Prey density affects strengths of density- and trait-mediated indirect interactions of predators on an algal community

• Density often varies in a natural population of marine animals.
• The strengths of indirect interactions were quantified in an intertidal food chain.
• Neither density- nor trait-mediated interactions were detected at high prey densities.
• Both interactions changed algal community structure at low prey densities.
• Prey density is a key determinant of the strengths of indirect interactions.

In trophic cascades, predators can indirectly affect resources by reducing prey density (density-mediated indirect interactions; DMIIs) or by changing prey traits, such as their behavior, morphology, physiology, or life history (trait-mediated indirect interactions; TMIIs). Although the importance of predator (e.g., foraging strategy) and resource characteristics (e.g., quantity) in these indirect interactions is well recognized, little attention has been paid to prey characteristics (e.g., density), especially in field studies. We focused on a marine trophic cascade involving the carnivorous snail Thais clavigera, its limpet prey Siphonaria sirius, and the algae Lithoderma sp. and Ulva sp. Using intertidal rocks as natural replicates, we experimentally evaluated the in situ effects of the DMIIs and TMIIs on the algal community under two density ranges of prey. The strengths of consumptive effects (CEs) and non-consumptive effects (NCEs) of the predator on the prey limpet were also monitored to elucidate the mechanisms of the indirect interactions. At high densities, CEs decreased the percentage of individual limpets that disappeared (that likely died); however, neither DMIIs nor TMIIs were detected. At low densities, both CEs and NCEs reduced the per capita feeding rate and CEs reduced the growth rate of limpets. Moreover, Lithoderma sp. was replaced by Ulva sp. through both DMIIs and TMIIs. These results suggest that prey density is a key determinant of the strengths of CEs and NCEs, and notably, of DMIIs and TMIIs.