Tipping Points, Positive Feedback Switches, and the Potential for Rapid Change in Arctic Ecosystems

In this talk, I will review the evidence for widespread ecosystem change in the Arctic, and explore the idea that Arctic ecosystems share a tendency to exhibit abrupt shifts in structure and function as climate warms. Improving our understanding of the causes and dynamics of rapid, nonlinear ecosystem change is an important goal common to both terrestrial and marine ecosystems.

Recent years have seen a rapid accumulation of evidence of pervasive change in Arctic ecosystems, marine and terrestrial. Many of these changes are consistent with ecological expectations that more southerly species will work their way northward as climate warms. On land, boreal tree species began moving northward decades ago in many parts of the arctic (e.g., Lloyd 1997), and the areal extent of tall shrub tundra has expanded since the mid-1900s (e.g., Tape et al. 2006). In the oceans, there is similar evidence that subarctic species such as gray whales, Pollock and pink salmon are moving northward into Arctic waters (e.g., Grebmeier et al. 2006). Although the weight of evidence suggests that ecological transformation is widespread, the temporal trajectory of ecosystem change remains more poorly known.

There is mounting evidence, however, that terrestrial and marine ecosystems in the Arctic share a high probability of exhibiting rapid, nonlinear responses to climate change. On land, for example, data suggest that the advance of tall, woody vegetation into low-stature tundra may initiate positive feedbacks that accelerate the rate of change (e.g., Sturm et al. 2005). Such 'positive-feedback switches' (Wilson and Agnew 1992) may allow initially gradual changes in species distribution or community composition to accelerate once thresholds in species abundance are surpassed. In marine systems, tipping points may occur as thresholds in key physical drivers (including water temperature, ice extent, and aragonite saturation) are crossed (e.g., Carmack et al. 2006, Grebemeier et al. 2006, Fabry et al. 2009). Accurate predictions of the future trajectory of change in Arctic ecosystems will require a better understanding of the processes that may promote abrupt, rapid shifts in ecosystem structure.