California wolf diet study shows that cattle are overpopulated
A California wolf diet study reveals just how thoroughly domestic cattle have displaced native prey on public lands.
A new study from the University of California Davis on the diet of the state’s expanding wolf population is causing quite a stir in the livestock industry, and also in wildlife conservation circles. The headline-grabbing finding was that beef has been found in 72% of wolf scat, while their natural prey were found in a smaller proportion of samples – 51% for small mammals and 45% for mule deer. Kind of makes it sound like wolves are concentrating their efforts on cattle in northern California, right? That’s the story that the livestock industry and its allies are running with. But a detailed analysis of the study itself reveals quite different (and surprising) conclusions.
Before we get into the details of the study, it’s important to point out that when wolves are eating beef, it doesn’t mean that they killed a cow. Wolves are happy to scavenge carcasses of animals killed by other causes, and a U.S. Department of Agriculture analysis show that natural causes like weather, disease, and poisoning are so common and prevalent that losses to predators of all species combined make up a very small proportion – only one-tenth of one percent – of cattle losses nationwide. That’s half as many cattle as are lost to cattle rustling. Of those 99.9% of cattle out there that have died of other causes, wolves would be expected to scavenge any carcass they find out in the mountains.
The recent UC Davis study focuses heavily on biomass that wolves are consuming, or the combined weight of meat from different species that wolves are eating compared to the total combined weight of prey animals in the northern California mountains. Beef made up 50.3% of the biomass of wolf scats. Mule deer were next most, with 15.2%. Third place in total biomass, surprisingly were montane voles, making up almost 10% of the total biomass of wolf scats. Here, the size of the carcass becomes key – a single dead cow could feed a wolf for weeks, while it would take a really large number of mice to make up a measurable proportion of the biomass of wolf diets.
Ever seen the movie Never Cry Wolf? Dial it up on your favorite streaming platform, and give it a watch. Its portrayal of wolf-prey dynamics proves prophetic, and bears directly on California’s experience (the political vilification of wolves by vested interests, wolves relying heavily on voles for sustenance). It’s fiction, sure, and put out by Disney. But as a former Institute for Arctic Biology scientist, I can vouch for its scientific realism.
The UC Davis study dances around this issue of domestic livestock starving out the mule deer in the Modoc Plateau, even accidentally (on purpose?) forgetting to list competition for forage with livestock as a major factor in California’s mule deer decline. The USDA Natural Resources Conservation Service, a federal bureau founded to support farmers and the cattle industry, is more forthcoming. According the NRCS statistics, each cow-calf pair eats the same amount of forage as two elk, or six mule deer. So for every 12 months a cow-calf pair grazes in the national forests of the northern Sierra Backcountry, there will be six fewer mule deer.
California mule deer are pretty small for this species. Female mule deer in this study weighed an average of 70 pounds apiece, and their fawns weighed 5 pounds. An average cow, by contrast, was estimated by this particular study to weigh 1,200 to 1,500 pounds, while calves weighed 300 to 500 pounds. That’s a big difference. So when a cow dies in the woods and the carcass is abandoned by its owner, that’s one big pile of meat for a wolf to discover!
While beef accounted for 60% and 50% of the biomass in wolf diets in 2022 and 2023, respectively, mule deer meat made up 8% and 15% of the biomass in wolf diets in 2022 and 2023. So the story of wolves eating beef revolves around the fears of the cattle industry, right?
Wrong conclusion! Mule deer are so rare, compared to cattle, that even though they make up a comparatively small proportion of the wolf diet, wolves seem to be actively selecting for mule deer, while eating beef less in proportion to its availability. The researchers concluded, “Despite the high consumption of cattle among ungulate diet items suggested by our dietary analysis (88% in 2022, 78% in 2023), consumption of cattle was slightly lower than expected based on availability, which we estimated at 93% of the prey biomass available in 2022 and 86% in 2023. In contrast, deer composed a higher proportion of ungulate biomass consumed (12% in 2022, 22% in 2023) than expected based on availability, which we estimated at 7% of the prey biomass available in 2022 and 14% in 2023.”
The California Davis study’s statistics on biomass make this point in very stark numbers.
So wolves in northern California are actively avoiding beef (even scavenging) and selecting for mule deer meat. But there’s almost nothing but beef in the woods. That’s due to the chronic overstocking and overgrazing of California’s national forests by domestic livestock, a choice the U.S. Forest Service has been making for more than a century, resulting in dwindling populations of mule deer and other native species of wildlife. And, really, to stock cattle so heavily on public lands that 93% of the prey biomass is cattle, in one of the years of the study, while all the other prey species of native wildlife combined only amount to 7% of the biomass? That’s not multiple use, or balanced management. That’s a national scandal.
When you look at the study from this perspective, the problem comes into stark focus. The crisis isn’t wolf predation on cattle, it’s the complete eradication of native wildlife on California national forests by a radical overpopulation of livestock. So let’s stop crying wolf, and address the real problem: Too many cattle on National Forest lands.
Erik Molvar is Executive Director of Western Watersheds Project, and also is a wildlife biologist who has authored peer-reviewed scientific studies on predator-prey dynamics and herbivore diet and ecology.
Erik, ty for this study. This analysis made me wonder whether there is a similar dynamic in wild-horse management. Just as cattle consumption by wolves looks very different once cattle’s overwhelming share of available prey biomass is considered, wild horses are often labeled “overpopulated” only after livestock has already been allocated much of the available forage.
In both cases, cattle dominance is treated as the ecological baseline rather than as a management choice shaping the outcome. Wolves are blamed for eating what management has made overwhelmingly available, while horses are blamed for exceeding what remains after livestock allocations are protected. The deeper parallel may be that cattle dominance helps create the very condition used to cast other species as the problem.
In the West, it’s people—esp ranchers—that are overpopulated.