Sea Otters Indirectly Promote Recovery of Seagrass Beds by Snacking on Crabs
Scientists have discovered that the recolonization of sea otters in one of California's largest estuaries has promoted the recovery of seagrass.
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The new study led by researchers at the University of California, Santa Cruz, states that there is a worldwide decline in seagrass meadows due to nutrient runoffs from farms and urban areas that spur the growth of algae on seagrass leaves and deprive them of sunlight. Seagrass meadows provide habitat for fish and protect the shorelines against extreme weather.
They also absorb the greenhouse causing gases.
"When we see seagrass beds recovering, especially in a degraded environment like Elkhorn Slough, people want to know why," said Brent Hughes, a Ph.D. candidate in ecology and evolutionary biology at UC Santa Cruz.
In 1984, when sea otters started moving back into the Elkhorn Slough, they indirectly started promoting the recovery of seagrass beds by snacking on large amount of crabs. By reducing the number of crabs in the water, population of grazing invertebrates like sea slugs grows in number. They feed on the algae that are thriving on the seagrass leaves and help in keeping them clean and healthy. The reduction in crab population benefits not just the sea slugs but also Idotea, a small crustacean, which is a crucial algae grazer.
"The seagrass is really green and thriving where there are lots of sea otters, even compared to seagrass in more pristine systems without excess nutrients," Hughes said.
The researchers refer to this type of food chain as 'trophic cascade'. Otters are known to benefit kelp forests in a similar manner. They prey on animals that graze on kelp. But this study focuses on the crucial role sea otters play in the estuarine ecosystem.
"This provides us with another example of how the strong interactions exerted by sea otters on their invertebrate prey can have cascading effects, leading to unexpected but profound changes at the base of the food web," Hughes said. "It's also a great reminder that the apex predators that have largely disappeared from so many ecosystems may play vitally important functions."
The population of sea otters in Elkhorn has fluctuated over the years and along with it the algae population also declined and grew, source HNGN.
Hughes concluded saying, "These are important coastal ecosystems that we're losing, and mostly that's been associated with bottom-up effects like nutrient loading. This study shows that these ecosystems are also being hit by top-down forces due to the loss of top predators."
The study will be published in the Proceedings of the National Academy of Sciences.