THE KEYSTONE SPECIES CONCEPT - VARIATION IN INTERACTION STRENGTH IN AROCKY INTERTIDAL HABITAT

The usefulness and generality of the keystone species concept has rece ntly been questioned. We investigated variation in interaction strengt h between the original keystone predator, the seastar Pisaster ochrace us, and its primary prey, mussels (Mytilus californianus and M. trossu lus). The study was prompted by differences in community structure at two low zone sites along the central Oregon coast, Boiler Bay (BB) and Strawberry Hill (SH). Predators, especially seastars, were larger and more abundant at SH than at BB. Further, sessile animals were more ab undant and macrophytes were less abundant at SH. Predators were more a bundant at wave-exposed sites at both sites, and at SH, sessile invert ebrates were more abundant at the wave-exposed location and sand cover was high at the wave-protected location. To test the hypothesis that variation in predation strength explained some of these differences, w e examined the seastar-mussel interaction at locations with high and l ow wave exposure at both sites. Predation intensity was quantified by determining the survival of mussels in clumps (50 mussels per clump, s hell length 4-7 cm) transplanted to large plots (18-163 M2) with or wi thout seastars in the low intertidal zone. Predation effects were quan tified by determining prey recolonization rates in marked quadrats in the same large plots. Spatial variation in interaction strength was qu antified by examining predation at scales of metres (among transplants within plots), 10's of metres (between replicate plots within each ex posure at each site), 100's of metres (between wave exposures within l ocations), and 10 000's of metres (between sites). Temporal variation was evaluated by performing the experiments in 1990 and 1991. The rela tion between prey (mussel) recruitment and growth to differences in co mmunity structure was evaluated by quantifying recruitment density in plastic mesh balls (collectors) and growth of individually marked tran splanted mussels, respectively, at each site x exposure x tide level c ombination each month for 4 yr. Predation intensity varied greatly at all spatial scales. At the two largest spatial scales (10's of kilomet res, 100's of metres), differences in both survival Of transplanted mu ssels and prey recolonization depended on variation in seastar abundan ce with site, wave exposure, prey recruitment and growth, and at SH pr otected, the extent of sand burial. Variation at the two smallest scal es (metres, 10's of metres) was high when seastars were scarce and low when seastars were abundant. Transplanted mussels suffered 100% morta lity in 2 wk at wave-exposed SH, but took > 52 wk at wave-protected BB . Seastar effects on prey recolonization were detected only at the SH wave-exposed site. Here, where prey recruitment and growth were unusua lly high, the mussel M. trossulus invaded and dominated space within 9 mo. After 14 mo, whelks, which increased in both size and abundance i n the absence of Pisaster, arrested this increase in mussel abundance. Similar changes did not occur at other site x exposure combinations, evidently because prey recruitment was low and possibly also due to wh elk predation on juveniles. Longer term results indicate that, as in W ashington state, seastars prevent large adult M. californianus from in vading lower intertidal regions, but only at wave-exposed, not wave-pr otected sites. Thus, three distinct predation regimes were observed: ( 1) strong keystone predation by seastars al wave-exposed headlands; (2 ) less-strong diffuse predation by seastars, whelks, and possibly othe r predators at a wave-protected cove, and (3) weak predation at a wave -protected site buried regularly by sand. Comparable experimental resu lts at four wave-exposed headlands (our two in Oregon and two others i n Washington), and similarities between these and communities on other West Coast headlands suggest keystone predation occurs broadly in thi s system. Results in wave-protected habitats, however, suggest it is n ot universal. In Oregon, keystone predation was evidently contingent o n conditions of high prey production (i.e., recruitment and growth), w hile diffuse predation occurred when prey production was low, and weak predation occurred when environmental stress was high. Combining our results with examples from other marine and non-marine habitats sugges ts a need to consider a broader range of models than just keystone pre dation. The predictive and explanatory value of an expanded set of mod els depends on identifying factors distinguishing them. Although evide nce is limited, a survey of 17 examples suggests (1) keystone predatio n is evidently not distinguished from diffuse predation by any of 11 p reviously proposed differences, but (2) may be distinguished by rates of prey production. Further, (3) differential predation on competitive ly dominant prey does not distinguish keystone from nonkeystone system s, since this interaction occurs in both types of community. Instead, differential predation on dominant prey evidently distinguishes strong -from weak-predation communities. While the keystone predation concept has been and will continue to be useful, a broadened focus on testing and developing more general models of community regulation is needed.