Predictable and severe disturbance from an ecosystem engineer controls aquatic insect phenology

Disturbance regimes are an important selective agent. In streams, for example, floods have been found to drive behavioral, morphological, and life-history adaptations. The activities of certain ecosystem engineers can be a substantial component of disturbance regimes. For example, in some coastal streams in western North America, salmon consistently disturb stream benthic habitats during nest-digging. We investigated if this bioturbation has led to local adaptation of aquatic insect life-histories. Specifically, we predicted that in streams with high salmon densities, the life-histories of aquatic insects are synchronized to salmon spawning, emerging from aquatic nymphs to terrestrial adults prior to salmon spawning, minimizing their exposure to bioturbation. We monitored emergence timing of aquatic insects in 11 streams in southwestern Alaska across a gradient in salmon densities. In streams with high densities of salmon, dominant aquatic insect taxa emerged immediately prior to salmon spawning but emerged approximately 4 weeks later in streams with low densities of salmon. Date of peak emergence of the most abundant mayfly taxa (Cinygmula spp) was significantly related to salmon density, with stream temperature explaining much of the remaining variance. These shifts in emergence timing were maintained in a common garden experiment—mayflies from a stream with high salmon densities still emerged significantly earlier than those from a stream with low salmon densities even in the absence of different environmental cues. These data suggest that spawning salmon can drive local adaptation of aquatic insect life-histories. Therefore, dominant ecosystem engineers can be an important selective force.