Long-term monitoring of Phengaris (Lepidoptera : Lycaenidae) butterflies in the Přelouč surroundings (Czech Republic) : is the waterway construction a serious threat?

The monitoring of sympatrically occurring Phengaris teleius and P. nausithous metapopulations in the surroundings of Přelouc, Czech Republic, was launched over a decade ago in connection with the planned waterway construction on the Elbe river. The mark-recapture surveys were initially restricted to 10 habitat patches on the right river bank, but in 2010 three new patches were discovered on the opposite bank. We use the monitoring results for 2011–2015 to assess how the discovery of additional populations alters the impact evaluation of the prospective construction. The overall abundance of P. teleius in the recently discovered populations was about twice as high as on the right bank (ca. 2,800 vs. 1,400 adults on average), while the numbers of P. nausithous on both banks proved to be balanced (at ca. 600 adults on average). Furthermore, we confirmed a substantial exchange of butterflies between population representing both banks, which indicates a well-integrated metapopulation in both species. Since the waterway would partly destroy the two largest populations on the right bank, the potential loss due to its construction is estimated at 40.1–64.3% P. teleius individuals and 20.2–47.4% P. nausithous individuals occurring there. Nevertheless, concerning the entire metapopulation, the predicted decline is considerably smaller, reaching 13.9–25.7% in P. teleius and 8.5–20.0% in P. nausithous. Consequently, a long-term survival of the species is likely even in the case of the waterway construction, as long as appropriate management is applied on the unaffected habitat patches.