As humans drive animals extinct across the globe, the big winners are rats. And the parasites and plagues they carry with them.
That cockroaches will inherit our despoiled earth is just a tired misconception. The real champions will be disease-carrying rats.
Even though cockroaches seem to be of inexhaustible supply, their invertebrate ilk are actually suffering a fairly rapid decline—and the rodents are rising up. In a recent and widely-discussed study in Science, researchers examined a process called defaunation—remember that term, it's likely to prove as vital as 'Arctic ice melt' or 'habitat loss' to understanding our planet's ecological collapse—that describes how the majority of the world's animals are vanishing at a rapid pace.
Led by Rodolfo Dirzo, a professor of biology at Stanford University, a team of scientists documented the rate that fauna are going extinct in the modern era. Since the year 1500 AD, at least 320 vertebrate species have been extinguished, primarily due to human activity. Those that remain have seen their total populations decline by 25 percent. Even more striking is the decline of insects: In the past 35 years alone, the scientists found that the number of invertebrates have plummeted 45 percent. The researchers cite the drops as further evidence that we are bearing witness to the unfurling of the Anthropocene Extinction event—the planet's sixth great mass extinction.
So who wins, besides humans, when the bees and the tigers and the bears lose? Rats.
"Where human density is high, you get high rates of defaunation, high incidence of rodents, and thus high levels of pathogens, which increases the risks of disease transmission," Dirzo said in a statement upon the study's publication. "Who would have thought that just defaunation would have all these dramatic consequences? But it can be a vicious circle."
Hilary Young, one of the study's authors, has conducted previous research examining how rodents thrived after a large species went extinct.
Rats could grow larger than sheep
"What we found was that these areas quickly experienced massive increases of rodents," Young told The Current. "All the grass and shrubs normally eaten by this megafauna was, instead, available for rodents—both as food and as shelter. Consequently, the number of rodents doubled—and so did the abundance of the disease-carrying ectoparasites that they harbored."
Twice the rats. And twice the ectoparasites. A 2013 study in the International Journal of Current Microbiology and Applied Sciences examined how parasite-carrying rats are instrumental in transporting disease: "Rodents together with arthropod ectoparasites can play an important role in the distribution of the arboviruses, streptococcal infections, choriomeningitis, plague, tularemia, leptospirosis, spirochaetosis etc.," the authors wrote.
"Ectoparasites include insects and acarnies (fleas and mites)," the 2013 study continued, "some of them are permanent like lice, while most of the mature ticks and fleas are temporary parasites. Rats are known to harbor four groups of arthropod ectoparasites: fleas, ticks, mites and lice... Some of the ectoparasites can biologically or mechanically transfer infectious agents to the human or animals and results in the spread of infection."
In other words, rats carry a lot of parasites, which carry a lot of diseases. Here, according to the Centers for Disease Control, is a quick list of the diseases rats are currently responsible for spreading in the United States:
- Hantavirus Pulmonary Syndrome
- Hemorrhagic Fever with Renal Syndrome
- Lassa Fever
- Lymphocytic Chorio-meningitis (LCM)
- Omsk Hemorrhagic Fever
- Rat-Bite Fever
- South American Arenaviruses (Argentine hemorrhagic fever, Bolivian hemorrhagic fever, Sabiá-associated hemorrhagic fever, Venezuelan hemorrhagic fever)
It's an ugly list. And in light of their impending dominance, it's worth remembering that rats played a key role in helping spread the bubonic plague during the Black Death. Crammed, unhygienic living conditions helped it become such a devastating killer, but it was an ectoparasite—a flea—that brought the plague.
"The bubonic plague, a disease still present in some areas of the world, is now known to have spread via fleas living on rats," Mark Ormrod, a professor of history at the University of York, wrote for the BBC.
Our hygiene and health-care are much improved from Medieval times, but we are headed towards a future marked by shared, maybe cramped, living spaces: More than half the world's population currently lives in cities, billions are slated to join them, and so, the megacities are growing. More urban living, paired with more rats, could beget similar, if not as deadly, health woes.
And Dirzo and his crew aren't the only ones who worry about the rise of the rats. In fact, just earlier this year, another group of scientists determined that rodents would be the species most likely to outlast all others.
Dr. Jan Zalasiewicz, a geologist at the University of Leicester, believes that rats are the animal best suited to repopulate the world in the event of a mass extinction.
"[Rats] are now on many, if not most, islands around the world," he explained, "and once there, have proved extraordinarily hard to eradicate. They're often there for good, essentially. Once there, they have out-competed many native species and at times have driven them to extinction. As a result, ecospace is being emptied—and rats are in a good position to re-fill a significant chunk of it, in the mid to far geological future."
As humans continue to knock out the larger fauna, and the number of rats "double" to fill the void, we can, theoretically, look forward to seeing more of all of the above. And even if you're not concerned with the health implications, there's the simple fact that we're hacking away at our immense, spectacular biodiversity, and trading it in for a deeply unpleasant, rat-centric monotony.
Beyond defaunation, there's evidence that climate change is improving conditions for rats in general in many regions, too. It's also probably worth adding at this point that warmer temperatures are causing some rat species to grow larger, too, thus adding another potential population booster. Zalasiewicz, for his part, imagines that once its competition is scarce, rats could become larger than sheep.
So that, then, is a foreboding slice of the Anthropocene: Giant, parasite-and-disease-carrying rats, multiplying in droves while everything else goes extinct.