Grooming the Kelp Forest

Is there a universal link between otters, urchins and kelp?

Early in the twentieth century, a rampant fur trade devastated sea otter populations along the west coast of North America. Decades later, several of these populations were restored in an effort to conserve the species. This ecological ‘disappearing act’ has provided scientists with a unique opportunity to examine what happens to a marine ecosystem when the apex (top) predator is removed and later reintroduced.

One leading theory suggests that removing sea otters triggers a cascade of changes through lower levels of the food web; specifically, by allowing their key prey, sea urchins, to flourish. The herbivorous urchins systematically graze on enormous amounts of seaweed, collapsing the kelp forest ecosystem and leaving behind only rocky ‘urchin barrens’. Kelp forests have been observed to return following the reintroduction of sea otters, which seems to support the idea that sea otters are a keystone species that plays a critical role in kelp forest ecosystems.

But is this theory of trophic cascade applicable to all kelp forests from Alaska to Washington? Do sea otters always reduce urchin populations and thus promote kelp growth? Or could other factors such as physical disturbance be equally important? What implications might this have for the way otter populations — and the lucrative commercial urchin fishery — are managed and conserved?

In an attempt to answer these and other questions, a group of scientists at the University of Washington recently tested the universality of trophic cascade theory as it applies to sea otters and kelp forests. Their study, authored by Sarah Carter, Glenn VanBlaricom and Brian Allen, was recently published in the journal Hydrobiologia.

Analyzing urchins
At a sheltered study site within San Juan Channel, the scientists simulated three levels of experimental urchin harvest: a monthly removal that mimicked near-constant sea otter predation; an annual removal that simulated the commercial harvest of the largest specimens; and a control site in which no urchins were harvested.

The first objective of the study was to determine whether the real effects of sea otter predation — i.e., those previously observed along outer (unprotected) coastlines — were the same as those simulated in the physically protected waters of San Juan Channel. Secondly, the scientists sought to clarify whether differences in the timing, duration and size-selectivity of otter predation (versus commercial harvesting) had an indirect bearing on the ecology of the kelp forest.

“The two experimental urchin removal treatments did not significantly increase the density of perennial or annual species of macroalgae [kelp] after two years, despite significant and persistent decreases in urchin densities,” the authors write, pointing to a litany of other factors that may have influenced the local ecosystem, including: grazing by other invertebrates, the density of certain perennial kelp species such as Agarum, and the frequency at which populations of kelp and invertebrates reproduce and grow.

These factors, they note, may significantly influence community structure in San Juan Channel as well as other physically protected marine waters that feature sea otters. Thus if the Washington sea otter population were to expand into San Juan Channel, the authors suggest it would likely not cause the same dramatic changes in kelp forest ecosystems that have been documented along exposed coastlines.

The critical difference between these habitats hinges on the relative composition of Agarum, a species of macroalgae. Agarum tends to form denser stands in sheltered areas than it does along turbulent, exposed coastlines. An established Agarum stand uses shading, abrasion and possibly chemical defenses to limit the growth of competing algae species and to resist grazing invertebrates, including urchins. Because of this chemical defense, a sheltered kelp forest (with dense Agarum stands) is less vulnerable to increases in urchin population than an exposed kelp forest, which is typically more diverse because of Agarum’s inability to tolerate heavy wave action. Thus the grazing of non-defended algae by urchins in these areas is more widespread and significant.

Conservation concerns
The question remains: are otters a keystone species? The results of this study support the suggestion that sea otters are only one of several factors that may regulate urchin populations in nearshore waters. They also refute the assertion that the sea otter–trophic cascade paradigm is universally applicable across locations or habitat types. From a conservation perspective, the authors encourage managers to ensure that any otter-urchin management policies are built on sound, reproducible science that is specific to the geographical region in question.

With their gradual re-emergence in west coast kelp forests, sea otters continue to provide scientists with ample opportunity to study the intricate relationship between this apex predator and its complex environment.

24 April 2007

Publication:
Testing the generality of the trophic cascade paradigm for sea otters: a case study with kelp forests in northern Washington, USA. Carter, S.K., G.R. VanBlaricom and B.L. Allen. 2007. Hydrobiolgia 579:233-249. abstract Trophic cascade hypotheses for biological communities, linking predation by upper trophic levels to major features of ecological structure and dynamics at lower trophic levels, are widely subscribed and may influence conservation policy. Few such hypotheses have been evaluated for temporal or spatial generality. Previous studies of sea otter (Enhydra lutris predation along the outer coast of North America suggest a pattern, often elevated to the status of paradigm, in which sea otter presence leads to reduced sea urchin (Strongylocentrotus spp.) biomass and rapid increases in abundance and diversity of annual algal species, followed by a decline in diversity as one or a few perennial algal species become dominant. Both sea otter predation and commercial sea urchin harvest are ecologically and economically important sources of urchin mortality in nearshore benthic systems in northern Washington marine waters. We recorded changes in density of macroalgae in San Juan Channel, a marine reserve in the physically protected inland waters of northern Washington, resulting from three levels of experimental urchin harvest: (1) simulated sea otter predation (monthly complete harvest of sea urchins), (2) simulated commercial urchin harvest (annual size-selective harvest of sea urchins), and (3) no harvest (control). The two experimental urchin removal treatments did not significantly increase the density of perennial (Agarum and Laminaria) or annual (Desmarestia, Costaria, Alaria and Nereocystisi) species of macroalgae after 2 years, despite significant and persistent decreases in urchin densities. Our results suggest that other factors such as grazing by other invertebrates, the presence of dense Agarum stands, and recruitment frequency of macroalgae and macroinvertebrates may play a large role in influencing community structure in San Juan Channel and other physically protected marine waters within the range of sea otters.