Intermediate habitat fragmentation reduces gastropod movement in experimental intertidal landscapes: implications for fragmentation–diversity relationships

Abstract

Abstract Context Understanding how habitat configuration influences biodiversity is central to landscape and seascape ecology, yet experimental tests that isolate fragmentation per se from habitat area remain rare. In an earlier experiment that disentangled the effects of habitat area and spatial configuration, we found a unimodal species–fragmentation relationship, where species richness peaked at intermediate fragmentation. Although the mechanisms were not identified then, dispersal limitation was proposed as a key driver underlying this pattern. Objectives Here, our goal was to test whether intermediate fragmentation generates the strongest dispersal limitation by reducing gastropod movement, using two complementary studies conducted in the same experimental landscapes. Methods We created experimental intertidal landscapes by attaching artificial habitat tiles to seawalls. The design crossed three levels of tile cover (7%, 14% and 21%) with three spatial configurations (single large patch, several small patches and many scattered tiles). In Study 1, we used drone surveys to quantify spatial aggregation of nerite snails across all nine treatment combinations. In Study 2, we used mark–recapture in the 14% tile-cover landscapes to compare movement distances of individuals among spatial configurations over time, focusing on the better-represented nerite and whelk species. Results Spatial configuration of habitat tiles affected nerite aggregation, but the direction of the effect depended on tile cover in the landscape plots. At 14% and 21% cover, aggregation tended to decline as habitat tiles became more fragmented as hypothesised, whereas at 7% tile cover the opposite pattern was observed. In the 14% tile-cover landscapes, movement distances of mobile snails were lowest in the several-small-patches configuration, especially for the better-represented nerites and whelks, consistent with our expectation that intermediate fragmentation generates stronger dispersal limitation. Conclusion Our results suggest that intermediate fragmentation can reduce gastropod movement in artificial intertidal landscapes. This provides empirical support for dispersal limitation as a plausible mechanism linking habitat configuration to the unimodal species richness–fragmentation relationship previously observed in this experimental system.