Climate change increases the variability of spider dispersal dynamics

Climate change affects the dispersal behavior of many species. Shifts in the migration timing of plants and birds have already been demonstrated but it is still unclear how climate change will affect other animal taxa. We used a unique long term data set to study the effect of changes in climatic conditions on spider migration. Spiders have the ability of a passive aerial dispersion (ballooning). Because ballooning depends on local meteorological conditions, one can expect that migration patterns are closely linked to global change. More than 15000 ballooning spiders were caught in weekly intervals by a 12 m high suction trap between 1994 and 2004. Meteorological data showed that the mean temperatures did not increase in the study area, but irradiation and temperature variability did. Nonetheless, analyses of the time series revealed that spiders tended to disperse earlier. This shift was accompanied by a change in spider dispersal dynamics. In the beginning of the study we could clearly distinguish between a first ballooning maximum in early summer and a second in autumn. This pattern changed toward a single ballooning peak in the middle of summer. Interestingly, the variability in abundance of ballooning spiders increased linearly with temperature variability over the years. In conclusion, our results indicate major shifts in the dispersal phenology of spiders in response to altered meteorological conditions. The observed trend of increased climatic variability may reduce the resilience of spider species and increase their vulnerability to local extinction.