Natural History

This past week there was a much talked about study that claimed that the reason why Lyme disease was on the increase is because Eastern coyotes are now killing all the red foxes.

Red foxes are a major predator of the main host for the Lyme disease- carrying tick, the white-footed and deer mice (Peromyscus).

Coyotes do kill red foxes whenever they encounter them, but to blame coyotes for the decrease in red fox numbers is a bit simplistic.

The study examined the number of red foxes, deer, and coyotes in the states of Wisconsin, Minnesota, Virginia, and Pennyslvania over a 30-year period. It found a correlation between the drop in red fox numbers and an increase in Lyme disease. Red fox numbers in all 4 states have dropped over this time period.

Of course, the first rule is that correlation does not imply causation.

There are lots of reasons why Lyme disease could be on the increase. I noticed that they didn’t try to examine the numbers of Peromyscus mice in the study– but it’s one that certainly key to understanding why Lyme disease might become more prevalent.

The study seems to be implying that the number of these mice is on the increase, yet it didn’t even look at them.

Of course, there are reasons why they might be in on the increase– and fox predation isn’t necessarily one of them. This entire region is experiencing longer breeding seasons for mice, which means they can have more litters during their lifetimes.  That wold mean that there could potentially be more Peromyscus running about in a given region now than in the past.

Further, milder winters are good for ticks.  More ticks can overwinter in egg, larval, and adult forms during mild winters than really  harsh ones. The winters in this entire region have been generally milder than they were in the past, which would tend to suggest that we should see an increase in Lyme disease.

I think that this factor is the more likely cause of the increase than a drop in red fox numbers.

However, even the assumption that coyotes are to blame for the drop in red fox numbers needs to be scrutinized.  Although Minnesota and Wisconsin have expansive farmland, Virginia and Pennsylvania have seen a decrease in farmland and increase in forest over this same time period. Dense forests are quite good for gray foxes (Urocyon), and although they are a bit smaller than red foxes on average, they are quite aggressive toward reds and drive them out of their territories. Grays prefer to have access to forests, for unlike the red fox, they readily take to the trees to escape predators.

Competition from gray foxes that are thriving as return of forested habitats seems to me a more likely reason for the red fox’s decline than coyote predation.

Red foxes have a distinct musky odor, and it takes quite a bit of training to get hounds to choose to hunt them over deer, and it seems very unlikely to me that coyotes would choose to hunt red foxes over other prey. They might kill them if they can catch them, but in general, they aren’t going to waste precious energy trying to kill red foxes when there are deer fawns lying around.

So although red fox numbers are on the decrease, it doesn’t make sense that the coyotes are entirely to blame for the decline.

I think the gray fox is a much more likely culprit, and its increase has happened only because forested habitat is expanding in much of East. If the land had remained largely agricultural, it is unlikely that gray foxes would be expanding their range, as they clearly are now.  The warming climate also has some effect here, for gray foxes are not as adapted to living in very cold climates as red foxes are.

But if gray fox numbers are on the increase, this really hurts the hypothesis that fox predation on Peromyscus mice is the main factor controlling Lyme disease. Gray foxes hunt mice just as readily as red ones do. The two animals are competitors for the same niche, and the gray has certain advantages over the red in forested habitats.  A gray fox is not limited to hunting prey on the ground. It can take to the trees to raid bird nests, and it is slightly more omnivorous than the red. If the two foxes have to compete in this environment, the gray can utilize more of the territory and thrive on a smaller acreage than a red fox.

Never mind that gray foxes will attack any red fox they encounter. They can also outcompete them for the same ecological niche.

So if the red foxes are in lower numbers than they were in the past, the mice are still being eaten by foxes. They are just being eaten by gray ones this time.

Further, coyotes are adept mousers. Before they came into the East and started hunting deer, they lived largely on rodents in the West. The idea that they are somehow less efficient predators of mice than red foxes are is pretty hard to justify. If anything, they likely eat more mice in a year than any red fox. Even if they do hunt deer and other larger animals, they still eat mice in order to augment their diet, especially if they aren’t pack-hunting coyotes.

And then there is the whole feral and free-roaming domestic cat factor.

Cats eat mice.

Everyone knows this, and cat numbers in the United States have never been higher.

Cats are likely killing scores more Peromyscus mice than ever before, and they could play a role in transmitting the deer ticks to humans.

Cats like to bring home their kills of white-footed and deer mice, and those little carcasses carry deer ticks, which then come off and attach themselves to the cats’ owners.

The authors of this study have not made a convincing case that coyotes are to blame for the rise in Lyme disease.

A much more likely culprit is the warming climate and the resultant mild winters.

Further, the authors haven’t shown that coyotes are the main reason why red fox numbers are down, when it is likely that a lot of more complex issues are at work here, including climate change, reafforestation, and the rise of the gray fox.

The idea that coyotes are to blame for this problem is quite reductionist.

Reductionist explanations, however, are exactly what lazy science journalism likes to use for headlines.

We should always be skeptical of such simplistic answers to what are more likely to be quite complex ecological issues.

Yes, coyotes do kill and compete with red foxes, but are they alone to blame for the decrease in red foxes and the rise of Lyme disease?

I really don’t think anyone can make that case.