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

TLDR: 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 were actually caused by “hidden treatments” that altered plant biomass and productivity.

Abstract: Interactions between biotic and abiotic pro- cesses complicate the design and interpretation of eco- logical experiments. Separating causality from simple correlation requires distinguishing among experimental treatments, experimental responses, and the many pro- cesses and properties that are correlated with either the treatments or the responses, or both. When an experi- mental manipulation has multiple components, but only one of them is identified as the experimental treatment, erroneous conclusions about cause and eAect relation- ships 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 treat- ment.'' Three types of hidden treatments are potential problems in biodiversity experiments: (1) abiotic condi- tions, such as resource levels, or biotic conditions, such as predation, which are intentionally or unintentionally altered in order to create diAerences in species numbers for ''diversity'' treatments; (2) non-random selection of species with particular attributes that produce treatment diAerences that exceed those due to ''diversity'' alone; and (3) the increased statistical probability of including a species with a dominant negative or positive eAect (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 inadver- tently attributed to variation in species diversity. Case studies re-evaluating three diAerent types of biodiversity experiments demonstrate that the increases found in such ecosystem properties as productivity, nutrient use eAciency, 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.