Sunday, 23 July 2017

Is the Dormouse an Endangered Species?

At first glance, this seems a fairly daft question. A moment's search on the internet will reveal that only is the dormouse not an endangered species, it isn't even under any significant threat of becoming one. It's about as safe as any species that doesn't specifically rely on human beings can be. There is no risk that we are going to, at any point in the foreseeable future, even come close to running out of dormice.

Look past the headlines though, and things do become a little more complicated. No, the dormouse is not an endangered species, but its population is under threat in certain parts of its range, particularly in the northwest. Some of these populations are isolated from the main bulk of the species in mainland Europe by the presence of various seas, and perhaps the most obvious of those is the British population. Since this population lives on a (relatively) northerly island, and since there are, apparently, risks to the dormouse in the north of its range, it is, in fact, reasonable to ask whether or not the dormouse is an endangered species in Britain.

The species as a whole may be fine, but the British population may not be. (Or, indeed, the Swedish one, but we'll stick to one country for today).

Perhaps we should begin though, by explaining exactly what animal we're talking about here. Dormice have their own taxonomic family, which is actually more closely related to squirrels than it is to animals such as true mice and voles. There are at least 28 species, spread across Europe, Asia, and Africa, although they have never crossed to the Americas. However, only two of those species live wild in Britain, and one of those is a historical introduction. The other, the genuine native species, is the hazel dormouse (Muscardinus avellanarius). This is also known as the "common dormouse", or, particularly in Britain where further distinction is rarely needed, simply as "the dormouse". Among other things, it's the animal Lewis Carroll was referring to in Alice in Wonderland.

The next question to ask, though, is "what exactly constitutes an 'endangered' species?" Species, or populations within species, may be classified as 'endangered' under any of a number of different laws or conventions applied to individual countries, or even internationally. Whether a species is, or is not, endangered, may vary between those laws, as do the requirements to be entered onto the relevant lists of animals needing protection. However, the most important such list internationally is the Red List, maintained by the International Union for the Conservation of Nature. (Although it's worth noting that the list maintained by the Convention on the International Trade in Endangered Species is also very important, especially from the point of view of international law).

The IUCN lists the threat to species on a seven-point scale, plus a couple of categories for "don't know" and "can't tell". Each of these categories has a range of criteria, which, every few years, specialist panels try to evaluate species against, to see where they fall. Under this scheme, for example, the dormouse, as a species, gets the lowest possible threat rating of "Least Concern" - the same rating that humanity itself gets. This is because dormice are found across a wide area, and are a numerous and common species across most of that area. Nor - outside of northwestern Europe - are there are any particular threats to their continued existence.

The next category up is "Near Threatened", meaning that the species isn't really in any danger, but there are enough credible threats that that could potentially change in the near to medium-term future. This is true, for example, of the garden dormouse, or lérot, (Eliomys quercinus) of continental Europe. Beyond that we get "Vulnerable", where the species is considered threatened, but not sufficiently so to be formally listed as an endangered species, then Endangered, Critically Endangered, Extinct in the Wild, and actually, definitively, too-late-to-do-anything-about-it, Extinct.

So, this allows us to frame our question more precisely: if dormice did not exist outside the British Isles, would their status there be sufficient to qualify, under IUCN rules, for the category of an "Endangered" species?

The formal definition of "Endangered" used by the IUCN runs to nearly 700 words, but it boils down to the species meeting one of three criteria: its population is too small, its population is more reasonably sized, but declining rapidly, or its population is restricted to a small enough area that it wouldn't take much to wipe it out. Dormice are found throughout southern and central England, Wales, and Cumbria in northern England, which, taken together, is easily enough to rule out that third criterion. Since, being such small animals, we also have good reason to suppose that their raw population numbers are high enough for them not to qualify on the first criterion alone, we are left with the question of how, and by how much, that population is changing.

How would we know the answer to that?

Enter, the National Dormouse Monitoring Program, a citizen-science project organised by the National Biodiversity Network charity. Since 1988, the NDMP has been setting up artificial dormouse nests at sites across England and Wales, and registered volunteers check the sites at least twice a year, before and after the breeding season, to count the number of dormice making use of them. The project has grown over time, but there are currently around 400 monitoring sites, some of them with data going back the full 29 years.

There are, of course, a number of limitations with this sort of study. Because it relies entirely on volunteers, it's not obvious that all locations have equally dedicated workers, so that the quality of the data might vary from place to place. Nor is it clear that the studies have been conducted in exactly the same way at every site, in every year, since the 1980s. And, of course, since it's all still growing, some of the sites just aren't old enough to have any kind of long-term data. Indeed, the number of sites prior to around 1992 probably wasn't enough to draw any firm conclusions about the national picture anyway, useful though they might be locally.

But those are all factors that we can allow for. The latest analysis of the results has done just that, using the large scale that such citizen-science projects allow to give us what is likely the most detailed picture we can get of the population trends of dormice in Great Britain.

And it's not good.

The headline conclusion is that, between 1993 and 2014, the number of dormice found at the study sites has, on average, decreased by 72%. The exact numbers vary from year to year, and there are even some specific sites where the number has increased, but the trend is very clearly down, and, while it's slowed a little over the last decade, is still continuing in that direction. If this reflects the overall British dormouse population (and the authors of the analysis conclude that, within a reasonable margin of error, it does) then clearly there's a pretty drastic underlying change.

The relevant bit of the IUCN scheme (criterion EN A2) states that a species is considered to be endangered if there is "an observed, estimated, inferred, or suspected population size reduction of [at least] 50% over the last ten years... where the reduction or its causes may not have ceased, or may not be understood, or may not be reversible." 72% over 20 years, especially if much of it was in the first decade of that period, probably doesn't quite meet this standard. But it's not far off, and it's surely within the range for a classification of "Vulnerable species", which requires a minimum of a 30% decline over the same period.

In fact, doing the maths, it's an average decline of almost 6% every year for the past two decades, and, any way you cut it, that has to be worthy of note. Aside from the obvious significance for the species itself, the abundance of dormice can be an indicator of wider scale issues. They require diverse woodland habitats, and so may be an indicator of woodland quality and the state of forest management. It has also been suggested that their sensitivity to summer and winter temperatures may also make them a marker for the effects of climate change.

Dormice don't seem to be under any threat in France, Italy, or eastern Europe, but their population is in decline in Britain, Sweden, Germany, Denmark, and the Netherlands. It's in these regions, on the edge of their territory, where such declines are both most likely to occur, and most likely to be problematic. The dormouse is already a protected species under British law, and it's unlikely that anyone is deliberately killing them in large numbers. But, if the population decline is worse than we suspected - and this is, of course, just one study, albeit a large one - then clearly that's not enough.

It's not clear what the solution to this is, or even if there is a good solution. But, the bottom line is that, while the dormouse may not be an endangered species in Britain yet, if things continue as they are, that may not be the case for much longer...

[Photo by Hectonichus, from Wikimedia Commons.]

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