Multiple, predictable selection pressures lead to life history canalization in a temporary pond caddisfly

Many metapopulations occur in habitat patches that vary in the magnitude of physical and biotic stressors, thus selecting for polymorphic or polyphenic flexibility in life histories strategies, morphologies, and/or behaviors. We investigated the role of multiple selection pressures on the life history and behaviour of Asynarchus nigriculus, a cased caddisfly that inhabits ponds across a permanence gradient of wetland habitats in the Rocky Mountains. In short-duration temporary ponds, slowly developing larvae had increased risk of mortality from desiccation. Development is synchronous in these species, and larvae that lagged behind the population mean had the highest mortality from cannibalism, suggesting that time constraints and biotic interactions should select for rapid development. In long duration temporary ponds, emergence occurred well before pond drying and coincided with the appearance of predatory beetle (Dytiscus) larvae.  Laboratory trials showed that the last two instars of beetle larvae pose a significant threat to Asynarchus,, but that threat declines after the caddisflies pupate. Thus, rapid development in Asynarchus should be selected for in these habitats even though drying constraints are relaxed. Surprisingly, we did not observe antipredator responses (reduced activity, large cases, accelerated development) by Asynarchus to Dytiscus, and caddisflies in predator-free ponds also emerged well before drying. This suggests that rapid development in Asynarchus is  a fixed trait, regardless of habitat type. We propose that predictable multiple selection pressures across different types of habitats have led to the canalization of rapid larval development in Asynarchus. The predictability of environmental selection pressures in this case may lead to specialization in life history, rather than the flexibility observed in other species.