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Factors shaping latitudinal patterns in communities of arboreal spiders in northern Europe

Kozlov, Mikhail V., Stańska, Marzena, Hajdamowicz, Izabela, Zverev, Vitali, Zvereva, Elena L.
Ecography 2015 v.38 no.10 pp. 1026-1035
Araneae, Betula pubescens, Picea abies, Pinus sylvestris, birds, climate change, conifers, forest trees, functional diversity, herbivores, intensive forestry, latitude, predator-prey relationships, predatory arthropods, species diversity, weather, Finland, Norway, Russia, Scandinavia
Predator effects on herbivores are often referred to as examples of biotic interactions that weaken with latitude, but more studies are needed to test for the generality of this pattern. To further the understanding of large‐scale geographical patterns in abundance and diversity of predatory arthropods, from 2008–2011 we explored spider communities in the canopies of primary forest trees of the boreal zone (Pinus sylvestris, Picea abies, Betula pubescens and B. pendula) along five latitudinal gradients in northern Europe, from 59 to 70°N and from 10 to 60°E. The abundance of arboreal spiders in Norway and Finland was about a half of that in Russia, presumably due to more intensive forest management in Scandinavia. The abundance, taxonomic and functional diversity of arboreal spiders generally decreased with latitude; however, actual weather conditions during the study years had little effect on this pattern. Coniferous species supported higher abundance and diversity of arboreal spiders than birches, but the poleward decrease in either abundance or taxonomic diversity of arboreal spiders did not differ between coniferous and deciduous tree species. In contrast, functional diversity on birches decreased with latitude greater than on coniferous trees. Euryphagous spiders showed stronger decrease with latitude in terms of both abundance and taxonomic diversity than more specialized (steno‐ and oligophagous) spiders. We attribute the general decrease in density and diversity of spiders with latitude to an increased pressure from apex predators (birds) rather than to direct effects of climate or changes in prey abundance. The abundance of spiders declined with the latitude to the greater extent than the densities of their potential prey did, suggesting a decrease in the strength of predator–prey interactions towards the north.