An individual based model for the conservation of the endangered Large Blue Butterfly, Maculinea arion (Lepidoptera: Lycaenidae)

European populations of the Large Blue Butterfly Maculinea arion have experienced severe declines during the 20th century, especially in the northern part of the species' range. This endangered lycaenid butterfly needs two resources for development: flower buds of specific plants (Thymus spp., Origanum vulgare), on which young caterpillars briefly feed, and red ants of the genus Myrmica, whose nests support caterpillars during a prolonged final instar. In order to improve understanding of those mechanisms that are most influential to population dynamics of the butterfly, we developed a stochastic population model. This individual based model implements three main biological components relevant for population dynamics of the butterfly: (i) life on the initial host plant, (ii) adoption of the caterpillars by host ants, and (iii) life of the caterpillars within host ant nests. The model explicitly describes the spatial distribution and abundance of the butterfly, the foodplant and the host ant. Life-history parameters of the butterfly were derived from literature. Habitat characteristics such as area size, Thymus cover, density of host ant nests and proportion of adoption by non-host Myrmica ant species were obtained from a population of M. arion in the Swabian Jura (Baden-Wurttemberg, Germany). The model was successfully tested on results from studies of this natural population. A highly significant correlation between the number of individuals marked in the field and the number of individuals predicted by the model was found. A sensitivity analysis was used to test and assess general conservation measures for the Swabian population and the species in general. The Monte Carlo simulations clearly indicate that the density of host ant nests and the proportion of caterpillars adopted by non-host ant Myrmica is more critical for the survival of the butterfly than the density of its initial foodplants. A population inhabiting an area of I ha is likely to be regarded as safe for the next 50 years under the following conditions: Thymus cover should not fall below 5%, the proportion of adoption of caterpillars by host ants should be greater than 20%, and a minimum nest density of host ants greater than 500 per ha should be assured. Maintenance or re-establishment of grazing or mowing was successfully tested as a suitable conservation measure for declining M. arion populations. (C) 2002 Elsevier Science B.V. All rights reserved.