Invasive plants as drivers of regime shifts: Identifying high priority invaders that alter feedback
relationships
Mirijam Gaertner, Reinette Biggs, Mariska Te Beest, Cang Hui, Jane Molofsky & David M.
Richardson
Appendix S1: Theoretical background on regime shifts
From a systems perspective, ecological regime shifts entail a switch within an ecosystem from
one domain of attraction to another due to a change in the ecological feedback processes that
structure the system (Biggs et al. 2012). All ecosystems have multiple reinforcing (positive) and
balancing (negative) feedback processes. Balancing feedbacks tend to counteract changes in the
system i.e. enhance ecosystem stability, whereas reinforcing feedbacks amplify changes i.e.
destabilise ecosystems (e.g. Ehrenfeld et al. 2005). The interaction and balance between all the
different feedbacks in a system, and in particular which feedbacks dominate, will determine
whether the net outcome is one of system stability or instability. For a particular set of external
conditions, there are typically one or more different configurations of feedbacks in which the
system can stabilize over time. These different possible configurations of dominant feedbacks
define the alternate regimes in which the system can exist, sometimes under the same set of
external conditions.
Regime shifts occur when there is a shift between these alternate configurations of
dominant ecosystem feedbacks, either due to a change in the balance between existing
feedbacks in the system or the introduction of new feedbacks to the system (Bennett et al.
2005). For a regime shift to occur there has to be at least one reinforcing feedback present in the
system that can assume sufficient dominance in the system for a period of time to drive a shift to
an alternate regime or feedback configuration. Regime shifts typically result from a combination
of gradual changes in exogenous drivers and an external shock (disturbance). Gradual changes
in drivers (e.g. altered nutrient cycling) may slowly weaken the dominant feedbacks that
maintain the system within a particular regime. Once the system is close to a critical threshold, a
shift to a different set of dominant feedbacks can be precipitated by even a small shock to the
system. Weakening of the dominant feedbacks (and hence a loss of ecosystem resilience)
typically leads to little or no obvious system changes until a critical threshold is reached where
one or more reinforcing feedbacks become dominant and drive the system towards an alternate
configuration or domain of attraction – i.e., a regime shift occurs. Loss of ecosystem resilience
therefore often goes unnoticed until a regime shift occurs.
Because different sets of dominant feedbacks are associated with different regimes and
since these feedbacks are often self-reinforcing, regime shifts are often hysteretic or “sticky”:
once the system is in a particular regime, it tends to remain there even if the exogenous drivers
that caused the shift are reduced or removed. The critical threshold for a shift from Regime 1 to
2 therefore differs from the critical threshold for a return shift from Regime 2 to 1, making many
regime shifts very difficult or even impossible to reverse. Restoration work aimed at reversing a
regime shift therefore often involves breaking unwanted feedbacks that have become dominant
and/or rebuilding or strengthening other feedbacks that promote organization of the system
around a different attractor (Suding & Hobbs 2009). The degree of hysteresis in a system
depends on the strength of the dominant system feedbacks, which is usually a function of
multiple factors (e. g. plant types, sediment type).
References
Bennett, E.M., Cumming, G.S. & Peterson, G.D. (2005). A systems model approach to determining
resilience surrogates for case studies. Ecosystems, 8, 945–957.
Biggs, R., Blenckner, T., Folke, C., Gordon, L. J., Norström, A., Nyström, M. & Peterson, G. D. (2012).
Regime shifts. In: Encyclopedia of theoretical ecology (ed by A. Hastings and L. Gross).
University of California Press, Berkeley, pp. 609-617.
Ehrenfeld, J.G., Ravit, B. & Elgersma, K. (2005). Feedback in the plant-soil system. Annual Review
of Environmental Research, 30, 75–115.
Suding, K.N. & Hobbs, R.J. 2009. Threshold models in restoration and conservation: A developing
framework. Trends in Ecology & Evolution, 24, 271-279.