The latest report from Michael Walter:
How much longer can it last? I have recorded no rain whatsoever since 7th June, and that was only 2.8mm. The last significant rainfall was on 29th May, nearly seven weeks ago, which is probably unprecedented in the 37 years that I’ve been keeping records here. Remarkably, most of the vegetation is standing up to the searing weather pretty well, although some of the birches are losing their leaves, and many of the beech trees are looking rather forlorn – a thin canopy with exceptionally small leaves, so the sky shows through in a way that is distinctly abnormal for a tree renowned for its heavy shade. Cow-wheat, the foodplant for our rare heath fritillary butterfly, is sensitive to dry weather, yet is still hanging on doggedly, but devil’s bit scabious is starting to look distinctly woebegone. It is a rather lovely plant, the pale blue flowers with a hint of mauve appearing later in July and August. In places it forms attractive stands along the ride edges, but at present the leaves are curling up and turning an unhealthy grey. This drought is certainly exceptional, but not unprecedented (some of you may recall the post of minister for drought that James Callaghan hurriedly created in 1976, to be replaced weeks later by the minister for floods!) so, if the vegetation could recover then, we can expect it to pull through again, with the help of one or more strategies. The scabious, for example, has fleshy roots that will be able to aestivate (the summer equivalent of hibernate) if all the leaves die off, ready to resume growth next year. Other plants will have old seed already in the soil (the seedbank), from which they can spring, phoenix-like, in 2019. However, should the drought persist for several more weeks, we could be entering unknown territory.
In the previous paragraph I mentioned the “rare heath fritillary”. We like to think we know what the words we use mean, but what is a “rare” butterfly? This season has been a good one for the heath fritillary, making it one of the reserve’s commonest butterflies! Compare that with the white admiral, a glorious butterfly related to the red admiral; widespread in southern England, and extending into the Midlands, it disappeared from Blean Woods sometime after the 1950s, but has recolonised the reserve in recent years. This season has been the best ever, with 32 already recorded on my weekly transect counts (and more to come), compared to my previous best season’s count of 10, and most years just one or two. But, 32? This year I have had daily heath fritillary counts of over 1000, so which is the rare butterfly? Rarity in wildlife is about more than just numbers: it also relates to geographical distribution which, for the heath fritillary is simply the Blean Woods complex, Exmoor, a couple of small sites in Cornwall, and a few reintroduced colonies in Essex. Plants and animals are mapped nationally on a 10-kilometre grid, and the heath fritillary occurs in just 14 of these squares in England, whereas the white admiral occupies about 230. Intertwined with distribution is the concept of the rarity or vulnerability of the preferred habitat: for white admirals the requirement is woodland, especially with oaks, where honeysuckle is present for the caterpillars to feed on, and a certain amount of direct sunlight. The heath fritillary’s needs are no more complex, but the cow-wheat that its caterpillars feed on can mysteriously disappear from former strongholds despite a continuation of the requisite coppice management, which itself is now increasingly uncommon away from nature reserves. Whereas the white admiral’s requirements are easily met, the heath fritillary’s habitat is perpetually on a knife-edge, with the ever-present risk of colonies going extinct. So, to return to the original question, and the meaning of rarity, the word can now be seen to be rather more nuanced than we might have imagined.
Michael Walter
michaelwalter434@gmail.com
01227 462491
White admiral courtesy of Glynn Crocker.
Drought- stressed beech courtesy of Michael Walter.
Wilting devil's bit scabious courtesy of Michael Walter