Effects of nutrient loading in North American bogs and fens: Multi-trophic effects of nutrient loading on a continental scale

Background/Question/Methods Ecologists have searched for regularities in food web structure through meta-analyses of diverse aquatic, terrestrial, and marine food webs. However, very few studies exist in which the structure of a single food web has been measured across a geographic gradient. Here, we report on a continental-scale survey of the invertebrate and microbial food web that inhabits water-filled leaves of the carnivorous pitcher plant Sarracenia purpurea. This brown web consists of captured prey as the resource base for abundant bacteria, which are consumed by a suite of bacterivores including protozoa, rotifers, and mites. Midges and flesh flies shred prey, and mosquitoes function as top predators. We surveyed the Sarracenia food web with a standardized sampling protocol used at 39 sites throughout its geographic range, from the Florida panhandle to central Canada. We synthesized these food web data with measurements of annual nitrogen deposition (from ammonium and nitrate) at 23 monitoring stations throughout the U.S. Across this range of sites, annual N deposition rates varied spatially over 20-fold during the year of the field census, and are a potentially important determinant of food web structure.
Results/Conclusions We found that standard food web metrics were not correlated with latitude longitude or elevation. However, web connectance and abundance of bacteria were both positively correlated with annual nitrogen deposition. These results at the continental scale are consistent with other small-scale experimental studies suggesting that bottom-up processes are important in controlling many aspects of the structure of the Sarracenia food web.