As the Bering Sea has warmed and lost winter ice cover over the last three decades, sub-Arctic fish and crustaceans have moved northwards, while the range of Arctic species has shrunk. However, an interesting thing happened when the Bering went through a relatively cold period during 2006-2013. Even though the warming trend was temporarily reversed, fish and crustaceans failed to respond in the expected way. The northward shift in the community that occurred during the warming period mostly failed to reverse itself during the cold years.

Ecologists call this kind of one-way response to disturbance “hysteresis”. This is a central concept in our attempts to understand the complexities of ecological change. But (like many ecological concepts!) it is surprisingly difficult to apply this idea to data from large ecosystems. With support from Alaska Sea Grant, we showed that early warning indicators were generated during the Bering Sea cold period, which in turn shows that the hysteresis concept does indeed fit this “one-way” response to warming and cooling periods.

Figure: One-way responses to temperature change. The left-hand axis shows if fish and crustaceans are found more to the south (towards the bottom) or more to the north (towards the top). As the Bering Sea warmed up (green arrows), the community moved northwards. But as it cooled down (blue arrows), the community failed to return to its previous southern distribution.

Figure: One-way responses to temperature change. The left-hand axis shows if fish and crustaceans are found more to the south (towards the bottom) or more to the north (towards the top). As the Bering Sea warmed up (green arrows), the community moved northwards. But as it cooled down (blue arrows), the community failed to return to its previous southern distribution.