Indirect Effects in Experimental Studies of Marine Soft-Sediment Communities

Most experimental studies that deal with the effects of bioticinteractions in marine soft sediments are designed to examinethe direct effects of one species (or group of species) on somesubset of the community. However, species within a communityinfluence one another through a complex web comprising bothdirect and indirect effects. Ignoring this complexity in thedevelopment of hypotheses and experimental designs will likelylead to unexpected or conflicting findings. Few studies of marinesoft-sediment communities have emphasized indirect effects andeven fewer have included experimental designs capable of testingfor them. To illustrate the potential importance of indirecteffects in marine soft sediments, I provide several examplesfrom the literature and describe the type of experimental designrequired to demonstrate a simple indirect effect involving threespecies. Perspective on indirect effects in a particular communitychanges as more information is collected about individual speciesand their effects. The influence of one species or guild maybe confounded by that of another and even effects expected tobe strong may appear weak or nonexistent as a result of unidentifiedindirect effects. Recognizing indirect effects in soft-sedimentcommunities may help to explain apparent paradoxical resultsin past experiments and provide an improved template for thedesign of future experiments.     

The Role of Fundulus heteroclitus in Salt Marsh Trophic Dynamics

The common mummichog, Fundulus heteroclitus, functions as bothpredator and prey in the trophic structure of east coast tidalmarshes. Although mummichogs are generally considered importantto energy transformations within marshes, few studies have convincinglydemonstrated that predation by F. heteroclitus affects the abundanceof salt marsh benthic invertebrates. Thus far investigationsof this type have dealt only with the direct effects of adultmummichogs. The results of recent experiments have suggestedthat by controlling smaller predators, mummichogs may indirectlyhave a positive effect on the densities of some infaunal marshinvertebrates. Our current knowledge of larval and juvenilemummichogs in their natural habitat is minimal. Unlike the adults,which can utilize the intertidal zone only when it is flooded,the young remain on the marsh even at low tide, inhabiting shallowpuddles of residual tidal water that form between clumps ofvegetation and around fiddler crab (Uca sp.) burrows. The importanceof F. heteroclitus in salt marsh communities will remain incompletelyunderstood unless future studies consider the role of larvaland juvenile mummichogs. Although many species of fishes andwading birds feed on mummichogs, the blue crab (Callinectessapidus) is probably the major predator of adult F. heteroclitusin the intertidal salt marsh. Predation by adult mummichogsand xanthid crabs (e.g., Eurytium limosum) may contribute tothe high mortality of larval and juvenile Fundulus.