Changing Communities: Impacts of the Pisaster ochraceus Collapse on Intertidal Communities

The ochre sea star (Pisaster ochraceus) is a keystone predator that can control the structure and maintain the diversity of rocky intertidal communities. In 2013, a densovirus instigated a large sea star die-off that caused populations of Pisaster across the West Coast of North America to collapse. In the wake of their absence, the rocky intertidal zone faces potential change in community structures. For example, prey populations, specifically mussels, could now expand without predation by Pisaster to restrict them. Field surveys and experiments examined the possible impacts of the die-offs of the ochre sea star on the intertidal zone. Specifically, impacts were measured on the California Mussel (Mytilus californianus), and two predatory whelks Ocinebrina circumtexta and Nucella analoga compressa. Eighteen 0.5 m2 plots within mussel beds at two sites, within the Lovers Point State Marine Reserve, in Monterey Bay, California, were photographed on seven dates between June 2014 and July 2015 to measure changes in mussel percent cover and tidal heights of the mussel beds. To examine whether whelks have the potential to replace P. ochraceus in controlling the M. californianus population, I conducted counts of the dominant species, Ocinebrina sp. & Nucella sp., in the plots on five dates and I performed a lab experiment to measure the mortality rates of M. californianus with and without the predatory whelk Nucella sp. Counts of P. ochraceus were also conducted at the two sites, on each monitoring date, and compared to J. Pearse’s (2010) historical counts done for the same areas since 1950. Percent mussel cover showed a small but significant increase over the 13-month monitoring. Plots that had a greater cover to begin with showed a slightly greater increase. However, some mussel patches with low cover but with recently recruited M. californianus grew faster because smaller mussels grow faster than larger ones. The lower limit of the mussel beds has shifted up to 32 cm lower in some plots, possibly due to recruitment and higher survival occurring at lower levels in the absence of sea stars. Whelk counts revealed a decrease in densities over the 15-month monitoring. The abundance of P. ochraceus is now the lowest it has been since 1950: sea star population size has seen a 90-95% decline over the past 25-50 years. Most of this mortality is not due to the 2013 outbreak of the sea star die-off. Finally, the presence of Nucella increases the mortality of M. californianus in laboratory feeding experiments, but there is little evidence for size selection. However, the potential for the whelks to fill in the niche left vacant by Pisaster population collapse seems limited based on my laboratory estimates of mortality from whelk predation and field estimates of whelk densities. Overall, the predicted mussel expansion did occur, though patchily. However, processes other than predation could drive or limit mussel expansion. Continued monitoring and field experiments are needed to examine possible changes in intertidal communities and elucidate their drivers.

Keywords:  Department of Biology
Biology
Marine Biology
Marine Ecology
Intertidal Community
Mytilus californianus
Pisaster ochraceus
Thesis
Advisor: Micheli Fiorenza
Denny Mark