Oh rats! Decline of large animals lets rodents thrive

Ducking for cover. IRRI, CC BY-NC-SA

Declining numbers of zebra, antelope and other big grazers cause rodent populations to rapidly increase, a new study has shown. Published in the Proceedings of the National Academy of Sciences, Hillary Young and colleagues from the Kenya Long-Term Exclosure Experiment show that the rise in rodent population brings with them fleas and various diseases that can be passed on to humans.

You might think it obvious that when big animals disappear, smaller ones become more abundant. However, the reason isn’t linked to the food chain. The reality is more subtle.

The heavy feet of big grazers trample everything underfoot. Their grazing also keeps the grass short. When these animals are around, rodents and other small mammals try not to get stepped on and hide from predators in the patches of long grass.

Without the big grazers, the trampling is gone and the grass grows tall, and rodents thrive. The other thing that becomes more abundant is the fleas that these rodents carry – and the pathogens carried by them.

In particular, the researchers looked at Bartonella, a globally distributed group of bacteria that causes a variety of human infections, some benign and some potentially fatal if untreated. They found that with decline of large mammals, the number of infected fleas and Mearn’s pocket mice (the most common rodent on the site) increased. While the infection rate among rodents stayed the same, because there was a bigger population of them, there were more infected mice in the same area.

Increased exposure?

Why does this matter? Should we be afraid of a sudden increase of rats carrying fleas and disease? Just because there are twice as many rodents, twice as many fleas and twice as many Bartonella microbes doesn’t mean that the infection risk doubles for people. This entirely depends on how much contact people have with rodents to begin with.

Most rodents are nocturnal, or crepuscular (they come out at dawn and dusk), while most humans are diurnal (they go out during the day). In the wild, local rodents are not used to humans and are likely to avoid them, in the same way that they avoid predators and large grazers.

Outside places like national parks, where large grazers are missing they are likely to have been replaced by cattle, sheep or goats, which will control rodent populations in the same way as the zebras, antelope, or deer did. Rodent populations also vary enormously year to year.

These caveats mean the increase in risk is probably negligible unless you frequently sleep outside, exposing yourself to noctural rodents at a site without any livestock or wild large grazers, in a year when rodent numbers are high. This profile might match some locals (perhaps hunters and hikers), but even if their risk actually doubled, we would want to know what the incidence of Bartonella transmission was before the loss of large herbivores. Unfortunately, a weakness of the study is that the local incidence or infection rate of Bartonella-caused diseases in humans is not reported.

Don’t get your tail in a spin

In any case, domestic cats and dogs are also common carriers of Bartonella, which means doubling the rodent population probably has little effect on human risk. So we should not be more worried than we were before about contracting a rodent-borne disease next time we go for a walk in a landscape missing its deer, zebra or wild camels.

Big animals have been becoming scarcer for at least 10,000 years, so any consequences for disease transmission from rodents have probably already happened. As large animals decline even further, we are talking about a shift in risk from one cause of death (being attacked by an upset large animal) to another (rodent-borne diseases).

Both of these risks are likely to be low. In an urban area of the Congo with Bartonella-infected rats and a significant immune-suppressed human population, zero identified cases of human bartonellosis have ever been reported, although this may be an underestimate. In Kenya, 200 people died from elephant attacks 2000-2007, giving an “infection” rate of one in nearly 2m people. Wilderness always has risk: take your pick.

Controlling rodent-borne diseases is still one of many good reasons to keep big wild animals around. When wild grazers go out, domestic ones often come in, but they are not good replacements. The presence of livestock can also control rodent populations, but livestock, or more specifically, their diseases, have evolved alongside humans and potentially fatal illnesses such as bird and swine flus are linked to them.

In other words, big wild animals can be important as disease buffers for humans. If in the future the species that get along well in human-dominated habitats – livestock, pets, some rodents and invertebrates – were the only ones to survive, bacteria and viruses would evolve to take advantage of the small number of host species and their close connections. Then we would really have an increased risk from disease.