Use of road verges by butterfly and burnet populations, and the effect of roads on adult dispersal and mortality.
In a survey of areas beside 12 main roads (with the road number prefix 'A') in the UK in June-September 1989, it was found that verges and central reservations supported a wide variety of (mostly common) Lepidoptera. The variation in the number, density and diversity of species 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 abundance of nectar. The amount of traffic had no apparent effect on populations on verges. It is suggested that road verges could be substantially improved as habitats by reducing the depth of topsoil and amount of fertilizer applied, planting with native seed mixes and shrubs, creating an irregular topography, surrounding with hedges, and making verges and central reservations as wide as possible. Wide busy roads were no barrier to the movement of species living in open populations, but slightly impeded those in closed populations. In 3 species, 10-30% of adults in closed populations crossed the road. 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 when compared with those caused by natural factors.