Positive and negative effects of intertidal algal canopies on recruitment and survival of barnacles

Abstract

Understanding the diverse ways that species interact in communities continues to pose significant challenges to experimental ecologists. By simultaneously (1) abrading the substrate (i.e. 'whiplash'), (2) altering local hydrodynamics and abiotic conditions and (3) influencing predator densities, marine algae can alter the recruitment and survival of the organisms with which they co-occur. I used predator exclusion cages and canopy manipulations in the barnacle-algae (Semibalanus balanoides-Ascophyllum nodosum) zone in Rhode Island (USA) intertidal habitats to demonstrate how to experimentally tease apart these types of multiple effects. In the absence of algal abrasion, barnacle recruitment was greater under the algal canopy than in cleared areas. This suggests that the canopy did not reduce the delivery of larvae but, rather, created a depositional environment where larvae accumulated. The overall effect of the canopy, however, was a reduction in recruitment compared to cleared areas because algal whiplash overwhelmed these positive, hydrodynamic effects. Predators had very little effect on the recruitment of barnacles. In contrast to recruitment, I found that the canopy positively influenced the early post-recruitment survival of S. balanoides. The reduction in survival caused by predators was offset by an increase in survival caused by both the physical presence of the canopy (probably by reducing thermal stress) and its subsequent mechanical abrasion of the substrate (probably by reducing the accumulation of sediments). For both recruitment and survival, the effects of algal whiplash were twice as large as those of modifying the habitat or influencing predation intensity. In general, my findings illustrate how a series of a priori, statistical comparisons within a multi-factorial experiment can be used to separate and quantify these types of multiple effects in plant-animal interactions.