Impact of grazing management on hibernating caterpillars of the butterfly Melitaea cinxia in calcareous grasslands

Abstract

Semi-natural grasslands are increasingly grazed by large herbivores for nature conservation purposes. For many insects such grazing is essential for the conservation of their habitat, but at the same time, populations decrease at high grazing intensity. We hypothesised that grazing management may cause increased butterfly mortality, especially for life-stages with low mobility, such as hibernating caterpillars. To test this, we measured the effect of sheep grazing on overwinter larval survival. We used the Glanville fritillary (Melitaea cinxia), which has gregarious caterpillars hibernating in silk nests, as a model species. Caterpillar nests were monitored throughout the hibernating period in calcareous grassland reserves with low and high intensity sheep grazing and in an ungrazed control treatment. After grazing, 64 % of the nests at the high intensity grazing treatment were damaged or missing, compared to 8 and 12 % at the ungrazed and low intensity grazing treatment, respectively. Nest volume and caterpillar survival were 50 % lower at the high intensity grazing treatment compared to both ungrazed and low intensity grazing treatments. Nest damage and increased mortality were mainly caused by incidental ingestion of the caterpillars by the sheep. It is likely that grazing similarly affects other invertebrates, depending on their location within the vegetation and their ability to actively avoid herbivores. This implies that the impact of grazing strongly depends on the timing of this management in relation to the phenology of the species. A greater focus on immature and inactive life-stages in conservation policy in general and particularly in action plans for endangered species is required to effectively preserve invertebrate diversity.

17 years of grassland management leads to parallel local and regional biodiversity shifts among a wide range of taxonomic groups

“…In addition, the management regime seems to have been insufficient to create favorable habitat conditions for some characteristic species, particularly carabid beetles and weevils. Intensive autumn management in Dutch calcareous grasslands may cause particular obstacles for their larval stages, as was previously demonstrated for ants (van Noordwijk et al 2012a) and butterflies (van Noordwijk et al 2012b). …”

“…Holometabolous species generally have a more strongly synchronised life-cycle and their immature stages are less mobile and need different environmental conditions than the adult stages. This makes holometabolous species particularly sensitive to management timing, intensity and scale (van Noordwijk et al 2012b;van Klink et al 2015), indicating that too intensive management may have hampered restoration of carabid beetle and weevil communities.…”

“…The small size of individual sites (\5 ha.) and the large distance between sites, make species with large population fluctuations especially prone to local extinction (Henle et al 2004;van Noordwijk et al 2015).…”

“…Characteristic species and food specialists are generally more vulnerable to habitat degradation (Römermann et al 2008;Ö ckinger et al 2010), but are also expected to respond more positively to renewed management than noncharacteristic and food generalist species, because habitat conditions improve most for them. Finally, a species' trophic position modulates its sensitivity to processes operating at larger spatial scales (Holt et al 1999;Vanbergen et al 2010;van Noordwijk et al 2015), making higher trophic levels more vulnerable to habitat fragmentation (Purtauf et al 2005;Krauss et al 2010;van Noordwijk et al 2015). These species characteristics vary between species and, even more so, between taxonomic groups.…”

Effects of large herbivores on grassland arthropod diversity

“…Aphids and ladybirds, for example have been shown to avoid ingestion by dropping off the plant when detecting the breath of large vertebrates (Gish, Dafni & Inbar, 2010; Ben-Ari & Inbar, 2013). However, in short vegetation with limited possibilities to escape, and during immobile life stages (eggs and larvae) ectophagous species also may be ingested (Van Noordwijk et al , 2012 b 2012b). These differences in vulnerability to incidental ingestion among arthropod guilds suggest a large potential for shifts in arthropod communities.…”