The Yellowstone Hysteresis: nature cannot always be restored

One of the most reassuring ideas underlying our relationship with nature is that destruction and restoration are, at least in principle, inverse processes. If we eliminate a species, we can reintroduce it; if we interrupt a process, we can reactivate it; if we give nature space once more, we can expect the ecosystem to recover what it had lost. A significant part of contemporary conservation is necessarily based on the possibility of restoration, because there are already altered environments in which intervention is required; the problem arises when we move from the possibility of achieving recovery to the idea that an ecosystem maintains a state of equilibrium towards which it will spontaneously tend as soon as we have removed the cause of the damage.

Ecology in recent decades has provided many reasons to be wary of this portrayal. Ecosystems have a history, and what happens during a disturbance can alter the conditions on which all subsequent developments will depend. A species that has gone extinct may have left behind a different community; a different community may have transformed the soil or water; those transformations may continue to exert their effects even after the cause that set the process in motion has been removed. Returning to the starting point therefore requires something other than simply retracing one’s steps, and in some cases that previous path may no longer be available.

Few places illustrate this problem better than Yellowstone, which for years has been used to tell precisely the opposite story.

At the beginning of the 20th century, wolves were systematically eradicated from the national park as part of predator control policies that were considered normal at the time. The last known pack was culled in the 1920s, and for around seventy years the wolf remained virtually absent from Yellowstone. Meanwhile, the elk – the large North American deer that is one of the wolf’s main prey species – exerted increasing pressure on the vegetation, particularly on willows and other plants in riparian areas.

When, in 1995, the United States decided to reintroduce wolves by transferring them from Canada, one of the most successful ecological stories of recent decades began to take shape. The wolves preyed on elk and reduced their numbers; their presence also altered the behaviour of their prey, which would frequent areas where the risk of being killed was greater less frequently; the willows were thus able to regrow, the beavers found food and material for their dams, the dams altered the course of the waterways once more, and the recovery of vegetation reduced bank erosion. By the end of this process, the return of a predator had even altered the rivers.

The scientific basis for this story was sound. In the early years following the reintroduction, several researchers investigated the possibility that Yellowstone was exhibiting a trophic cascade – that is, the propagation of effects exerted by a predator through the lower levels of the food web. Other studies focused on the ‘ecology of fear’, according to which the presence of the predator might have altered the way prey used the space even before reducing their numbers. These were reasonable ecological hypotheses, subject to normal scientific debate and difficult to distinguish from the other processes affecting the park at the same time.

The transformation took place when this complexity was condensed into a linear narrative.

In 2013, George Monbiot took Yellowstone to the TEDGlobal stage during a talk dedicated to rewilding – the restoration of ecological processes through the reintroduction of extinct species and the reduction of human intervention. The case of the wolves featured prominently in the talk: after seventy years of absence, the return of this predator would have reduced the deer population, encouraged the regrowth of trees along watercourses and allowed beavers to return, ultimately leading to changes in the very shape of the rivers.

The following year, that section of the speech was turned into the video "How Wolves Change Rivers", featuring spectacular footage of Yellowstone, Monbiot’s voiceover and a narrative flow so straightforward that it could be followed without any knowledge of ecology. The video reached tens of millions of people, and its central argument became part of environmental outreach, educational materials and popular conservation culture.

Its strength lay in the perfect symmetry of the narrative. Man had removed the wolf and Yellowstone had deteriorated; man had reintroduced the wolf and Yellowstone had begun to recover. What had been taken away was restored, and the system resumed its path towards its previous state. The removal and reintroduction appeared as two inverse operations, separated by seventy years and linked by the same causal chain, traversed first in one direction and then in the other.

This idea coincided perfectly with a much more general belief: that nature possesses a state of its own, from which we can cause it to deviate, but to which it would be able to return as soon as we restore what we have taken away. The ability of ecosystems to reorganise themselves was thus interpreted as a capacity for self-repair, attributing to nature a sort of memory of its previous state.

Yellowstone seemed to provide the evidence that was missing to support this belief.

Data collected in subsequent years painted a different picture. Wolves certainly preyed on elk and contributed to the decline in their population, alongside other large carnivores and human hunting outside the park’s boundaries. The behavioural effects attributed to fear of predators proved to be variable and context-dependent, as a herbivore must continue to forage and chooses where to do so based on the distribution of resources as well as the risk of being killed. Vegetation showed recovery in some areas and much more modest responses in others.

Daniel Stahler, who heads the Yellowstone wolf programme, described these effects using a phrase far less suited to a viral video: rather than a major ‘trophic cascade’, in many parts of the system a ‘trophic trickle’ is observed.

The most significant finding comes from an experiment that began in 2001 and continued for twenty years, published in Ecological Monographs by N. Thompson Hobbs and colleagues. The authors studied willow communities in the northern part of Yellowstone to understand why the return of large carnivores had not restored the riparian environment that existed prior to their elimination.

To understand this, one needs to look at what happened during the decades when the wolves were absent.

The willows in riparian areas have been linked to beaver activity through a relationship that has endured over time. Beavers used the willows both as food and as building material; their dams slowed the flow of water and raised the water table locally, creating conditions favourable to the growth of the willows themselves. The vegetation therefore supported the beavers’ activity, which in turn helped to maintain the environment necessary for the vegetation.

The pressure exerted by the elk kept the willows short for a long time and reduced the resources available to beavers. As the number of dams decreased, the watercourses also changed: river channels became deeper, the water table fell in several areas, and conditions favourable to the growth of riparian plants diminished. A process that began with a change in the food web had thus gradually transformed the physical environment as well.

When the wolves returned to Yellowstone, they found the result of those seventy years.

Hobbs and colleagues experimentally separated the two main factors. In some plots, they prevented large herbivores from reaching the willows by erecting fences; in others, they built artificial dams that mimicked the hydrological effects produced by beavers. The design allowed for a comparison between normal conditions and those in which grazing was reduced, and each situation could be studied with or without the restoration of the hydrological conditions associated with the dams.

The question was crucial. If the problem had essentially stemmed from pressure from the elk, reducing grazing should have allowed the willows to return to their previous condition. This was precisely the outcome that the reintroduction of large carnivores was intended to promote.

After twenty years of experimentation, simply excluding herbivores improved the growth of the willows without restoring the historic community. Altering the hydrological conditions also had a significant effect. The willows reached the heights characteristic of the environment being restored when the two measures were combined: reducing grazing whilst simultaneously restoring the conditions previously created by beaver dams.

The return of the predators had not erased what had happened during their absence, because in the meantime the system had acquired new constraints.

In ecology, this phenomenon is described by the concept of hysteresis. When a system exhibits hysteresis, the trajectory it follows after a disturbance depends on its previous history, because certain variables have changed during the transition and continue to influence what may happen subsequently. Returning the factor that caused the change to its initial value does not, therefore, force the system to retrace its steps.

The model proposed by Hobbs and colleagues illustrates the difference well. In a very simple food chain, consisting of a predator, a herbivore and a plant, one can imagine that the removal of the predator would increase the number of herbivores and reduce the number of plants, whilst the return of the predator could rapidly reverse that sequence. Real-world ecosystems comprise organisms that alter the environment in which they live and processes that operate over different timescales. During the absence of predators, herbivores can therefore contribute to changes in the habitat that persist even after the predators return.

Yellowstone provides a particularly clear example of why the beaver is an ‘ecosystem engineer’: through its dams, it directly alters the hydrology. When its activity declines, this changes a variable that will continue to influence the vegetation even if the pressure from the elk is subsequently reduced.

Meanwhile, the herbivore community has also changed. The elk population has declined sharply compared with the 1990s, whilst the bison population has increased and now accounts for a much larger proportion of the biomass of large herbivores. Bison are far less vulnerable to wolves and continue to exert pressure on the vegetation. Consequently, the food web into which the predator has been reintroduced is also different from the one that existed before its elimination.

Talking about hysteresis does not mean claiming that every transformation is definitively irreversible. Hobbs’s experiment shows that, by intervening in hydrology and herbivore pressure, it is possible to shift the system once again and encourage strong growth in willows. It means recognising that restoration does not consist of undoing the initial intervention, because the system we are acting upon today is the product of what has happened in the meantime.

This distinction profoundly alters the meaning of the expression ‘nature’s self-repair’. A disturbed ecosystem continues to organise itself according to the prevailing conditions and may stabilise in a configuration different from the previous one. Nature does not retain a blueprint of the lost state and does not work to restore it. It retains the consequences of the processes that have taken place.

A carved-out watercourse remains carved out even as the elk population declines; a lowered water table continues to affect the vegetation even after the wolves have returned. If beavers cannot find enough willow, they do not build the dams that would encourage the growth of the willow itself. The past thus becomes part of the causality of the present.

Even the concept of balance in nature becomes dangerous when it is used as if it were describing a general property of ecosystems. An ecological system can remain in different configurations for a long time and can develop mechanisms that cause a state resulting from a previous disturbance to persist. Stability offers no guarantee that the stabilised state will be the one we wish to preserve.

The implications for conservation are very serious. If degradation and restoration were symmetrical processes, we could regard the loss of a species or the transformation of a habitat as temporary damage, because it would be sufficient to subsequently eliminate the original cause and leave it to the system to rebuild what had been lost. Hysteresis removes this guarantee, because during degradation the ecosystem continues to change, and every change alters the conditions on which recovery will depend.

Restoration may therefore require interventions that no one would have foreseen at the start of the transformation and may depend on processes that have become difficult to reconstruct. In some systems, thresholds may be exceeded beyond which a return to the previous state requires conditions that no longer exist.

Preservation remains a top priority in this regard – one that no promise of restoration can replace.

The Yellowstone case also shows just how easily a sound scientific question can turn into a false certainty when it encounters a narrative that confirms what we already wish to believe. Trophic cascades do exist and the ecological effects of large predators are real; the oversimplification arose when a network of time-dependent relationships was reduced to a linear chain in which each step could be traversed in both directions.

The success of "How Wolves Change Rivers" also stemmed from this implicit reversibility. The story suggested that by removing one element, we had caused degradation; by reintroducing that same element, we would set the entire system in motion in the opposite direction. It was a perfect fit for our confirmation bias, because it transformed the complexity of ecology into a demonstration of nature’s innate ability to restore its own balance.

Twenty years of experimentation have taught us a different lesson. Ecosystems do not wait for us to intervene in order to continue changing, nor do they revert to their past state when we remove the cause that had transformed them. Every phase of their history shapes the conditions of the next phase.

The wolf has returned to Yellowstone. But the time that has passed during its absence has not.

And it is precisely for this reason that destroying and rebuilding are not equivalent actions, whilst preserving what still exists remains the safest form of ecological intervention possible.