Tipping points, Thresholds and Change
What are tipping points and thresholds, and who cares? A threshold is the boundary separating different behaviours or states. We are all familiar with thresholds – you cross a threshold and things change, and can do so abruptly. A tipping point is a threshold, but not all thresholds are tipping points. This is an important concept to grasp. We often misuse or interchange these two terms.
Once a threshold is crossed things change. A good river example is sediment movement - when flow velocity exceeds some critical point sediment at rest on the river bed starts moving. However, once flow velocity drops below this critical level sediment will stop moving. The important concept here is the return to the previous state. This a common phenomenon that we can identify with. We get sick, get help- take medicine-rehabilitate and, then get better – we return to where we were before getting sick. The time for recovery may vary depending of the extent of the sickness, or disturbance, but eventually we expect to return to our previous or ‘normal’ state or behaviour. The concept of recovery and return to ‘normal’ condition is deeply ingrained in how we view rivers and manage them.
Tipping point is often used in public (and sometimes scientific) discourse to refer to impending doom, or at least major change with uncertain and potentially negative impacts. The concept of tipping points has been used to highlight the significance of climate change, ecosystem collapse, regime shifts and a range of other phenomena. Not all tipping points are associated with negative impacts on humans. Nor are they associated with direct or indirect human activities; Earth history is marked by numerous tipping points, periods of thresholds of change and regime shifts. From 2010 to 2020 there have been at least 713 published scientific articles with the term ‘tipping point’ and 821 with ‘regime shift’ in the title (according to the Web of Science database, as of 18 August 2021). These articles span the disciplines of science, technology, and engineering, and river science is well represented.
There is one important difference between a tipping point and a threshold. Once you exceed a tipping point and move to a new state or set of behaviours there is no return, even if the cause of the change is removed. Recognising that river systems will not recover once a tipping point is exceeded is critical to how we study and manage these important landscapes and the ecosystems contained within them. Our research on river geomorphology and food webs clearly illustrates recovery back to ‘normal’ conditions does not occur if you cross a tipping point. Non-recovery to prior states or behaviours has implications for river restoration.
River restoration is big business. Every year we invest billions of dollars globally to restore and protect the many highly valued ecosystem services that rivers provide us; services like clean water and fishes. Our research on the Illinois River, USA, shows, for the first time, the unpredictable and complex response of a highly modified large river ecosystem to system-wide restoration measures. Despite improvements in water quality in the Illinois River, food webs did not respond or conform to accepted recovery models. Although fish diversity (a function of the total number of fishes, the number of fish species, and the community distribution) did improve, it did not do so to pre-disturbance levels. Far from it, the fishes of the Illinois River are still very suppressed compared to the early 20th Century. The patterns of fish diversity display a variety of response trajectories over the 60-year period of monitoring. The diversity of some types of fish increased while others decreased. Others decreased and then increased. The Illinois River has crossed a tipping point and, as a result is in a new regime with a different set of ecosystem structures, behaviours and sets of interactions.
Check out some of our research in this area.
DeBoer, J.A., Thoms, M.C., Delong, M.D. (in press). Ecosystem response through a resilience lens: Do differences in large river function over 150 y indicate regime shifts? Journal of Geophysical Research: Biogeosciences
Kreiling, R.M., Thoms, M.C., Bartsch, L.A, Larson, J.H., Christiansen, V.G., (2020). Modelling land use effects on sediment nutrient processes in a modified watershed using Structural Equation Models. Water Resources Research, 56, e2019WR026655. DOI:10.1029/2019WR026655
Thoms, M.C., Delong, M.D. (2018). Ecosystem responses to water resource developments in a large dryland river. Water Resources Research, 54, 6643-6655. DOI: 10.1029/2018WR022956