Hidden treatments in ecological experiments: re-evaluating the ecosystem function of biodiversity

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 "hidden treatments" that altered plant biomass and productivity.

The Influence of Tropical Plant Diversity and Composition on Soil Microbial Communities

“…As with studies attempting to relate species diversity to ecosystem function, we cannot attribute the changes we observed in microbial communities along the diversity gradient to the number of plant species per se [24,26,48,50]. The likelihood that a study site contained a species that had a strong effect on microbial community composition increased with plant diversity in this study (i.e., the sampling effect [57]).…”

Integrating plant ecological responses to climate extremes from individual to ecosystem levels

“…Both the sampling and niche complementarity effect of biodiversity have been reported to decrease the resistance of ecosystem productivity to climate extremes. The sampling probability effect suggests that biodiversity and ecosystem function relationships may be often driven by the chance of a plant community containing a highly productive species [110]. Yet just as more species-rich communities may have a greater probability of containing highly productive species, such highly productive species may exhibit functional trade-offs and thus be highly sensitive to a particular climate extreme.…”

Benthic assemblages associated with native and non-native oysters are similar

“…The abundance of all taxa was quantified by estimating %-cover on each plate. was not included as response variable for multivariate analyses to avoid confounding independent (manipulated) and dependent (response) variables (Huston, 1997). Analyses were carried out on all taxa initially and subsequently data were also classified into taxonomical groups (class, order) and re-analyzed, to test for general effects on functional diversity based on taxonomically distinct groups.…”