Maculinea arion as an indicator of rare niches in semi-natural acid grasslands in South West England and the role of Myrmica species of ant

Abstract

Anecdotal evidence suggests that habitat management for the Large Blue butterfly, Maculinea arion, benefits other rare species. Selected plant and invertebrate species were monitored on a degraded site which was under going restoration management for the future re-introduction or natural recolonisation of M. arion. This study measured the shift of the floral and invertebrate community in order to understand the species composition and rate of successional changes in the re-establishment of early seral, semi-natural, acid grassland. Several UK Red Data Book species increased on the restoration site including the Pearl-bordered Fritillary, Boloria euphrosyne and the Small Pearl-bordered Fritillary, Boloria selene. The increase in these two butterfly species coincided with an increase in Viola riviniana, a food plant of these two Boloria species, and ants of the genus Myrmica. A detailed study was undertaken to investigate the processes explaining the increases in Viola and Boloria species, with emphasis on their possible interactions with various Myrmica species. A key question was: Does management for M. arion merely create a scarce niche shared by a guild of other species, or is there a direct impact due to the increase in Myrmica sabuleti or other Myrmica? There was a positive Myrmica affect on Viola density, due to the elaiosomes on the seeds of both Viola species, which were highly attractive to all Myrmica species tested but not to the other ants (Formica spp.) inhabiting the study sites. Myrmica scabrinodis (HOT) preferred Viola lactea seeds, the species of Viola that coincides most within its niche in the field. The relative density of Viola riviniana to V. lactea was primarily determined by microclimate, with V. lactea replacing V. riviniana in the hottest niches. Boloria euphrosyne oviposition sites are also influenced by microclimate and coincide with the niche of M. sabuleti, straddling the warmest microclimates occupied by V. riviniana and the coolest ones occupied by V. lactea. B. selene inhabits a cooler niche, ovipositing mainly on V. riviniana near M. ruginodis. Habitat management for M. arion increases the larval food plant and niches of both Boloria species, but especially that of B. euphrosyne. This is partially due to the creation of a scarce niche that is shared by a guild of species and partly due to the activity of Myrmica species ants acting as ecosystem engineers.

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