Oviposition site selection of an endangered butterfly at local spatial scales
Résumé
As pre-hibernating larvae of the marsh fritillary (Euphydryas aurinia) have limited mobility essential resources need to be available at a very local scale. We surveyed larval webs (2011-2013), the host plant devil's bit scabious (Succisa pratensis) (2012), and derived variables from digital orthophotos and digital elevation models (Normalized Differenced Vegetation Index, accumulated sun hours, slope, aspect) to explain the presence-absence and abundance of larval webs at three different spatial grain sizes (5 x 2.5 m, 10 x 10 m, 25 x 25 m) across seven study sites in northern Jutland, Denmark. Two-component hurdle models indicated that host plant abundance was the only important predictor of presence-absence and abundance of larval webs across the seven sites. The strength of the host plant effect on larval web prevalence increased when enlarging spatial grain size. For presence-absence (and less for abundance), the effect of host plants on larval webs varied across study sites. Using mixed effects models, we additionally analysed presence-absence of larval webs (in 1 x 1 m plots) in relation to detailed host plant measurements (abundance and size), vegetation height, and environmental variables (soil temperature, air temperature and soil moisture) across four of the sites. This showed that larval webs were located in the densest parts of the host plant patches. Given the low mobility of pre-hibernating larvae (< 0.5 m), our results suggest that females select dense parts within large patches of host plants as oviposition sites. Future management should concentrate on establishing large patches of the larval host plant.