Epigeal spider responses to fertilization and plant litter: testing biodiversity theory at the ground level

Recent studies of nutrient additions to terrestrial ecosystems have focused on the “aerial” portion of the food web associated with living plants. These studies showed nutrient loading increased arthropod abundance and biomass, but decreased diversity. However, none of these studies explicitly examined nutrient loading effects on epigeal arthropods. To test nutrient loading effects on epigeal spiders and on individual species within a temperate-latitude grassland community, we used pitfall traps to sample spiders for four years within 24 large (314 m2) plots in which we manipulated nutrients (NPK fertilizer) and plant litter (litter removed or left in place). We measured the diversity, abundance, biomass, and community structure responses of the spider community, and of wolf spiders (Lycosidae) and linyphiid spiders (Linyphiidae), as well as the abundance and biomass responses of the six most common species. We hypothesized increased nutrient loading would increase epigeal spider abundance and biomass but decrease diversity. Contrary to predictions, spider species richness, diversity, and biomass were not significantly affected by fertilization, while fertilization resulted in significantly increased abundance. Also contrary to predictions, plant litter did not affect any of these variables. Linyphiid spiders had the strongest responses to fertilization, with significantly increased abundance and biomass, and, contrary to predictions, increased species richness in fertilized plots. Wolf spiders responded more closely to predictions. Our results indicate that the epigeal spider community does not respond as would be predicted by biodiversity-productivity theory. This underscores the need to integrate the largely detritus-based epigeal community into current biodiversity-productivity theory.