Oecologia (1997) 110:449-460 © Springer-Verlag 1997 Michael A. Huston Hidden treatments in ecological experiments: re-evaluating the ecosystem function of biodiversity Received: 16 December 1996 /Accepted: 2 March 1997 Abstract Interactions between biotic and abiotic processes complicate the design and interpretation of ecological experiments. Separating causality from simple correlation requires distinguishing among experimental treatments, experimental responses, and the many processes and properties that are correlated with either the treatments or the responses, or both. When an experimental manipulation has multiple components, but only one of them is identified as the experimental treatment, erroneous conclusions about cause and effect relationships are likely because the actual cause of any observed response may be ignored in the interpretation of the experimental results. This unrecognized cause of an observed response can be considered a "hidden treatment." Three types of hidden treatments are potential problems in biodiversity experiments: (1) abiotic conditions, such as resource levels, or biotic conditions, such as predation, which are intentionally or unintentionally altered in order to create differences in species numbers for "diversity" treatments; (2) non-random selection of species with particular attributes that produce treatment differences that exceed those due to "diversity" alone; and (3) the increased statistical probability of including a species with a dominant negative or positive effect (e.g., dense shade, or nitrogen fixation) in randomly selected groups of species of increasing number or "diversity." In each of these cases, treatment responses that are actually the result of the "hidden treatment" may be inadvertently attributed to variation in species diversity. Case studies re-evaluating three different types of biodiversity experiments demonstrate that the increases found in such ecosystem properties as productivity, nutrient use efficiency, and stability (all of which were attributed to higher levels of species diversity) were actually caused by Michael A. Huston Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6335 USA Fax: 423-574-2232; e-mail: mhu@ornl.gov "hidden treatments" that altered plant biomass and productivity. Key words Species diversity • Experiment • Productivity Stability • Resources Introduction The field of ecology is distinguished by the complexity of the processes and interactions that are its primary focus and also the primary excuse for ecologists' failure to effectively address major environmental problems. Repeated calls for more rigorous and relevant ecology (e.g., Suter 1981; Peters 1991; Shrader-Frechette and McCoy 1994; Sarkar 1996) have led to an increasing emphasis on the need for "mechanistic models". (Schoener 1986; Tilman 1987a) and the use of an experimental approach that manipulates such factors as resources and predation rates (Hairston 1989; Underwood 1996). Rigorous experimental tests of hypotheses are essential as ecology increasingly addresses issues of political, social, and economic importance. Experimental results typically have a much greater impact than new theory, models, or observations. However, poorly designed experiments or misinterpretations of experimental results have the potential to mislead scientists and policy makers alike. Biodiversity has recently emerged as an issue of both scientific (Wilson 1988; Ehrlich and Wilson 1991; Peters and Lovejoy 1992) and political (United Nations Environment Programme 1992; World Resources Institute 1992; Heywood and Watson 1995) concern primarily because of an increase in extinction rates caused by human activities (Myers 1979; Ehrlich and Ehrlich 1981; Lawton and May 1995; Pimm et al. 1995). Biodiversity is considered to be important for a variety of reasons (Oldfield 1989; Randall 1994; Rolston 1994), but recent attention has focused on its potential importance for the adequate functioning of the Earth's ecosystems (Schulze and Mooney 1993). This concern about the environmental consequences of biodiversity loss (and thus, a O__________________________________________________ _____________________________________________________________________________________________________________________________________________________________________ ________________________________________________ ___