Introduction

As a rule of thumb, the older the habitat, the greater the specialisation and diversification of species that live within it, and lowland calcareous grassland is one of the most ancient semi-natural habitats in Europe. This exceptionally diverse grassland has accumulated species over millennia of low-intensity farming, but the rapid development of agricultural practices in the past century has greatly diminished its distribution. Habitat-specific organisms have suffered as a result, and species that were once widespread across Europe are now under threat of extinction. The reversion of farmland into livestock-grazed grassland has restored that low-intensity relationship between agriculture and habitat. This has facilitated the recovery of chalk grassland species and offered a vital lifeline to some bizarrely fastidious plants and insects.

Bastard toadflax (Thesium humifusum)

The bastard toadflax is one of 1,000 global species that belong to the sandalwood family (Santalaceae), but it is the only member found in the UK. Its somewhat unfortunate name derives from the Dutch word “bastaard”, meaning hybrid, which is a reflection of its similarity to the much more prevalent toadflax genus, Linaria – an entirely separate group of plants in the plantain family (Plantaginaceae).

It is a slender and woody-stemmed perennial that grows along the ground and has tiny 4mm flowers (made up of five white triangular petals) that emerge from June to August. It is also a specialist of grazed grassland on lowland chalk soil, and like many other small plants living in nutrient-poor environments, the bastard toadflax is a hemiparasite. This means that it is capable of photosynthesising, albeit poorly as its leaves are only 1mm by 8mm, but it obtains water and nutrients by tapping into the sap of other plants – much like mistletoe.

The distribution of the bastard toadflax in the UK is restricted to a southerly triangle bounded by Dorchester, Gloucester and Brighton (plus a few instances near Cambridge). However, the vast majority of all official reports of this species are in South Wiltshire – in particular, Salisbury Plain. As a grazed, lowland, species-rich calcareous grassland habitat just outside of the Salisbury Plain, Coombe Bissett Down nature reserve is optimum location for the bastard toadflax. According to Wiltshire & Swindon Biological Records Centre (WSBRC), consistent field records of this species have been collected on the nature reserve since the 1980s, but the reports fall short as of August 2011.

Down shieldbug (Canthophorus impressus)

Rarer than a specialist plant is a specialist of a specialist plant: the down shieldbug (or bastard-toadflax bug) is an insect that feeds only on the sap of the bastard toadflax. It belongs to a family called burrowing bugs (Cydnidae), which is a group of roughly 750 sap-sucking species loosely related to the shield bugs or ‘stink bugs’ (Pentatomidae).

As an adult, this oval-shaped insect is a dark metallic blue with a cream margin on its back, silvery-brown wings, and a banded rear end, however, its immature form is a vivid black and red. It may only be 7mm long, but the down shieldbug lives in large communal groups, and the brightly coloured nymphs, which typically emerge around July, are usually found living among adults from the previous year.

This bug only exists within the triangular distribution of its host, the bastard toadflax, with isolated records on chalk sites in Oxfordshire, Wiltshire, Hampshire and the Isle of Wight. Nevertheless, its unexpected reappearances in areas such as East Sussex offers hope to those in search for this tiny insect. WSBRC holds records of the down shieldbug on Coombe Bissett Down from 1996 to 2004, with counts ranging from just 1 adult to 12-strong clusters of nymphs.

Conclusion

It seems counterintuitive to focus conservation efforts on just a few rare specialists, but their requirements fall within that of thousands of other species. If given the opportunity, entire ecosystems can be restored simply by agreeing to the terms set out by a few fussy organisms, and the combination of low-intensity grazing and nutrient-poor soil at Coombe Bissett Down does just that. As increasing amounts of land is urbanised or used for agriculture, this nature reserve may well be one of the few remaining refuges for the bastard toadflax and down shieldbug. Therefore, it is imperative for their survival that their specific needs and restricted distributions are recognised and accounted for. 

If you spot these species on one of our nature reserves, let us known by sharing your sightings with Wiltshire & Swindon Biological Records Centre (http://wsbrc.org.uk/).