Grazing intensity negatively affects the maintenance of Gentiana pneumonanthe and the survival of Phengaris alcon egg-laying

Abstract

Phengaris alcon is an endangered butterfly that is subject to several conservation programs in Europe. Our study aims at evaluating how grazing pressure by domestic animals affects the maintenance of Gentiana pneumonanthe stems, larval plant host of Phengaris alcon and the survival of Phengaris alcon's eggs. We selected 304 stems of G. pneumonanthe (including 125 with P. alcon eggs) that were distributed over 4 ungrazed plots and 18 plots grazed by cattle or horses, for which we collected a grazing pressure index (Livestock Units days grazed ha−1). We monitored each stem over seven sampling sessions at 10 days intervals during the summer (from 02 July to 12 September 2018). We demonstrated that the G. pneumonanthe stems browsing probability was two times higher in the grazed plots than in the ungrazed plots. The number of eggs sharply decreased in the grazed plots, whereas it remained constant in the ungrazed plots during successive sampling sessions. Grazing, even at low intensity (10 LU days grazed ha−1), induced defoliation of G. pneumonanthe stems. The probability of the maintenance of G. pneumonanthe stems and the probability of survival of P. alcon eggs is close to zero above 20 LU days grazed ha−1. In order to protect the P. alcon population, we recommend extensive and early grazing (before mid-July) before the growth of G. pneumonanthe stems, or late grazing (after early September) avoiding the development period of G. pneumonanthe, the oviposition period and the early caterpillar development of P. alcon. Alternatively, we suggest the establishment of temporary exclosures on high densities areas of G. pneumonanthe between mid-July to early September.