The obligate socially parasitic Alcon Blue butterfly (Maculinea alcon) uses the Marsh Gentian (Gentiana pneumonanthe) as its initial host plant, on which caterpillars develop during the first larval instars. In the last larval stage, M. alcon lives as a brood parasite in colonies of specific Myrmica host ants by using chemical and acoustic mimicry to coerce the ant workers to feed and nurture the caterpillar preferentially over their own brood. Maculinea alcon is thus an extreme dietary specialist as absence of just one of the hosts precludes presence of the butterfly and as a result it is highly sensitive to the ongoing environmental change caused by humans. The main drivers for its decline are changed land-use and associated habitat loss or fragmentation, and (in more recent times) drainage, increased eutrophication and lack of appropriate management, but future climate change may further enhance the risk of extinction. Maculinea alcon is selected as an umbrella for conservation and numerous aspects of its biology has been studied extensively.
This thesis explores the spatio-temporal impact of the tight biotic dependence in this tritrophic interaction system and integrates the large body of research with novel methods such as spatial population genetics and macroecological modelling.
The results presented in this PhD thesis suggest that inclusion of biotic interactions in macrecological models may completely alter predictions of species’ response to environmental change. The cross-methodological approaches offer new insight into the conservation of Maculinea alcon and thus aid the future management of this unique butterfly.
In chapter I we found two syntopic populations of M. alcon to be genetically differentiated caused by their host plant flower phenology, not differences in host ant use as expected, and is an example of a genetic barrier operating on a temporal scale rather than spatial. In chapter II, we developed habitat suitability models for M. alcon and G. pneumonanthe potentially useful in locating undocumented populations and for improving management of them at the landscape scale. In chapter III, we integrated a model from chapter II but did not find an effect of landscapes resistance on gene flow between populations, independent of geographic distance. Instead our study showed that populations of Maculinea alcon are likely very sensitive to both past and current anthropogenic change and that conservation should focus on preserving or establishing functional network of habitats to support viable metapopulations. In chapter IV, we found that the suitability of M. alcon in Europe is likely to be geographically stable because of its tight dependence on the climatically stable hosts, contrary to what strict climate-based model suggests. In appendix I is a draft argument to uplist the red status of M. alcon in Denmark to Endangered (EN) from Vulnerable (VU) based on temporal trends in distribution data for the butterfly and its host plant.