Recovering Apex Predator Lessens Nutrient-flooding Affects on Seagrass

For coastal vegetation it has been shown that nutrient loading (the accumulation of excess nutrients perhaps through runoff processes) can lead to major disturbances of primary producers, sometimes leading to the loss of dominant vegetation like sea grasses. In the unusual four-level trophic cascade present in Elkorn Slough, recent studies have shown that restoring an apex (or top) predator can mitigate these bottom-up influences.

Nutrient loading is considered to be a bottom-up influence as it directly impacts producers like sea grass which then leads to an influence on those organisms that consume the sea grass; these indirect affects carry on through the trophic levels of the ecosystem, similar to that of a domino effect—knock over one domino and it disturbs the last in the chain.

The producer affected by nutrient loading in the Elkorn Slough—a highly nutrient-loaded estuary on the coast of California—is the sea grass commonly referred to as eelgrass (Zostera marina). The diminishing apex predator, reintroduced into this ecosystem to combat nutrient-flooding, is the sea otter (Enhydra lutris).

 

Image

Sea otter floating in California bay 21 February 2007. Image by “Mike” Michael L. Baird.

 

 

The sea otter colonized Elkorn Slough in 1984 in a time that eelgrass was at an all-time low. Prior to 1984, eelgrass had steadily been decreasing as nutrient levels rose—not uncommon.

However, once sea otters inserted themselves into the system an anomaly occurred. Despite severe nutrient-loading and an expected dramatic loss in eelgrass, the Elkorn Slough sea grass actually expanded.

The eelgrass bed experienced a 600% increase following initial sea otter introduction. As Hughes pointed out, “sea otter densities were significantly correlated with extent of eelgrass.”

Although a remarkable finding, this abnormality puzzled scientist. How could an apex predator like the sea otter not only mitigate nutrient loading on sea grass but actually help these producers expand?

The truth lies within the otter’s diet of marine invertebrates like sea urchins, various molluscs and crustaceans, and some fish. Specifically, the crab is a crustacean that both feeds on eelgrass and is food to the sea otter.

With the influx of otters there was “a significant decline in the biomass and size of crabs in the estuary” (Hughes, et al.). Through these indirect means (preying on crabs), sea otters lessened the predation threat experienced by eelgrass.

How could such a crab population decline be attributed to only the sea otter?  Scientist claim sea otters were most likely the cause of declines in crab populations because sea otters were expanding during a period when other crab predators, such as sharks and rays, were in a state of decline, due to overfishing.

It makes sense then that predation on crabs increased with sea otter population growth, but so should the rate that otters are preyed upon too—so one would think. The truth is that sea otters are not really preyed on.

Sea otters pungent scent glands make them quite unappealing to eat. This, however, does not stop the sheer killing of some otters by their limited predators: the great white shark, killer whales, and sea lions.

Therefore, with the colonization of the Elkorn Slough by sea otters, crabs were greatly reduced in not only biomass but also size, resulting in lesser nutrient-loading felt by eelgrass. Thus (re)colonizing a region with an apex predator can mitigate bottom-up influences.

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