Geir Ottersen and Jess Melbourne-Thomas

PNAS September 10, 2019 116 (37) 18157-18158

There is growing evidence indicating that variability and extremes in conditions in the marine environment are as (or more) important as changes in the mean for determining threats to biodiversity, impacts on ecosystem services, and consequences for human systems (1⇓⇓–4). With respect to ocean temperature, long-term persistent warming has been accompanied by an increased frequency of discrete periods of extreme regional ocean warming (marine heatwaves) (5). This poses a threat to biodiversity and ecosystem services, including impacts on foundation species (corals, seagrasses, and kelps) (1, 4). The potential of human and natural systems to adapt to such changes remains unclear. In PNAS, Pershing et al. (6) show that an increasing frequency of extreme heat events—or “surprises”—is challenging autonomous modes of adaptation that rely on historical experience. The authors contrast reactive adaptation that is guided by experiences of past events with proactive adaptation based on forward-looking decision making. They use ocean ecosystems as a case study and, based on mathematical models, consider how temperature trends and the frequency of surprise (high) temperature events could impact natural and human communities under different adaptation strategies.

Pershing et al. (6) define a temperature surprise as an annual mean temperature that is 2 SDs above the mean, where the mean and SD are determined by the prior 30 y of temperature records (a rolling mean), at the scale of 65 large marine ecosystems (LMEs) (7). “Surprising surprises” are those events that are in excess of the number expected, based on the probability of a surprise determined from the rolling mean approach. Importantly, the authors found that the frequency of such surprises is increasing faster than expected and will continue to rise. Indeed, the cumulative increase may be even higher than estimated by Pershing et al. (6). They show that the increase is particularly high in the Arctic but exclude the Antarctic LME (due to missing data), which encompasses one of the fastest-warming regions globally (8).

Environmental conditions that fall outside the typical range of experience have a high potential to drive change in socioecological systems. Pershing et al. (6) use simple models to consider what an increasing frequency of surprises might mean for humans and for ecosystems, under different strategies for adapting to such change. These models suggest that, for humans, there is a higher payoff for strategies that are forward-looking than those that are based on a backward-looking approach (Fig. 1). For ecosystems, an increasing frequency of surprises may lead to a homogenization of the species mix, with specialists being replaced by generalists. The success of generalist species under future climate conditions has also been demonstrated separately in mesocosm experiments (9). Interestingly, based on Pershing et al.’s models (6), the occurrence of surprises appears to be more important for human systems, while the trend in the mean is more important for ecosystem responses to environmental change.

See more: https://www.pnas.org/content/116/37/18157

Figure: Summary of implications for human communities and ocean ecosystems of adaptation approaches that are based on information from the past (left-hand side: fewer surprises) versus those that look forward and consider temperature trends (right-hand side: more surprises), based on findings from Pershing et al. (6). For human systems, there is a higher return for strategies that are responsive to temperature trends, while for ecosystems, increasing ocean temperatures and more surprises may lead to the replacement of specialist species with generalists, and a consequent decrease in biodiversity. Image courtesy of Stacey McCormack (University of Tasmania, Hobart, Australia).