Forest detrital food webs in multiple-use landscapes: Bottom-up nutrient enrichment by overabundant deer and top-down predator effects

White-tailed deer (Odocoileus virginianus) are “over-abundant” in many areas of the eastern United States.  This over-abundance is often related to landscape composition whereby food resources are derived from agricultural land and adjacent forest is utilized for diurnal concealment. When primary food sources for deer are external to the forest, the result may be a net transport of nutrients from agricultural to forest ecosystems and a nutrient subsidy to the forest detrital food web.  I experimentally address (1) the bottom-up impact of supplemental N (i.e. deer urine) on the detrital food web, (2) the interaction of nutrient subsidies with top-down effects of vertebrate predation, and (3) the effect of concurrent top-down and bottom-up effects on ecosystem processes. Experimental treatments include vertebrate exclusion, nutrient enrichment, and combined exclusion and enrichment in a forest ecosystem in Baltimore County, Maryland.  Relative to controls, vertebrate exclusion increased collembola and orabatid mite populations by 58% and 99%, respectively. These increases result from vertebrate predation decreasing invertebrate predators (e.g., spiders) of collembola and mites.  Nutrient enrichment increased mite populations by 99%, but collembola populations changed little.  The combined effects of these treatments (collembolan: +429%; orabatid mites: +189%) indicate a synergistic interaction between bottom-up and top-down processes that increased short-term leaf litter decomposition rate (+3.6% over control conditions).  In addition to demonstrating synergistic effects of top-down and bottom-up population control on ecosystem processes, this study provides evidence that local detrital food web and ecosystem processes are mediated by landscape composition and landscape-scale foraging by an overabundant herbivore.  The need for a holistic view of white-tailed deer management and forest ecosystem restoration are highlighted in a conceptual model of multi-faceted deer impacts on forest nutrients, the detrital food web, forest floor ecosystem processes, and forest floor structure.