over experimental methodology and interpretation of research results. The debate burst into the public sphere in 2000 when a group of critics of the biodiversity-ecosystem function experiments accused proponents of misrepresenting the scientific debate to the public for political purposes. One media source described it as a “full war among ecologists” [Kaiser, 2000]. Recent writings have been more conciliatory in tone, but the incident points to a broader socio-political context that has played an important role in both motivating and enabling research on biodiversity-ecosystem function relationships, a context that connects research in this field to debates in conservation science and environmental policy. A comprehensive overview of this debate needs to take account of this socio-political context. Young ecologists beginning their research careers are often unaware of the in- tellectual history of their field, or the relevance of this history for understanding the scientific and socio-political environment within which their work is situated. The primary aim of this paper is to provide an historical and conceptual overview of the biodiversity-ecosystem function debate that will help to illuminate research that is currently being conducted in this field. Handbook of the Philosophy of Science. Volume 11: Philosophy of Ecology. Volume editors: Kevin deLaplante, Bryson Brown and Kent A. Peacock. General editors: Dov M. Gabbay, Paul Thagard and John Woods. c 2011 Elsevier BV. All rights reserved. Author's personal copy 170 Kevin deLaplante and Valentin Picasso The biodiversity-ecosystem function literature employs concepts like “biodiver- sity”, “ecosystem” and “function” that are themselves subjects of considerable debate in the foundational literature in ecology and in the philosophy of ecology and biology. It is a central thesis of this paper that a proper understanding of the biodiversity-ecosystem function debate requires an appreciation of this broader intellectual history. Consequently, one of the tasks of the paper is to critically assess the status of these concepts as they are used in ecology generally and in the biodiversity-ecosystem function literature in particular. In this respect the paper serves not only as an introduction to the biodiversity-ecosystem function debate, but also as an introduction to a number of central debates in the philosophy of ecology more broadly. 2 BACKGROUND: THE DIVERSITY-STABILITY DEBATE The contemporary biodiversity-ecosystem function debate is best viewed against the background of the long-standing debate in ecology over the relationship be- tween the diversity and stability of ecological systems. Commentators on the history of the diversity-stability debate commonly dis- tinguish three historical periods in the history of ecology, each characterized by a particular theoretical and empirical perspective on diversity-stability relationships, with the most recent third period identified with a shift toward what we now call “biodiversity-ecosystem function” relationships [Ives, 2005; McCann, 2005]. As we will see, the history of the diversity-stability debate has important lessons for contemporary research on biodiversity and ecosystem function. 2.1 The 1950s and 1960s The view that diversity is positively correlated with stability was endorsed by a number of prominent ecologists in the 1950s and 1960s, including Eugene Odum [1953], Robert MacArthur [1955] and Charles Elton [1958]. Odum related the notions of diversity and stability to the flow of energy through the trophic links in an ecological network. A system with greater redundancy in energetic pathways will be more stable than one with lesser redundancy. For Odum, diversity is interpreted as diversity of network connections, and stability as stability of energetic throughput and organizational structure—the more stable system is the one that suffers the least change in energy flow with the removal of a random species. However, these ecosystem concepts have a rough correspondence to population and community concepts via the identification of network nodes with species populations and network connections with trophic links. MacArthur [1955] followed Odum in understanding stability as a measure of “the amount of choice which the energy has in following the paths up through the food web”. He sketched a series of food webs and described the ramifications of energy partitioning for stability using information theory. Formally, MacArthur’s notion of stability is a measure of the response of a community to a perturbation Author's personal copy The Biodiversity–Ecosystem Function Debate in Ecology 171 that influences the density of at least one of the species. MacArthur gives a semi- formal argument that recapitulates Odum’s conclusion—in general, more diverse communities will be more stable than less diverse communities. Elton’s [1958] arguments draw on a wider range of theoretical and empirical ev- idence, but he agrees that diversity and stability are positively correlated. Elton noted that both simple Lotka-Volterra models and simple laboratory microcosms suffered from instability, and argued that simpler food webs are more vulnerable to invaders. Elton’s definitions of stability vacillate within his discussion, but they re- flect his general interest in dynamic instabilities that drive destructive oscillations and population explosions in food webs. To sum up, the broad consensus during this period was that stability of ecolog- ical systems is positively correlated with diversity, and indeed that diversity is a causal factor in generating stability. 2.2 The 1970s and 1980s This consensus did not survive the next two decades. By the end of the 1980s the general consensus was that diversity is not, in general, positively correlated with stability. How did this shift in attitudes come about? In the early 1970s mathematical ecologists began to systematically study diversity-stability relationships in model communities [Gardner and Ashby 1970; May 1973; Pimm 1980]. The conclusion of these studies undermined the conven- tional wisdom about diversity and stability. The most influential work of this period was Robert May’s seminal 1973 book Stability and Complexity in Model Ecosystems. May argued that diversity actually begets instability. More specifically, he showed that the chances of a randomly con- structed Lotka-Volterra community being stable decreases with both the number of species in the community and the connectance among species, where connectance is measured by the probability that a pair of species interacts. May’s argument employed a very specific definition of stability: it is the prob- ability that the population size of every species in the community would return to equilibrium if there were an arbitrarily small perturbation in the population size(s) of one of the species. It is important to note that this so-called “neigh- borhood stability” (or “Lyapunov stability”) is an all-or-nothing property; for a given perturbation, either every population returns to equilibrium or