Spatial synchrony in sub‐arctic geometrid moth outbreaks reflects dispersal in larval and adult life cycle stages

• Published in: The Journal of animal ecology Vol. 88; no. 8; pp. 1134 - 1145
• Main Authors: Vindstad, Ole Petter Laksforsmo, Jepsen, Jane Uhd, Yoccoz, Nigel Gilles, Bjørnstad, Ottar N., Mesquita, Michel d. S., Ims, Rolf Anker, Zytynska, Sharon
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01-08-2019; Wiley
• Subjects: Animal behavior; Animals; Anisotropy; ballooning; Butterflies & moths; Disease Outbreaks; Dispersal; dispersal barrier; Environmental factors; Epirrita autumnata; Female; Females; Insects; interspecies comparison; Larva; Larvae; Life Cycle Stages; Life cycles; Matematikk og Naturvitenskap: 400; Mathematics and natural science: 400; Moths; Mountains; Natural populations; Norway; Operophtera brumata; Outbreaks; Pest outbreaks; Polar environments; population cycle; Population Dynamics; Population studies; Populations; Synchronism; Traveling waves; travelling wave; VDP; Weather; Wind direction; wind‐driven dispersal; Zoogeografi: 486; Zoogeography: 486; Zoologiske og botaniske fag: 480; Zoology and botany: 480; Norway; population cycle; ballooning; Epirrita autumnata; interspecies comparison; dispersal barrier; Operophtera brumata; travelling wave; wind-driven dispersal
• ISSN: 0021-8790; 1365-2656; 1365-2656
• DOI: 10.1111/1365-2656.12959

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.
The goal of the present study was to evaluate the evidence for dispersal as a cause of meso‐scale (distances of tens of kilometres) 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.
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).
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 to 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.
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.
The study of dispersal‐induced spatial synchrony in population dynamics has relied heavily on theory and laboratory experiments. The current long‐term field study indicates that both larval and adult dispersal are important synchronizing mechanisms in natural populations of two sympatric geometrid moths, and highlights wind as determining the directionality of synchrony.