Explaining the changing spatial synchrony of UK aphid outbreaks with wavelets
The Moran effect is the well-known mechanism by which spatially synchronous environmental fluctuations can result in spatially synchronous ecological behaviour. Depending on the timescale of fluctuation successive years may be similar or dis-similar. We introduce wavelet techniques to demonstrate the Moran effect in action in UK aphids at long timescales.
We examine 35 years of variation in UK climate and aphid data from 11 sites, including counts and phenological data for 20 important species. We apply a Morlet wavelet transform to the local fluctuations and evaluate their spatial synchrony. We look for statistically significant relationships between the aphid and climate transforms at long and short timescales. We examine how synchrony differs between timescales, and between the early and late period of measurement.
We find statistically significant relationships between the winter climate and aphid outbreaks (as previously observed in, for example, the Green Spruce aphid, Elatobium Abietinum, and the Grain aphid, Sitobion Avenae) for 18 of the species. We introduce a Moran theorem for wavelets and show that approximately 80% of the spatial synchrony of long timescale fluctuations in these 18 species’ first flight times can be explained by the relationship with local winter temperature fluctuations.
Species for which the phenology is found to be strongly related to winter temperatures show greater spatial synchrony than those with a weak relationship. A reduction in the spatial synchrony of long timescale winter temperature fluctuations since 1993 has been accompanied by a proportionate reduction in the spatial synchrony of aphid phenology. Long timescale UK temperature fluctuations are closely related to the North Atlantic Oscillation climate index; ongoing fluctuations in the NAO thus have implications for the synchrony and predictability of UK aphid phenology.