Thresholds and tipping points? - Arctic lake 'regime shifts'

For the next post I'm going to be reviewing an article by Smol et al (2005) entitled "Climate-driven regime shifts in the biological communities of Arctic lakes". So before I launch into that, I thought it would be best to have a review of the ecological theory surrounding what they are calling 'regime shifts' by looking at the article they reference. Ecological shifts can occur naturally (such as through fires) or, as is more likely in recent times, due to human activities. Click 'Read more' for a proper explanation...

Conceptual diagram of regime shifts and alternative
stable states. Direct from Scheffer et al (2001)

Ecosystems respond to a very large number of environmental and ecological variables, making them all extremely complex systems. Therefore, ecosystems are a product of all these influences and are subsequently sensitive to changes in any of them. Intuitively, one would assume a simple linear or normal response to changes in both directions. For example, a lake increasing in primary productivity in step with nutrient loading and decreasing in a similar manner once these have been removed.

However, as can be seen in the conceptual diagram above, some systems do not operate as one of these 'two-way roads' where it is easily possible to return to previous conditions. The diagram is taken from a paper by Scheffer et al (2001) published in Nature called "Catastrophic shifts in ecosystems" (which is referenced by the Smol et al article) and describes the theory behind 'alternative stable state' regime shifts.

Firstly, on the 'slice' nearest to us, the ecosystem is in a very stable state where even a very large perturbation would not permanently change the ecosystem structure. As the environmental conditions change (moving backwards) it is clear that a perturbation would be able to move the system over to an alternative (stable) state (by moving the hall over the peak). Eventually the condition in this conceptual example reaches the point at F2, where a perturbation moves the system to another state.

Notice, however, that once in this state, even if the conditions are reversed, the 'Ecosystem state' does not necessarily return to it's previous point, perhaps requiring management intervention. In this way, changes to an ecosystem can happen suddenly and unpredictably, and may be difficult to reverse. Gradual changes in environmental variables may cause little obvious ecosystem change but are reducing the systems capacity to 'deal with' a perturbation. In our Arctic case, climate change may not be directly causing the changes, but merely reducing system resilience.

Of course, this all depends on the type of ecosystem. For example, shallow lakes subject to eutrophication (nutrient loading) exhibit this phenomenon very strongly due to complex buffering mechanisms related to macrophytes (underwater plants) and zooplankton. However, the relevance to this in the case of Arctic lakes may respond in a more straightforward way. These may not have similar buffering mechanisms, and thus not shift as rapidly or as seemingly irreversibly as others. It is likely however that they will experience changes in their ecosystem function, complexity and dynamics as a result of Climate Change.

So, have Arctic lakes and ponds responded in such a complex way to recent climatic changes, or is their response 'simpler'? If they are not responding, is it likely that their resilience has been reduced? Stay tuned to find out...