Early warning indicators are one of the fastest-growing fields in applied ecology. These indicators are based on theoretical predictions that key ecosystem parameters should follow predictable trends – like becoming more variable – as a sudden ecosystem reorganization becomes more likely. Monitoring changes in system properties such as population variability is therefore expected to give managers "early warning” that the system is approaching a tipping point into a less-desired state.

Early warning indicators have mostly been developed with mathematical models, laboratory experiments and studies of simple ecosystems like freshwater ponds. Our research concentrates on the next frontier - determining whether early warning indicators will work in the large, complicated settings that managers are concerned with, like ocean ecosystems. Support from The Pew Charitable Trusts allowed us to demonstrate, in a forthcoming paper in Ecosphere, which kinds of ecosystem change are most likely to produce early warnings. Additionally, a new project funded by the NOAA Fisheries and the Environment Program will summarize available time series from the Bering Sea, Gulf of Alaska and California Current to develop early indicators for biological response to the “warm blob” and other ecosystem perturbations.

Figure: The explosion in published studies on early warning indicators during 2006-2015.

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.

The Gulf of Alaska is an excellent “test bed” for understanding linkages between climate, commercial fishing, and ecological responses. Long-term data sets are available for many populations, and relatively well-understood warm and cold “regimes” governed the biology of this ecosystem for much of the 20th century.

With support from Alaska Sea Grant, we showed that historical collapses of shrimp and crab fisheries in the Gulf (and the Bering Sea) were proceeded by periods of rising variability in commercial landings – one of the only demonstrations of early warning indicators in commercial fisheries data. (See the paper in Ecological Applications.) With support from the National Science Foundation, we are attempting to take understanding of climate-biology links beyond the warm regime/cold regime paradigm. This study is focused on nonlinear climate variability – relationships between fundamental climate processes that change over time – and how the complex patterns of climate variability that result force changes in the ecosystem.