Spatial synchrony in sub-arctic geometrid moth outbreaks reflects dispersal in larval and adult lifecycle stages

1. Spatial synchrony in population dynamics can be caused by dispersal or spatially correlated variation in environmental factors like weather (Moran effect). Distinguishing between these mechanisms is challenging for natural populations, and the study of dispersal-induced synchrony in particular has been dominated by theoretical modelling and laboratory experiments. 2. The goal of the present study was to evaluate the evidence for dispersal as a cause of meso-scale (distances of tens of kilometers) spatial synchrony in natural populations of the two cyclic geometrid moths Epirrita autumnata and Operophtera brumata in sub-arctic mountain birch forest in northern Norway. 3. To infer the role of dispersal in geometrid synchrony, we applied three complementary approaches, namely estimating the effect of design-based dispersal barriers (open sea) on synchrony, comparing the strength of synchrony between E. autumnata (winged adults) and the less dispersive O. brumata (wingless adult females), and relating the directionality (anisotropy) of synchrony to the predominant wind directions during spring, when geometrid larvae engage in windborne dispersal (ballooning). 4. The estimated effect of dispersal barriers on synchrony was almost three times stronger for the less dispersive O. brumata than E. autumnata. Inter-site synchrony was also weakest for O. brumata at all spatial lags. Both observations argue for adult dispersal as an important synchronizing mechanism at the spatial scales considered. Further, synchrony in both moth species showed distinct anisotropy and was most spatially extensive parallel to the east-west axis, coinciding closely with the overall dominant wind direction. This argues for a synchronizing effect of windborne larval dispersal. Congruent with most extensive dispersal along the east-west axis, E. autumnata also showed evidence for a travelling wave moving southwards at a speed of 50-80 km/year. 5. Our results suggest that dispersal processes can leave clear signatures in both the strength and directionality of synchrony in field populations, and highlight wind-driven dispersal as promising avenue for further research on spatial synchrony in natural insect populations.

1. 种群动态（population dynamics）中的空间同步性（spatial synchrony）可由扩散（dispersal）或天气等环境因子的空间相关变异引发，即莫兰效应（Moran effect）。对于自然种群而言，区分这两种作用机制颇具挑战，而针对扩散诱导同步性的研究，长期以来多以理论建模与室内实验为主。
2. 本研究旨在评估挪威北部亚北极山地桦木林（sub-arctic mountain birch forest）内，两种周期性尺蛾（geometrid moths）——秋寒尺蛾（Epirrita autumnata）与冬尺蛾（Operophtera brumata）的自然种群中，扩散作为中尺度（meso-scale，数十千米范围）空间同步性驱动因子的相关证据。
3. 为推断扩散在尺蛾种群同步性中的作用，本研究采用三种互补研究方法：一是估算基于实验设计的扩散屏障（开阔海域）对空间同步性的影响；二是比较具翅成虫的秋寒尺蛾（E. autumnata）与扩散能力较弱、雌成虫无翅的冬尺蛾（O. brumata）二者的同步性强度差异；三是将空间同步性的方向性（各向异性（anisotropy））与春季主导风向相关联——春季正是尺蛾幼虫开展风载扩散（ballooning）的时期。
4. 扩散屏障对同步性的估算效应，在扩散能力较弱的冬尺蛾中几乎是秋寒尺蛾的三倍。在所有空间滞后（spatial lags）尺度下，冬尺蛾的种群间同步性也均为最弱。上述两项观测结果均表明，成虫扩散在本次研究的空间尺度下是一种重要的种群同步化机制。进一步来看，两种尺蛾的空间同步性均表现出显著的各向异性，其空间分布范围在东西向轴向上最为广泛，与区域主导风向高度吻合，这说明幼虫的风载扩散具有同步化效应。与东西向扩散最为广泛的结论一致，秋寒尺蛾还表现出以50-80千米/年的速度向南移动的种群行波（travelling wave）证据。
5. 本研究结果表明，扩散过程可在野外种群的同步性强度与方向性两方面留下清晰的特征印记，同时凸显了风驱扩散（wind-driven dispersal）作为自然昆虫种群空间同步性后续研究的极具前景的方向。