Species recovery and recolonization of past habitats: lessons for science and conservation from sea otters in estuaries.

Kerstin Wasson,Tracy Grimes ,Brian R Silliman,Katharyn E Boyer,Robert Scoles,Ellen Hines,Jane Rudebusch,Benjamin H Becker,Susan L Williams,Lilian Carswell ,M Tim Tinker,Ron Eby,Kathryn Beheshti ,Michael W Beck,Lisa A Needles ,Margot Hessing‐Lewis ,Michelle M Staedler ,Brent B Hughes,Sarah Espinosa,Joseph A Tomoleoni,Erin U Foster

doi:10.7717/peerj.8100

Abstract

Recovering species are often limited to much smaller areas than they historically occupied. Conservation planning for the recovering species is often based on this limited range, which may simply be an artifact of where the surviving population persisted. Southern sea otters (Enhydra lutris nereis) were hunted nearly to extinction but recovered from a small remnant population on a remote stretch of the California outer coast, where most of their recovery has occurred. However, studies of recently-recolonized estuaries have revealed that estuaries can provide southern sea otters with high quality habitats featuring shallow waters, high production and ample food, limited predators, and protected haul-out opportunities. Moreover, sea otters can have strong effects on estuarine ecosystems, fostering seagrass resilience through their consumption of invertebrate prey. Using a combination of literature reviews, population modeling, and prey surveys we explored the former estuarine habitats outside the current southern sea otter range to determine if these estuarine habitats can support healthy sea otter populations. We found the majority of studies and conservation efforts have focused on populations in exposed, rocky coastal habitats. Yet historical evidence indicates that sea otters were also formerly ubiquitous in estuaries. Our habitat-specific population growth model for California’s largest estuary—San Francisco Bay—determined that it alone can support about 6,600 sea otters, more than double the 2018 California population. Prey surveys in estuaries currently with (Elkhorn Slough and Morro Bay) and without (San Francisco Bay and Drakes Estero) sea otters indicated that the availability of prey, especially crabs, is sufficient to support healthy sea otter populations. Combining historical evidence with our results, we show that conservation practitioners could consider former estuarine habitats as targets for sea otter and ecosystem restoration. This study reveals the importance of understanding how recovering species interact with all the ecosystems they historically occupied, both for improved conservation of the recovering species and for successful restoration of ecosystem functions and processes.

Highlights

Widespread declines of top predator populations have profoundly altered ecological relationships, resulting in the degradation or collapse of many ecosystems (Jackson et al, 2001; Estes et al, 2011; Ripple et al, 2014)
State-space model of sea otter growth and equilibrium abundance in a California Estuary At the present time Elkhorn Slough is the only estuary in California in which a distinct and self-sustaining population of sea otters has become established (Kvitek et al, 1988; Maldini et al, 2012; Hughes et al, 2013; Eby et al, 2017), with continued range expansion it is likely that other estuaries will eventually be colonized given the current population is
Historical research effort on estuaries as sea otter habitat While many studies continue to be published on outer coast sea otters, our Web of Science search resulted in few investigations examining estuarine sea otters (Fig. 1B; Dataset S1; Code S2), and most of the estuarine studies were based on northern sea otters (E. lutris kenyoni)

Summary

Introduction

Widespread declines of top predator populations have profoundly altered ecological relationships, resulting in the degradation or collapse of many ecosystems (Jackson et al, 2001; Estes et al, 2011; Ripple et al, 2014). Because many of these losses occurred well before systematic ecological studies could document their effects, our understanding of ecological baselines has been fundamentally skewed (Jackson et al, 2001). Sea otters (Enhydra lutris) provide an excellent example of a top predator whose decline has dramatically altered ecological relationships. As sea otters have recovered from near extinction over the past decades, they have been the subject of intensive research aimed at furthering their recovery and understanding more fully their role as keystone predators in coastal ecosystems

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Discussion

Conclusion