Bottom-up when it is not top-down: Predators and plants control biomass of grassland arthropods

1) We investigate where bottom-up and top-down control regulates
ecological communities as a mechanism linking ecological gradients to the
geography of consumer abundance and biomass. We use standardized surveys
of 54 North American grasslands to test alternate hypotheses predicting
100-fold shifts in the biomass of four common grassland arthropod
taxa—Auchenorrhyncha, sucking herbivores, Acrididae, chewing herbivores,
Tettigoniidae, omnivores, and Araneae, predators. 2) Bottom-up models
predict that consumer biomass tracks plant quantity (e.g. productivity and
standing biomass) and quality (nutrient content) and that ectotherm access
to food increases with temperature. Each of the focal trophic groups
responded differently to these drivers: the biomass of sucking herbivores
and omnivores increased with plant biomass; that of chewing herbivores
tracked plant quality; and predator biomass did not depend on plant
quality, plant quantity, or temperature. 3) The exploitation ecosystem
hypothesis (EEH) is a top-down hypothesis that predicts a shift from
resource limitation of herbivores when plant production is low, to
predator limitation when plant production is high. In grasslands where
spider biomass was low, herbivore biomass increased with plant biomass,
whereas bottom-up structuring was not evident when spiders were abundant.
Furthermore, neither predator biomass nor trophic position (via stable
isotope analysis) increased with plant biomass, suggesting predators
themselves are top-down limited. 4) Stable isotope analysis revealed that
trophic position of the chewing herbivore and omnivore increased
significantly with plant biomass, suggesting these groups increased
scavenging and meat consumption in grasslands with higher carbohydrate
availability. 5) Taken together, our snapshot sampling documents gradients
of food web structure across 54 grasslands, consistent with multiple
hypotheses of bottom-up and top-down regulation. 10-Jan-2020