Giant Kelp Forest Resilience to Ocean Warming; Historical and Modern Systems

Giant kelp forests of the North Pacific are iconic among cool temperate marine communities. They are also amongst the most productive marine ecosystems, harboring significant biodiversity and supporting coastal economies. The fate of these systems over the next eight decades is uncertain, given the variance of outcomes for global warming scenarios and the complexities of ecological forecasting. There is a growing consensus that giant kelp will be susceptible to warming, leading to a decline of the communities. Kelp forest dynamics, however, are also controlled by biotic interactions. In the North Pacific, the main biotic factors today are kelp herbivory, especially by grazing sea urchins, and predatory control of the urchins, particularly by sea otters and sunflower sea stars. A recent study demonstrated that as late as the 18th century, the now extinct mega herbivore Steller’s sea cow, an obligate kelp browser, had a significant impact on North Pacific kelp forest dynamics. Mathematical models comparing the historical community to modern ones indicated that the sea cow would have increased forest resilience against reductions of predation of urchins caused by multiple types of perturbation. In addition to epidemics and temperature anomalies, it is expected that global temperatures will increase 2-4 degrees Celsius by the year 2100. Here we use the mathematical models to explore forest dynamics under various warming scenarios, including the changing impacts of epidemics and the frequency and intensity of warming anomalies. Preliminary results suggest that in contrast to a stable temperature regime, warming to 4 degrees increases chaotic dynamics, extinction of both sea urchins and sunflower sea stars, and abundance of understory algae. Those results also suggest that chaos and algal increase would be lessened in the presence of the extinct herbivore. These findings bear implications for future kelp forest management, conservation, and human economic exploitation.