Coastal dunes as important strongholds for the survival of the rare Niobe fritillary (Argynnis niobe)

We studied the oviposition and larval habitat preferences of the Niobe fritillary (Argynnis niobe) in the dunes of the east Frisian Island Langeoog (German North Sea). By ascertaining habitat quality we are able to assess the minimum habitat size for populations of A. niobe in dune islands. The preferred oviposition and larval habitats were best characterised by a combination of (1) host-plant abundance, (2) host-plant quality and (3) vegetation structure. The oviposition and larval habitats of A. niobe were characterised by low-growing vegetation with bare ground and a warm microclimate. In contrast to the also common V. tricolor ssp. curtisii, the leaves of the host plant V. canina showed a lower C/N ratio, suggesting that differences in plant quality may account for host-plant use. A. niobe seems to depend on very large areas of potential larval habitat, thus explaining its ongoing decline in the increasingly fragmented Central European landscape. Our data indicate that grass encroachment is a major threat for A. niobe in coastal dunes due to its negative impact on microclimate and violet germination. Mosaic top-soil removal and low-intensity grazing should be suitable tools to promote rejuvenation of V. canina.