Date of Award

1-1-2012

Document Type

Campus Access Dissertation

Department

Biological Sciences

First Advisor

Thomas J Hilbish

Abstract

Mechanisms controlling species' biogeographic ranges have been of interest for centuries, as biologists explore the suite of factors that constrain a species to a certain distribution. These factors include abiotic environmental variables such as temperature and precipitation and biotic factors such as competition and predation. Understanding what factors affect the distribution of a species helps us better understand its life history and potential impacts from external forces such as climate change, habitat destruction, and the introduction of invasive species. The closely-related marine mussels Mytilus edulis, M. trossulus, and M. galloprovincialis provide an ideal study system for the effects of a changing environment on the biogeography of species. These three species form a biogeographic replacement series with respect to temperature, as one species replaces another as climate varies with latitude. We examined several of the mechanisms constraining the distribution of these species. The physiological energetics of each of the three species correlated with their current range distributions. M. galloprovincialis, the most warm-water species, was able to maintain a positive energy budget, energy available for growth and reproduction to sustain a population, at higher temperatures than M. edulis or M. trossulus. The warm end of each species' range correlated strongly with the point at which that species' energy budget became negative in summer and fall, while energetics at cold temperatures did not predict the cold end of the species' ranges.

A case study of the distribution of M. edulis and M. galloprovincialis in Great Britain revealed significant sea surface temperature (SST) warming over the past three decades, but no large-scale shifts in species distribution. There were fine-scale changes in species composition that depended on the genetic architecture of the populations. Populations with higher frequencies of pure species genotypes showed larger changes over time than populations that were more genetically-mixed. Factors such as wave exposure and ocean currents likely impacted these local-scale changes. Even with increasing SST over time, the temperatures around Great Britain were not warm enough to restrict these species physiologically. However, in both Great Britain and along the mainland coast of Europe, the northern limit of M. galloprovincialis appeared to be constrained by winter SST less than about 10°C. This constraint is not due to adult energetics, and is likely due to reproductive or larval success of the species.

We developed mechanistic species distribution models that used SST and chlorophyll a concentrations as inputs based on the energy budgets measured for these three species. These models determined areas along the coastlines in the eastern Pacific, eastern Atlantic, and western Atlantic where the annual energy budgets of each species would remain positive, suggesting that species could sustain a viable population. The models' effectiveness varied with species and location, suggesting that different factors constrain different parts of species' ranges. As SST increases with a changing climate, these factors will influence the future ranges of these marine mussels.

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