When Carnivores Clash: Evidence of Competition and Prey-shifting by Brown Bears During a Period of Sea Otter Recovery

Sea otters are an important component of the northeastern Pacific nearshore ecosystem because when present, they have dramatic effects on nearshore subtidal and intertidal community structure and dynamics. However, commercial exploitation in the 18^th and 19^th centuries eliminated sea otters from the majority of their historic range, allowing their invertebrate prey populations to proliferate. Prior to substantial recovery of the sea otter population along the coast of Katmai National Park and Preserve within southcentral Alaska, brown bears utilized the abundant intertidal clam resources available here. However, in recent decades, the Katmai sea otter population has increased by approximately seven-fold. In the early 1990’s, brown bears along the Katmai coast were commonly observed foraging on clams in the intertidal. At that time, the Katmai sea otter population was just beginning its recovery and was estimated to be less than 1000 animals. By 2006 the sea otter population had grown to approximately 7000 animals and appears to have peaked or stabilized, with clams being their primary prey. Since 2006, sea otter energy recovery rates have declined suggesting the otter population is approaching carrying capacity. In contrast, brown bears monitored from 2015-2017 showed reduced utilization of intertidal clam resources. Our results suggest sea otters out-competed bears for intertidal resources by reducing clam abundance (and thus the calories bears could consume per unit time) to the point where most bears switched to other resources. With sea otters restored to the ecosystem, indirect effects on the nearshore community likely include increased canopy and understory kelps, which could enhance salmon smolt rearing habitat and positively influence availability of salmon, the single most important food resource of Katmai brown bears. This work adds to a growing body of evidence for the cascading ecosystem effects of top-level carnivores and highlights interactions between top-level marine and terrestrial predators that have not been documented elsewhere.