Temperature-Associated Population Diversity in Salmon Confers Benefits to Mobile Consumers

Habitat heterogeneity can generate intraspecific diversity through local adaptation of populations. While it is becoming increasingly clear that population diversity can increase stability in species abundance, little is known of how population diversity can mediate important interactions among species. Here we demonstrate cascading effects of thermal heterogeneity on trout - salmon interactions in streams. Water temperature in a third order stream varied spatially from 5^oC to 17.5^oC and spawning sockeye salmon showed local adaptation to this thermal heterogeneity.  Individuals that spawned early in cool regions of the 5 km stream ecosystem were genetically differentiated from those spawning later in the season in warmer regions. Sockeye spawning generates a pulsed resource subsidy that accounts for the majority of seasonal growth in stream-dwelling rainbow trout and other consumers. The spatial and temporal structuring of sockeye spawn timing in our focal stream extended the duration of the pulsed subsidy compared to a thermally homogeneous stream with a single population of salmon, and rainbow trout adopted movement strategies that exploited these multiple pulses of egg subsidies. Fish that moved to track the resource pulse had growth rates up to 2.5 times higher those that remained stationary or trout in the reference stream with a single seasonal pulse of eggs. Our results demonstrate that habitat heterogeneity can have important effects on the population diversity of dominant species, in turn influencing their interactions with other species. Therefore, habitat homogenization may have farther reaching ecological effects than previously considered.