Top-down effects of sea otters mitigate negative effects of nutrient loading

A California Sea Otter enjoying its sea grass habitat.

A California Sea Otter enjoying its sea grass habitat.

In the latest sea otter science development, it has been found that sea otters contribute to the resilience of sea grass ecosystems.

Published in the Proceedings of the National Academy of Sciences (PNAS) by Hughes et al. (including Tim Tinker), the study examines the role of sea otters in a highly nutrient-loaded estuary in California. Hughes et al. found that through a 4-part trophic cascade, the sea otter mitigates the negative effects of nutrient loading.

A common question in ecology is whether an ecosystem is driven primarily by “top-down” or “bottom-up” processes. Top-down processes include things like predation or competition that essentially “trickle down” to affect lower trophic levels. Two classic examples in marine ecology are sea stars as a keystone predator in the intertidal and sea otters in kelp forests. Robert Paine found that predation by the sea star Pisaster allows for mussels and barnacles, competitors for space in the intertidal, to coexist (Paine, 1974). In another classic study, Estes & Palmisano (1974) found that sea otter predation on sea urchins indirectly leads to increased kelp productivity  by decreasing herbivory on the kelp. Bottom-up processes affect the lower trophic levels by changing the availability of nutrients and other resources.

Sea-otter-collecting-sea-urchin

Here is the low down on how the 4-part trophic cascade works:

Sea otters in the estuary eat primarily crabs. These crabs eat primarily sea hares (sea slugs) and isopods, which in turn eat algae. In an ecosystem with nutrient loading, the excess of nutrients causes a boom in algae growth. When this happens in a sea grass ecosystem, this algae covers the sea grass blades and prevents them from receiving sunlight and causes them to die. Without the sea otters, there would be more (and larger) crabs, less isopods & sea hares, and therefore more harmful algae and less healthy sea grass.

A few reasons why I find this paper particularly interesting:

1) Sea otters are traditionally thought to prefer kelp canopy as habitat, and are not traditionally thought to be an important player in sea grass ecosystems. This study demonstrates that they are not only important, but that they also drive the ecosystem through top-down effects.

2) Top-down effects of sea otters selectively consuming crabs dominates the bottom-up effects of eutrophication in shaping the sea grass ecosystem.

3) Sea otter diet specialization may have wide scale implications for the marine ecosystem as a whole,  which is something that I am examining currently with sea otters off the Washington coast.

Literature

Hughes et al. 2013. Recovery of a top predator mediates negative eutrophic effects on seagrass. http://www.pnas.org/content/early/2013/08/23/1302805110

Paine, 1974. Intertidal Community Structure. Experimental Studies on the Relationship between a Dominant Competitor and Its Principal Predator.  http://www.jstor.org.offcampus.lib.washington.edu/stable/4214949

Estes & Palmisano, Sea Otters: Their Role in Structuring Nearshore Communities. http://www.jstor.org.offcampus.lib.washington.edu/stable/1738455

Sea Otters: Your Defence Against The Algal Apocalypse by Ed Yong for National Geographic

Sea otter return boosts ailing seagrass in California on BBC News

Otters Aid Seagrass Recovery by Conservation Magazine

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