The small blue butterfly Cupido minimus is the UK’s smallest native butterfly and it’s a scarce and declining UK species. The London Borough of Sutton has a small population, restricted to a few nature reserves on the North Downs.
The small blue doesn’t help itself, unfortunately. The caterpillar is a fussy little fellow, only eating a type of wild pea called kidney vetch Anthyllis vulneraria (the ‘larval host plant’). Kidney vetch, here in the south east at least, is predominantly restricted to the chalk deposits making up the North and South Downs. The low nutrient and low water retention properties of the chalk mean that being a plant on this substrate is very tough. However, perhaps counter-intuitively, the harsh growing conditions for many plants mean that the species diversity is extremely high (up to around 50 species per square meter), as each species finds a particular way to eke out its living. Where nutrient loads are higher, coarse species like nettles and brambles dominate.
As open areas on chalk evolved through man’s low intensity grazing over thousands of years, species also adapted to these new grasslands, to resist grazing pressure by only producing low growing basal rosette leaves or by exploiting the mechanical damage done to the soil surface as grazing animals moved across the landscape. When the topsoil of a grassland is removed through animal trampling, in a process known as ‘poaching’, the bare subsoil or substrate can play home to species that may not have had a chance to set seed previously, due to the competition from the extant species or simply because they smothered the ground surface.
Kidney vetch is one of those species that requires poaching, as it is a pioneer species on bare ground. Kidney vetch is able (predominantly due to being a ‘pea’ and thus a nitrogen fixer) to make a living on bare chalk where many other species cannot. As it goes through its life processes of living and dying, it gradually enriches the surrounding soil and provides a greater nutrient base for other species to gain a foothold (or should that be ‘roothold’?) and thus outcompete the kidney vetch.
Kidney vetch is therefore a short-lived perennial plant, requiring transportation via grazing animals to the next bare patch of chalk substrate. As kidney vetch is transitory, it follows that the small blue must be, to some extent, transitory to follow the larval host plant. The large-scale loss of natural grasslands to commercial agriculture, housing or recreation (parks, golf courses etc.) means that the wider scale processes previously so vital to kidney vetch generation have been lost.
This is all a rather roundabout way of saying that to help preserve kidney vetch and therefore the small blue butterfly so reliant on it, we replicate the creation of bare ground on chalk grasslands. Usually, this is on a small-scale by hand, digging off the topsoil and scattering a few kidney vetch seeds. Today, however, saw the start of a SITA funded project to help the small blue butterfly on the North Downs. Thanks to our friends at the Old Surrey Downs Project and their successful funding bid, a 1.5 tonne excavator was brought onto Roundshaw Downs Local Nature Reserve to create large areas of bare chalk (‘scapes’) and to use the spoil from the scrapes to build small linear mounds (‘bunds’). The bunds are aligned east-west, creating small north and south-facing slopes. Roundshaw Downs is a rather flat and featureless site, dipping to the north off the Downs, so the addition of these small bunds adds a much-needed microtopography. Having north and south-facing slopes, these bunds can create different thermal conditions, which are vital for a variety of insect species, as well as creating basking areas for common lizards Zootoca vivipara.
Two scrapes and bunds were created today and we are confident that once seeded with kidney vetch, they will provide a significant increase in the total amount of kidney vetch on site and thus provide conditions suitable for take up by the small blue butterfly. We’ll be creating more scrapes and bunds over the next few years, thereby ensuring a variety in age structure of the vetch and healthy habitat for the small blue.
If you wish to discuss the small blue project at Roundshaw Downs or are even keen to grow kidney vetch in your garden to help the small blue, please contact firstname.lastname@example.org.