Biodiversity and Ecosystem Functioning: Synthesis and Perspectives

M. Loreau, S. Naeem, and P. Inchausti, editors. 2002. Oxford University Press, New York, 294 pages, $60.00, ISBN 0–19–851571–5 (soft cover). Does diversity matter in restored communities? This edited book explores the diversity–function debate, which has raged in ecology during the past several decades. The diversity–function question is relevant in restoration, but the question has not yet received a straightforward hearing in the field of restoration science. None of the authors of this book explicitly relate their ideas to restoration, but the book can educate restorationists about the most recent developments in the diversity–function debate. It is also an excellent book for students to learn about the recent developments and historical background of the diversity–function debate and can be used for graduate-level seminars in restoration or plant ecology. The diversity–function debate in ecology tackles the question of whether increased numbers of species enhance function. A common question within diversity–function research is: “Do communities with more species have more biomass production?” Nested within this question is the issue of whether all species matter, or whether there are interchangeable groups of functionally equivalent species. Some of the chapters of this edited book effectively argue the relative merits of functional-effect group approaches, that is, the idea that groups of species have the same function as others (chapters by Schmid et al. and Hooper et al.), and others work from a more species-based perspective (chapters by Tilman et al., Naeem et al., and Hector et al.). In the field of restoration, research that directly supports either functional-effect group or species-based views is nearly absent; however, more precise answers to the role of species in the diversity–function question are critical to the future of restoration. The perspective that “each species matters” is supported in this book by results from plot-based research projects on grasslands (chapters by Tilman et al., Naeem et al., and Hector et al.) showing that production is higher in communities with higher biodiversity. The explanation for this diversity–function relationship is based on both the advantages of a large species pool and niche differentiation between species, so that production and utilization of resources are maximized. The debate continues with chapters that soundly criticize these plot-based research projects (chapters by Huston & McBride and Schmid et al.). Some criticism of plot-based research is certainly deserved because of the statistical limitations inherent in the plot-based approach. These studies have no replication on a species level, because numbers of species, and not particular species, are replicated in the experiments. Certain combinations of species, for example, legumes and C4 grasses, outproduce others when put in species-rich combinations; yet these combinations are not distinguished from other sets of species in these experiments (chapter by Huston & McBride). Heterogeneity of soil and other environmental factors produce unacknowledged block effects within the grasslands used in these experiments, and these heterogeneity factors further confound the statistics in plot-based studies (chapter by Huston & McBride; see also Gough et al. 1994). While plot-based studies have been criticized, these studies form the core of the diversity–function debate as covered in the first several chapters of the book. Other chapters review the history of diversity–function questions in ecology (chapter by Schmid et al.) and historical links of the diversity–function debate to the diversity/stability question (chapters by Loreau et al. and Hughes et al.). The earlier view that diversity begets stability (Odum 1969) was followed by the later opposite view that complex systems are less stable (Gardner & Ashby 1970). From these older stability discussions spring the current research debate regarding species invasion and its relationship to resilience and species packing (chapter by Levine et al.). Later chapters explore the diversity–function issue from perspectives of soil microbes, algal systems (Petchey et al.), mycorrhizae (van der Hejden & Cornelissen), decomposers (Mikola et al.), plant–soil relationships (Wardle & van der Putten), multitrophic dynamics (energy and nutrient flow relationships; Raffaelli et al.), agroecosystems (Vandermeer et al.), and marine ecology (Emmerson & Huxley). Although diversity–function research typically is conducted at small scales, ultimately, politicians and policymakers may be most persuaded toward conservation action by biodiversity loss at regional and landscape scales (chapter by Bengtsson et al.). As Bengtsson et al. point out, the species loss issue is the likely environmentalist cause forming the basis of the diversity–function debate. Bad science for a good cause has never advanced conservation issues. We would like to point out that restoration ecologists also need to carefully divorce themselves from any rationalization for unsupported science justifying their recommendations. The main message relevant to restoration ecology in this book is that the relationship between diversity and function is complex. As restorationists, we need to remember that, thus far, the research in plant ecology does not either solidly support or refute the idea that larger numbers of species enhance the function of restoration sites. While restoration sites are places where we can address the species loss issue by creating more available habitat, we need to be careful not to ascribe functional benefits to this procedure unless specific research ultimately supports the idea. Those of us in the restoration field need to pay close attention to the diversity–function debate in the coming years, because the issue forms the theoretical and philosophical underpinnings of our work. Cleverly designed research projects in the field of restoration ecology could greatly illuminate this question so close to the heart of restoration. As we attempt to restore habitats in increasingly fragmented landscapes with ongoing changes in climate, the answer to this diversity–function question is timely for restoration ecology.