Potential positive effect of the ant species Lasius niger on linyphiid spiders

Abstract

Ants are highly abundant generalist predators and important ecosystem engineers which can strongly affect the composition of animal communities. We manipulated the density of the ant species Lasius niger with baits in a small-scale field experiment to study the role of intraguild predation, top-down control and bottom-up effects of ants in a dry grassland surrounded by agricultural fields. Two different kinds of baits (honey and tuna) were presented near to the nests and at a distance of 2 m from six L. niger colonies in a dry grassland habitat, where L. niger was a highly abundant, omnipresent species. The experiments were performed for 1 month in spring. Additionally, the natural abundance of L. niger varying with the distance to their nests was used to study the effects on spiders and potential prey groups. The activity of L. niger was significantly higher at tuna baits compared with that at honey baits and empty control dishes. We found no effects of higher activity of L. niger on the arthropod community. However, there is evidence for a facilitation effect of ants on Collembola near to their colonies, probably due to habitat modification, which also influenced the density of Linyphiidae. Both groups had up to four times higher denisities next to L. niger colonies than at a distance of 2 m. Furthermore, delta(13)C values demonstrated that linyphiid spiders and L. niger predominantly feed on Collembola. We conclude that there is no evidence of top-down effects of L. niger in a grassland in spring, but we found a facilitation of linyphiid spiders and their prey by the ants, which acted as ecosystem engineers.