Managing calcareous grassland for the declining Duke of Burgundy Hamearis lucina butterfly: effects of grazing management on Primula host plants

Many butterfly species are declining in range and abundance, sometimes to the point of becoming vulnerable to extinction. Several traits increase a species' vulnerability to population decline through stochastic processes, including high larval specificity and poor dispersal rate. The Duke of Burgundy Hamearis lucina relies on Primula as its sole larval host plant. This monophagus dependency, coupled with susceptibility to environmental stochasticity, low dispersal and poor recolonization potential, means it is vital that sites supporting this rapidly-declining species are managed optimally. Here, we use two calcareous grassland sites in the UK with different grazing systems to identify optimal grazing management for Primula abundance and, for the first time, Primula characteristics linked previously to Duke oviposting preference and success: size, condition, succulence and surrounding sward height. We find that autumn and winter grazing intensity are both positively associated with Primula abundance, but there is a trade-off for winter grazing with negative effects on plant size, condition and succulence. Winter grazing also decreased the sward height below the optimum. Plants were bigger and better at the site managed using continuous (free-roaming) grazing versus the site managed using rotational (paddock-based) grazing. We recommend moderately high grazing intensity during autumn using a free-roaming system where possible to attain abundant Primula, with a reduction in grazing intensity or grazing removal during winter to ensure suitability of individual plants. This management would also benefit other host plants, such as bird's foot trefoil Lotus corniculatus, which are vital for other declining butterflies that frequently co-occur with Dukes.