Barriers to salmon migration impact body condition, offspring size, and life history variation in an avian consumer

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The impacts of barriers, such as dams, and associated stream effects on the migration of salmon have been well documented, but there is much less information on consequences for terrestrial vertebrates in these freshwater systems. Salmon themselves provide food resources to higher trophic levels and deposit rich marine-derived nutrients (MDN) that benefit the base of freshwater food webs. We studied a higher-order terrestrial consumer of aquatic prey, the American dipper Cinclus mexicanus, in a riparian food web, to test the hypothesis that barriers to salmon migration negatively impact body condition, and ultimately life history. We predicted that, compared to dippers breeding above barriers, those in areas with salmon would receive MDN and thus be in better condition, have greater breeding effort (frequency of multiple brood attempts), produce larger offspring in better condition, be more likely to maintain year-round territories, and have higher annual survival. Using stable isotopes as signatures of MDN, we found that 13C and 15N were more enriched in dippers below barriers, confirming access to MDN. Female dippers below barriers had greater mass corrected for body size, were more likely to attempt multiple broods within a season and produced larger female offspring. Furthermore, dippers below barriers were more likely to maintain year-round territories and had higher annual survival. Our results demonstrate how river barriers such as dams influence vital rates and life histories of higher-order consumers.

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