ABSTRACT
Trophic interactions may strongly depend on body size and environmental variation, but this prediction has been seldom tested in nature. Many spiders are generalist predators that use webs to intercept flying prey. The size and mesh of orb webs increases with spider size, allowing a more efficient predation on larger prey. We studied to this extent the orb-weaving spider Araneus diadematus inhabiting forest fragments differing in edge distance, tree diversity, and tree species. These environmental variables are known to correlate with insect composition, richness, and abundance. We anticipated these forest characteristics to be a principle driver of prey consumption. We additionally hypothesized them to impact spider size at maturity and expect shifts toward larger prey size distributions in larger individuals independently from the environmental context. We quantified spider diet by means of metabarcoding of nearly 1,000 A. diadematus from a total of 53 forest plots. This approach allowed a massive screening of consumption dynamics in nature, though at the cost of identifying the exact prey identity, as well as their abundance and putative intraspecific variation. Our study confirmed A. diadematus as a generalist predator, with more than 300 prey ZOTUs detected in total. At the individual level, we found large spiders to consume fewer different species, but adding larger species to their diet. Tree species composition affected both prey species richness and size in the spider's diet, although tree diversity per se had no influence on the consumed prey. Edges had an indirect effect on the spider diet as spiders closer to the forest edge were larger and therefore consumed larger prey. We conclude that both intraspecific size variation and tree species composition shape the consumed prey of this generalist predator.
ABSTRACT
Forest fragments in highly disturbed landscapes provide important ecosystem services ranging from acting as biodiversity reservoir to providing timber or regulating hydrology. Managing the tree species richness and composition of these fragments to optimize their functioning and the deliverance of multiple ecosystem services is of great practical relevance. However, both the strength and direction of tree species richness and tree species composition effects on forest ecosystem multifunctionality may depend on the landscape context in which these forest remnants are embedded. Taking advantage of an observatory network of 53 temperate forest plots varying in tree species richness, tree species composition, and fragmentation intensity we measured 24 ecosystem functions spanning multiple trophic levels and analyzed how tree species diversity-multifunctionality relationships changed with fragmentation intensity. Our results show that fragmentation generally increases multifunctionality and strengthens its positive relationship with diversity, possibly due to edge effects. In addition, different tree species combinations optimize functioning under different fragmentation levels. We conclude that management and restoration of forest fragments aimed at maximizing ecosystem multifunctionality should be tailored to the specific landscape context. As forest fragmentation will continue, tree diversity will become increasingly important to maintain forest functioning.