Wolf Urine, Deer Fences and Trophic Cascades
by Dr. Meredith Palmer
What the heck are these guys even up to?
Our study uses a clever experimental manipulation to reexamine the role that top predators like wolves play in shaping the environments that they live in. You may be familiar with the story of Yellowstone National Park, where gray wolves were reintroduced in 1994 after almost a century of local extinction. These new wolves were thought to have restored historic willow and aspen groves by scaring resident elk away from browsing on tasty saplings. We call this phenomenon a behaviorally-mediated trophic cascade: elk perform specific behaviors to avoid getting eaten by wolves (such as staying away from areas where wolves hunt or becoming increasing vigilant at the expense of foraging), which in turn alter how elk interact with and impact the overall landscape. This is a wonderful story, but one that is increasingly being called into question by new research.
Traditionally, ecologists focused only on the consumptive effect of predators – the effects that predators have on ecological processes by chowing down on and thinning prey populations. In the last few decades, evidence has emerged that non-consumptive predator effects, including these behavioral changes induced by the fear of being eaten, can have even stronger impacts on prey survival, reproduction, and how prey interact with the broader ecological community. Short-term experiments with spiders and grasshoppers suggested that predators shaped environments not by eating prey, but by scaring them. This was followed by a flurry of research looking for these “fear effects” in the wild, including at the Yellowstone National Park. The first batch of data to come out appear to support that yes, indeed, fear was the mechanism responsible for the drastic ecosystem changes following wolf restoration. Later research was not so certain – was it fear? Was it actually our throw-back hypothesis, consumption? Or were wolves and elk even responsible for these new trees at all? (some studies have thrust beavers into the limelight as being the true ecosystem engineers responsible for these dramatic vegetation changes) In other wildlife communities across North American and around the world, the findings were just as ambiguous. Clearly, there was a great need to reevaluate fear’s place as a structuring force in nature.
This is the exact question we tackled at the Cedar Creek Ecosystem Science Reserve. Cedar Creek presents an ideal ecosystem to evaluate the strength of the behaviorally-mediated trophic cascade pathway due to its special history with wolves. While part of the gray wolf’s historic stomping ground, wolves had not been present at Cedar Creek for over a hundred years until very recently. In 2014, a small family of wolves ventured down from northern Minnesota and set up shop in the reserve, denning and over the course of the subsequent years, starting a small family. This is great for wolf conservation, but likely not so exciting for local deer populations that led a predator-free existence for multiple generations. Deer readjusted to this renewed predation pressure, learning to survive and coexist again with this apex predator. The Cedar Creek wolves were then removed in 2017, creating a highly unique situation where deer had experience surviving with predators but not predators were locally present. That is to say, the consumptive effects of predators were gone, but if we could get deer to think there were still wolves around, non-consumptive effects might be induced…
The good news is that wolves have been sighted in the reserve since our experiment took place, giving us hope that they might once again call Cedar Creek home!
So how do we generate the fear of wolves in a landscape lacking predators, and thereby isolate that behaviorally-mediated pathway? First, we located over a dozen grassland sites across Cedar Creek that were closely matched in terms of plant communities and other environmental factors. Half of these study locations were randomly selected to be “safe” spots untainted by wolf cues, while the other half received a weekly dose of wolf urine to simulate the territorial markings of a resident wolf pack. This created a scenario with hotspots of "wolf" activity and safe areas avoided by "wolves".
You can order anything on the internet these days.
Based on classical theory, one would expect deer to predominantly hang out in the spots where they feel safe from predators and, by chowing down on all the vegetation in these safe areas but not in places they perceived to be dangerous, unevenly impact plants and lower trophic levels. To test this theory, we had to keep close tabs on deer behavior and assess whether there were any cascading effects of fear responses on plant community composition, vegetation biomass, and soil nutrients.
Deer behavior is easy – call in the camera traps! Who hasn’t seen a deer during their time on Eyes on the Wild? We used images from our camera trap grid to count daily visitors to safe and dangerous areas and check out what deer were doing at each location. When did they come? How long did they eat? Did mothers feel safe enough to bring their young? We anticipated that deer would be less likely to visit the places frequented by “wolves” and, when venturing into these risky areas, spend less time eating and more time watching for predators. The advantage of using cameras instead of human observers is that a) I didn’t have to stand in a field 24 hours a day for three months watching deer (just imagine!) and b) cameras don’t disturb natural animal behavior like human presence is known to do.
Roadmap for evaluating behaviorally-mediated trophic cascades.
Cascading effects on plants and soils were a little trickier to quantify. We anticipated that deer would eat less in areas they thought were dangerous, reducing the amount of vegetation in safe areas compared to dangerous ones, and affecting nutrient uptake by foraged vs. unforaged plants. First, we compared “safe” vs. “dangerous” locations by sampling and weighing vegetation within set plots and deploying probes that measured ions in the soils. We also performed in-site comparisons to see whether vegetation exposure to deer foraging decreased relative to plants we protected from herbivore consumption using the fences you so often see in the camera trap footage!
A mother deer and her baby have a snack - potentially perpetuating a trophic cascade - at one of our experimental plots.
At the end of the growing season, I classified all the images from the trail cameras - this was in the months before the Dec 2018 launch of the Zooniverse site, so the volunteer help we now rely on wasn’t yet available – and compiled the data from our plant and soil measurements. Then we sat back and crunched the numbers: is the fear of predators strong enough to restructure the landscape by causing a behaviorally-mediated trophic cascade?
… No! Not in this system at least. Our wolf sites and our safe sites were visited by the same number of deer, who came as frequently and ate as much, and consequentially, had comparable impacts on lower trophic levels. However, if we only looked at these metrics, we might incorrectly assume that deer didn’t perceive or respond to predation risk at all – and this is not the case. Traditional theory assumes that prey consistently avoid areas they perceive as dangerous. It is this complete abstinence that results in the strong cascading effects on what they eat and the soil nutrients being used. However, we did not observe this total avoidance of unsafe areas: we found that deer were congregating in safe areas during times when wolves would be actively hunting but venturing into "dangerous" areas when wolves would typically be sleeping or otherwise inactive. Remember that while there was no wolf pack in this system when the study was going on, almost all the deer at Cedar Creek had personal experience with the wolf pack that lived there the previous three years. So essentially, while the traditional theory only assumes prey respond to where predators are in space, leading to a "trophic cascade" on plants and soils, we find that prey respond to predator activity in both space AND time, spreading their impact across the entire landscape and thereby attenuating top-down predator effects on the broader ecological community.
This is an exciting experiment because it challenges a lot of strongly-held theory about the role that top predators play in shaping their ecological communities. We present an interesting mechanism that could help explain why trophic cascades are so often predicted but rarely materialize in terrestrial ecosystems!
Research like this is only possible with help from an amazing team of collaborators: many thanks to the amazing research team at Cedar Creek, including Drs. Forest Isbell, Caitlin Potter, and L. David Mech, the newly-minted Dr. Cristy Portales-Reyes who assisted with this work as a graduate student, and the innumerable interns and staff who helped collect data. We also cannot stress how invaluable citizen scientists are in classifying camera trap photos for projects like this – your help is an invaluable part of the scientific process, and will greatly speed up our ability to suss out patterns like this at Cedar Creek, now that the Zooniverse interface is chugging along smoothly!
Still curious? Check out my previous post or listen to our ‘Lunch with a Scientist’ presentation on this exciting experiment!
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