A Field Notebook: Keeled skimmers, why Bickershaw?

The hot weather this week has made for some great dragonfly hunting at Bickershaw, though I tend to go in the morning at the moment and avoid the severe heat of the afternoon. Hot, literally, on the heels of last weeks' Norfolk hawkers, the past couple of days have seen the even more remarkable emergence of keeled skimmers.

Remarkable because in the north of it's range this is a species mainly associated with upland peat bogs. How it comes to be here, apparently breeding at an old industrial site in the lowlands, complete with slag heaps is a mystery. Following my discovery of these dragonflies here last year, the Lancashire and Greater Manchester (VC59 & VC60) dragonfly recorder Steve White commented:

"The keeled Skimmers are amazing. I know of only one other record in GM. Several years ago in the West Pennines. Well established in Lancs but confined to a quite small area of Bowland and showing no signs of range expansion. So all previous VC59 & 60 records have been in the uplands." I believe that there is also one record from the Sefton Coast.

Photo: female keeled skimmer.

The "Field Guide to the Dragonflies of Britain and Europe" gives the habitat requirements as follows: Running waters such as streams and ditches. In north of range [inc. UK] mainly runnels in boggy areas [acidic, peat].

Photo: teneral male keeled skimmer.

Given that last years records were only the second for Greater Manchester it seems very unlikely that these are simply individuals dispersing from breeding sites in the uplands, especially since there is only one known (small) site in Lancashire and that's miles away in Bowland. What are the odds of the insect appearing by chance at Bickershaw again this year?

Last year I saw a male and female in tandem and then today I saw a newly emerged teneral male. Unlikely as it may seem, the species is obviously breeding at Bickershaw.

So how does a dragonfly associated with upland peat bogs end up breeding here?

Well first of all it's worth noting that there are other species present at New Water which are also often associated with peat bogs, such as common hawker, black darter and emerald damselfly. There's obviously something going on here which makes this area suitable for these types of species and makes it in someway comparable to a peat bog.

I'm not really sure what the answer is, but it's well known that ex-industrial sites such as Bickershaw can be oases in a desert of surrounding farmland and can hold habitats and species quite unique to the local areas in which they are.

For example, the slag which is associated with the ex-coal mine contains abundant unrefined, reduced ferrous iron as a result of the previous industrial process, which creates ideal conditions for iron-oxidizing bacteria.

The bacteria help create the orange substrate which can be seen in some of the ditches, ponds and streams at Bickershaw, though the water itself can be crystal clear. These types of slag heaps are often highly acidic which seems to tick at least one of the boxes for keeled skimmer since the peat bogs on which they usually occur are also highly acidic.

The epicentre for keeled skimmers at Bickershaw appears to be the stream between New Water and Nevisons. On moorlands, keeled skimmers love these type of ditches.

There are also a few acidic loving plants here as well, including wavy hair grass. This is a plant which prefers acidic soils and is often found in areas where keeled skimmers occur. I don't know of anywhere else in the area where this grass grows, not even at the superficially similar and adjacent Pennington Flash, which also has former industrial slag heaps. I guess that Pennington Flash is much further along the process of being a Country Park and acidic streams with orange substrate do not fit nicely with the manicured version of nature that most people seem to want. Any that did once exist have probably been "purified".

There are probably other factors involved as well, but it seems likely that at some point in the past few years, a dispersal has taken place of keeled skimmers from their moorland strongholds during suitable weather conditions and a few have turned up at Bickershaw. Fortunately they have found a habitat to their liking and have started breeding here. How long this breeding colony has been here or how long it can persist is unknown.

Of course, if you want to take this even further, we could ask the question "How can it be that a species like Norfolk hawker which relies so heavily on the aquatic plant water-soldier (absent from Bickershaw) and usually lives in nutrient rich non-acidic fens, can now be an established breeder at Bickershaw on two pools which are connected by a highly acidic stream on which keeled skimmers breed?". That's way beyond my remit, but somebody, please find out!