Reinforcing abiotic and biotic time constraints facilitate the broad distribution of a generalist with fixed traits

Abstract

Many species are habitat specialists along environmental gradients as a result of contrasting selection pressures, but others maintain broad distributions along such gradients. Phenotypic plasticity explains the persistence of some generalists, but not the broad distributions of species with fixed traits. We combined comparative and experimental data to investigate the role of multiple selection pressures on the distribution of a cased caddisfly (Asynarchus nigriculus) across a pond permanence gradient in the Mexican Cut Nature Preserve, Elk Mountains, Colorado, USA. Rapid development in this species facilitates the exploitation of short-duration vernal pools. Comparative data document that slowly growing individuals die from desiccation, suggesting an ongoing selection for rapid development. Surprisingly, development is as fast or faster in long-duration, autumnal ponds where emergence occurs long before drying, and overlaps with the appearance of beetle (Dytiscus) predators. In field experiments we found that the last two instars of beetle larvae pose a significant mortality threat to Asynarchus, but that threat declines after caddisfly pupation. In natural populations, the caddisflies pupate and emerge just as large beetle instars appear in the ponds. Experimental manipulation of caddisfly size suggests that rapid development in autumnal ponds will both facilitate intraguild predation on other caddisflies and reduce Asynarchus cannibalism. Both types of caddisfly interactions should have a positive feedback effect on rapid development via a protein supplement to their detrital diet. All of these biotic time constraints should select for rapid Asynarchus development in autumnal habitats, despite relaxed drying time constraints. Asynarchus did not display flexible antipredator responses to beetles (no changes in activity rates, morphology, or development), suggesting that the traits that lead to rapid development are fixed, regardless of habitat type and presence of predators. We propose that different, but convergent, selection pressures across different habitat types have led to fixed specialized traits that enable a broad distribution along this environmental gradient. These selection pressures are dependent on the relative phenologies of interacting species and appear to trump the trade-offs between other types of physical and biotic constraints across habitats.