Podcast: Why the UK's moths are under pressure

Small elephant hawk-moth

Small elephant hawk-moth

25 November 2014 by Sue Nelson

This week in the Planet Earth podcast, Richard Fox of Butterfly Conservation and Tom Oliver of the Centre for Ecology & Hydrology (CEH) explain how records of millions of moths, citizen science and expert analysis are showing that populations of large moths are under increasing pressure.

To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.

Sue Nelson: This time in the Planet Earth podcast, how millions of moths, citizen science and expert analysis is showing that populations of large moths are under pressure.

Hello, I'm Sue Nelson and I'm in Newton Abbott in Devon in someone's back garden. Not anyone's back garden of course, but belonging to Richard Fox, who's the surveys manager for butterfly conservation.

Richard was part of a recent study involving the biggest ever moth survey in Britain. There's a huge contraption in front of us, which looks like you've got a large casserole dish with a see-through lid, a black bucket upturned on top of it, and inside I can see through that lid, some egg boxes. What is it?

Richard Fox: Yes, it's a strange looking contraption, isn't it? So this is a moth trap. Most of the records that we gather for the National Moth Recording Scheme from citizen scientists and members of the public across the UK come from moth traps like this and similar, in people's back gardens and indeed out in the countryside as well.

So effectively what a moth trap is, is a big container, uses a light source to attract moths, nocturnal moths flying at night. It'll effectively suck moths in, in this case from the garden and moths that might be passing through. They fall down into the box. If I take this bucket, the bucket's not really part of it; that was just to keep things inside...

Sue Nelson: Oh yes, and now you can see that there's big light bulb.

Richard Fox: Yeah, so there's a big light there, and moths will fly in and they'll fall down through the small holes there below the light bulb. The only point for the big container and all the egg boxes is just to provide a lot of surface area and volume inside.

So it really just works on the same principle as a lobster pot; that there is a small way in, and once the creature is inside there, moths in this case, and other insects, then they can get out, there's nothing preventing them getting out, but the way out is quite small, there are lots of lovely nooks and crannies, nice rough surfaces to hold onto, provided by these egg boxes, so they tend to stay in, and then you can come out in the morning and have a look and find out what you've got.

Sue Nelson: Brilliant. Well, I know you've run this trap overnight especially for me, which I'm very grateful for, so shall we have a look inside and see what you've got?

Richard Fox: Yeah let's have a look. Moths fly all through the year in Britain, and there's a whole group of moths that fly particularly in autumn, so you find more moths, more species, greater diversity, and usually a richer variety of colour in the middle of summer than you would in autumn, when most of the moths are camouflaged to look like dead leaves.

Sue Nelson: So basically you're dampening my expectations.

Richard Fox: [Laughs]. I'm easing you in, unless of course it was raining torrentially for a large part of last night. So there are...

Sue Nelson: Oh yes they there are.

Richard Fox: There's one just leaving. I think that was a Common Marbled Carpet. Some moths are very easy to wake and they will fly off at the slightest disturbance, but many of them are now firmly asleep in the daytime, and so we'll be able to have a look. So this part comes off.

Sue Nelson: So that's the little cone. I love the fact that inside the egg boxes, instead of eggs, you've got lovely, individual little moths as if they've set up home...

Richard Fox: Yeah they're like little rooms for moths.

Sue Nelson: ...for the night, like a moth hotel.

Richard Fox: Yeah, absolutely, it is like a moth hotel.

Sue Nelson: I'm immediately drawn to actually this small one here. They're all not much bigger than my thumbnail...

Richard Fox: No that's right.

Sue Nelson: But it's the green marbling on it that's...

Richard Fox: So that one is red green carpet, which will go through the winter now like that. Next to it, in this next little moth bedroom as you put it, is a black rustic, which is quite a striking moth, sort of jet black with little white spots. This is a lunar underwing, and this little guy here is a micro moth, Tachystola acroxantha...

Sue Nelson: That is tiny.

Richard Fox: Big name for a small moth, and this is a broad-bordered yellow underwing, so again it's employing a strategy here of being quite camouflaged, but underneath here there are some bright yellow ones...

Sue Nelson: [Sharp intake of breath]. That's gorgeous. Orange and black underneath its wings.

Richard Fox: Yeah, so that's the hind wing, so it's keeping that tucked out of the way when it's hiding. If a predator got too close and it took flight, it would show a bright flash of that bright sort of orangey yellow and black, which would presumably act as a bit of a shock tactic to a would-be predator.

Sue Wilson: So how many people did these moth traps?

Richard Fox: So there are many thousands of people taking part. We started gathering data in 2007, but we're collecting not just contemporary records, but also historical records, and we now have over 17 million moth records in the database.

So each record is of species at a place on a date. Depending on how many species we've got in the trap here, I'm guessing we've probably got about 20 species; that would be 20 records.

Sue Nelson: And I can see how, because as you say, they're quite dopey, you're able to pick them up and examine them and they're not flying away unless you disturb them. Then for the citizen scientists taking part, they can easily identify them, get a book, look at them and take their time.

Richard Fox: Exactly.

Sue Nelson: So it must be fairly accurate information [overtalking].

Richard Fox: Yes it is. We ensure the accuracy of records in the National Moth Recording Scheme by filtering all the records through a county expert, so it might be a group of people actually. It might be a little team of volunteers. So these are amateur experts, citizen scientists who've got often decades of experience identifying moths, who'll help to weed out unlikely or just plain wrong records.

The National Moth Recording Scheme so far has only concentrated on the larger moths, but nevertheless that's about 800 or 900 species.

Sue Nelson: So you have all this moth data and the next stage is to analyse it, and I went to the Centre for Ecology & Hydrology's biological records centre in Wallingford, Oxfordshire to meet Doctor Tom Oliver. He's an ecological modeller and I began by asking him how much of that data he used.

Dr Tom Oliver: The 17 million records is the whole moth data set that we have, so that's a great resource, spreading the whole of the UK, going back several decades. For this analysis we used about 11 million of those records, and that's because we filtered the species to exclude species that are not resident to the UK, and also species for which the recording methodology's slightly changed, for example pheromone lures have been introduced. That might be reflected in the changes in the number of records for those species.

Sue Nelson: Eleven million's still a huge amount. You've got your computer, you've got to number crunch them; how do you actually go about doing that?

Dr Tom Oliver: Because they are effectively, we call them unstructured in the sense that we don't have a measure of the amount of effort people put in, in collecting data, so how long those volunteers had their moth traps out for.

Sue Nelson: Because that would affect the number wouldn't it, because if your moth trap's open for four hours, you're probably going to catch more moths than if it was just open one hour.

Dr Tom Oliver: Exactly, yeah, and also the absence of moth records in a certain place might reflect there being few moths there, but it could also reflect there being no recorder to record the moths.

So we adopted a new method to analyse these data, which creates a benchmark list of species that you might find in that location if you had recorded it very well, and so the way that's done is we use actually the plant data set to find a set of locations in the nearby region which are ecologically similar, they have the same plant species composition, and then from that neighbourhood we create a reference list of possible moth species that you might find.

So the idea is if you went to that site and you did record reasonably well, you would have found all your benchmark moth species relatively quickly, and so that gives us a measure of the sites that are well-recorded, and then that allows us to analyse the trends in those data.

Sue Nelson: Did you find any specific increases or declines?

Dr Tom Oliver: Of the 673 species that we looked at, there were 260 that were significantly declining. So it wasn't all bad news. There were 160 that were increasing, but the numbers declining outweighed those that were increasing by about a factor of 1·6.

Sue Nelson: Did you find anything in common with a species that either declined or increased?

Dr Tom Oliver: Yes, so we split our moths into different groups based on the regions that they occurred. For example there were northern species which occur only in the northern half of the UK, southern species in the southern half, and then species which were geographically widespread across the whole country.

And what we found is that the widespread species were overall declining, but interestingly they showed greater declines in the southern half of their ranges. So that's consistent really with a greater intensification of land use that's occurred in the south of the country, so large farms with lots of fertilisers put down on the fields, whereas the northern half of the country's relatively less intensive.

So that was the pattern for the widespread species. For those northern species, there are only a few of those species, 17; they're associated with the colder northern half of the country, and they showed declines over time, and we think that's a climate change signal, where because the climate in the UK's warm, it's become less suitable for those moth species.

And then the southern species, although they prefer warmer conditions, a lot of those species occur in Europe and the UK's just about suitable, so you'd expect them to be expanding in the UK, but actually they're also expanding in the southern half of the UK which has this intensive land use. There's a sort of conflicting pattern of what you expect from climate and the land use, and there we saw no change overall in those moths species.

Sue Nelson: Were you able to actually understand why certain species had improved?

Dr Tom Oliver: Yeah, we were able to look at the host plants that some of the moths feed on for a subset that only fed on a single host plant in the caterpillar stage, and we found two results.

One is that moths that feed on host plants that occur in low fertility environments, so this is places where there's been less nitrogen deposition, farmers add fertiliser to their fields; that fertiliser doesn't just go onto the crop, but it can also spread into the surrounding environment and cause growth of high nitrogen plants such as stinging nettles for example.

But there's a lot of plants which can't live in those conditions. They're out-competed by the high nitrogen plants, and it's those species that have really declined in our environment recently, and that's had knock-on effects onto the moth populations.

The other result was we found that moths that feed on plants which grow in more open light conditions have also suffered, and this fits with results from, for example, a study called the Countryside Survey, which has looked at plants across the whole country and found that woodlands have become more shady as the management of those woodlands has declined. Plants that occur in open light conditions have tended also to decline.

Sue Nelson: Tom Oliver. Well I'm still with Richard Fox in his back garden and it's starting to rain, so I'll keep this very short. Richard, were you surprised by those results?

Richard Fox: Well sadly not. We know from the studies of butterflies we've done over the past few decades that British butterflies have declined very severely, and we think that butterflies are good indicators for other invertebrate wildlife as well, terrestrial wildlife. Although moths are a much more speciose group, much more diverse group, we suspected that they would have declined.

There is also some population data on moths in Britain from the Rothamsted Insect Survey, and that also shows declines in moth populations. So when you put all that together, really it would have been quite amazing if moths hadn't declined.

There are some species that are doing well and that are expanding their ranges in Britain or that are increasing in abundance, but overall the picture is a fairly gloomy one.

Sue Nelson: Richard Fox, thank you very much indeed for allowing me to witness some of your beautiful moths. That's it from the Planet Earth podcast from the Natural Environment Research Council, and you can see some pictures of today's recordings on our Facebook page and Twitter feed. I'm Sue Nelson. Thanks for listening.