Experimental separation of effects of consumers on sessile prey in the low zone of a rocky shore in the Bay of Panama: direct and indirect consequences of food web complexity

Abstract The effects of predation by a diverse assemblage of consumers on community structure of sessile prey was evaluated in the low rocky intertidal zone at Taboguilla Island in the Bay of Panama. Four functional groups of consumers were defined: (1) large fishes, (2) small fishes and crabs, (3) herbivorous molluscs, and (4) predaceous gastropods, (l) and (2) included fast-moving consumers and (3) and (4) included slow-moving consumers. Experimental treatments were: no consumers deleted (all groups present), most combinations of deletions of single groups (i.e., one group absent, three present), pairs of groups deleted (two absent, two present), trios of groups deleted (three absent, one present), and the entire consumer assemblage deleted (all groups absent). Changes in abundance (percent cover) of crustose algae, solitary sessile invertebrates, foliose algae, and colonial sessile invertebrates were quantified periodically in 2–4 plots of each treatment from February 1977 to January 1980 after the initiation of the experiment in January 1977. Space on this shore is normally dominated by crustose algae; foliose algae, solitary sessile invertebrates, and colonial sessile invertebrates are all rare. After deletion of all consumers, ephemeral green algae increased from 0 to nearly 70% cover. Thereafter, a succession of spatial dominants occurred, with peak abundances as follows: the foliose coralline alga Jania spp. by July 1977, the barnacle Balanus inexpectatus by April 1978, and the rock oyster Chama echinata by January 1980. Although no longer occupying primary rock space, Jania persisted as a dominant or co-dominant turf species (with the brown alga Giffordia mitchelliae and/or the hydrozoan Abietinaria sp.) by colonizing shells of sessile animals as they became abundant instead of the rock surface. Multivariate analysis variance (MANOVA) indicated that the effect of each group was as follows. Molluscan herbivores grazed foliose algae down to the grazer-resistant, but competitively inferior algal crusts, altered the relative abundances of the crusts, and inhibited recruitment of sessile invertebrates. Predaceous gastropods reduced the abundance of solitary sessile animals. Small fishes and crabs, and large fishes reduced the cover of solitary and colonial sessile animals and foliose algae, although they were incapable of grazing the foliose algae down to the rock surface. Many of the effects of each consumer group on prey groups or species were indirect; some effects were positive and some were negative. The variety of these indirect effects was due to both consumer-prey interactions among the consumers, and competitive or commensalistic interactions among the sessile prey. Comparison of the sum of the effects of each of the single consumer groups (i.e., the sum of the effect observed in treatments with one group absent, three present) with the total effects of all consumers (i.e., the effect observed in the treatment with all groups absent) indicates that a “keystone” consumer was not present in this community. Rather, the impacts of the consumer groups were similar but, due to dietary overlap and compensatory changes among the consumers, not readily detected in deletions of single consumer groups. The normally observed dominance of space by crustose algae is thus maintained by persistent, intense predation by a diverse assemblage of consumers on potentially dominant sessile animals and foliose algae. The large difference in structure between this and temperate intertidal communities seems due to differences in degree, not kind of ecological processes which produce the structure.