A multi-scale investigation of distributional limits in the mussel Myitlus Californianus

Background/Question/Methods

Explanation of distributional limits is a fundamental goal of ecological research, for many species causes of distributional patterns across scales are poorly understood. Many explanations of range limits consist of correlations between changes in population density and changes in environmental factors in the absence of models describing how environmental variation impacts population growth rates. The present study investigates two questions: 1. How do environmental factors thought to drive changes in population density impact population growth rates via repression of specific vital rates across distributional limits? 2. Are the same environmental factors driving declines in population growth rates at small scales responsible for changes in population growth rates at larger scales? In order to answer these questions demographic models were parameterized at multiple scales across several distributional limits by transplantation of individuals and subsequent monitoring of growth, survival, and recruitment rates.

Results/Conclusions

At the largest scale investigated across a range limit recruitment limitation appears to drive the decline in density, growth and survival of transplanted individuals did not vary significantly from rates measured in more central extant populations. At smaller scales a more complicated picture including declines in recruitment, growth, and survival along gradients of declining submergence time and decreasing wave exposure collectively lead to changes in density. Previous hypotheses explaining the range limit which depended on increases in experienced temperature due to declines in wave exposure and changes in the timing of low tides appear to be superseded by the effect of recruitment limitation. This result indicates the value of comparing vital rates across range limits and their relation to changes in environmental factors. Across all scales variation in recruitment rates was found to play an important role, but small scale patterns were also driven by changes in growth and survival highlighting scale dependent variation in how distributional patterns are developed.