Geology topics

Andrew B. Johnson

Publications and source records attributed to Andrew B. Johnson.

2 recordsLinked to original sources

USGS2019

Surrogate rearing a keystone species to enhance population and ecosystem restoration

Translocation and rehabilitation programs are critical tools for wildlife conservation. These methods achieve greater impact when integrated in a combined strategy for enhancing population or ecosystem restoration. From 2002-2016, we reared 37 orphaned southern sea otter (Enhydra lutris nereis) pups, using captive sea otters as surrogate mothers, then released them into a degraded coastal estuary. As a keystone species, observed increases in the local sea otter population unsurprisingly brought many ecosystem benefits. The role that surrogate-reared otters played in this success story, however, remained uncertain. To resolve this question, we developed an individual-based model (IBM) of the local population using surveyed individual fates (survival and reproduction) of surrogate-reared and wild-captured otters, and modeled estimates of immigration. Estimates derived from a decade of population monitoring indicated that surrogate-reared and wild sea otters experienced similar reproductive and survival rates. This was true for males and females, across all ages (1-13 years) and locations evaluated. The IBM simulations indicated that reconstructed counts of the wild population are best explained by surrogate-reared otters combined with low levels of unassisted immigration. In addition, the model shows that 55% of observed population growth over this period is attributable to surrogate-reared otters and their wild progeny. Together, our results indicate that the integration of surrogacy methods and reintroduction of juvenile sea otters helped establish a biologically successful population and restore a once-impaired ecosystem.

California↗

USGS2018

Gaps in kelp cover may threaten the recovery of California sea otters

Despite more than a century of federal protection, the California sea otter Enhydra lutris nereis remains threatened under the U.S. Endangered Species Act (ESA), and the population has not appreciably expanded its range in two decades. Here, we examine a novel dataset of 725 sea otter live strandings from 1984–2015 to gain insights into demographic and environmental factors underlying threats to sea otter recovery. Using multinomial logistic regression to evaluate spatiotemporal patterns of stranding causes, we demonstrate that increases in stranding rates, particularly outside the range center, are related to a substantial increase in shark bites. By contrast, trauma linked to human activities has declined dramatically, and now accounts for less than 5% of stranding cases. Within the range core, where the sea otter population seems regulated by prey availability, symptoms of energetic stress represent more than 63% of all strandings and are strongly associated with high sea otter density. Conversely, in range peripheries, the majority of strandings are caused by shark bite and neurological disease. Notably, these threats are virtually absent where nearshore habitat is characterized by at least 10% kelp canopy cover. Our analyses reveal that declining kelp cover may therefore constrain the population's spatial expansion and recovery in two key ways. Absence of kelp intensifies density‐independent threats in the range peripheries, and likely limits dispersal of reproductive females, which depend on kelp canopy for nursery habitat. These results highlight the significance of both top‐down and bottom‐up processes in population dynamics, and inform an ecosystem‐based approach to conservation planning.

California↗