Ecological determinants of rockweed performance: interactions among herbivores, epiphytes and tides

Abstract

Ecology seeks to understand how biotic and abiotic factors influence species distribution and abundance in order to make predictions about the outcome of interactions between species and their environment. The rocky intertidal zone is characterized by strong environmental gradients over small spatial scales, making it an ideal habitat to study how changes in abiotic and biotic factors influence the distribution of species. The rockweed </Fucus distichus> is an important benthic primary producer and habitat forming species that experiences gradients of tidal immersion, herbivory, and epiphytes across its vertical range in the intertidal zone. Since these factors act together to influence </F. distichus> performance, studying their effects singly may not adequately characterize the outcomes of their combined interactions. This research examines three interaction scenarios through mesocosm and field experiments: 1) effect of herbivores (</Littorina sitkana>) on </F. distichus> performance, mediated indirectly through inducible defenses, 2) multiple stressor effects of herbivores, epiphytes, and tidal immersion on </F. distichus> performance, and 3) direct and indirect interactions between herbivory and </F. distichus>, mediated by epiphytes. In contrast to some other fucoid algae, </F. distichus> did not produce inducible defenses in response to waterborne cues from snails feeding. However, </F. distichus> grown with snail feeding cues had lower carbon to nitrogen ratios and higher subsequent consumption by snails. Thus, uptake of snail metabolic wastes by </F. distichus> may influence subsequent feeding preferences. Multiple stressor results demonstrate that herbivores and epiphytes negatively affect </F. distichus> performance, and each is additive with tidal effects. </F. distichus> grew slower when always immersed, even when protected from herbivores or epiphytes, pointing to immersion as a determinant of the lower limit of </F. distichus>. Finally, when snail herbivores fed on both epiphytes and </F. distichus>, the net effect of the snails on </F. distichus> switched from a positive indirect interaction to a negative direct interaction with increased snail density, mediated by epiphyte load. The interactive effects of immersion, herbivory, and epiphytes on </F. distichus> are complex but clarified by this research involving careful experimental manipulation of environmental factors that vary across the intertidal zone.