No detectable trophic cascade in a high-Arctic arthropod food web

Abstract

In so-called trophic cascades, a predator may influence the non-adjacent trophic level by suppressing the abundance and/or altering the traits of its prey. In the Arctic, the occurrence of trophic cascades seems particularly likely due to the restricted number of interacting species. Spiders are abundant, and their predation of herbivorous insects has been recognized as a potentially important structuring force of the community. To test whether variation in spider abundance results in trophic cascades through an arctic food chain, we explored how the abundance of the wolf spider Pardosa glacialis is reflected in herbivore damage on the plants of a high-Arctic locality: the Zackenberg Valley in Northeast Greenland. We first established observational transects to reveal that local variation in spider densities is indeed reflected in local variation in caterpillar herbivory on mountain avens (Dryas octopetala x integrifolia) and arctic willow (Salix arctica). We then used two types of enclosures to manipulate local densities of P. glacialis and/or of the herbivore Sympistis nigrita. While these treatments were successful in altering predator and herbivore densities, the manipulation of spider densities had no detectable effect on levels of plant damage or seed production. The results indicate that spiders as predators are unable to suppress the herbivores in Northeast Greenland. While surprising, this lack of cascading effects might be attributable to the general diet of wolf spiders. P. glacialis may feed on the predator, the herbivore, the decomposer and/or the pollinator guilds of the food web, thereby diluting the predation pressure in multiple directions. Thus, the terrestrial food webs of the Arctic seem both complex and robust against cascading effects from fluctuations in the densities of single species.