Clear-cuts support the metapopulation of a critically endangered butterfly

Abstract

Clear-cutting is believed to have a negative impact on forest biodiversity, but numerous studies have also shown its positive effects, especially in temperate and boreal regions. This phenomenon is mainly related to the creation of early successional habitats that serve as temporary meadows. Among the species that benefit the most are those associated with semi-natural grasslands, i.e. biotopes that have been disappearing from the landscape due to agricultural intensification. A good example is the Danube Clouded Yellow Colias myrmidone, one of the most endangered butterflies in Europe. Its last lowland metapopulation in the EU exists in the Knyszyn Forest (north-eastern Poland), but the species occurrence is limited to the eastern parts of this large forest complex. All currently known habitats are forest clear-cuts located mainly on former agricultural land, i.e. mostly extensively used pastures that were afforested in the 20th century. Chamaecytisus ruthenicus, the only local larval food plant of C. myrmidone, is currently found almost exclusively in first-generation forests. Logging allows the host plants to flourish, and clear-cuts are colonized by the butterfly. In 2022, intensive mark-release-recapture studies were carried out in the focal metapopulation. The estimated metapopulation size was 4643 adults, indicating a six-fold increase compared to previous studies conducted in 2017. This was plausibly mainly due to the increase in the area of potential habitats, as butterfly densities were very similar in both seasons. The improved connectivity between habitat patches resulted in a very high proportion of dispersing individuals successfully reaching other patches, which is in contrast to the dispersal mortality of ca. 50% observed five years earlier. A relatively high mobility was also observed in the case of the much smaller spring generation. To our knowledge, this is the first study to document a reversal of selection against dispersal and it demonstrates that such changes can occur relatively quickly, i.e. within a few generations. In order to ensure the long-term persistence of the viable C. myrmidone metapopulation and mitigate the negative effects of climate change, a network of stable forest-steppe-like habitats in combination with the creation of clear-cuts in forest stands with presence of C. ruthenicus would be optimal. To achieve this, we recommend leaving some selected clear-cuts without replanting to restore grasslands (through grazing and removal of excess trees, shrubs and invasive plants), while the suitability of ephemeral habitats could be improved through appropriate silvicultural measures.