Date of Award

1-1-2010

Document Type

Campus Access Dissertation

Department

Biological Sciences

First Advisor

David S Wethey

Abstract

Latitudinal distributions of many organisms are limited by temperature. In such cases, the poleward boundaries are limited by cold intolerance, while the equatorward boundaries are mediated by heat intolerance. Thus, with warming climatic conditions, thermal tolerances at the equatorward range edge should be exceeded, causing poleward biogeographic contractions. The marine intertidal zone is a physically rigorous habitat that may serve as a model system for the effects of climate on biogeography. Organisms residing here are exposed to a suite of stressful abiotic factors, are exposed to both water and aerial conditions, and extreme temperatures may be experienced. Sessile marine invertebrates, such as mussels and barnacles, are excellent model organisms because their response to the environment is unmitigated by behavior. Furthermore, they are dominant primary space occupiers with discreet latitudinal distributions and well-studied range edges, which provides baseline data from which to determine changes in biogeography. Mytilus edulis and southern limit: The upper lethal thermal limits, for both air and water, were determined via laboratory experiments. Field experiments indicated that mortality in the intertidal occurs at rates expected from laboratory responses, and hindcasts of historical conditions suggest mortality is occurring earlier in the year. The combined data suggest that the historical southern limit of M. edulis near Cape Hatteras, North Carolina is limited by intolerance of high temperature, and that this range edge may be susceptible to warming climatic conditions.

M. edulis poleward range contraction: Field transplant experiments were conducted at three locations along the US Atlantic coast. Survival and heat shock protein 70 expression were determined at biweekly intervals, and air and water temperature profiles were used to determine rates of temperature change and to model current and historical patterns of mortality. Since 1960, summer sea surface temperature increases have exceeded the upper lethal limits of this organism south of Lewes, Delaware (38.8°N). This has caused a geographic contraction of the southern, equatorward range edge of M. edulis, shifting the range edge approximately 350 km north of the previous limit. Barnacle range shifts: Using a combination of survey and transplant data with thermal modeling, we investigated whether (i) the southern range edge of the north temperate barnacle Semibalanus balanoides was contracting polewards, and (ii) the northern range edge of the subtropical barnacle Chthamalus fragilis was expanding polewards. The equatorward limit of S. balanoides has contracted approximately 350 km to the north. Two range expansions have also been documented: (i) the northern range edge of C. fragilis is north of Cape Cod, and (ii) the invasive barnacle Megabalanus coccopoma extendend its range along the southeastern United States.

Modeling Mytilus distributions: Using a mechanistic model, survival of M. edulis from the east coast of the United States was predicted on a geographic scale. The observed southern limit contraction due to high summer temperatures limiting survival was predicted. However, geographic survival modeling of Mytilus sp. for North America and Europe indicate that mechanisms controlling these species’ distributions vary, and caution must be used when taking such approaches to predicting changes of biogeography.

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