Evaluating the relative roles of positive and negative interactions in communities: Shade, herbivory and physiological stress in the rocky intertidal zone

Evaluating the relative influences of positive and negative interactions in shaping communities is a major topic in modern ecology. Facilitative interactions between basal species are important in habitats with intense predation pressure or severe abiotic stresses. However, few studies address the potential for positive interactions between trophic levels to influence community structure by altering patterns of predation.; I investigated whether the association between the canopy-forming alga Hedophyllum sessile and the herbivorous chiton Katharina tunicata was due to the provision of a refuge from predation, a preference for Hedophyllum as a food item, or amelioration of abiotic conditions. In a field experiment, Katharina were not affected by predation or Hedophyllum thalli, but showed a strong behavioral selection for shaded areas during summertime low tides. By providing shade, Hedophyllum controls the distribution of the system's major herbivore.; In a second field experiment I evaluated the relative effects of shade and Katharina on the rest of the community. Shade had strong positive effects on a suite of consumers, increasing abundances of seven animal groups relative to unshaded areas. Shade and Katharina had quantitatively equal negative effects on the abundance of basal species, but their effects were qualitatively very different. The positive interaction between Hedophyllum and Katharina affects the entire community by altering patterns of herbivory. Such complex networks of positive, negative, direct and indirect interactions can produce deceptively simple patterns in natural systems.; I used field experiments and laboratory analyses to evaluate potential physiological benefits of this positive interaction on Katharina. Levels of heat shock protein 70 isoforms in field populations were greater in summer than in winter, suggesting that Katharina are experiencing seasonal sub-lethal stress. Although shade did not affect Hsp70 levels in Katharina maintained in field enclosures, amelioration of abiotic stresses through positive biotic interactions could have direct physiological consequences for beneficiary species.; These studies provide strong evidence that positive interactions between trophic levels can profoundly affect the physiology of individuals, the distribution and abundance of populations, and the structure of communities. I present a conceptual model to summarize predictions of the importance of these multi-level positive interactions in structuring communities.