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."

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. What are the odds of the insect appearing by chance at Bickershaw again this year? 

Last year I saw a male and female mating and the 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 acidic peat bogs end up breeding here? I'm not really sure, but there are a few clues. 

Firstly, the main area they seem to favour is a small stream between New Water and Nevison's Flash. So that ticks one box. 

Though the water in the stream is very clear, the substrate is orange. In fact there are several streams and ditches in the area of New Water which are orange, but new water itself is not orange, and neither is Nevisons which the stream flows into, emphasising again the point that it is the substrate which is orange, NOT the water.

Orange substrate with crystal clear water is caused by iron-oxidizing bacteria. Slag heaps such as those at Bickershaw, contain abundant unrefined, reduced ferrous iron as a result of the previous industrial process, which creates ideal conditions for these bacteria. These types of slag heaps are acidic, occasionally highly so.

In my experience, the habitat choice of keeled skimmer in the UK is always acidic peat bogs, even in lowland areas such as the New Forest. So a highly acidic stream such as this is actually probably a decent habitat for keeled skimmer.

Other evidence of this being acidic is the presence last year at New Water of common hawker, black darter and emerald damselfly, all of which are species often associated with acidic areas, especially peat bogs.

Wavy hair grass also grows here, a plant which prefers acidic soils. 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".

Once you realise that this is not actually pollution and is in fact a living bacteria providing a unique habitat for a unique array of species in the area, you can start to view it as quite a beautiful thing!