Schultz JA et al. (2016), Evidence for a trophic cascade on rocky reefs f...

ECB-ART-44679

PeerJ 2016 Jan 01;4:e1980. doi: 10.7717/peerj.1980.

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Evidence for a trophic cascade on rocky reefs following sea star mass mortality in British Columbia.

Schultz JA , Cloutier RN , Côté IM .

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Echinoderm population collapses, driven by disease outbreaks and climatic events, may be important drivers of population dynamics, ecological shifts and biodiversity. The northeast Pacific recently experienced a mass mortality of sea stars. In Howe Sound, British Columbia, the sunflower star Pycnopodia helianthoides-a previously abundant predator of bottom-dwelling invertebrates-began to show signs of a wasting syndrome in early September 2013, and dense aggregations disappeared from many sites in a matter of weeks. Here, we assess changes in subtidal community composition by comparing the abundance of fish, invertebrates and macroalgae at 20 sites in Howe Sound before and after the 2013 sea star mortality to evaluate evidence for a trophic cascade. We observed changes in the abundance of several species after the sea star mortality, most notably a four-fold increase in the number of green sea urchins, Strongylocentrotus droebachiensis, and a significant decline in kelp cover, which are together consistent with a trophic cascade. Qualitative data on the abundance of sunflower stars and green urchins from a citizen science database show that the patterns of echinoderm abundance detected at our study sites reflected wider local trends. The trophic cascade evident at the scale of Howe Sound was observed at half of the study sites. It remains unclear whether the urchin response was triggered directly, via a reduction in urchin mortality, or indirectly, via a shift in urchin distribution into areas previously occupied by the predatory sea stars. Understanding the ecological implications of sudden and extreme population declines may further elucidate the role of echinoderms in temperate seas, and provide insight into the resilience of marine ecosystems to biological disturbances.

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Genes referenced: LOC100887844

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	Figure 1. Progression of sea star wasting disease.(A) A healthy-looking specimen of P. helianthoides moves across the kelp, Agarum fimbriatum. (B) Afflicted sea stars exhibit a loss of turgor pressure and body wall ruptures, followed by (C) limb autotomization, disintegration and death. Photos by Donna Gibbs.
	Figure 2. Rocky reef survey sites in Howe Sound, British Columbia.Benthic community composition was assessed at each of the 20 sites once in 2009 or 2010 and again in 2014. A mass mortality of sea stars occurred in the summer and fall of 2013 in this area. A site-level trophic cascade following the mortality was detectable at some sites (solid circles) but not others (open circles). (Map data © 2015 WorldMap).
	Figure 3. Mortality of sea stars, and subsequent change in urchin abundance and kelp cover after sea star mortality.Mean abundance (per m2) of (A) sunflower stars and (B) green sea urchins, and (C) percent cover of kelp on rocky reefs in Howe Sound, British Columbia, on 80 transects before and after the mass mortality of sea stars in 2013. Error bars represent standard error. The dominant kelp was the sea colander kelp, Agarum fimbriatum.
	Figure 4. Sunflower star and green sea urchin abundance trajectories.Sixty-day running average abundance scores for green sea urchins (Strongylocentrotus droebachiensis; green solid line) and sunflower stars (Pycnopodia helianthoides; purple dashed line) recorded in REEF surveys from January 2010 to November 2014 in Washington and British Columbia (n = 1568 surveys). Grey bands indicate 95% confidence intervals of the running average. The vertical red dotted line indicates the date of the first recorded observation of sea star wasting syndrome (7 June 2013), which was on the Olympic coast of WA.
	Figure 5. Rocky reef species assemblages before and after sea star mortality.Multidimensional scaling plot of benthic community composition on rocky reefs before (blue triangles) and after (inverted red triangles) the 2013 sea star mass mortality event in Howe Sound, British Columbia. The analysis included 18 fish and invertebrate taxa at 20 sites, surveyed both in 2009/2010 and 2014. The associated stress value (0.13) suggests some distortion in the multivariate representation of the data.
	Figure 6. Relative difference in abundance of sea stars, urchins and kelp by site.The relative difference in total count of sunflower stars (blue triangles) and green urchins (green circles), and the relative difference in the mean percent cover of algae (red squares) before and after the sea star mass mortality. Open symbols indicate sites where population density was zero both before and after the mass mortality. Relative difference was calculated as the change in abundance divided by the mean abundance of both time periods. A relative difference of −2 indicates the population declined to zero. Sites are numbered chronologically according to the order in which they were surveyed, from June to August, 2014. The geographic location of these sites is shown in Fig. 2.

References [+] :

Duggins, Interspecific facilitation in a guild of benthic marine herbivores. 1981, Pubmed