Diffusion modeling reveals effects of multiple release sites and human activity on a recolonizing apex predator

Abstract

Background Reintroducing predators is a promising conservation tool to help remedy human-caused ecosystem changes. However, the growth and spread of a reintroduced population is a spatiotemporal process that is driven by a suite of factors, such as habitat change, human activity, and prey availability. Sea otters (Enhydra lutris) are apex predators of nearshore marine ecosystems that had declined nearly to extinction across much of their range by the early 20th century. In Southeast Alaska, which is comprised of a diverse matrix of nearshore habitat and managed areas, reintroduction of 413 individuals in the late 1960s initiated the growth and spread of a population that now exceeds 25,000. Methods Periodic aerial surveys in the region provide a time series of spatially-explicit data to investigate factors influencing this successful and ongoing recovery. We integrated an ecological diffusion model that accounted for spatially-variable motility and density-dependent population growth, as well as multiple population epicenters, into a Bayesian hierarchical framework to help understand the factors influencing the success of this recovery. Results Our results indicated that sea otters exhibited higher residence time as well as greater equilibrium abundance in Glacier Bay, a protected area, and in areas where there is limited or no commercial fishing. Asymptotic spread rates suggested sea otters colonized Southeast Alaska at rates of 1–8 km/yr with lower rates occurring in areas correlated with higher residence time, which primarily included areas near shore and closed to commercial fishing. Further, we found that the intrinsic growth rate of sea otters may be higher than previous estimates suggested. Conclusions This study shows how predator recolonization can occur from multiple population epicenters. Additionally, our results suggest spatial heterogeneity in the physical environment as well as human activity and management can influence recolonization processes, both in terms of movement (or motility) and density dependence.

Macronutrient composition of sea otter diet with respect to recolonization, life history, and season in southern Southeast Alaska

“…Foraging records for the areas where sea otters had been present the longest (zone 1, colonized for >30 years) had an EIR (5.7 kcal/min) that was comparable to previous studies where sea otters were at carrying capacity (Coletti et al, 2016 ; Rechsteiner et al, 2019 ; Tinker et al, 2008 , 2021 ). This shows that the sea otters in zone 1 of POW are likely at or near carrying capacity, which is in line with modeling from aerial survey data from the region (Eisaguirre et al, 2021 ; Tinker, Gill, et al, 2019 ).…”

“…Foraging records for the areas where sea otters had been present the longest (zone 1, colonized for >30 years) had an EIR (5.7 kcal/min) that was comparable to previous studies where sea otters were at carrying capacity (Coletti et al, 2016;Rechsteiner et al, 2019;Tinker et al, 2008Tinker et al, , 2021. This shows that the sea otters in zone 1 of POW are likely at or near carrying capacity, which is in line with modeling from aerial survey data from the region (Eisaguirre et al, 2021;Tinker, Gill, et al, 2019). When considering sea otter diets and how sea otter invertebrate removal can affect the nearshore system, it is important to look at diet variation across recolonization zones as diets in the newly colonized areas were less diverse, which is similar to past studies in Alaska (Hoyt, 2015).…”

“…More frequent surveys are needed to better understand the population size and distribution. There was an 11-year gap between the current Southeast Alaska population surveys and the one previous (Eisaguirre et al, 2021;Schutte et al, 2023;Tinker, Gill, et al, 2019). Because of the cost-prohibited nature of large-scale survey efforts, more frequent surveys on regional levels may be the most efficient route to monitor the changing sea otter distribution.…”

Range reexpansion after long stasis: Italian otters (Lutra lutra) at their northern edge

“…One of the most studied cases is the range expansion of the sea otter ( Enhydra lutria ) across North Pacific following translocations (Tinker et al, 2008 ). Mechanistic models of ecological diffusion have been used to estimate spatiotemporal changes in occupancy and abundance of this gregarious carnivore from counts of individuals (Eisaguirre et al, 2021 ; Lubina & Levin, 1988 ; Williams et al, 2017 , 2019 ).…”

Revealing the extent of sea otter impacts on bivalve prey through multi‐trophic monitoring and mechanistic models

“…Glacier Bay, a marine protected area with limited commercial fishing (Nielsen & Seitz, 2017) and no sea otter harvest, is unique in Southeast Alaska (Eisaguirre et al, 2021). Moreover, it is geographically well‐defined, and the subject of extensive long‐term monitoring and stewardship efforts (e.g.…”