Grazing management impacts on the viability of the threatened bog fritillary butterfly Proclossiana eunomia

Abstract

Management of semi-natural open ecosystems has become a necessity for biodiversity conservation in Europe. However, management methods are most often experience-based, rather than science-based. Here, we explore the impact of extensive (light) grazing, which is often the only practical management solution to stop ecological succession towards forest. A metapopulation of the bog fritillary butterfly ( Proclossiana eunomia), a wet meadow specialist, was monitored for 15 generations: 11 before and four after the introduction of cattle grazing in some habitat patches. We relate the observed changes in P. eunomia abundance and recruitment to the effect of grazing on vegetation descriptors linked to habitat quality for the butterfly. Cattle strongly decreased the abundance of the butterfly host plant, destroyed grass tussocks used by caterpillars for sun basking, and altered vegetation composition and physiognomy. Consequently, grazed habitat patches are less suitable for P. eunomia: recruitment rate and butterfly use decreased by 2/3, whereas emigration rate increased drastically; this is likely to alter the viability of the metapopulation. We therefore propose modifications to its current implementation to limit its negative side-effects. More generally, it is of prime importance that conservation biologists and site managers collaborate in ensuring that sound ecological evaluations become an integral part of the design and application of management plans. This implies the monitoring of crucial indicators of the system state and variability before the beginning of the management, and the maintenance of unmanaged areas as biological preserves in case of negative impact and as controls for statistical tests of impact.