Wolves and Trophic Cascades: What is All the Controversy About?
article by Douglas W. Smith
The wolf reintroduction to the American West has been described by some as the conservation event of the century – a restorative event for nature. Others have viewed it as a disaster. They say the idea of pristine ecosystems is no longer valid – it’s a human dominated world out there and pretending that we can fix nature is naive. So can wolves really fix landscapes damaged by humans? This idea has caught on and is a rallying cry for expansion of wolf recovery to other regions, but is it really true? The scientific debate on both wolves and trophic cascades has been intense, with researchers claiming everything from ‘trophic cascades don’t exist’, to ‘they do, but the explanation of how they work is just wrong’. So how do wolves actually impact ecosystems? Or, what really is a trophic cascade?
Trophic cascades are indirect ecological interactions, events in nature that move through multiple trophic levels (remember the food chain or pyramid: vegetation – herbivores – predators, as a simple example); that makes these cascades the ecological equivalent of the well-known ‘trickle down’ effect in economics. One research paper described them as ‘the impact of a predator on its prey’s ecology trickles one more feeding level to affect the density and/or behavior of the prey’s prey”. So in terms of the current debate in Yellowstone, it’s about wolves affecting elk and elk affecting vegetation (wolves therefore affect vegetation indirectly). In this case we are only referring to woody vegetation: willow shrubs and aspen and cottonwood trees. After decades of suppression, some of Yellowstone’s woody plants are now growing again (in biological terms they have “released”) and many attribute this change in growth to wolf restoration and its impact on elk. The story in Yellowstone is seductive as it is one of the few examples in today’s world of something getting better in nature — yet it is hotly debated.
It’s debated because understanding how ecosystems function and are organized is one of the holiest of grails in all of ecology. Simply described one camp believes in “bottom-up” forces impacting ecosystems – sun into plants, plants into herbivores, and herbivores into carnivores; or the reverse could be true, carnivores eat herbivores, impacting how many herbivores eat plants, or what is called “top-down”. So which is it? Or maybe it’s both and switches between these two conditions over time? These opposing views offer very different interpretations of the essence of ecosystem function and this is at the heart of the battle.
Another issue that comes up as one tries to understand the trophic cascade debate for wolves relates to interpretation of what is written on the subject. Often researchers are not clear when they write, or reporters perhaps distort what they are reporting about. For example, when a writer seems to show opposition to this theory, is the criticism intended to add more detail to past research and better describe how a trophic cascade works, or is it actually intended to reject trophic cascades out of hand? One researcher, for example, may believe that trophic cascades exist, but they support a view that increased vegetation is due primarily to reduced herbivore numbers due to predation – this is referred to as a numeric affect. Others also support trophic cascades, but claim it is the result of changes in herbivore behavior. Since wolves were restored to Yellowstone, elk can no longer forage anywhere and anytime they want. This has caused some vegetation to grow better in terrain which has more risk posed by the presence of wolves. This was described with the very catchy term ‘landscape of fear’ (ecologists like sexy labels as much as anybody else) and is a behavioral response that does not necessarily require fewer elk. Which one of these is primary (or both again) is the subject of intense and legitimate debate, since we do not have an answer yet.
Another criticism of Yellowstone’s trophic cascades relates to those claiming all changes are attributed to wolves, when there are other important predators – like cougars and bears—in the system. Multiple studies have proven that it usually takes more than just one predator to significantly reduce prey populations, so attributing all ecological change to wolves ignores other important players. This is another very legitimate criticism. Furthermore, other factors come into play like site characteristics and water availability. Nature is complex and it is difficult to ascribe change to just one factor. Another criticism is that there has not been enough time for the ecosystem to really change – predator (wolf, cougar, bear) recovery in Yellowstone has only been underway for a few decades and this is not enough time for the ecosystem to be ‘fixed’. In scientific parlance ecological states are stubborn – they do not change overnight.
None of these criticisms refute the phenomena of trophic cascades – they are just arguing about how they work. If trophic cascades are rejected as a whole, then what is the alternative explanation? An all too common ill in our world today is people often criticizing ideas without offering a better explanation. One possibility is that all the changes we see may be due to weather variations. This theory posits that there are periods of favorable weather conditions that allow for productive plant growth and essentially the plants can out-grow the herbivores (elk in Yellowstone) no matter how many are eating them. This is a viable alternative, but it is not well-supported by the data, little has been published on it, and the researchers supporting it have not subjected their views to open and widespread debate – so it has not caught on.
Some researchers want set themselves apart in these debates; after all one does not get famous by agreeing with everyone. Science is essentially based on scientists arguing to clarify concepts, and trophic cascades have generated a lot of argument. The race to be famous, present in all scientific endeavors, involves setting yourself apart by discovering and publicizing the nugget that no one else figured out. Reading the literature on trophic cascades closely reveals that there is much fundamental agreement, but subtle disagreement. Statements such as ‘decades without wolves changed Yellowstone too much to undo’ does not actually discount that trophic cascades exist, but questions whether they will return to historic patterns. I was tempted to say to this critic ‘what about cougars and bears?’, and point out to them that they are essentially making an argument in favor of trophic cascades, while at the same time explicitly stating ‘It’s not true’. This article confused so many people that I received dozens of emails asking ‘Are trophic cascades true at all?’. In reality this was just another essay aimed at improving our understanding of Yellowstone’s ecosystem function, AND making a splash.
Finally, I do agree that wolves are not the panacea to fix all environmental ills. As mentioned, it takes more than just wolves to create change, and as importantly, wolves and other predators must occur at their ‘natural’ density. What do I mean by ‘natural’ density? Density set by what the landscape can support, and in this case it would be wolf prey. One of the problems this presents is that rarely outside of parks and remote wilderness do we see predators at their ‘natural’ density. Because of conflicts and competition with humans predators are usually reduced below the density of what the landscape can support, to a point often referred to as “social carrying capacity”, or the number humans will tolerate. It is not just predator (wolf) presence on the landscape, but their number which determines their ecological impact. After all, it is a human dominated world out there and most of what we see happening to ecosystems is due to our own behavior: agriculture, hunting, development, logging, mining, and the like. This does not have anything to do with trophic cascades unless humans are counted as top predator.
So the final valid criticism of trophic cascades: In only a precious few places will nature function unfettered by humans.
Douglas W. Smith is the project leader for the Yellowstone Gray Wolf Restoration Project in Yellowstone National Park. He has been with the program since its inception in 1994, and has studied wolves for more than 30 years.