Subsidized predation on imperiled species: Gulls, landfills, and salmonids

Background/Question/Methods

Subsidies can disrupt predator-prey relationships. Specifically, when a novel food source subsidizes the predator, predator populations can increase and can potentially depress prey species to unsustainable levels. The consequences of subsidized predation are further exacerbated when the prey species is already imperiled. Despite such a possibility, there remains little understanding of the role of subsidies in imperiled species conservation. We examined one system—subsidies from landfills to gulls that also feed on threatened steelhead and endangered coho salmon juveniles. In central California, local western gull (Larus occidentalis) populations are increasing, perhaps in part due to subsidies from anthropogenic food sources such as landfills. We conducted stable isotope analyses to examine how gull diets have changed over the last 80 years. We also quantified minimum gull predation rates by scanning nearby western gull nesting areas for salmonid PIT (passive integrated transponder) tags that were regurgitated or defecated by gulls that had eaten a tagged salmonid. Minimum predation rates were used in conjunction with a large-scale experiment to calculate overall consumption rates of salmonids by gulls. To further examine the impact of gull predation on salmonids, we investigated the characteristics of eaten salmonids to identify characteristics that increase susceptibility to predators.

Results/Conclusions

Results from our research suggest western gull predation is a substantial source of mortality for salmonids and anthropogenic sources may be contributing to salmon decline. From 2002 to 2010, the recovery of PIT tags on gull nesting areas in conjunction with estimates of tag transportation to nesting grounds suggest that gulls consume approximately 27% of out-migrating juvenile salmonids. The characteristics of eaten salmonids are characteristics that typically lend to high ocean survival (large size and lagoon rearing life histories), which suggests gull predation is eliminating juveniles that may otherwise be successful returning adults. In addition, stable isotope analysis of historic and contemporary gull feathers indicate gull diets have shifted over the last 80 years. On average, western gulls have shifted away from marine sources such as fish and toward human food, as evidenced by significantly less enriched in δ¹⁵N than in the early 1900’s. Furthermore, isotopic signatures from contemporary gull feathers are also more variable than historic samples suggesting that anthropogenic sources may be increasing available food sources for gulls. This study thus suggests that subsidies may strengthen the top-down impacts of a generalist consumer on their prey, a potentially important but overlooked factor in the conservation of small populations.