Climate change brings Europe more boars

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There are more boars than ever in Europe. While that might seem like a good thing from a conservation perspective, not everyone is happy about it. That’s because the wild pigs (Sus scrofa) have a habit of wandering into agricultural fields in search of food, and finding a decadent buffet of tasty things to eat. If wildlife managers are to better equip farms with a means for avoiding losing too many of their crops to hungry boars, it would help to know just why their populations are increasing in the first place.

That’s the task taken up by researchers Sebastian G. Vetter, Thomas Ruf, Claudia Bieber, and Walter Arnold, from Vienna’s University of Veterinary Medicine. They suspect it has something to do with climate change.

As the world changes, some species respond by shifting their ranges, moving farther north or south to find more suitable habitats, climbing up mountainsides, or diving into deeper seas. But most of those studies have centered on small or migratory animals, like birds and fish. Not much is known about how large mammals are equipped to deal with climate change, and whether different populations in different places respond differently.

Boars are among the most widely-distributed mammals in the world, with populations surviving in a wide range of habitat types, so there is the potential that different groups could be better or more poorly equipped for climate change. The researchers combined boar hunting information from 12 European countries, some of which had data going back 150 years, with monthly weather data. To that, they added information regarding food availability, both in terms of agriculture and wild plants (like beech) that are regularly consumed by boars, information from boar-related traffic accidents.

First, Vetter and his group had to ensure that hunting counts were a reliable method for estimating the boars’ population size. It could be that certain boars are more susceptible to being killed by hunters, or that certain populations are somehow less able to avoid being shot, or that certain human populations are better hunters, and so on. Roadkill data, on the other hand, is somewhat more random and less subject to individual differences among boars. Since there was a strong correlation between traffic accidents and successful hunts, both over time and space, the researchers concluding that the changes in the number of successful hunts from year to year is a good proxy for the boars’ population size.

When it came to the climate-related information, Vetter discovered that severe winters were usually followed by local population declines, in particular by affecting juvenile mortality. It’s not necessarily that fewer boars were being born, but that those that were had a tougher time surviving their first year. Therefore, he reasoned, increasingly mild winters in Europe could be responsible for the boars’ population increase.

And even the occasional severe winter isn’t all that effective at hampering the boars’ population growth, because all the other mild years leave enough food, like beech nuts and acorns, that they can rely upon through the chilly winter months. That extra food, afforded by the increasing number of mild winters and early springs, allows juveniles enough nutrition to last the winter, and adults enough energy reserves to reproduce after the winter passes.

The data also suggest that an increase in agricultural food sources is probably not associated with the increase in boar populations. For one thing, those crops are only available for brief periods of time during the year. In addition, the researchers didn’t find any correlations between boar population increases and the amount of corn and potato crops available to them. The boars are increasing regardless of access to farmland. Which probably doesn’t provide all that much comfort to farmers losing their crops to the pigs.

While the researchers were able to observe similar population dynamics for even the warmest parts of Europe, where winters are already fairly mild, the different populations grew in different ways. A boar population only increases if the average temperature during the winter reaches a certain threshold. And those thresholds are higher in the warmer south than they are in the cooler north. Although boars are increasing through the continent, they’re doing it at different rates.

“Flexible and generalist species like the wild boar are generally more likely to cope with or even benefit from climate change compared to specialist and less flexible species,” writes the team. But that doesn’t mean that the effects of climate change on one population can be extrapolated and applied to another. It does mean, however, that farmers will need to figure out how to live in a world with more boars. – Jason G. Goldman | 19 August 2015

Vetter, Sebastian G., et al. “What Is a Mild Winter? Regional Differences in Within-Species Responses to Climate Change.” PLoS ONE 10.7 (2015): e0132178. DOI: 10.1371/journal.pone.0132178.

Header image: Sebastian Vetter/Vetmeduni Vienna