The influence of sea otters (Enhydra lutris) on nearshore marine communities has been widely studied in kelp-forest ecosystems, however less is known about their impacts in other systems. In Southeast Alaska, the recolonization by sea otters into areas where they have been long absent (~100 years), provides a natural experiment with which to examine the response of soft-substrate intertidal prey communities to the return of a top predator. Sea otter abundance in Glacier Bay, Alaska increased from zero in 1993 to >8,500 individuals in 2012. Their aggregated diet from intertidal foraging bouts consisted predominantly of clam (56%), horse mussels (15%) and urchins (18%) and was found to diversify over space and time as sea otters colonized and persisted. To assess intertidal community response to sea otters 45 intertidal sites were sampled prior to, or concurrent with sea otter colonization, and again up to 12 years later. Using GIS and aerial surveys I estimated the cumulative density of the sea otters at each site. At a sea otter density of roughly 6-otters/km^{2 clam populations were found to have 50% probability of population decline, and preferred prey species such as Saxidomus giganteus were found to likely decline at sea otter densities <2-otters/km2. At sites where sea otters were at low densities or absent, clam biomass generally remained unchanged or increased. Energy recovery rates decreased significantly with cumulative otter density from roughly 20 kcal/min at 0.36 otters/km2 to 9 kcal/min at 4.54 otters/km2, requiring individual otters to increase the amount of energy they expend foraging. The effect of sea otters on intertidal clam populations in soft-sediment was consistent with findings in other habitats, where reduced densities and sizes of prey were documented. This study established the occurrence of top-down structuring in soft-sediment systems over a multi-decadal time scale.}

Case

Animal space use determines access to resources, such as food and mates, and has implications for animal mating systems. Polygynous mating systems emerge when males, who typically exhibit low parental investment, monopolize females, who typically exhibit high parental investment, by defending patches of resources or aggregations of females. Defense of resource-containing territories incurs energetic costs and detracts time from essential activities such as foraging. Understanding mating systems is therefore key to interpreting processes that limit population growth in threatened species such as sea otters. Reproductive success of male sea otters may be related to defense of small territories (2–6 km2) in shallow coastal waters. The absence of strong seasonal synchrony in female estrus requires that males defend territories year-round, suggesting a high cost to territory defense. My objectives were to: (1) characterize male space use, (2) quantify male reproductive success, and (3) describe the sea otter mating system. This research provided a new method of estimating animal space use in restricted habitats. The method estimates the probability of space use based on features of the physical environment (e.g. water depth) and effectively excludes unsuitable areas from home range estimates. I applied this method to VHF radio-telemetry data of 72 male sea otters in California and distinguished between three distinct space-use tactics, constituting territorial, satellite, and transient tactics. Tissue samples were collected from 67 males and 215 females in Monterey, California in 1999–2012. I assigned 37 microsatellite nuclear DNA markers to these samples, conducted paternity analyses, and provided the first estimate of male reproductive success in sea otters. Paternity analyses matched 40 father-pup pairs. Paternity assignments were low overall; individual males were assigned a maximum of 3 pups. Reproductive success peaked at seven years of age, and continued to 14. Territorial males who defended abundant kelp canopy experienced elevated siring success, but satellite males also sired pups. The sea otter mating system is thus characterized by resource defense polygyny. This work provides a foundation for synthesizing the energetic and reproductive tradeoffs of space-use tactics and better understanding the population biology of sea otters.

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