Life history traits and population processes in marine bivalve molluscs
In this thesis, I investigated the how the life history characteristics of the clam Mya arenaria determine the population response to chronic contaminant exposure. To predict the potential responses of a broadcast-spawning life history such as that of M. arenaria, I surveyed the literature on a variety of bivalve species. By incorporating information on growth, survival, and reproduction into matrix population models I could evaluate the relative contributions of these factors to fitness. For broadcast-spawners, long life is an important factor enabling them to gamble on rare, large recruitment events. Another important aspect of the broadcast spawning strategy is the possibility of high variation in larval settlement from year to year. I evaluated the role that this variability plays using a stochastic matrix model, and showed that it tends to increase population growth because of the larger size of rarer, successful recruitment events. With an understanding of how the life history traits of M. arenaria might control its responses to change in the environment, I analyzed the vital rates of clams at clean and contaminated sites. The effects of contaminants measured in the lab do not necessarily predict population condition in the field. Since surviving with a long life span contributes the most to fitness in broadcast-spawning bivalves, effects on reproductive output and juvenile survival, which are strong in many lab studies, may not necessarily playa large role in field populations. The life history of this clam, with natural variation in recruitment from year to year, further reduces the population dependence on high reproductive output and larval survival. The combination of little population-level relevance of the strongest contaminant effects, and potential contaminant effects on very important clam predators, could explain why populations at contaminated sites are observed to be growing the fastest. The interaction of contaminant exposure and normal ecological processes determines the overall impact on the population.
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Massachusetts Institute of Technology and Woods Hole Oceanographic Institution