Populations of animals are not always stable from year to year, even ignoring the effects of direct human activity or climate change. If there is a prolonged drought, plants may die off, reducing the population of herbivores, which, in turn, reduces the number of predators that feed on them. This much is part of the natural cycle of life and death.
Perhaps the most famous example, sufficiently so to turn up in some school textbooks, is the case of the Canada lynx. This eats almost nothing but showshoe hares, so when the supply of the hares runs low, the lynx are more likely to be malnourished, so that mothers give birth to smaller litters. This means that there are fewer lynx around, which leads to the snowshoe hare booming, which means more food for the lynx, so they have larger litters, their population increases, they eat the hares so that the hare population goes down... and so on. Because the effect works through the litter size of the lynx, which take time to reach maturity, the lynx population rises and falls about a year behind that of the hares, and the entire cycle takes a decade or so to come full circle. We have records of this going back to the early 19th century from the fur-trappers of the Hudson Bay Company, and it doubtless goes back well into prehistory.
This is not the only example but it shows how such things can be perfectly natural. But that's not always the case; sometimes human activity can destabilise animal populations, leading to boom and bust cycles, without actually endangering them and driving them to extinction - although it may, of course, make such a thing more likely. For a couple of examples outside the world of mammals, we can look to the way that wild salmon populations fluctuate due to parasite loads, which they probably picked up from farmed salmon, or the way that pollution and overfishing lead to a boom-bust cycle in starfish and sea urchin populations.
For a more mammalian example, we can turn to the prairie dog.
Prairie dogs (Cynomys spp.) are a type of burrowing ground squirrel, most closely related to the marmots - and only much more distantly to the gophers, which are not squirrels and are also physically quite a bit smaller. There are five species of prairie dog, one of which is found only in northern Mexico, with the other four found across the Great Plains of the US. Of these four, one lives from southern Utah and Colorado down to New Mexico and Arizona, one lives immediately north of that, just about crossing the southern border of Montana, and a third is an endangered species restricted to only one small area in Utah between the first two.
The remaining species is the most widespread. This is the black-tailed prairie dog (Cynomys ludovicianus) and it is generally found further to the east than the other three American species, although there is considerable overlap in the middle. The Rio Grande forms an effective barrier to the species in Texas, but they do cross over the border just south of eastern Arizona and are found in a broad band northward from there all the way to Montana and North Dakota, with the very northern extent of their range just across the Canadian border in Saskatchewan.
Prior to the arrival of Europeans, black-tailed prairie dogs (and presumably the other species) are thought to have been present in high numbers across their range, occupying wide stretches of the landscape. It's harder to tell how common they were during the 18th and 19th centuries, since they weren't the sort of thing that people in the area tended to make more than passing mention of and there certainly weren't detailed surveys. But, absent much in the way of human interference across that part of the continent at the time, it seems likely that they were still common, and had stable populations.
That changed in the early 20th century.
In 1901, the US Department of Agriculture published a calculation that suggested the animals ate so much forage that eradicating them would allow an additional 1,500,000 cattle to be reared in Texas alone. It's likely that Texan farmers didn't need this encouragement to try and get rid of the perceived pests, but in 1916, the federal government stepped in and started sponsoring official campaigns to poison and kill off the animals. The program continued, under various guises, until it was stopped by presidential order in 1972 but poisoning prairie dogs remains perfectly legal for individual ranchers.
This, it has to be said, has in no way led to any existential threat to the species' survival (and the endangered Utah species is protected by law). There are thought to be around 18 million black-tailed prairie dogs across the US today and, while their population may have declined in some areas, overall it's likely held fairly steady over the last few decades. Over shorter timescales, though, it undergoes a dramatic boom and bust cycle.
And it isn't the poisoning campaigns that are doing it.
Instead, the main reason that the population declines so dramatically at the start of the 20th century, and continues to fluctuate wildly today is the introduction of a foreign disease to which the prairie dogs (and American animals in general) had no natural immunity. That disease is the plague.
Yes, the actual disease that caused the Black Death. It isn't called "bubonic plague" or any of the other human-related terms when rodents catch it and is instead called "sylvatic plague". But it's caused by exactly the same bacterium (Yersinia pestis) and is, like the human version, transmitted through flea bites - although, in the case of prairie dogs it's likely that other routes of transmission, such as infected carcasses or breathing in droplets breathed out by other small mammals, may be at least as, if not more, important.
Whatever the precise details, colonies of black-tailed prairie dogs are regularly hit by outbreaks of plague, causing rapid population crashes that only slowly recover. The time between outbreaks seems to vary between five and fifteen years, sufficiently common to restructure and reduce the prairie dog populations. Even between outbreaks, a low level of plague seems to persist in the population, perhaps forming a reservoir from which the next one will arise.
Understanding this is important for a couple of reasons. For one thing, ranchers are probably not wrong when they say that high populations of prairie dogs cause problems for them; they really do reduce the available forage for cattle (or sheep) although this may affect the cattle less than one might think.
From a conservation point of view, however, it's significant that black-tailed prairie dogs are a "keystone species", one whose existence is disproportionately important to maintaining an entire ecosystem. Indirectly, the presence of prairie dogs reshapes the grasslands in ways that are beneficial to burrowing owls, mountain plovers, and pronghorn, among others. More directly, they provide an important food source for many local predators, most notably various species of hawk. If they keep dying off, clearly this is going to be an issue.
One area in which this has been studied is the Thunder Basin National Grassland in northeastern Wyoming. For 21 years, between 1997 and 2018, scientists mapped the extent and number of black-tailed prairie dog burrows across the area as waves of the plague came and went. During this time period, there were three local outbreaks of plague. A major outbreak in 2002 almost wiped out the local animals, which had only partially recovered when a second wave hit in 2005. This was smaller, affecting primarily the colonies in the southwestern corner of the study area, but continued for a full three years before petering out. A long slow, rebuilding followed so that, by 2017, prairie dog colonies covered more of the land than they had before the first outbreak; the following year, the third wave hit and wiped almost all of them out again.
The fact that the second outbreak was on a different scale than the other two, and the changes in colony size over the intervening years means that it's not simply a matter of population density reaching some critical value before the plague starts to spread again. This makes it difficult to predict when waves of the plague will hit, or how long it will take for the prairie dogs to recover. But one pattern that was observed, and which supported previous findings, related to the weather.
During dry years, the prairie dogs cannot feed on food as lush and nutritious as they might like, and consequently end up underfed, unfit, and susceptible to disease. Under normal circumstances, they can recover from that, but if the following year is unusually hot and damp, fleas spread and an outbreak becomes more likely. So the conditions in a single year might not have much effect, but two of the right sort in succession create a problem.
Perhaps more significantly, the study also showed that the periodic crashes in population and loss of entire colonies meant that just 8% of the study area was inhabited by the animals for over seven years at a time. The seven-year cut-off was chosen because previous work had shown that that is roughly how long it takes for prairie dogs to have a longlasting effect on the local plantlife and to fulfil their role as a keystone species. So, in other words, despite the animals being present, in over 90% of the area studied they kept dying out before their efforts could really bear fruit. It's also likely that, even in the very best years, immediately before a crash, the population never reached the heights it had in the 19th century before the plague reached the Americas from Asia.
That which does not kill you does not always make you stronger.
[Photo by "Musicaline" from Wikimedia Commons.]
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