Disturbances such as disease can reshape communities through interruption of ecological
interactions. Changes to population demographics alter how effectively a species
performs its ecological role. While a population may recover in density, this may
not translate to recovery of ecological function. In 2013, a sea star wasting syndrome
outbreak caused mass mortality of the keystone predator Pisaster ochraceus on the
North American Pacific coast. We analyzed sea star counts, biomass, size distributions,
and recruitment from long-term
intertidal monitoring sites from San Diego to
Alaska to assess regional trends in sea star recovery following the outbreak.
Recruitment, an indicator of population recovery, has been spatially patchy and varied
within and among regions of the coast. Despite sea star counts approaching predisease
numbers, sea star biomass, a measure of predation potential on the mussel
Mytilus californianus, has remained low. This indicates that post-outbreak populations
have not regained their full predation pressure. The regional variability in percent of
recovering sites suggested differences in factors promoting sea star recovery between
regions but did not show consistent patterns in postoutbreak recruitment on
a coast-wide scale. These results shape predictions of where changes in community
composition are likely to occur in years following the disease outbreak and provide
insight into how populations of keystone species resume their ecological roles following
mortality-inducing
disturbances.
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Testing ecological release as a compensating mechanism for mass mortality in a keystone predator
Top predator decline has been ubiquitous across systems over the past decades and centuries, and predicting changes in resultant community dynamics is a major challenge for ecologists and managers. Ecological release predicts that loss of a limiting factor, such as a dominant competitor or predator, can release a species from control, thus allowing increases in its size, density, and/or distribution. The 2014 sea star wasting syndrome (SSWS) outbreak decimated populations of the keystone predator Pisaster ochraceus along the Oregon coast, USA. This event provided an opportunity to test the predictions of ecological release across a broad spatial scale and determine the role of competitive dynamics in top predator recovery. We hypothesized that after P. ochraceus loss, populations of the subordinate sea star Leptasterias sp. would grow larger, more abundant, and move downshore. We based these predictions on prior research in Washington State showing that Leptasterias sp. competed with P. ochraceus for food. Further, we predicted that ecological release of Leptasterias sp. could provide a bottleneck to P. ochraceus recovery. Using field surveys, we found no clear change in density or distribution in Leptasterias sp. populations post-SSWS, and decreases in body size. In a field experiment, we found no evidence of competition between similar-sized Leptasterias sp. and P. ochraceus . Thus, the mechanisms underlying our predictions were not in effect along the Oregon coast, which we attribute to differences in habitat overlap and food availability between the 2 regions. Our results suggest that response to the loss of a dominant competitor can be unpredictable even when based in theory and previous research.
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- NSF-PAR ID:
- 10154633
- Date Published:
- Journal Name:
- Marine Ecology Progress Series
- Volume:
- 637
- ISSN:
- 0171-8630
- Page Range / eLocation ID:
- 59 to 69
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Disease outbreaks can have substantial impacts on wild populations, but the often patchy or anecdotal evidence of these impacts impedes our ability to understand outbreak dynamics. Recently however, a severe disease outbreak occurred in a group of very well-studied organisms±sea stars along the west coast of North America. We analyzed nearly two decades of data from a coordinated monitoring effort at 88 sites ranging from southern British Columbia to San Diego, California along with 2 sites near Sitka, Alaska to better understand the effects of sea star wasting disease (SSWD) on the keystone intertidal predator, Pisaster ochraceus. Quantitative surveys revealed unprecedented declines of P. ochraceus in 2014 and 2015 across nearly the entire geographic range of the species. The intensity of the impact of SSWD was not uniform across the affected area, with proportionally greater population declines in the southern regions relative to the north. The degree of population decline was unrelated to pre-outbreak P. ochraceus density, although these factors have been linked in other well-documented disease events. While elevated seawater temperatures were not broadly linked to the initial emergence of SSWD, anomalously high seawater temperatures in 2014 and 2015 might have exacerbated the disease's impact. Both before and after the onset of the SSWD outbreak, we documented higher recruitment of P. ochraceus in the north than in the south, and while some juveniles are surviving (as evidenced by transition of recruitment pulses to larger size classes), post-SSWD survivorship is lower than during pre-SSWD periods. In hindsight, our data suggest that the SSWD event defied prediction based on two factors found to be important in other marine disease events, sea water temperature and population density, and illustrate the importance of surveillance of natural populations as one element of an integrated approach to marine disease ecology. Low levels of SSWD-symptomatic sea stars are still present throughout the impacted range, thus the outlook for population recovery is uncertain.more » « less
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In recent years, a massive mortality event has killed millions of sea stars, of many different species, along the Pacific coast of North America. This disease event, known as ‘sea star wasting disease’ (SSWD), is linked to viral infection. In one affected sea star (
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3354/meps13227
