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.

中文翻译：

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放牧强度对龙胆的维持和Phengaris alcon产卵的存活产生负面影响

Phengaris alcon 是一种濒临灭绝的蝴蝶，在欧洲受到多项保护计划的约束。我们的研究旨在评估家畜的放牧压力如何影响 Gentiana pneumonanthe 茎的维持、Phengaris alcon 的幼虫植物宿主和 Phengaris alcon 卵的存活。我们选择了 304 根 G. pneumonanthe（包括 125 根带有 P. alcon 卵的茎），分布在 4 个未放牧地和 18 个由牛或马放牧的地块，我们收集了放牧压力指数（放牧单位天数 ha−1） . 我们在夏季（2018 年 7 月 2 日至 9 月 12 日）以 10 天为间隔在 7 个采样阶段监测每个茎。我们证明，放牧地块中的 G. pneumonanthe 茎浏览概率是未放牧地块的两倍。在连续采样期间，放牧地块中的鸡蛋数量急剧减少，而未放牧地块中的鸡蛋数量保持不变。放牧，即使在低强度下（10 LU 天放牧 ha-1），也会导致 G. pneumonanthe 茎的落叶。G. pneumonanthe 茎的维持概率和 P. alcon 卵的存活概率在放牧 ha-1 的 20 LU 天以上接近于零。为了保护 P. alcon 种群，我们建议在 G. pneumonanthe 茎生长之前进行广泛和早期放牧（7 月中旬之前），或避免 G. pneumonanthe 产卵期的晚放牧（9 月初之后）时期和 P. alcon 的早期毛毛虫发育。或者，我们建议在 G 的高密度区域建立临时围栏。