Abstract
Characterizing the habitat associated with predation events can inform on predator-prey dynamics. Despite being evaluated extensively in terrestrial systems, quantifying and characterizing the role predation plays in upper trophic marine ecosystems is challenging due to the cryptic nature of pelagic predators and the difficulty of observing predatory behavior. We developed a multi-step method to characterize habitat associated with predation that integrates data from post-mortem pop-up style mortality transmitters, and data from traditional external tracking devices, both of which use the Argos satellite system. In our case study with juvenile Steller sea lions (SSL, Eumetopias jubatus) in the Gulf of Alaska, 20 mortality events were previously described, of which 18 were attributed to predation. The locations of 13 of these at-sea predation events with post-mortem tracking data were estimated, with spatial uncertainty calculated using movement-based approaches of backwards step-length and state-space modeling. We then generated a Mortality Occurrence Probability Distribution (MOPD), resampled points within the MOPD based on isopleth weighting, and extracted habitat variables (i.e. slope, depth, distance to haulout-rookery) associated with these locations. This final dataset represented ‘case’ points (n=115) in terms of predation and was compared to the habitat associated with ‘control’ points (n=1000), locations within juvenile SSL distribution in this region (i.e. population home range), in a resource-selection function (GAM). Predation events were associated with habitats characterized by greater depths and moderate distances from SSL haulouts and rookeries. This information enabled us to generate a risk-map for juvenile SSL in the Gulf of Alaska, spatially representing areas of high predation probability. Our study provides important information about threats to this vulnerable age-class, and establishes a novel approach to characterizing risk in marine ecosystems that can be applied to other management and ecosystem concerns.
Highlights
Predation is one of the primary risks animals must balance when acquiring resources (Lima and Dill, 1990)
We present a case-study exploring the habitat features associated with direct predation risk by apex predators on an upper trophic level marine meso-predator, the juvenile Steller sea lion (SSL, Eumetopias jubatus)
When combined into a Mortality Occurrence Probability Distribution (MOPD), the 10–95% isopleths for the case/mortalities combined covered a total area of 13,230.9 km2 (Table 2, Figures 4A,B)
Summary
Predation is one of the primary risks animals must balance when acquiring resources (Lima and Dill, 1990). A study investigating the mortality sites of roe deer, Capreolus capreolus, found that risk of predation from lynx, Lynx lynx, and risk of human hunting mortality shared divergent relationships with habitat cover (Lone et al, 2014). Such interactions between species, habitat, and consumptive predation risk can inform our understanding of population trajectories (Horning and Mellish, 2012), trophic cascades (Ripple et al, 2016), or the response of ecosystems to rapidly changing environmental conditions
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