Biotic diversity and ecosystem processes: using the Ecotron to study a complex relationship Shahid Naeem, John I-I. Lawton, Lindsey J. Thompson, Sharon F? Lawler and Richard M. Woodfin Global change research examines a variety of effects that changing environmental conditions may have on ecosystems. One aspect, a decline in species richness (biodiversity) observable in most ecosystems, has been less well studied, partly due to the complexity and long time necessary for experimentation. Model ecosystems (micro- or mesocosms), however, provide a feasible means for testing the many hypothetical relationships proposed to exist between diversity and ecosystem behaviour that observation and theory support. This article examines these hypotheses and shows how mesocosm studies conducted in the Ecotron, a controlled environmental facility, provide insights into this complex issue. Shahid Naeem, Ph.D. Is a graduate of the University of California, Berkeley, USA and obtained his Ph.D. from the same institute in 1988. He is presently an assistant professor in the Department of Ecology, Evolution and Behavior at the University of Minnesota, Twin Cities Campus. His interests concern theory, obser- vation, and experimental investigations of the factors that regulate biotic diversity. John H. Lawton, Ph.D. Is director of the NERC Centre for Population Bioloav and has been Professor of Communitv Ecology at Imperial College of Science, Technology and Medicine, London, since 1989. He holds hon- orary positions at the Natural History Museum, London, and at the Institute of Ecosystem Studies, New York. His current interests include theoretical and experimental population biology, community ecology and the impact of global environmental change. Lindsey J. Thompson, Ph.D. Is a graduate of the University of Wales, Bangor, and obtained her Ph.D. for the same institute. She is currently a research scientist at the NERC Centre for Population Biology, Imperial College of Science, Technology and Medicine. Her current interests include investigations into the impact of global en- vironmental changes at the individual, population and ecosystem level. Sharon P. Lawler, Ph.D. Is a graduate of Lehigh University, Pennsylvania, USA and obtained her Ph.D. from Rutgers University, New Jersey in 1992. She is presently an assistant professor in the Department of Entomology, University of California, Davis. Her interests concern community ecology and popula- tion dynamics. Richard M. Woodfin, MSc. Graduated in 1982 from the University of Hull and obtained his M.Sc. at the University of Birmingham in 1983. His interests include plant biology and seed conservation, plus the design and develop- ment of controlled-environmental facilities. 58 Characteristic of every region is a rich diversity of plants, animals and microbial species. The aesthetic and economic import- ance of this diversity is well established, and nations agree that global, biotic diver- sity is a vanishing resource that should be wisely managed [l-4], even if they disagree about what ‘managed wisely’ means or who should effect it. Aside from the aesthetic and economic reasons for preserving biodiversity, what are the ecological values of biodiversity? Is diversity necessary for the maintenance of environmental conditions? Although the issues are broadly discussed by ecologists [S-lo], surprisingly, the ecological values of diversity are still largely hypothetical in nature [11,12]. The area where ecologists feel that diver- sity and environment are most likely to interact is at the level of the ecosystem. An ecosystem consists of a community of organisms and its associated energy and nutrient flows (Figure 1). These flows - sometimes referred to as ‘ecosystem processes’, ‘ecosystem functions’ or ‘bio- geochemical activities’ - ultimately con- tribute to local and global environmental conditions [13]. Clearly, if a system has no populations of any species within it, then such an inanimate system would settle to some physical/chemical equilibrium most likely inhospitable to life [13]. What is unclear, however, is how the addition of species changes the behaviour of such a system. If we were to measure ecosystem behaviour by monitoring its biogeochemical activity, how would ecosystem behaviour change if a community went from low diversity to high diversity, or conversely, from high to low diversity as is currently occurring globally (Figure 2)? Observation and theory support a wide array of conflicting hypothetical relation- ships between diversity and ecosystem functioning (for convenience, we will refer to these hypotheses as ‘diversity/ecosystem hypotheses’). If we were to plot diversity along one axis of a bivariate plot and ecosystem activity along the other axis, a large number of plausible relationships between the two factors are possible (Figure 3). The interpretation of currently available data supports several of these views, rang- ing from those that claim that ecosystem processes are insensitive to changing diver- sity (for example, ‘redundant species’ hypotheses in Figure 3) to those that claim that even small changes in diversity could have dramatic, unpredictable effects on ecosystem activity (for example, ‘idiosyn- cratic response’ hypothesis in Figure 3). Presently, there is little agreement about how to predict the consequences of declin- ing diversity as described in Figure 2. When observation and theory cannot resolve a scientific debate, experimental testing of the ideas is generally the next step. Only recently, however, have exper- imental tests of diversity/ecosystem hypotheses begun [14-171. Why have these hypotheses, some traceable back to Darwin [US], remained largely untested? Two ob- stacles to experimental analyses have hin- dered progress. First, manipulating popula- tions of even one species while holding environmental variables constant among replicates is difficult under field conditions; to manipulate dozens of species in environ- mentally replicated natural systems is almost impossible. Secondly, most organ- isms either follow seasonal trends or are long-lived: effective studies of plants and animals in nature could take years. Perhaps 0 1995 Elsevier Science Ltd 0160-9327/95/$09.50