Size‐Specific Shelter Limitation in Stone Crabs: A Test of The Demographic Bottleneck Hypothesis

Habitat structural complexity can facilitate species persistence by providing refuges and the availability of refuges may control the size of many populations. In organisms that increase greatly in size during ontogeny the size and abundance of refuges is particularly important, because population—regulating factors may operate strongly on only one size class or ontogenetic stage within a population, creating a demographic bottleneck. I test the demographic bottleneck hypothesis in stone crabs by first supplementing shelter with five different sizes of polyvinyl chloride (PVC) pipe at eight sites in St. Joseph Bay, Florida to determine the size class most likely to be sheltered limited. I then used tethering experiments, cage studies, and natural observations to ask if habitat structural complexity regulates the stone crab population through its size—specific effects on growth, survival, and fecundity. Shelter availability appears to create a demographic bottleneck that affects the growth and fecundity of large crabs. In both 1991 and 1992, the largest PVC pipes were occupied in significantly greater proportions than smaller pipes, suggesting that this shelter size class is limiting. Predation in size specific; smaller individuals were at significant risk but no large crabs were taken by predators. However, shelter is particularly important for the growth of large crabs. There was a fivefold increase in molting individuals on shelter—supplemented sites relative to natural densities suggesting that the bottleneck size class has been identified correctly and that shelter additions can alleviate the bottleneck. Supplemental shelters also particularly attracted gravid females and, in tethering experiments, females with shelter produce egg masses twice as quickly as females without shelter. These results differ from most prior results in marine populations in two ways. First, it is factors affecting adults and not juveniles that may regulate the stone crab populations. Second, refuge use in those size classes likely to be shelter limited is not explained primarily by predation, but by effects on growth and fecundity. The evidence suggests the merits of the demographic bottleneck hypothesis; habitat structural complexity may affect both the size of populations and the size of individuals within the population.