PATTERNS IN THE EVOLUTION OF PHYSIOLOGICAL SPECIALIZATION IN SALT-MARSH ANIMALS

Patterns of tolerance to stressful abiotic variables among animals that have colonized salt marshes provide models for the study of the evolution of physiological specialization. We provide an overview of some closely related pairs or groups of species that show hierarchical gradations in tolerance, and thereby physiological mechanisms of compensation, along a line perpendicular to the shore. However, a full understanding of the evolution of physiological specialization requires that we evaluate the role of abiotic factors in light of the biotic interactions in which salt-marsh species are embedded. The common occurrence of salt-marsh endemics in a species-poor environment must be related to the unusual character of this habitat, in which the reduced importance of some biotic forces is paired with highly stressful abiotic factors. The salt marsh is probably not a refuge for competitively inferior forms so much as it is a highly selective environment that admits only those specialized species that can muster the necessary physiological adaptations. Thus the apparently lower competitive ability of salt-marsh species under conditions that are not abiotically stressful is likely to be a secondary consequence of the costs of mechanisms conferring tolerance, and of life in a species-poor habitat. Salt marshes provide an arena in which it is possible to experimentally evaluate the evolutionary and ecological importance of abiotic stress, especially in relation to parapatric or allotopic distributions. Yet very few studies have provided such a rigorous approach, despite a long-standing recognition that abiotic factors interact strongly with biotic forces in the salt marsh.