Trophic cascades in benthic marine ecosystems: lessons for fisheries and protected-area management

An important principle of environmental science is that changes in single c omponents of systems are likely to have consequences elsewhere in the same systems. In the sea, food web data are one of the few foundations for predi cting such indirect effects, whether of fishery exploitation or following r ecovery in marine protected areas (MPAs). We review the available literatur e on one type of indirect interaction in benthic marine ecosystems, namely trophic cascades, which involve three or more trophic levels connected by p redation, Because many indirect effects have been revealed through fishery exploitation, in some cases we include humans as trophic levels. Our purpos e is to establish how widespread cascades might be, and infer how likely th ey are to affect the properties of communities following the implementation of MPAs or intensive resource exploitation We review 39 documented cascade s (eight of which include humans as a trophic level) from 21 locations arou nd the world; all but two of the cascades are from shallow systems underlai n by hard substrata (kelp forests, rocky subtidal, coral reefs and rocky in tertidal), We argue that these systems are well represented because they ar e accessible and also amenable to the type of work that is necessary, Ninet een examples come from the central-eastern and north-eastern Pacific, while no well-substantiated benthic cascades have been reported from the NE, CE or SW Atlantic, the Southern Oceans, E Indian Ocean or NW Pacific, The abse nce of examples from those zones is probably due to lack of study. Sea urch ins are very prominent in the subtidal examples, and gastropods, especially limpets, in the intertidal examples; we suggest that this may reflect thei r predation by fewer specialist predators than is the case with fishes, but also their conspicuousness to investigators. The variation in ecological r esolution amongst studies, and in intensity of study amongst systems and re gions, indicates that more cascades will likely be identified in due course . Broadening the concept of cascades to include pathogenic interactions wou ld immediately increase the number of examples. The existing evidence is th at cascade effects are to be expected when hard-substratum systems are subj ect to artisanal resource exploitation, but that the particular problems of macroalgal overgrowth on Caribbean reefs and the expansion of coralline ba rrens in the Mediterranean rocky-sublittoral will not be readily reversed i n MPAs, probably because factors other than predation-based cascades have c ontributed to them in the first place. More cascade effects are likely to b e found in the soft-substratum systems that are crucial to so many large-sc ale fisheries, when opportunities such as those of MPAs and fishing gradien ts become available for study of such systems, and the search is widened to less conspicuous focal organisms such as polychaetes and crustaceans.