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A new UK Butterfly monitoring scheme (UKBMS) transect at Embsay reservoir.

Keeping a detailed inventory of all of our wildlife is beyond our limited capacity to do, but by counting key “indicator” species we are able to assess the “state of nature” overall. Butterflies are one such group, and they have been monitored under the UKBMS since 1976. Relatively easy and fun to count, the UK has 59 species of breeding butterfly. 

The first official route or “transect” was walked at Potton Wood NNR in Bedfordshire on April 10^th. In that first year, 39 sites were monitored across England and Wales. The scheme now includes over 3000 sites, and from the 1^st of April this year, we have started a new transect at Embsay reservoir. This is in an area that is relatively poorly monitored yet has some interesting and important species and habitats.

UKBMS monitoring starts in April and continues until the end of September. Transects are walked once a week, weather permitting. Walking at a constant, relatively slow, pace you record the number of individual butterflies of each species seen in each section of the transect. These must be within a “bubble” of 2.5 metres to either side and 5 metres ahead. It is often useful to have a small pair of binoculars to identify butterflies that flash past while in the “bubble” but then settle some distance away. Recording is done between 10:45am and 3:45pm, when there is a temperature of 11 Celsius or more in upland areas such as ours and with sunshine for at least 60% of the time. However, overcast days are acceptable at temperatures of 17 Celsius or above. Showery days are permissible, but it must stay dry while the transect is walked. Finally, the wind must be no stronger than 5 on the Beaufort scale, a condition that may often be challenging to meet at Embsay as it is so exposed. Particularly early and late in the season, it is inevitable that some weeks are not recorded because of the weather!

The Embsay transect is divided up into 10 sections and goes anticlockwise around the reservoir and a short distance up towards the crag (see below). The route includes a number of different habits, from the grassy pastures on the steep slopes to the right side of the first section, to a sunny, flower-rich bank about ½ way around and on to the damp moorland towards the crag. With this variety of habits, we hope to record 22 species, with the possibility of one or two more.  

We already know that the transect is home to some nationally scarce or declining species, including wall brown and small heath, two grassland butterflies. The “wall” is believed to be particularly hard hit by our changing climate, while the small heath has declined along with the “unimproved” grasslands that their caterpillars feed upon.

We also know that there are small numbers of the pretty common blue, another butterfly that has decreased in recent years. Finally, it is possible that there are some rare small pearl-bordered fritillary at Embsay too.

To understand the ecology of any plant or animal it is necessary to know something about their lifecycle. Like other insects, butterflies undergo metamorphosis, with the eggs laid by the adult imago hatching into caterpillars or larvae, which eventually pupate to undergo their final transformation into adults again. While the adult stage is the most obvious, every year the survival of the species depends upon the successful completion of every stage of the lifecycle. While adult butterflies must find flowers to feed upon, most will nectar upon a wide range of species. Caterpillars, on the other hand, are far more selective about the plants they feed upon, with each species only eating one or a few (see table for some examples). Likewise, pupae can often require quite restricted conditions to survive. Climate change, leading to increasingly erratic and extreme weather, is proving to be a challenge for many butterflies, often with one or more of their lifecycle stages affected.

The adults of different butterfly species are on the wing at different times from spring to autumn. Those that over winter as adults including brimstone, peacock and small tortoise shells are first on the scene in spring, followed by others that emerge as adults in the spring including the white butterflies, green hairstreak and small copper.

Others, including many of our grassland species such as the small heath and ringlet, are summer specialists. Some of our butterflies go through multiple rounds of their lifecycle in the course of a year, while others only one. With the increasing vagaries of our weather, it is no wonder that the numbers of butterflies varies so much from year to year.  

While we have been surveying butterflies for 15 years, this is the first time we have set up a new transect. We have visited Embsay reservoir fairly often over the past 7 years but have not done so regularly. We are looking forward to walking the transect weekly as it means that we will get to know the plants and animals there really well. Extensive tree planting to the north of the reservoir recently also means that the wildlife, including butterflies, will inevitably change over the years to come. One benefit could be that some sections will become more protected from the prevailing winds, something that many butterflies – and transect walkers – are likely to appreciate! Now seems like a particularly good time to start, before the trees become more established so that we have a chance to follow any changes that occur.

Starting a new transect is an exciting venture and although we feel fairly sure that we have a good idea of the reservoir’s wildlife, there will no doubt be some surprises to come. Can you identify butterflies accurately, or would you like to learn? Would you like to help us to survey them at Embsay reservoir? Contact Ian at devilsbit18@gmail.com for more details.