INDIRECT EFFECTS IN MARINE ROCKY INTERTIDAL INTERACTION WEBS - PATTERNS AND IMPORTANCE

To determine the patterns of occurrence and importance of indirect eff ects relative to direct effects in natural communities, I analyzed exp erimentally based studies from 23 rocky intertidal habitats. The vehic le of analysis was the construction of interaction webs, or the subset of species in food webs involved in strong interactions. The analysis focused on indirect effects involving changes in abundance, or intera ction chains, since little information was available on other types of indirect effects (behavioral, chemical response, environmental). As e xpected, number of direct (= strong) interactions, indirect effects, i nteraction sequences producing indirect effects, and types of indirect effects (e.g., keystone predation, apparent competition, etc.) all in creased with web species richness. Less expected, when these measures were adjusted to a per species basis, positive relationships with spec ies richness were still observed for all measures but the number of ty pes. In other words, with increasing web diversity, each species inter acted strongly with more species, was involved in more indirect effect s, and was part of more interaction pathways. The analysis identified 83 subtypes of indirect effect, including the seven previously identif ied types. Many of the 76 additional types could be reclassified into the seven types if the original definitions of these ''classic'' types were expanded to include interactions having similar effects but diff ering in the specific mechanism (e.g., both interference competition a nd inhibition of recruitment [preemption] have negative effects involv ing a spatial resource). Two new types of indirect effect, termed ''ap parent predation'' and ''indirect defense'' were also identified, prod ucing a total of 9 general types of indirect effect divided among 565 specific indirect effects. Of these, keystone predation (35%) and appa rent competition (25%) were most common and exploitation competition ( 2.8%) was least common in these webs. Two methods of analysis suggeste d that indirect effects accounted for approximate to 40% of the change in community structure resulting from manipulations, with a range of 24-61%. The proportion of change due to indirect effects was constant with web species richness, indicating that strong direct interactions and indirect effects produce roughly the same level of alteration of c ommunity structure regardless of the lever of web complexity. Several potential artifacts and biases were evaluated. Most importantly, neith er variation in level of taxonomic resolution nor intensity of experim entation varied significantly with web size (species richness). Despit e a bias toward manipulation of consumers over manipulation of basal s pecies, some predator-initiated indirect effect types were scarce whil e some basal species-initiated types were common. While the frequency of exploitation competition may have been underestimated, it is unlike ly that the frequency of this indirect effect would change dramaticall y: changes due to this effect should have been detected in many of the studies and reported: and the most intensively studied individual web s did not report frequencies differing much from the average. This ana lysis suggests investigators effectively identified and first manipula ted those species responsible for most indirect effects and that more experiments added decreasing numbers of indirect effects. Moreover, th e frequencies and importance of indirect effects may be more predictab le than expected on the basis of theory.