Confounding in field experiments: Direct and indirect effects of artifacts due to the manipulation of limpets and macroalgae

This study addresses questions about the problem of artifacts in field experiments on plant-animal interactions on rocky shores. In a previous study we suggested that grazing by limpets on filamentous algae facilitated the establishment of the red alga Rissoella verruculosa at mid-shore levels on the rocky coast south of Livorno (Italy). This model predicted similar patterns of recovery of Rissoella in plots exposed to limpets and in plots where both limpets and filamentous algae were removed. Here we explore this prediction through a 2-way factorial experiment in which limpets and filamentous algae were manipulated by means of barriers of epoxy-putty painted with copper paint and manual removal, respectively. The experimental results did not support this prediction as the brown crust Ralfsia verrucosa, rather than Rissoella, dominated in plots were both herbivores and the filamentous algae had been removed. These results, however, appeared confounded by artifacts due to the experimental procedure. The effects of barriers were tested by comparing patterns of recovery of species in plots surrounded by frames of epoxy-putty painted with discontinuous bands of copper paint, against control plots. This experiment revealed a significant positive effect of barriers on Ralfsia and a trend toward a negative effect on Rissoella. An additional experiment indicated that overgrowth by Ralfsia significantly reduced the expansion of the encrusting base of Rissoella. overall, these patterns suggested the occurrence of indirect effects of artifacts: barriers, by increasing the abundance of Ralfsia indirectly prevented the establishment of Rissoella in plots where both limpets and the filamentous algae were removed. This study offers an example of how artifacts propagating through indirect pathways may further confound the results of field experiments. Copyright (C) 1997 Elsevier Science B.V.