Introduction - What Are Alternative Stable States? What Is a Regime Shift?

Take a look at Figure 1. How can it be that two lakes side-by-side with similar conditions look so different from one another? The answer is that these two lakes are examples of alternative stable states. Systems can shift radically and suddenly into fundamentally different configurations (in this case, a clear-water lake and a turbid, cyanobacteria-dominated lake) depending on the type and magnitude of disturbances inflicted on the system. The theory of alternative stable states is related to the concept of hysteresis, the idea that sometimes the way to reverse a change is different than just reversing what caused the change in the first place.  Excess nutrient pollution from sources such as agricultural runoff can lead to lake eutrophication, or the turbid appearance visible in Figure 1 (Wilkinson et al. 2018). However, returning to the clear-water lake (on the right below) may require the removal of more nutrients than originally led to the catastrophic shift from a clear to a turbid lake. Given these facts and the presence of hysteresis in the turbid lake, it is possible for the two lakes to have the same amount of nutrients present, yet be in different states. Clear-water states and turbid, algae- or cyanobacteria-dominated states in freshwater lakes are a common example of alternative stable states exhibited by complex systems.

The theory of alternative stable states utilizes three main concepts to explain the dynamic nature of ecosystems: 1) alternative stable states or regimes, 2) thresholds of change, and 3) regime shifts between these alternative stable states.

Alternative stable states are defined as differing arrangements of an ecosystem’s characteristics, such as its functions, processes, components, and interrelationships. These differing arrangements are maintained through different stabilizing feedbacks with abrupt shifts between these arrangements. The arrangement of an ecosystem at a given time is its state. In the lakes example, one alternative stable state is the clear-water lake and the other alternative stable state is the turbid lake.

Thresholds of change are the borders between alternative stable states. If a disturbance pushes an ecosystem beyond that border line, then the ecosystem will switch into an alternative stable state. These are also referred to as “tipping points”. In the lakes example this is the amount of nutrient loading, or the quantity of nutrients, that the clear lake must surpass to “tip” into the turbid lake alternative stable state.

Regime shifts are the transition from one alternative stable state to another through the passing of a threshold. It is caused by some form of internal feedback or external perturbation over timescales of varying length (Angeler and Allen 2016). In the lakes example this is the moment and process of shifting from the clear lake alternative stable state to the turbid lake alternative stable state.

Therefore, alternative stable state theory suggests that multiple states may exist for one ecosystem. Over time this ecosystem may transition from one state to another due to either external or internal influences, fundamentally altering the structure and function of the ecosystem.

The ability to understand the range of alternative states one ecosystem may move between is crucial to understanding and managing ecosystems overall. The theory of alternative stable states and regime shifts predates the 1973 inception of ecological resilience by Holling, instead originating in 1969 with Dr. Richard Lewontin, an evolutionary biologist. Scheffer et al. (2001) describes three response curves an ecosystem may have as a result of changes in external conditions:

1. An ecosystem may respond in a “smooth, continuous” way
2. An ecosystem may be relatively static over a certain range of these changes, before responding in a more sudden and strong manner at a certain point
3. Or, an ecosystem’s response curve may be “folded” in on itself, demonstrating two alternative stable states that can exist for the same external conditions

These response curves and the feedback strength associated with them are further described in Figure 2, demonstrating how for a continuous, linear change in an external condition the three response curves may result, with the final one showing strong positive feedbacks that result in alternative stable states and increased hysteresis:

The shape of the third response curve, or the curve that demonstrates the theory of alternative stable states, is notable as it demonstrates how these regime shifts can occur rapidly and with little warning as they cross a threshold. The second notable feature of this response curve is the presence of hysteresis, i.e. the ecosystem cannot be returned to the previous regime by simply returning to the same external conditions. The third notable feature is that for the same conditions on the x-axis, two different ecosystems states could occur. This is the fundamental idea behind alternative stable states: fundamentally different ecosystem structures can occur under the same external conditions. Figure 3 demonstrates the two alternative stable states under changing external conditions, with the hill representing the threshold between these states.