The refugia hypothesis maintains that the stable coexistence of predator and prey depends on the either spatial refuges, regions in the distribution of prey that are inaccessible to the predators, or age/size refuges, in which the prey have attained sufficient size or strength to resist attack. Although first proposed in the context of laboratory populations and now applied to predator-prey systems from insects to mammals, the hypothesis was first field-tested in experiments investigating the zonation of barnacles and mussels. On rocky coast of the Pacific Northwest, the barnacles S. glandula do not attain sufficient shell thickness to resist attacks by dog whelks Nucella spp., but the barnacles form a densely populated zone high on the shore above the foraging ambits ofthe whelks, in a spatial refuge. S. cariosus forms a zone below S. glandula, within the range of the foraging range of the whelks, but it has the capacity to grow a thick shell impenetrable by the whelks, an age/size refuge.
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