Positive interactions between foundation species and their associated species are expected to be influenced by the degree of environmental stress as well as trait variations of the species involved. However, there is scarce empirical evidence regarding how these two factors interact and shape the intensity of facilitation. To test how facilitation varies with stress, a colonization experiment using artificial algal units that varied in a functional trait (morphological complexity) was conducted at different intertidal height levels. The isolated effects of shore-height and host-complexity are as expected: the number of species and overall abundance of epifauna decreased with lower complexity host units and greater exposure to environmental stress. However, in contrast to the initially proposed model, based on the Stress Gradient Hypothesis, the magnitude of positive interactions increased at lower tidal heights (those levels with, presumably, milder environmental conditions). An additional predation exclusion experiment ruled out the effects of predation pressure as a major factor driving the structure of mobile epifaunal assemblages in these intertidal habitats. Thus, further studies are needed to identify the factors that explain the patterns of facilitation intensity found in the present study.

The upload files contains the abundance of each taxa obtained throught the study (both the main experiment and the additional predation experiment).