Trait-mediated indirect interactions among residents of rocky shore tidepools

Trait-mediated indirect interactions (TMIIs) are an important component of food web structure and dynamics. We determined whether TMIIs occur in rocky tidepool communities on the west coast of the USA. In the laboratory, both adults and juveniles of the keystone predator Pisaster ochraceus and adults of a smaller predatory seastar Leptasterias spp. caused the abundant herbivorous snail Tegula funebralis to stop foraging and flee the water, inducing a positive TMII on micro-and macroalgae. Snails preferred 3 common species of macroalgae (Ulva lactuca, Cladophora columbiana and Porphyra spp.) over 4 others, indicating that seastars might provide the strongest benefits to these species in tidepools. In the laboratory, snails responded rapidly to both species of predatory seastars and many more snails responded than could be eaten; thus, there is a potential for TMIIs to occur in natural populations. Snails responded to waterborne cues from P. ochraceus by reducing grazing and leaving still water, and reducing grazing in laminar flow (0.5 l min(-1)), resulting in TMII effects at least as far as 75 cm away. Adult P. ochraceus and Leptasterias spp. introduced to tidepools during low tide induced many snails to flee the tidepools. Considerable individual variation occurred in the responses of snails. Medium and large snails mediated TMIIs and hungry snails were marginally less responsive to seastars potentially altering TMII strength in nature. Thus, we demonstrated that TMIIs could occur in natural tidepools and showed how predator and algal identity, predator and prey size, water flow and prey hunger level may influence these TMIIs.