Kristen Weiss: Sea otters, kelp and ocean tipping points
Through September 2, Monterey Bay Aquarium and Monterey Bay National Marine Sanctuary are hosting Big Blue Live – an unprecedented series of live natural history broadcasts from PBS and the BBC. Big Blue Live highlights the remarkable marine life that gathers in Monterey Bay each summer, and celebrates an ocean conservation success story of global significance. We’re publishing guest commentaries about conservation efforts that contribute to the health of the bay and our ocean planet. This is from Kristen Weiss, an early career science fellow at the Center for Ocean Solutions. The Center is a collaboration among the Stanford Woods Institute for the Environment and Hopkins Marine Station of Stanford University, the Aquarium and the Monterey Bay Aquarium Research Institute.
The story of sea otter loss and recovery has had dramatic consequences for the health of Monterey Bay’s kelp forests. Less than 100 years ago, sea otters were thought to be extinct along the California coast as the result of rampant overhunting by fur traders. While otter hunting was officially banned in 1911, there seemed little hope of recovery at the time.
Then, in 1938, a small population of otters was discovered off the Big Sur coast just south of Monterey. Since then, sea otters have made a modest comeback (about 3,000 individuals) thanks to their protected status.
They’re now a common sight in the Monterey Bay National Marine Sanctuary where they have helped catalyze the regrowth of kelp habitat. As Dr. Steve Palumbi of Stanford University’s Hopkins Marine Station writes in his book The Death and Life of Monterey Bay, once otters recolonized Monterey Bay “they fed happily on sea urchins, and they left luxuriant kelp forest growing in their wake.”
Kelp Forests reach a tipping point
In nature, one plus one does not always equal two. Sometimes, small changes in human pressures or environmental conditions can result in disproportionately large responses in the ecosystem – potentially even collapse. Ecosystems can respond to stressors in nonlinear ways, displaying ecological thresholds (also known as “tipping points”) beyond which systems undergo dramatic change. The Center for Ocean Solutions is a collaborator in the Ocean Tipping Points project that aims to understand and predict where ecological thresholds might exist in marine habitats such as kelp forests.
As was the case in Monterey Bay, the loss of sea otters typically marks an abrupt tipping point for kelp forest habitat. As a keystone species, otters maintain kelp habitat by eating sea urchins, the main consumers of kelp. In the absence of otters, urchin populations can grow unchecked, their out-of-control grazing undermining kelp forests and creating “urchin barrens” devoid of the shelter and biodiversity that kelp ecosystems typically offer. Where kelp once harbored diverse assemblages of juvenile and adult fishes, invertebrates like urchins and shellfish now dominate a simplified habitat.
When such tipping points occur, the distribution of ecosystem benefits to humans can shift considerably. For example, kelp habitats support important commercial fish species and attract diving and snorkeling tourism. However, in the absence of otters, urchin fishermen often gain substantial benefits and may be opposed to management interventions aimed at otter reestablishment. These types of trade-offs highlight the difficulty of balancing social and ecological values in marine management.
To help managers address social and ecological complexity, Ocean Tipping Points project collaborators have outlined seven principles for managing ecosystems that are prone to tipping points (see infographic below), so that managers can better predict and prevent unwanted tipping points.
Hope for sea otters in Monterey Bay
In Monterey Bay, marine managers, scientists, and conservationists are working to promote sea otter recovery through research and active management. The Monterey Bay Aquarium’s Sea Otter Program has been active for over 30 years, conducting important research on sea otters as well as caring for injured or stranded otters. To maintain healthy populations of otters, we need healthy marine environments.
At the Center for Ocean Solutions, researchers are working on projects such as the Kelp Forest Array and Environmental DNA to collect important information about water quality and species biodiversity in kelp forest habitat. These projects are helping identify what human-caused and natural threats may be impacting the bay so that we can better protect sea otter habitat into the future.
The expansion of sea otters along the Monterey coastline “left behind a string of changed shorelines and restored bay,” writes Palumbi. “Where there was once subtidal rock bristling with urchin spines, there now bloomed kelp forest with sea urchins and abalone restricted to crevices in the rock. Where kelp bloomed, there now thrived a bustling community of fish and invertebrates.”
While sea otters still have a long way to go to reach numbers comparable to historical population sizes, their initial recovery along California’s central coast and the related comeback of healthy kelp forest habitat here in Monterey Bay – offers hope for other species and regions affected by significant human activity.