The Effects of Nitrogen Deposition on Cascading Interspecific Interactions in a Mutualistic/Antagonistic Food Web

EPA Grant Number: FP917309
Title: The Effects of Nitrogen Deposition on Cascading Interspecific Interactions in a Mutualistic/Antagonistic Food Web
Investigators: Grinath, Joshua B
Institution: Florida State University
EPA Project Officer: Lee, Sonja
Project Period: August 1, 2011 through July 31, 2014
Project Amount: $126,000
RFA: STAR Graduate Fellowships (2011) RFA Text |  Recipients Lists
Research Category: Academic Fellowships , Fellowship - Ecosystem Services: Terrestrial Systems Animal Ecology


Humans have dramatically altered earth’s nitrogen cycle by increasing the availability and mobility of biologically useful forms of nitrogen; specifically, atmospheric nitrogen deposition has increased due to oxides of nitrogen emitted from fossil fuel combustion and ammonia and ammonium emitted from agricultural operations. In nitrogen-limited ecosystems, the food quality hypothesis predicts that enhanced food quality due to nitrogen pollution will cause greater magnitudes of top-down trophic cascades, whereby upper trophic levels (predators/omnivores) have greater effects on lower trophic levels (herbivores and plants). This study asks: (1) does nitrogen deposition cause stronger cascading interspecific interactions? and (2) do the interactions become stronger as N:C stoichiometric mismatches decrease between consumers and resources?


This study is being conducted in a nitrogen-poor mountain meadow in Colorado, and focuses on cascading effects in a food web composed of antagonistic (negative) interactions between two honeydew-producing herbivores and their shared host plant, and mutualistic (positive) interactions between an ant and these honeydew-producers. This study will conduct a nested factorial field experiment manipulating nitrogen availability to mimic pollution levels elsewhere in the western United States and the presence or biomass of each of the four focal species within 36 single ant nest enclosures. The magnitude of the cascading interactions between ants and plants will be measured at both the per capita and total population levels, and interaction strengths will be compared between fertilized and unfertilized food webs using structural equation modeling (SEM) and ANOVA analyses. Also, stoichiometric N:C in the four focal species and in honeydew resources will be analyzed to test how N:C mismatches relate to cascading interaction strengths and to track the flux of nitrogen through the food web.

Expected Results:

This experiment will test how interspecific interaction strengths are affected by increased nitrogen deposition, where cascading effects are expected to become stronger due to higher food quality and decreased stoichiometric mismatches between consumers and resources in nitrogen-polluted food webs. This study also tackles the challenge of integrating mutualistic and antagonistic interactions into a single food web and uses multiple ecological currencies and metrics to compare interaction strengths to other studies and to examine how they affect the interpretation of interaction strength. It is hoped to continue nitrogen treatments in the experimental plots to ask how the strength of these interactions will change due to increased nitrogen deposition in the long term.

Potential to Further Environmental/ Human Health Protection

This study has the potential to contribute to regulatory decisions for pollutants causing nitrogen deposition. Representing fluxes of energy and nutrients among species, the interaction strengths measured in this study have important consequences for species productivity and dominance. By using an experimental nitrogen treatment mimicking high levels of nitrogen deposition occurring in the western United States, this study will provide evidence for how atmospheric nitrogen pollution is affecting ecosystem functioning in a widespread type of system.

Supplemental Keywords:

nitrogen pollution, atmospheric nitrogen deposition, trophic cascade, plant quality hypothesis, ecological stoichiometry, honeydew mutualism, interspecific interaction strength, structural equation modeling, food web

Progress and Final Reports:

  • 2012
  • 2013
  • Final