Ants and Spiders in Grassland Food Webs

A challenge in ecology is to uncover the link between food web interactions and ecosystem function. Recent studies have shown that top-down forces by invertebrate predators on their prey and cascade effects on plants play an important role in structuring terrestrial communities. The strength of such trophic cascades is hypothesised to be affected by habitat structure, predator diversity and predator identity. Ants and spiders are among the Earth s most abundant terrestrial arthropod predators and most are generalists, feeding on a wide variety of prey, including other predators. To study the role of predation by ants and spiders in six field experiments the density and diversity of predator groups, the habitat structure and habitat fragmentation were manipulated. In addition to gain a better understanding of the positions of spiders and ants in the studied food web and important trophic links, we used a stable isotope analysis (15N and 13C). The analysis of the stable isotope 15N revealed a higher trophic position in the food web for hunting spiders as intraguild predators than for web-builders and an intermediate position for the studied ant species as omnivores. We found that spiders had a strong predatory effect on decomposers (Collembola) and herbivores. One experiment revealed strong intraguild interactions between ants and spiders. In contrast, in a small-scale field experiment we found that the presence of the ant species Lasius niger positively affected spiders and their prey. A similar positive influence of ants on spiders and Collembola was found in another experiment studying the predatory impact of web-building spiders, hunting spiders and ants. Web-builders responded positively to ant treatment based on higher prey densities (Collembola), which profit from the nutrient input by ants. Hunting spiders had a positive influence on the density of ants. The assemblage of web-builders and hunting spiders reduced the density of the most abundant herbivore and of decomposers by 50 %, while the presence of ants in the predator guilds reduced the predatory impact.The loss in species diversity and the resulting change in food web dynamics were studied using predatory effects of web-builders and hunting spiders. Field experiments were conducted for a natural and a depleted diversity level: consisting of three or one species of web-builders and hunting spiders, respectively. Top-down effects were weaker and plant biomass 20 % lower in systems with lower spider diversity. If intraguild predators such as hunting spiders were included, the per capita effects and top-down effects on Auchenorrhyncha population declined with increasing predator abundance and diversity. In order to test if differences in structural complexity of habitats modify the extent of top-down forces, densities of predators and the complexity of habitat structure were manipulated. Habitat structure was changed by cutting the vegetation to half its height, removing leaf litter and using natural differences. The predatory effect on herbivores was stronger in lower-structured vegetation. The presence of predators had a positive effect on plant- and leafhopper species diversity, probably by reducing competition among the herbivores.To study the effects of small-scale fragmentation on the species diversity of web-builders and hunting spiders grassland fragments of different size and varying distance to the surrounding habitat were created by frequent mowing in seven grassland habitats. Spider abundance and diversity responded to the treatment effect of fragment size. Especially hunting spiders were more diverse on small grass-land fragments. These studies demonstrate that predators can interact positive and negative, depending strongly on habitat structure, the predator identity and diversity, and that these interactions can change the strength of top-down forces.