Use of Road Verges by Butterfly and Burnet Populations, and the Effect of Roads on Adult Dispersal and Mortality

1. Transects made beside 12 main roads in Dorset and Hampshire (UK) showed that verges and central reservations supported a wide variety of butterflies and burnets (Zygaenidae). One site had 23 species of butterfly (=40% of British species), while the average was nine (16%) species per 100-m transect. Most were common species, but some rarities were present. 2. Mark-recapture estimates of adult densities on verges were up to 2774 adults ha-1for Maniola jurtina. Populations of Melanargia galathea, Pieris rapae and Polyommatus icarus were large or medium-sized for these species. 3. Variation in the number of species, density and diversity of butterflies and burnets depended on the range of breeding habitats on verges. The density of adults and number of species were correlated with verge width, while diversity was correlated with the abundance of nectar. The amount of traffic had no apparent effect on populations on verges. 4. Road verges could be substantially improved for butterflies and burnets by reducing the depth of top soil and amount of fertilizer applied, by planting with native seed mixes and shrubs, by creating an irregular topography and surrounding them with hedges, and by making verges and central reservations as wide as possible. 5. Wide busy roads were no barrier to the movements of species living in open populations, but slightly impeded those with closed populations. Mark-recapture showed that 10-30% of adults of three species with closed populations crossed the road. Roads cannot be considered as a barrier to gene flow in any species in this study. 6. Vehicles killed 0.6-1.9% of adults of species from closed populations, and about 7% of those from open populations. These mortalities were insignificant compared to those caused by natural factors.