| This thesis investigates the population and community consequences of abiotic processes (physical forcing mechanisms) in both exposed and protected rocky intertidal habitats in New England, USA using research done from 1993–1997. Section 1 investigates the ways that predictable variation in tidal currents influences intertidal communities in the Damariscotta River estuary in Maine. Through a series of experiments, it was discovered that the relative importance of bottom-up (i.e. resource) processes and top-down (i.e. predation) processes varied between high flow habitats and low flow habitats. Differences in community structure between sites were largely due to flow-enhanced recruitment and growth of barnacles and mussels at high flow sites versus severe mortality of these organisms at low flow sites by crab predation. In addition, barnacles extend only a short distance into the upper intertidal zone at low flow sites. This was a result of low larval delivery and subsequently high mortality from crab predators and thermal stress. Blue mussels, another important food source for crabs in these habitats, were also experimentally shown to exhibit inducible defenses (shell thickening) in response to these predators.;Section 2 investigates how species interactions in exposed rocky intertidal habitats vary as a function of thermal stress across Cape Cod. This was done by testing the hypothesis that large, canopy-forming seaweeds, would facilitate barnacle survival at sites south of Cape Cod, where thermal stress is high and predators are uncommon, but reduce survival at sites to the north of Cape Cod, where the opposite conditions prevail. Through a series of manipulative experiments, it was found that algal canopies at southern sites consistently increased barnacle survival. However, they only increased reproductive fitness potential (survival x fecundity) during 1995, a slightly hotter year than 1996. The mechanism by which these algal canopies altered the early pattern of barnacle recruitment was mechanical abrasion of the substrate rather than a reduction in the delivery of larvae. These findings highlight the numerous direct and indirect ways abiotic factors influence the population biology and community ecology of these dynamic intertidal habitats. |
