The deer population decline after 1924 refers to the famous Kaibab Plateau deer disaster in Arizona, where a mule deer herd that had grown rapidly over nearly two decades crashed due to overgrazing, food depletion, and harsh winters. The story is one of the most cited examples in ecology textbooks, though the real numbers behind it are more complicated than the classic version suggests.
What Happened on the Kaibab Plateau
The Kaibab Plateau sits on the North Rim of the Grand Canyon in Arizona, part of the Kaibab National Forest. Starting around 1906, the federal government launched an aggressive campaign to protect the deer herd there. Predators like wolves, mountain lions, and coyotes were systematically killed, and livestock that competed with deer for food were removed from the range.
The results were dramatic. The mule deer population, estimated at roughly 4,000 in 1906, grew rapidly over the next two decades. By the early 1920s, the herd had ballooned to somewhere between 30,000 and 100,000 deer, depending on who was doing the counting. With predators gone and no competition for food, the population exploded well beyond what the land could support.
Why the Population Crashed
The core problem was simple: too many deer, not enough food. As the herd grew, the animals stripped the plateau’s vegetation faster than it could recover. Browse lines became visible on trees and shrubs, with everything eaten up to the height a deer could reach. The range deteriorated badly.
Then two severe winters hit back to back, in 1924-1925 and 1925-1926. A weakened, overpopulated herd on degraded land couldn’t survive the cold and food scarcity. Mass die-offs followed. By 1939, the population had reportedly fallen to fewer than 10,000 deer. The textbook version of the story puts the death toll at roughly 60 percent of the herd in just a couple of years.
The combination of factors created a classic ecological overshoot and collapse. Without predators to keep numbers in check, the deer reproduced beyond the carrying capacity of their habitat. Once the food ran out, the population had nowhere to go but down, and harsh weather accelerated the decline.
The Role of Predator Removal
The Kaibab story became famous largely because of ecologist Aldo Leopold, who used it as a cautionary tale about what happens when humans remove predators from an ecosystem. In his view, the wolves and mountain lions had been keeping the deer population in balance with its food supply. Remove the predators, and the population spirals out of control until nature corrects it through starvation and disease.
This interpretation shaped wildlife management for decades. It became a foundational example of why predator-prey relationships matter and why top-down population control is essential to healthy ecosystems. The Kaibab case was taught in biology classrooms as a clean, compelling illustration of these principles.
The Numbers Are More Disputed Than Textbooks Suggest
The classic story, while useful as a teaching tool, rests on shaky data. Ecologist Graeme Caughley published an influential critique in 1970, arguing that the evidence Leopold relied on was “unreliable and inconsistent” and that the factors behind the deer population changes were “hopelessly confounded.” In other words, it wasn’t possible to cleanly separate the effects of predator removal from livestock removal, changing land management practices, and other variables.
The population estimates themselves varied wildly. Different biologists and visitors to the plateau put the 1924 peak anywhere from 30,000 to 100,000 deer, and none of these figures came from rigorous scientific surveys. The often-cited 60 percent crash was actually extrapolated from a single visitor’s estimate that the herd dropped from 50,000 to 20,000 between 1924 and 1926.
Perhaps most telling, the forest supervisor who tracked the herd most consistently over this period reported a population of about 30,000 deer in 1923 and a stable population of roughly 30,000 deer from 1924 through 1929, the exact period when the catastrophic crash supposedly occurred. This suggests the decline may have been more gradual than sudden, and the peak population lower than the dramatic figures in textbooks.
What the Kaibab Story Still Gets Right
Even with the disputed numbers, the underlying ecological lesson holds up. The deer herd did grow significantly after predator removal, the range did suffer visible damage from overgrazing, and the population did eventually decline. Whether the peak was 30,000 or 100,000, the pattern of overshoot and decline was real.
The Kaibab case remains valuable because it illustrates a principle that has been confirmed repeatedly in other ecosystems: removing top predators can trigger cascading effects through a food web, leading to overpopulation of herbivores, degradation of plant communities, and eventual population crashes. The specifics of the Kaibab numbers may be debatable, but the ecological dynamics it represents are well established. Modern wildlife management continues to use the broad lesson, even as ecologists acknowledge that the original story was far messier and more uncertain than the tidy graphs in textbooks made it appear.