Mechanisms underlying the effects of marine herbivores : implications for a low intertidal kelp community

Rocky low intertidal communities structured by the canopy forming kelp Hedophyllum sessile and the generalist herbivore Katharina tunicata are typical of semiexposed coasts of British Columbia. Katharina tunicata is capable of removing the community supporting canopy formed by H. sessile . Mechanisms which determine and mediate the interaction between this alga and herbivore are investigated. Densities of Katharina tunicata were manipulated within the range of densities found on the shore, without the use of artificial barriers, and the percent cover, density, population structure and demography of Hedophyllum sessile were monitored to determine the effects of these manipulations. Differential susceptibility of H. sessile holdfast size classes are shown to account for the rapid decline of H. sessile abundance in areas of high chiton density. These data are used to quantify the mechanism referred to as 'an escape in size.' Several lines of evidence suggest that the mechanism of K. tunicata interaction with H. sessile adults is an indirect effect whereby holdfast integrity is degraded, making individuals of H. sessile more susceptible to wave-induced mortality. The implications of this mechanism for observed geographic variation in K. tunicata interaction strength are discussed. The potential role of polyphenolics (anti-herbivore secondary metabolites) in the interaction between Hedophyllum sessile and Katharina tunicata is examined. Three experiments determined that: (1) H. sessile is a phenolic-rich species (5-7% H. sessile dry weight), (2) induction of phenolic production does not occur within three days of simulated wounding, (3) individuals of H. sessile from areas of naturally high and low herbivore densities do not differ in phenolic content, and (4) juvenile H. sessile tissue contains significantly more phenolics than adult vegetative tissue but adult reproductive and vegetative tissues do not differ. These results are discussed in terms of both present-day and evolutionary ecological importance. By monitoring the Hedophyllum sessile understory community during the chiton density manipulation experiment, and performing descriptive investigations during the same time period, several direct and indirect interactions were revealed which are potentially important determinants of the structure of this community. These interactions are used to construct three interaction webs which distinguish between the interactions experienced by juvenile and adult H. sessile. A model of the annual successional trajectory of this low intertidal kelp community is used to summarize the findings of this thesis.Science, Faculty ofBotany, Department ofGraduat