Changes in algal community structure via density- and trait-mediated indirect interactions
In various ecosystems, predators indirectly affect resources by reducing density of the prey (density-mediated indirect interactions; DMIIs) or by changing their behavior, life history, or morphology (trait-mediated indirect interactions; TMIIs). However, the strengths of DMIIs and TMIIs, especially in shaping plant community structure, have rarely been evaluated in situ. Moreover, although characteristics of predators and resources in affecting such indirect interactions are well documented, little attention has been paid to the prey’s characteristics. We considered that the prey’s density influences both components of indirect interactions because it intermediates the top-down effects of the predator to the plant communities. We conducted two field experiments in a three-level marine food chain involving the carnivore snail Thais clavigera, its prey limpet Siphonaria sirius, and the food algae Lithoderma sp. and Ulva sp. The limpet has the home range in which most rock surface is covered with the Lithoderma sp. In the first experiment, we measured strengths of DMIIs and TMIIs, and how algal community changes, under natural predation pressure by T. clavigera on S. sirius. In the second experiment, we experimentally manipulated the density of the limpets to make low- and high-density plots, and measured strengths of DMIIs and TMIIs in each condition.
In the first experiment, the strengths of DMIIs and TMIIs in terms of percent cover of the algae were of similar magnitudes. Both DMIIs and TMIIs increased percent cover of Ulva sp. and decreased cover of Lithoderma sp., because the carnivore snail reduced the limpet’s preferential feeding on the competitively dominant Ulva sp. over Lithoderma sp. These results suggest that both DMIIs and TMIIs have similar effects on the changes in resource community structure under natural conditions in the study system. In the second experiment, Lithoderma sp. was replaced by Ulva sp. through both DMIIs and TMIIs at low limpet densities, as in the first experiment. However, neither DMIIs nor TMIIs affected the percent cover of Lithoderma sp. and Ulva sp. at high limpet densities. These results suggest that the prey’s density is a key determinant of the strengths of DMIIs and TMIIs. Overall, we advocate the necessity of field experiments under natural conditions and the importance of prey’s characteristics, including density, in evaluating the strengths of DMIIs and TMIIs.