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Sea Otters Protect Marine Grass From Invasive Algae By Munching Troublesome Crabs; Kick-Start Chain Reaction Called 'Trophic Cascade'

By Rebekah Marcarelli r.marcarelli@hngn.com | Aug 26, 2013 03:54 PM EDT

Fed Offers $21,000 Reward to Catch Sea Otter Killer
The US Fish and Wildlife Service (FWS) announced on Friday that they will be giving a $21,000 reward for anyone who could help find those responsible in the killing three southern sea otters. (Photo : Reuters)

Otters may have helped crucial California sea grass make a comeback.

Nutrient runoff from farms and urban areas has spurred the excessive growth of algae, which can cover sea grass leaves and starve them of sunlight, a University of California, Santa Cruz press release reported. The sea grass meadows are important habitats for fish and other species, and a loss of the vital marine greens could be devastating.

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Sea grass beds host a number of fish including:  "Pacific herring, halibut, and salmon." They also protect shorelines from extreme weather conditions, and soak up climate change-causing greenhouse gases.

"When we see seagrass beds recovering, especially in a degraded environment like Elkhorn Slough, people want to know why," Brent Hughes, a Ph.D. candidate in ecology and evolutionary biology at UC Santa Cruz and first author of the study, said.

 Elkhorn Slough is one of "California's largest estuaries."

When otters started moving into the area in 1984 they prompted a sea grass-saving chain reaction. The otters eat a large number of the estuaries' crabs, which in turn don't eat as many of the algae-grazing sea slugs and other invertebrates.

The otters also save an important crustacean called the Idotea from getting munched on. These small crustaceans do a big part in cleaning algae off the sea grass.

"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.

This type of chain reaction is known as a "trophic cascade" in the science world. Otters have also been observed to provide the same service to kelp forests.

"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. It's also a great reminder that the apex predators that have largely disappeared from so many ecosystems may play vitally important functions," Tim Tinker, a wildlife biologist with the U.S. Geological Survey, who participated in the study, said.

The Elkhorn Slough otter populations have fluctuated over the years, and the algae population has dipped and peaked with it.

"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," Hughes said.

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