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

Abstract

An important principle of environmental science is that changes in single components 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 predicting such indirect effects, whether of fishery exploitation or following recovery in marine protected areas (MPAs). We review the available literature on one type of indirect interaction in benthic marine ecosystems, namely trophic cascades, which involve three or more trophic levels connected by predation. Because many indirect effects have been revealed through fishery exploitation, in some cases we include humans as trophic levels. Our purpose is to establish how widespread cascades might be, and infer how likely they are to affect the properties of communities following the implementation of MPAs or intensive resource exploitation. We review 39 documented cascades (eight of which include humans as a trophic level) from 21 locations around the world; all but two of the cascades are from shallow systems underlain by hard substrata (kelp forests, rocky subtidal, coral reefs and rocky intertidal). We argue that these systems are well represented because they are accessible and also amenable to the type of work that is necessary. Nineteen 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 absence of examples from those zones is probably due to lack of study. Sea urchins are very prominent in the subtidal examples, and gastropods, especially limpets, in the intertidal examples; we suggest that this may reflect their predation by fewer specialist predators than is the case with fishes, but also their conspicuousness to investigators. The variation in ecological resolution amongst studies, and in intensity of study amongst systems and regions, indicates that more cascades will likely be identified in due course. Broadening the concept of cascades to include pathogenic interactions would immediately increase the number of examples. The existing evidence is that cascade effects are to be expected when hard-substratum systems are subject to artisanal resource exploitation, but that the particular problems of macroalgal overgrowth on Caribbean reefs and the expansion of coralline barrens in the Mediterranean rocky-sublittoral will not be readily reversed in MPAs, probably because factors other than predation-based cascades have contributed to them in the first place. More cascade effects are likely to be found in the soft-substratum systems that are crucial to so many large-scale fisheries, when opportunities such as those of MPAs and fishing gradients become available for study of such systems, and the search is widened to less conspicuous focal organisms such as polychaetes and crustaceans.

Trophic restructuring of coral reef fish communities in a large marine reserve

“…Restructuring of fish communities has been revealed by the reintroduction of top predatory species in a variety of marine systems (Pace et al 1999, Pinnegar et al 2000. Here we observed evidence for community restructuring based on both body size and trophic score.…”

“…Trophic cascades have been observed in a variety of marine communities (Steneck 1998, Daskalov 2002, Graham et al 2003, Frank et al 2005, Mumby et al 2006, though the most distinct examples come from relatively simple systems with few trophic levels and low species diversity (Strong 1992, Pinnegar et al 2000. Indeed, the buffering effects of varied and differentiated consumptive relationships, including omnivory and ontogenetic trophic level shifts, as well as multiple prey refuges, may prevent strong trophic cascades in more speciose systems (Strong 1992).…”

Trajectories and magnitude of change in coral reef fish populations in Philippine marine reserves: a meta-analysis

“…The pattern of response (i.e., reserve effects) of top-trophic level species is expected to correlate positively with the age, size, and enforcement capacity of the reserves under consideration Russ 2006;Guidetti et al 2008;Claudet et al 2008). On the contrary, nonexploited prey species in the reserves are expected to have minimal response or even respond negatively due to concomitant increase in predation pressure (Pinnegar et al 2002;Graham et al 2003).…”

Latitudinal, seasonal, and small-scale spatial differences of the giant kelp, Macrocystis pyrifera, and an herbivore at their southern range limit in the northern hemisphere

“…It has been previously suggested that populations of giant kelp and sea urchins are coupled in nature (Mann and Breen 1971, Dayton et al 1992, Pinnegar et al 2000, and purple sea urchins are thought to be very closely connected to the habitat of algal populations for their primary food source Reed 1985, Pearse 2006). In the Baja California region, the purple sea urchin is often associated with giant kelp forests (Palleiro-Nayar et al 2008, Salgado-Rogel et al 2006.…”

“…Authors have previously suggested that populations of giant kelp and sea urchins are coupled in nature (Breen and Mann 1976, Dayton et al 1992, Pinnegar et al 2000, showing similar responses to environmental changes, as urchins are closely tied to algal habitat for their primary food source (Harrold and Reed 1985, Graham 2004, Pearse 2006. However, most of this work has been performed in northern temperate populations for sea urchins and for the giant kelp, Macrocystis pyrifera, whereas the kelp forest populations in Baja California Sur are known to respond to and recover from warming events such as El Niño in a different manner (Ladah et al 1999, Edwards 2004, Ebert 2010.…”