Award: DEB-1554702

This award investigated the impacts of climate change on the ecological communities that occupy rocky shores along the Oregon coast. Such "rocky intertidal" communities have long served as a model system for the investigation and generation of important ecological principles such as keystone predators, biodiversity, and succession. Reasons making the system favorable for such research include:

(1) the organisms are sessile or slow-moving,

(2) they are in a sweet-spot in terms of size, small relative to the investigator's size, yet large enough to be studied without special equipment,

(3) they have relatively "fast" lives, living from months to several years,

(4) they are compressed to a vertical scale of several meters, not the kilometer scales one might see on mountainsides,

(5) they recover from disturbances quickly, facilitating study of dynamics, and

(6) they are readily manipulable.

This research was based on 20+ years of previous research, knowledge of which enabled the initiation of studies focused on how the system responded to annual perturbations. In 2010, we established two experiments, one in the mussel-dominated mid-shore, the other in the seaweed-dominated low-shore. In both cases, small areas of the rock were cleared of mussels or seaweeds, then allowed to recover for 12 months, after which the same small area was recleared. Nearby areas of the same size were left uncleared as "controls." Experiments were conducted from 2010 to 2019. Because prior research had shown that the recovery responses in such experiments varied in space in relation to coastal currents, air and water temperature, and the amount of particulate "food" in the water for filter-feeding organisms, the studies were conducted at seven sites spread across three regions along 260 km of the Oregon coast.

Results showed that responses in both experiments varied through time, and that the variation was associated with environmental change. Specifically, recovery from disturbance slowed during the decade of research in both experiments. Further, the recovery rates increased in variability, indicating that initially, cleared plots recovered at similar rates while as time went on, recovery trajectories among the cleared plots grew increasingly different. Analyses of these results in the context of environmental data indicated that they were strongly related to increasing air and seawater temperature, and in particular, to the severe "marine heat wave" that occurred from 2015 to 2017.

A second key element of this research was the collapse of keystone predator sea star populations. This 2014 event was due to "sea star wasting." Wasting caused a 60-90% decline in sea star abundance. Although new baby sea stars settled on the shore at most sites the following year, the loss of large adults meant that their role in controlling mussel and barnacle prey was vacated. Further, despite high densities of now mostly small sea stars, recovery of their role as a predator in the system has been slow.

All these changes are indicative that the stability of the ecosystem is decreasing. That is, the loss of the keystone predator role of sea stars at most sites, and the slowed recovery of the system from the annual perturbations noted above, are all consistent with ecological theory regarding the conditions leading a system towards a "tipping" point into a different state or configuration. Since such different states are typically considered to be less "desirable", we believe that we are in the process of documenting the stages that an ecosystem might go through as it loses stability. Further, the evidence we have accumulated is consistent with the hypothesis that the changes are likely due to climate change, in this case warming of the ocean and the air above it. Although the organisms living in the intertidal region have adapted to severe conditions (intense wave action, highly variable temperatures), many intertidal species are known to be already close to their upper tolerance limits in terms of temperature.

We note, however, that although these changes in dynamics (recovery rates, growth rates, reproductive rates) are in decline, the system still "looks" similar to what it has been historically. Further, it is possible that these changes could be reversed if future environmental conditions return to more favorable states. Hence, we intend to continue our studies in an effort to determine which trajectory the system follows: further decline, or gradual recovery.

This work was conducted by a team of researchers located at Oregon State University. The project supported a full-time technician and benefitted from assistance by undergraduates, post-bac individuals, and a post-doctoral scholar. In all,  at least 16 individuals were involved in the research. Most of these individuals were authors, co-authors, presenters or co-presenters of published papers or talks. Products included 11 meeting presentations and public talks, and 16 publications. Two graduate student theses were also produced. Finally, the research also benefitted from the participation of undergraduate classes in marine biology taught at OSU.

Last Modified: 03/30/2021
Modified by: Bruce A Menge