Data from: Limited emigration from an outbreak of a forest pest insect

Population density and individual dispersal behaviour affect species´ distribution dynamics. Population densities vary over time, and some species occasionally increase to very high numbers, for example during outbreaks. In such situations populations are expected to expand into new areas due to density-dependent dispersal and sometimes even result in range expansion. A local population of the northern pine processionary moth Thaumetopoea pinivora has recently reached outbreak densities at the edge of its northern range at the southern tip of Gotland Island in the Baltic Sea. We first investigated if the outbreak had resulted in establishment of populations in suitable habitats on Gotland Island outside of the outbreak area. Six small populations were found that could potentially have originated from the outbreak area. However, data from 12 microsatellite markers strongly suggest that these populations did not originate from the recent outbreak. Genetic variability was not reduced in these small, isolated populations and there were several unique alleles, indicating instead a different population history and that there has been no recent range expansion. In addition, there was apparent genetic isolation by geographic distance, implying that despite that the outbreak population reached a high enough density to cause density-dependent dispersal, significant gene flow has not occurred.

种群密度与个体扩散行为会影响物种的分布动态。种群密度随时间波动，部分物种在暴发期偶尔会达到极高的种群数量。在此类情境下，种群会因密度依赖扩散（density-dependent dispersal）向新区域扩张，有时甚至会引发分布范围扩张（range expansion）。近日，波罗的海哥特兰岛南端、该物种北部分布范围边缘的一处北松异舟蛾（Thaumetopoea pinivora）本地种群达到了暴发种群密度。本研究首先针对此次暴发是否促使种群在暴发区域外的哥特兰岛适宜生境中建立新种群展开探究。最终共发现6个小型种群，其理论上可能源自该暴发种群。然而，基于12个微卫星标记（microsatellite markers）的遗传学数据强烈表明，这些小型种群并非源自此次近期暴发种群。这些小型孤立种群的遗传变异（genetic variability）水平并未降低，且存在多个独特等位基因（unique alleles），这反而表明它们拥有不同的种群历史，且近期未发生分布范围扩张。此外，种群间存在显著的由地理距离导致的遗传隔离（genetic isolation by geographic distance），这意味着尽管该暴发种群已达到足以引发密度依赖扩散的种群密度，但并未发生显著的基因流（gene flow）。