Award: OCE-1735607

This project was aimed at testing the ecosystem impact of the sea star wasting epidemic-driven, coast-wide decimation of populations of the famous keystone predator, the ochre sea star. Prior local-scale research had shown that in its absence, mid intertidal mussel populations expanded their range downward to the low intertidal region, sharply reducing diversity of the wide variety of seaweeds and invertebrates that dominated the lower shore. Thus, through its predatory activities, the ochre star was thought to maintain a state of high species diversity. While shocking, the wasting event offered an opportunity to test these ideas on a much larger, coast-wide scale. We hypothesized that in the near-absence of the ochre star, mussels would invade the lower shore and reduce the abundance and diversity of space-occupying intertidal species. Alternatively, this may not occur, and we also tested factors that might lead to a different result. More generally, this scenario offered the opportunity to study the ability of an iconic ecosystem to resist perturbations (i.e., test its "resistance" to change) and learn what factors might enable it to recover quickly or not (i.e., test its "resilience" to change). Components of the study were: (1) quantifying changes in the vertical distribution of mussels, (2) quantifying the response of sea star populations to the loss of most adults, (3) test the possibility that alternative predators could take over some or all of the role of top predator, and (4) determine if the supply of new mussels (recruits) might drive the response of mussels to the loss of the ochre star. Results showed that, to our surprise, mussels only rarely became abundant on the low shore. At most sites, little change occurred in their vertical distribution. Of the possible mechanisms that might have prevented this change, only a lack of mussel recruits plus the relatively fast recovery of sea star populations seemed likely. Experiments testing the effects of either larger predators such as seagulls and crabs, or smaller predators like whelks and another small sea star species found little to no support for the idea that these groups expanded their role and helped prevent mussel takeover of the lower shore. Importantly, this study coincided with two extreme environmental events: the occurrence of the 3rd strongest El Niño in recorded history and a massive "Marine Heat Wave" which overlapped and occurred from about 2014 to 2017. These events co-occurred with the collapse of mussel recruitment during this time period, which with relatively rapid recovery of sea star populations seems the most likely explanation for the failure of mussels to invade the lower shore. We conclude that this ecosystem was resistant to the wasting perturbation primarily because of the major warming event that coincided with the period of time that mussels might respond to sea star loss. In other words the resistance was driven by a succession of negative environmental impacts, the first affecting the predator, the second affecting mussels, the sea star’s primary prey. This research provides insight into possible future states of this system as climate change accelerates. Specifically, the work suggests that mussels hold the key to the future – their ability to persist in the face of continued warming will determine if rocky shores can maintain their current structure, or shift to a mussel-absent, lower-diversity system dominated by more thermally resistant seaweeds and barnacles. Last Modified: 05/05/2022 Submitted by: Peter T Raimondi