Changing Communities: Impacts of the Pisaster ochraceus Collapse on Intertidal Communities An Honors Thesis Submitted to the Department of Biology in partial fulfillment of the Honors Program STANFORD UNIVERSITY by Roberto Guzman November 2015 Acknowledgments I’d like to thank Terry Root and the Woods Institute for their MUIR Grant that made this all possible. And also for teaching me on the importance of an interdisciplinary education. I’d also like to thank my advisor, Fiorenza Micheli, for her assistance, expertise, patience, and ideas that helped throughout this project, every step of the way. Whether it was helping with setting up the projects, analyzing the results with me, or just coming out to the intertidal zone with me to help with field work, without you, this project and my thesis would not exist. Thank you Mark Denny for your contribution to my thesis. I appreciate not only your help as a second reader, but as someone who was able to contribute fresh eyes to my thesis, providing me with valuable insight of things I may have missed after working on the thesis. I am also grateful for the assistance of James Watanabe. Whether it was his expertise in the biology of the intertidal zone, or his quadrat camera setup, the success of the project lends itself to his efforts and generosity. A lot of appreciation goes to Steve Palumbi for providing me with financial assistance this year. You have not only helped me with it, but also my family. Your generosity will not be forgotten. For assisting me in fieldwork and making it more enjoyable, I’d like to thank Gracie Singer. Your positivity and conversations lit up the lab. I’d also like to thank Maria Castro ‘17 for serving as my unofficial assistant for the first summer of the project. Your help and uplifting energy in the field and in the lab made sure I stayed on top of my things, but it also made it pleasant. Sincerely, thank you and good luck in your academic future. 3 Table of Contents List of Figures & Tables……………………………………………………………………..…..5 Abstract………………………………………………………………………………………...…7 Introduction…………………………………………………………………………………..…..8 Materials & Methods……………………………………………………………….………..…12 Results…………………………………………………………………………………………..16 Conclusion/Discussion………………………………………………………………………….34 References……………………………………………………………………………………….45 4 List of Figures & Tables Figure 1: Map of the sites off of Point Cabrillo and of the Hopkins Marine Station. Figure 2: Percentage of total quadrat space covered by mussel beds average over all of the plots by date. Figure 3: Percentage of mussel bed cover of East and West quadrats categorized by initial size. Figure 4: Percentage of mussel bed cover of East quadrats categorized by initial size. Figure 5: Percentage of mussel bed cover of West quadrats categorized by initial size. Figure 6a: Percent coverage of quadrat by mussel beds by individual plots in the East site. 2 Regression lines, R ​ values, and p­values for each plot are included. Asterisks indicate plots that ​ are significant after applying a Bonferroni correction to account for multiple tests (N=18 & p<0.003). Figure 6b: Percent coverage of quadrat by mussel beds and individual plots in the West site. 2 Regression lines, R ​ values, and p­values for each plot are included. Asterisks indicate plots that ​ are significant with the Bonferroni correction (N=18 & p<0.003). Figure 7a: Distance of the mussels averaged for each date done for East plots. Distance was measured by the distance between the mussels to the fixed line between the eye­bolts. Figure 7b: Distance of the mussels averaged for each date done for West plots. Distance was measured by the distance between the mussels to the fixed line between the eye­bolts. Figure 8a: Heights of the mussels averaged for each date done for the East A&B plots. Figure 8b: Heights of the mussels averaged for each date done for the West A&B plots. Figure 9: Heights of the individual mussels measured in Plot 2A. Figure 10a: Heights of the lowest mussels found averaged for each date done for the East plots. 5 Figure 10b: Heights of the lowest mussels found averaged for each date done for the West plots. Figure 11a: Lengths of the lowest mussels averaged for each date done for the East plots. Figure 11b: Lengths of the lowest mussels averaged for each date done for the West plots. Figure 12: Total counts of the whelks Ocinebrina sp. & Nucella sp. in both East and West plots. ​ ​ Figure 13a: Total counts of the whelks Ocinebrina sp. & Nucella sp. in East plots. ​ ​ Figure 13b: Total counts of the whelks Ocinebrina sp. & Nucella sp. in West plots. ​ ​ Figure 14: Number of dead mussels found in experimental containers by treatment. Figure 15: Number of dead mussels in the treatment boxes by their length. Figure 16: Total counts of Pisaster ochraceus by date. ​ ​ Figure 17: Pisaster ochraceus counts done by Pearse (2010) averaged for every 5­year periods ​ ​ from 1950­2010 with my counts for 2015 added. Figure 18: Drill holes made by the predatory whelk Nucella sp. found on the dead mussels in the ​ ​ red boxes. Table 1: Two­Factor ANOVA with Plots and Dates as the factors. 6 Abstract 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 die­off 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 die­offs of the ochre sea star on the intertidal zone. Specifically, impacts were measured on the California Mussel (Mytilus californianus), and two predatory ​ ​ ​ ​ 2 whelks Ocinebrina circumtexta and Nucella analoga compressa. Eighteen 0.5 m ​ 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 13­month 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 ​ 7 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 15­month monitoring. The abundance of P. ochraceus is now the ​ ​ lowest it has been since 1950: sea star population size has seen a 90­95% decline over the past 25­50 years. Most of this mortality is not due to the 2013 outbreak of the sea star die­off. 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. Introduction A suite of abiotic and biotic factors, and their interactions, shape the structure and diversity of marine communities (Paine, 1974; Somero 2002). In rocky shores, fluctuating tidal heights change the physical conditions for intertidal communities and serve as the major driver of the distribution of organisms (Somero, 2002). The different tidal heights differ in their exposure to different biotic and abiotic factors. Above the waterline, organisms are exposed to desiccation, direct sunlight, high temperature, and crashing waves. The tolerance to these factors by different organisms dictate at what tidal height they can survive. Thus, rocky intertidal communities often exhibit distinct patterns across tidal heights (Paine, 1974; Somero 2002). 8 The rocky intertidal zone is an ecosystem where the most valuable resource is space. Having sessile organisms make up most of the community found in the intertidal zone, space on a rocky substrate quickly becomes the most limited and important resource (Dayton, 1971). Thus, competition for space is a major driver of diversity and community composition (Robles, 2002). Predation can control competitively dominant species, thereby mediating competitive interactions among intertidal species and allowing for the persistence of competitive inferiors (Paine, 1974). Predators that maintain high diversity in intertidal communities by controlling competitively dominant species are referred to as keystone species (Mills, 1993). Keystone species maintain diversity by indirectly facilitating other species by preying on organisms that would otherwise eliminate them (Mills, 1993). On the West Coast of North America, the Ochre Starfish, Pisaster ochraceus, can be ​ ​ keystone predator in its interaction with the California Mussel, Mytilus californianus.