Trophic role of the mesopredatory three-spined stickleback in habitats of varying complexity

Abstract

Mesopredators must contend with both top-down (predation pressure by top predators) and bottom-up (food availability) drivers when selecting habitat. We studied the distribution and trophic dynamics of an abundant mesopredator (the three-spined stickleback; Gasterosteus aculeatus) in three habitats of differing complexity: a macroalgal bed (Fucus vesiculosus), a seagrass meadow (Zostera marina), and a sand flat. We found that stickleback abundance and food availability were highest in the macroalgae, intermediate in the seagrass, and very low in the sand flat. We then tested in situ predation pressure on isopods and amphipods by tethering live prey in the three habitats. Predation on amphipods was very high in early summer, when it was highest in the sand flat, intermediate in the seagrass, and lowest in the macroalgae. However, small-scale within-habitat differences in physical complexity (shoot density) or location (interior vs. edge) did not affect predation rates. Predation rates on amphipods decreased with prey size, but predation on isopods was always low. Laboratory experiments using amphipod prey and stickleback predators supported the patterns observed in the field, where predation success was highest in the sand and lowest in the Fucus. Overall, sticklebacks were more abundant in, and preferentially chose, the two habitats with highest structural complexity (the macroalgae bed and seagrass meadow), potentially trading off low predation success for higher food availability and increased shelter from top predators. These results confirm the critical importance of these structured habitats for coastal trophic networks.