KEYSTONE PREDATION AND INTERACTION STRENGTH - INTERACTIVE EFFECTS OF PREDATORS ON THEIR MAIN PREY

The application of basic ecological concepts to fields of conservation biology and applied environmental sciences is a healthy sign, but bef ore these concepts are widely used, ecology must provide operational d efinitions and quantifiable methods. Keystone species and interaction strength are concepts with deep practical and theoretical implications . We studied the strength of predation on mussels (Mytilus trossulus) by the keystone seastar Pisaster ochraceus and the whelks Nucella emar ginata and N. canaliculata under different environmental conditions in the Oregon intertidal zone. We attempted to determine: (1) the sensit ivity of keystone predation to the presence of other predators in the system; (2) the role of other predators in the presence and absence of a keystone species; and (3) the per capita and population-level varia bility in interaction strengths of strong (keystone) vs. weak interact ors. Predation intensity on mussels was measured by recording the surv ival of mussels transplanted to areas from which seastars, whelks, or both, had been either manually removed or left undisturbed at natural densities. Whelk experimental units were nested within those for the s eastar treatment to account for the much larger body size and greater mobility of seastars. Each combination of seastar and whelk treatment was replicated four times in both wave-exposed and wave-protected habi tats of two sites that differed in predator densities, primary product ivity, and recruitment and growth rates of prey species. Predation int ensity by the keystone predator was strong under all site X wave expos ure combinations, and was unaffected by the presence of whelks. Whelks , in contrast, had ecologically important effects on mussel survival i n the absence, but not in the presence, of the keystone predator. Popu lation (total) interaction strength between seastars and mussels was 2 -10 times stronger than that between whelks and mussels across sites a nd wave exposures. Per capita interaction strength of seastars was two to three orders of magnitude larger than that of whelks. However, per capita effects of seastars were more variable between sites and wave exposures, probably because simple density values grossly underestimat e the ability of mobile predators to localize prey. Such interactive e ffects and variability in interaction strengths between keystone and w eak predators may characterize all keystone predator-dominated systems , but data currently are insufficient to test this proposition. Negati ve effects of seastars on whelk density were observed <4 mo following initiation of Pisaster removals. Seastars also had a negative effect o n whelk sizes, which took longer to appear, after 6 mo of continuous P isaster removal. Negative effects of seastars on whelks appeared to be stronger in places with higher densities of predators, partially expl aining the reduced predation intensity of whelks observed in the prese nce of seastars. Our results support the idea that in keystone-dominat ed systems, species other than the keystone species have only minor, i f any, effects on the rest of the community, and thus might be cited b y some as ''redundant species.'' However, our results also indicate th at, after the loss of a keystone species, previously ''redundant'' spe cies can partially compensate for the reduced predation and adopt a ma jor role in the altered system. Such responses are potentially an impo rtant force in stabilizing communities. Further, such possible compens atory capabilities of alternative consumers suggests that, at least fo r predators, the term ''redundant species'' conveys an inaccurate imag e of the potential importance of weak interactors and should be abando ned.