Species monitoring

Keith Steggall, National Trust Wiltshire Landscape Ranger Keith Steggall National Trust Wiltshire Landscape Ranger
Burnt tip orchid

Calstone and Cherhill Downs is an outstanding area of chalk grassland habitat covering 202 hectares. Part of the role of the Ranger for the area is to keep a close eye on the condition of the site to make sure that management is working, and that species are thriving.

With such an extensive landscape and vast amount of species of flora and fauna, this is quite a daunting undertaking.  The way that we monitor is to pick out key species and use them as a barometer for the whole of the downs. 

We have several orchid species on the downs, most of which are present in good numbers and so no formal recording happens each year.  Our rarest orchid is the burnt tip, found in one location on the western down in late May, and in very low numbers. Management of the downs ensures that the cattle aren’t grazing whilst the orchid is in flower to help it survive.

Butterflies are a good indicator of the condition of chalk grassland as they are dependent upon the right flora for the foodplant for their larvae.  We are fortunate to have a volunteer who goes out to record butterfly species around the downs from May through to September.  Over the last few years, it has been great to see the marsh fritillary is doing so well, back in 2008 we were recording less than 20 butterflies, recently numbers have risen so that we are recording butterflies in the hundreds.  A good site for blue species, there is a strong population of Adonis which needs short turf for its foodplant, horseshoe vetch.  Getting the grazing right to provide good habitat has been key to helping these butterflies to thrive here.

The rarest species that we have is found in the Calstone coombes, the wart biter bush cricket is only found at four other sites in the country.  A delicate balance is required for this species that needs bare ground for egg laying, areas of short turf with a diverse flora, but also a varied sward structure with areas of longer grass as cover from predators such as kestrels.  A survey is done each year in late summer which counts the number of males by locating them by their call or stridulation, this is the only way to find this elusive species.  This species was rediscovered here in the early 1990s when its survival was in doubt, since then with careful management it has recovered so that now it has a stable population covering an ever-expanding area. 

A threat to the chalk grassland habitat is the presence of tor grass, a dense, rough grass which smothers other flora and which the cattle don’t like grazing.  The extent of tor grass is monitored by taking fixed point photos. Last year some targeted grazing with cattle in the Autumn on one of the banks looks to have produced some good results in opening up the grassland, this is something that will continue and will hopefully create even better habitat for our chalk grassland species.