A common, usually implicit, assumption of the present epistemology of experimental natural science is the following: so long as any necessary experimental intervention is identically applied to each treatment, then the effects of artifacts of that intervention will be constant across all treatments. This constancy of artifacts of intervention thus allows unbiased assessment of differences between and among treatments because effects of any such artifacts contribute equally to each treatment and therefore cancel out in contrasts. Unfortunately, this assumption will be violated whenever an artifact of intervention and the experimental treatment interact. As one illustration of the lack of appreciation of this assumption, we review those marine ecological studies that employ tethering of mobile prey organisms as a technique by which to assess the relative intensity of predation as a function of changing habitat. That review reveals that: (1) only 55% of the 22 studies even include discussion of artifacts of tethering; (2) only 9% acknowledge the possibility that the magnitude or direction of the between-habitat difference in predation as estimated from mortality of prey on tethers could be inaccurate if the artifactual enhancement of predation rate induced by tethering is not constant across habitats; and (3) no study actually tests the assumption that tethering artifacts are independent of habitat (the experimental treatment). If different consumers are present in different proportions in the habitats being compared, as is often the case, it is possible and even likely that the magnitude of artifactual enhancement of predation induced by tethering otherwise mobile organisms will fail to remain constant across habitats. This represents but one example of a general lack of recognition that artifacts of experimental intervention may interact with treatments, a concern that applies also to use of enclosures and cages in field experiments and aquaria and containers in the laboratory. Because the experimental intervention that induces the artifact is typically essential for conduct of the experiment, clever indirect techniques may be needed to allow experimentalists to assess the importance of nonadditive artifacts of intervention.
