Sex and occupation time influence niche space of a recovering keystone predator

Predators exert strong effects on ecological communities, particularly when they re-occupy areas after decades of extirpation. Within species, such effects can vary over time and by sex and cascade across trophic levels. We used a space-for-time substitution to make foraging observations of sea otters (Enhydra lutris) across a gradient of reoccupation time (1-30 years), and nonmetric multidimensional scaling (nMDS) analysis to ask whether (a) sea otter niche space varies as a function of occupation time and (b) whether niche space varies by sex. We found that niche space varied among areas of different occupation times. Dietary niches at short occupation times were dominated by urchins (Mesocentrotus and Strongylocentrotus spp; >60% of diets) in open habitats at 10-40 m depths. At longer occupation times, niches were dominated by small clams (Veneroida; >30% diet), mussels (Mytilus spp; >20% diet), and crab (Decapoda; >10% diet) in shallow (<10 m) kelp habitats. Diet diversity was lowest (H' = 1.46) but energy rich (~37 kcal/min) at the earliest occupied area and highest, but energy poor (H' = 2.63, ~9 kcal/min) at the longest occupied area. A similar transition occurred through time at a recently occupied area. We found that niche space also differed between sexes, with bachelor males consuming large clams (>60%), and urchins (~25%) from deep waters (>40 m), and females and territorial males consuming smaller, varied prey from shallow waters (<10 m). Bachelor male diets were less diverse (H' = 2.21) but more energy rich (~27 kcal/min) than territorial males (H' = 2.54, ~13 kcal/min) and females (H' = 2.74, ~11 kcal/min). Given recovering predators require adequate food and space, and the ecological interactions they elicit, we emphasize the importance of investigating niche space over the duration of recovery and considering sex-based differences in these interactions.