Decline of a Rare Moth at Its Last Known English Site: Causes and Lessons for Conservation

The conditions required by rare species are often only approximately known. Monitoring such species over time can help refine management of their protected areas. We report population trends of a rare moth, the Dark Bordered Beauty Epione vespertaria (Linnaeus, 1767) (Lepidoptera: Geometridae) at its last known English site on a protected lowland heath, and those of its host-plant, Salix repens (L.) (Malpighiales: Salicaceae). Between 2007 and 2014, adult moth density reduced by an average of 30–35% annually over the monitored area, and its range over the monitored area contracted in concert. By comparing data from before this decline (2005) with data taken in 2013, we show that the density of host-plants over the monitored area reduced three-fold overall, and ten-fold in the areas of highest host-plant density. In addition, plants were significantly smaller in 2013. In 2005, moth larvae tended to be found on plants that were significantly larger than average at the time. By 2013, far fewer plants were of an equivalent size. This suggests that the rapid decline of the moth population coincides with, and is likely driven by, changes in the host-plant population. Why the host-plant population has changed remains less certain, but fire, frost damage and grazing damage have probably contributed. It is likely that a reduction in grazing pressure in parts of the site would aid host-plant recovery, although grazing remains an important site management activity. Our work confirms the value of constant monitoring of rare or priority insect species, of the risks posed to species with few populations even when their populations are large, of the potential conflict between bespoke management for species and generic management of habitats, and hence the value of refining our knowledge of rare species’ requirements so that their needs can be incorporated into the management of protected areas.珍稀物种所需的生境条件往往仅能被近似获知。长期监测这类物种，有助于优化其保护地的管理工作。本研究记录了稀有蛾类——暗边美尺蛾（*Epione vespertaria* (Linnaeus, 1767)，鳞翅目(Lepidoptera): 尺蛾科(Geometridae)）已知的英国最后一处分布地（一处受保护的低地荒原）的种群动态，同时记录了其寄主植物银叶柳（*Salix repens* (L.)，金虎尾目(Malpighiales): 杨柳科(Salicaceae)）的种群变化。2007年至2014年间，监测区域内的成虫蛾类密度年均下降30%~35%，其在该区域内的分布范围也同步收缩。通过对比该种群下降前（2005年）与2013年的监测数据，研究发现监测区域内寄主植物的总密度降至原水平的1/3，而在寄主植物密度最高的区域，其密度仅为此前的1/10。此外，2013年的寄主植物个体显著小于2005年。2005年时，蛾类幼虫多栖息于彼时个体显著大于平均水平的寄主植株上；而到2013年，符合该体型标准的寄主植株已大幅减少。这表明蛾类种群的快速衰退与寄主植物种群的变化高度吻合，且极有可能由后者的变化所驱动。目前尚不清楚寄主植物种群发生变化的具体原因，但火灾、冻害与放牧损害可能是相关诱因。尽管放牧仍是该保护地的重要管理手段，但在部分区域降低放牧强度，或可助力寄主植物种群的恢复。本研究证实了持续监测珍稀或优先保护昆虫物种的重要价值，证实了即便种群规模看似庞大，但分布种群数量稀少的物种仍面临生存风险；同时也揭示了针对单一物种的定制化管理与栖息地通用管理之间可能存在的潜在冲突，故而进一步明确了深化珍稀物种需求认知的价值——唯有将其生存需求纳入保护地管理体系，方能切实保护这类物种。