Sea Otters Homogenize Mussel Beds and Reduce Habitat Provisioning in a Rocky Intertidal Ecosystem

Abstract

Sea otters (Enhydra lutris) are keystone predators that consume a variety of benthic invertebrates, including the intertidal mussel, Mytilus californianus. By virtue of their competitive dominance, large size, and longevity, M. californianus are ecosystem engineers that form structurally complex beds that provide habitat for diverse invertebrate communities. We investigated whether otters affect mussel bed characteristics (i.e. mussel length distributions, mussel bed depth, and biomass) and associated community structure (i.e. biomass, alpha and beta diversity) by comparing four regions that varied in their histories of sea otter occupancy on the west coast of British Columbia and northern Washington. Mussel bed depth and average mussel lengths were 1.5 times lower in regions occupied by otters for >20 years than those occupied for <5 yrs. Diversity of mussel bed associated communities did not differ between regions; however, the total biomass of species associated with mussel beds was more than three-times higher where sea otters were absent. We examined alternative explanations for differences in mussel bed community structure, including among-region variation in oceanographic conditions and abundance of the predatory sea star Pisaster ochraceus. We cannot discount multiple drivers shaping mussel beds, but our findings indicate the sea otters are an important one. We conclude that, similar to their effects on subtidal benthic invertebrates, sea otters reduce the size distributions of intertidal mussels and, thereby, habitat available to support associated communities. Our study indicates that by reducing populations of habitat-providing intertidal mussels, sea otters may have substantial indirect effects on associated communities.

Making the most of fragments: a method for estimating shell length from fragmentary mussels (Mytilus californianus and Mytilus trossulus) on the Pacific Coast of North America

“…Of particular ecological relevance is evaluating the size distribution in relation to the long-term presence of sea otters which have been shown to homogenize mussel sizes through regular harvesting and elimination of larger and older M. californianus (Singh et al, 2013). Sea otters (Enhydra lutris) are voracious consumers of mussels (VanBlaricom, 1988), and the methods we provide can potentially be used to evaluate if mussel size distributions were altered when sea otter remains are found in temporally associated midden strata (e.g., Simenstad et al, 1978;Szpak et al, 2012).…”

“…1, Supplemental materials). Mussels were obtained from the mid-to-low tide on wave-exposed rocky shores as part of a marine ecology research project seeking to control for growth variation along this latitudinal gradient (Helmuth et al, 2006;Singh, 2010;Singh et al, 2013). This broad spatial scale is relevant as it accounts for multiple environmental and growing conditions across space and therefore serves as a proxy for variability in growing conditions across broad temporal scales (e.g., Pickett, 1989).…”

“…Ecologically, M. californianus populations respond to a variety of factors such as climate and oceanography (Menge et al, 2008;Smith et al, 2006), temperature (Ford et al, 2010;Harley, 2011), and intertidal predation by other marine predators such as sea stars (Pisaster sp.) and sea otters (Enhydra lutris) (Menge et al, 1994;Singh et al, 2013). Given the ecological and economic importance of mussels, further investigation into the characteristics of archaeological assemblages has the potential to enrich understanding of coastal adaptations, humaneenvironment interactions, and paleoecology.…”

Kelp forest size alters microbial community structure and function on Vancouver Island, Canada

“…Upwelling brings nutrient-rich water to the surface and can stimulate productivity. The 1990-2010 mean annual upwelling indices were stronger in the small kelp forest region than the large kelp forest region (Singh et al 2013). However, the mean daily upwelling index over our sampling period was 48.17 6 6.23 m 3 Ás À1 Á100 m À1 of coastline in Kyuquot Sound, the large kelp forest region, and 22.23 6 2.62 m 3 Ás À1 Á100 m À1 in Barkley Sound, the small kelp forest region, suggesting that the large kelp forest region was experiencing stronger upwelling than the other region.…”

“…Higher abundances and growth are typically found in warmer waters (Wetzel 2001, Sarmento et al 2010, Vezzulli et al 2013. Over a 10-year period (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009), sea surface temperatures (SST) were consistently 18C warmer in Barkley Sound, the small kelp forest region, than in Kyuquot (Singh et al 2013). SST was 0.6168C warmer in Barkley Sound than Kyuquot at the times that we sampled.…”