Yellowstone, wolves, elk, and aspens: a tale too good to be true?

If you were to say something in passing to someone about “the Yellowstone story”, they would probably think you were referring to the television series “Yellowstone”. Set in the ranching country outside Yellowstone National Park, it is a good story, with excellent acting and beautiful cinematic values, and has received a lot of attention in America. This would be a reasonable association to make. If, however, you were talking to a biologist, and especially an ecologist, they would probably think you were referring to a true story, set in Yellowstone park itself, which began in 1995, thus 30 years ago.

Here, I recount that second tale. Like the fictional story, it is enjoyable and interesting but has even larger resonances, bearing on some of the biggest issues in ecology. It also illustrates the difficulties of exploring (and understanding) complex systems. These are systems with many elements, which interact in multiple ways, neither predictable in advance nor always fully understood in retrospect. Such systems are at the heart of biological science – indeed, every individual organism is a complex system in itself and embedded within a complex ecological system. The Yellowstone story illustrates the problems of investigating and understanding this kind of complexity.

The Yellowstone National Park was created by federal legislation in 1872, only a little more than 80 years after the United States had come into existence. The American West, at this point, still consisted largely of wilderness areas, lightly populated with Native Americans and rapidly growing but still relatively small numbers of European descendants. However, the shape of the future, with its prospects of massive settlement and development in the West, could be discerned. As far-sighted people realized, if areas of the West were to be saved in their full natural beauty, they would need protection by the government. Yellowstone National Park was the initial step, with other national parks established in the following decades. In addition to preserving the land and the forests in such national parks, a major goal was to protect their wildlife from the mass slaughter of wild animals already taking place in the 1860s and ‘70s.

Such protection, however, could not extend beyond the parks’ boundaries and, inevitably, the animal populations within the national parks were affected by what was happening to their conspecifics outside the parks. After all, the animals, in their movements, did not observe the park boundaries. Great reductions in the numbers of those animals outside the parks would therefore, inevitably, be reflected in the numbers within them. The emblematic case of mass wildlife slaughter was the American bison, whose numbers went in a few decades from millions to hundreds, being close to extinction. Special herds of bison were established within a few parks, in particular Yellowstone, to save them from the ravages outside the parks.

Furthermore, animals were not always protected within the parks. In particular, wolves were considered odious by ranchers and farmers because they killed cattle and sheep. They were not only shot wholesale outside the parks but even killed within them, by hunting and poisoning, for decades. By the 1930s, wolves had been hugely reduced in numbers from much of the contiguous 48 states and were entirely gone from Yellowstone National Park.

By the 1970s, however, attitudes toward wild animals had changed from earlier times, as knowledge about them and their ecosystems had grown. This period marked the beginning of the environmentalist movement as a public phenomenon, as symbolized by the passing of the Endangered Species Act. It also featured a new appreciation of the role of predators in the economy of nature. It was understood that in the absence of certain predators, populations of herbivores could increase explosively and do tremendous damage to various valued plant species, not just crops but also trees. In Yellowstone, for instance, it was clear by this point that the elk and deer populations had grown far beyond desirable limits. The elk population was on the order of 20-30.000, numbers far beyond the desirable carrying capacity of the park.

Accordingly, in 1972, a plan was hatched within the Nixon administration, to reintroduce wolves to Yellowstone. It took considerable time and planning to fully develop and, of course, there were questions about it, including those from ranchers raising livestock on lands outside the park boundaries. Yet by the mid-1990s, the scheme was ready to be tried. In 1995 and 1996, a total of 31 wolves, imported from Canada, were released into the park, the first wolf presence there in about 70 years. Within a few short years, it was apparent that the wolves were thriving and their numbers increasing.

This was an experiment and, by definition, an experiment’s outcome is always uncertain. It was not clear how it would go but the initial interest was primarily in how well the wolves would do and how the elk populations would respond to their reintroduction. Even after only five to six years, however, it was clear that this experiment was a success: the wolf numbers were increasing, elk and deer numbers had somewhat declined, and the park’s ecosystems were not disturbed in any undesirable way. There was, however, a lot more going on.

The biggest unexpected change was in the vegetation. Much of the land that had been overgrazed by the elk had begun to regenerate its plant growth; both bushes and new trees began to appear. In particular, the aspens and willows began to flourish along the banks of the rivers and streams. These trees had been abundant there in earlier times. Even more strikingly, fairly sandy areas flanking the rivers, which had previously hosted a healthy vegetation began to do so again. This in turn stabilized the banks, which permitted the waters to flow more directly, without meanders and oxbows. Oxbows and shallow pools began to reduce and be replaced by faster running rivers.

As a result, soon there were visible effects on the animal life. More bushes meant more berries and the bears definitely benefitted and increased in number. More trees meant more places for birds to roost and soon the bird populations were growing. Decreased grazing of the young trees also allowed trees to grow taller, with the result that their leaves began to be out of reach of the elk, allowing their further growth. In short, there was a true regeneration of willow and aspen forests.

These were not the only changes. Increased numbers of trees also provided more building material for beavers. By the early 2000s, there were more beaver dams and deeper areas of the rivers and streams, promoting conditions for more fish and more diverse fish populations. Mountain lions had previously begun to re-infiltrate the park on their own, in the 1980s, but their numbers also were increasing, alongside those of the wolves and the bears. Despite the increased number of mammalian predators, the elk and deer populations did not crash but simply seemed to reach lower but still large and sustainable levels.¹

In short, this early picture seemed to indicate that the introduction of wolves to Yellowstone had triggered an ecological renaissance for the park, with both increased species diversity and increased numbers of each population, both the plants and the animals. Increased species richness means increased ecological resilience and stability. Any restoration project that changes an ecosystem in this way is a success. For Yellowstone National Park specifically, this was a hopeful sign for the long-term future of the park.

For ecologists and for friends of the American national park system, what was there not to like about this picture? Not only had a species been successfully re-introduced to its natural ecosystem, and had thrived, but there had been a cascade of positive effects, enriching and diversifying both the plant and animal life of the park. Indeed, there was a term to describe this effect: a “trophic cascade”. Change one important element higher up in the food chain and you catalyze a whole sequence of changes, affecting the nutritional status and numbers of organisms lower down in the chain.

Most trophic cascades that had been demonstrated had involved smaller animals, often invertebrates, and frequently on a smaller scale or in artificial, experimental circumstances. Yet, one dramatic prior example had a parallel with the Yellowstone story. This had involved sea otters on the Pacific coast. They had been hunted to near extinction by the early 20^th century for their luxurious fur, vital to their survival in the frigid waters of the Pacific but valued as a fashion item in the Western world. In the 20^th century, however, the loss of the sea otter had been understood as a real loss. They had begun to be seriously protected and their numbers had rebounded.

A consequence of this sea otter resurgence had been the restoration of the marine kelp forests along this coast, many of which had declined sharply in their absence. The reason for that decline was the proliferation of sea urchins, a major prey item for the otters that took place after the sea otters had mostly vanished from the northern Pacific coast. Sea urchins feed on kelp, especially their roots from which they grow. Largely released from predation by the otters, the sea urchins gnawed on and killed much of the kelp forests. Since kelp provide a rich ecosystem for fish and other animals, the kelp ecosystem as a whole had gone into steep decline with the near disappearance of the sea otter. As the otters came back, the sea urchin populations were checked, enabling the kelp to rebound and, with them, all the marine animal species that they provide shelter for.

The Yellowstone National Park example was an even more dramatic, or at least more visible, example of ecosystem restoration, taking place in a terrestrial system where observation and counting of animals was considerably easier. It was soon being celebrated by the environmental movement. Yet, was it too good to be true? – a whole ecosystem being revived and flourishing as the result of one species’ (wolves) introduction? So often, when something seems to be too good to be true, it is not or at least not as much as first believed.

In this case, the end-result – a richer, more diverse, more stable ecosystem in Yellowstone Park– was not in doubt; there was too much evidence for it to be denied. The question concerned the explanation: was this ecological renaissance really attributable to wolf reintroduction? Certainly the timing seemed to suggest this but that was hardly proof. As we know, correlations are not proof, indeed they are the bane of attempted explanations. If one wants clear causal explanations of a change you need additional evidence to support your claim.

In particular, the timing of the tree rebound seemed too quick. Recall that the original number of introduced wolves was 31 yet the prey population of elk was at least 20,000. Even with wolf numbers increasing due to births of wolf pups each year, the suite of ecological changes seemed to be far too rapid to be attributable to the killing of elk by wolves, given the disparity between elk and wolf numbers.

Perhaps it was not the active predation by the wolves that was causing the change but simply their presence that was causing a change in the behavior of the prey, the elks, which was at the heart of the explanation? This was the suggestion from the ecologist William Ripple and his colleagues, who published their idea in the early 2000s.

They proposed that it was the presence of wolves and their psychological effect on the elk that had been responsible. The wolves presumably had introduced a “the landscape of fear” amongst the elks, which caused them to flee the exposed terrain along the rivers, seeking shelter in the more protected upland areas. With elk largely self-removed into the highlands of the Park, away from its valleys and rivers, the vegetation, in particular the willows and oaks, had had a chance to recover from the over-grazing by the elk. In principle, even relatively small numbers of wolves might have had this outsized effect on elk psychology with the cascade of effects described above.

This was a plausible explanation and, for those of us who think about animal behavior and regard many animals (particularly mammals and birds) as sentient beings (even if not having the cognitive capacities of human beings) a very appealing one. Many animals clearly are aware of their environment – no animal in the wild would survive long if it did not – and some degree of agency in responding to its changes. Yet, in general, in science, it is the explanations that most appeal to one that one should be the most suspicious of. Correspondingly, it is the “good” stories that one should be most ready to test, to see if they hold up.

One of the wonderful aspects of this elk-wolf-aspen-rivers story is that it has been amenable to much observation, data-gathering and, therefore, a degree of testing. Ecological stories require time to unfold but, over 30 years, since the initial wolf reintroduction in 1995, there has been time and plenty of opportunity for observations and measurements of the different variables, using all the tools of modern ecology.

The results, which I will now briefly describe, show that things are more complicated than any single-factor hypothesis proposes and that “the landscape of fear” explanation is insufficient as the main factor in explaining the ecological restoration of the Yellowstone’s rivers.²

First, many observations of the park’s elk in the proximity of the wolves is that they are not terrified of the latter. They are wary, yes, watchful and observant, yes, but not terrified. I have not seen this myself but I have watched equivalent situations on the savannahs of Africa, where the various kinds of antelopes are in relatively close proximity to resting prides of lions. For herbivores, predators are a fact of life, creatures to be aware of, especially if they get too close or make sudden moves, but not to live in terror of. Indeed, if prey mammals were in a constant state of fear of wolves, bears, cougars (mountain lions), or coyotes, they would be in perpetual stress. We know enough about stress in humans to be sure that constant stress in other mammals would be health-destroying and life-shortening. (The species that showed the most visible stress around the wolves were the coyotes whose numbers in the park did decline.)

Second, while elk numbers did reduce by more than 50% between 1997 and 2007, the wolves were not the chief cause. Rather, hunting by humans which was allowed in the park careful studies from 1997 onwards strongly indicate that hunting by humans, allowed to thin the elk herds, was responsible for the largest share of the decline. Other predators that contributed to the decline were the two bear species in the park, Grizzly bears and brown bears, and cougars.

Third, there has been much dispute on how extensively throughout the park, the willows and aspens regenerated from their 1997 state. The argument has been as much about the best way to measure the regrowth of the trees as about its extent. It appears to be patchy in fact, with some areas showing less than others. (This is not surprising.) Unlike some scientific arguments, which become heated and even personal, this one, between the two principal scientists involved in this work, Elaine Brice and William Ripple, has remained completely civilized and, indeed, constructive, with both parties agreeing that the matter of causative factors and the details of the dynamics of reforestation need further work.³

Where does this leave us in terms of understanding this particular Yellowstone story? There are several relatively safe conclusions at this point. First, that major ecosystem restoration in Yellowstone National Park did take place within a large area of the park following the restoration of the wolves. Second, while the wolves played a part, they may not have been the major causative factor. Third, “the landscape of fear” hypothesis is probably not the entire explanation of the wolves’ beneficial effect although it, of course, may have been a contributing factor. (Unsurprisingly, Ripple, who proposed it, and Brice, who opposed it, differ on whether it has been disproved.) Fourth, all of this work has substantiated the idea that predators do play an important part in the health of the ecosystems in which they act. Keeping herbivores within certain limits is vital for that. This Yellowstone story has inspired other predator reintroduction schemes in other parts of the world and so far, they seem to be a good thing. In a world dominated by stories of destruction of the natural world, it is heartening to see stories like this, about the resilience of Nature, when given a chance.

Last but not least, the whole story illustrates the exploration of biological complexity at its best. New information comes in; people think about it and propose new ideas; some realize that these new explanations can be explored and tested; the latter is done; not everyone agrees on the interpretations and relative importance of all the contributing factors and more work is generated; and, the whole time, understanding gradually grows.

1. There are a lot of popular articles on this topic, just google “Wolves, Yellowstone” and you will find such. For a more cinematic experience of this tale of wolf introduction, look at the You Tube videos, “How Wolves Change Rivers” (narrated by George Monbiot, a British political commentator with a background in zoology) and “Wolves saved Yellowstone National Park – The Northern Range.”

2. For a short news account, see “ ‘Landscape of fear’ had little impact on Yellowstone ecology” by Virginia Morell, Science 386, p. 366. It is accompanied a picture of a herd of elk, near a small group of wolves; the elk do not seem particularly scared.

3. See Brice, E.M. et al. (2022). Sampling bias exaggerates a textbook example of a trophic cascade, Ecology Letters 25: 177-188, and Beschta, R.L. et al. (2023). Revisiting trophic cascades and aspen recovery in northern Yellowstone.

doi.org/ 10.1016/j.fooweb.2023.e00276