A novel trophic cascade between cougars and feral donkeys shapes desert wetlands (2022) Lundgren et al., Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.13766

Image credit: CHUCAO, CC BY-SA 3.0, via Wikimedia Commons

The Crux

Trophic cascades (see Did You Know?) are an important part of many ecological systems. However, most of the world’s large predator species were lost around 10,000 years ago (potentially due to human impacts), thus limiting the role that predators could play in driving trophic cascades. Though large predators were lost, many large herbivores are still around, which means it is difficult for a smaller predator to take down/consume these herbivores, much less have an effect large enough to drive a trophic cascade.

In the United States, large felines such as cougars (Puma concolor) are known to predate large equid species (such as feral horses or donkeys), but much of the ecological literature assumes/claims that cougars do no exert a strong enough pressure to consider them “significant” predators of these equid species. Specifically, some reports state that these species don’t have any natural predators, and other reports echo the claim. Today’s authors report on a novel trophic cascade between the cougar, feral donkeys (Equus africanus asinus), and wetland vegetation.

Did You Know: Trophic cascades

We’ve mentioned them before on Ecology for the Masses, but because trophic cascades are such an important part of today’s study it is worth coming back to them. A trophic cascade is when an organisms at the upper or lower parts of a food web causes an effect that cascades downwards or upwards to affect the other organisms in the food web. The most famous example of this was the reintroduction of wolves to Yellowstone National Park in the USA. When the wolves came back to the park, their deer prey began avoiding areas where the wolves were active. Because the deer stopped eating some of the vegetation, erosion was limited, which stabilized riverbanks, which eventually changed the course of the river itself!

What They Did

The authors conducted an observational study in Death Valley National Park, an area home to both feral donkeys and cougars. Luckily, cougars are protected from hunting in this area, making it easier to relate their presence to any effects on donkeys. First, they quantified the temporal activity of both donkeys and cougars, noting when and where each species was active. Then, they related the activity of the donkeys to the presence or absence of cougars. Cougar presence was defined by noting cougar kills, cougar trails, or direct evidence from camera traps. Finally, the authors quantified the effect of the feral donkeys on wetlands by noting how much of the ground was trampled and how much of the vegetation was consumed/destroyed by the donkeys.

What They Found

Donkeys changed their activity patterns in relation to cougar activity. Specifically, in areas where cougars and donkey kills were absent, donkeys were active throughout the day and night, but in areas where cougars were active the donkeys were almost exclusively diurnal, meaning they were active during the day. This switch to diurnal activity allowed the donkeys to avoid peak cougar activity, limiting their exposure to this dangerous predator.

The effects of cougar presence extended beyond the donkeys to the vegetation around wetlands, as their was more canopy cover, more vegetation around the water itself, fewer donkey trails, and less trampled ground. These effects were not explained by the presence of cougars alone, nor various geographic variables, solidifying the evidence for a predator-mediated effect on prey species allowing the wetland vegetation to flourish.

Problems?

As in any study with large species like the feral donkey and cougar, it was difficult for the authors to directly observe any of the predation events (though there was a great photo from the camera trap, see below). Because cougars are such a cryptic species, much of the data on cougar presence/kills in an area came from donkey carcasses. These carcasses may have been the result of a donkey dying from other causes, but then having a cougar scavenge the body, thus overestimating the presence/effect of cougars. Though this may have happened, the authors used statistical methods to determine that such events were likely not the results of density-dependent processes (such as disease), which when combined with the evidence from camera traps and cougar trails themselves, makes these carcasses the likely victims of a cougar attack.

So What?

This study is super cool for two reasons. One, it gives us another great example of a trophic cascade in North America being driven by large predators, much like the effect that wolves had in Yellowstone on their prey (which extended to many other aspects of the local environment). Second, the evidence this paper provides for this novel trophic cascade gives us a glimpse into the food webs of 10,000 years ago, with large predators driving trophic cascades via their effects on large prey species. Further work in this system can shed more light on the services that such large predators and prey species provide for ecosystem function and diversity.

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Dr. Adam Hasik is an evolutionary ecologist and Zuckerman Postdoctoral Fellow interested in the ecological and evolutionary dynamics of host-parasite interactions who has always wanted to work with large predators. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.