Document: JEN-3-48-21

The importance of shade to community structure: Positive interactions in the low rocky intertidal zone.

BURNAFORD, J.L.*

Oregon State University, Corvallis OR 97331, USA ¹

Abstract:
The shade provided by plant canopies has been shown to have major effects on understory plant abundance and diversity in terrestrial and salt marsh systems. However, few studies on canopy effects have been conducted in marine systems, and fewer have addressed the effect of canopies on herbivores. I used field manipulations to examine the effect of an algal canopy on a low rocky intertidal zone community in Washington State. In previous studies, removal of the canopy-forming kelp Hedophyllum sessile resulted in dramatic changes in the understory algal assemblage and the abundance of the system's major herbivore, the chiton Katharina tunicata. Canopy removal can expose understory organisms to predation by birds and increased heat stress. To determine the mechanism behind the maintenance of the original understory community, I monitored five experimental treatments (natural Hedophyllum canopy, artificial shades, bird exclosures, manipulation controls, and open plots) over two years. During the spring and summer, mobile invertebrates, including the chiton Katharina, were more abundant in plots with shade (artificial shade and natural Hedophyllum canopy) than in unshaded plots. During the fall and winter, invertebrate abundances did not differ between treatments. The algal understory assemblage was different in plots with and without shade in all seasons, but was similar in plots with natural Hedophyllum canopy and artificial shade. Birds had no effect on invertebrate abundance or the algal assemblage in any season. These results show that shade is the important factor in maintaining the low zone Hedophyllum-associated community, and the seasonal importance of shade to mobile animals suggests that the amelioration of heat stress is the mechanism behind these patterns. Positive interactions are considered important structuring forces for communities in harsh environments; this example of an important community-level effect of a plant canopy is a unique example of a positive interaction in a "low stress" habitat.

Keywords: canopy, positive interactions, herbivory, intertidal, heat stress, Katharina tunicata, Hedophyllum sessile

This abstract is being presented at: 10:45 AM in session:
Oral Session #37: Phytoplankton.