Nitrogen inputs suppress plant diversity by overriding consumer control

Area of Science:

• Ecology
• Environmental Science
• Plant-Community Ecology

Background:

• Anthropogenic nitrogen (N) deposition is a global environmental issue impacting terrestrial ecosystems.
• Nitrogen deposition is predicted to increase plant productivity but decrease plant diversity by favoring dominant species.
• Herbivores are hypothesized to counteract N-deposition effects by consuming excess plant biomass.

Purpose of the Study:

• To investigate the impact of nitrogen addition on herbivore top-down control and plant diversity in a grassland ecosystem.
• To determine how nitrogen enrichment influences the interaction between herbivores and plant communities.

Main Methods:

• A multifactorial field experiment was conducted to assess the effects of nitrogen addition and herbivore presence.
• Measurements included total plant biomass, community dominance, herbivore abundance, and plant Shannon diversity.
• Mechanisms of interaction, including plant chemistry and architecture effects on herbivores and predators, were examined.

Main Results:

• At ambient nitrogen levels, grasshoppers suppressed biomass and dominance, increasing plant diversity (top-down control).
• Without grasshoppers, nitrogen addition increased biomass but did not alter plant diversity.
• With grasshoppers, nitrogen addition eliminated herbivore control, increased biomass and dominance, and reduced plant diversity by 21%.

Conclusions:

• Nitrogen deposition can disrupt the top-down control exerted by herbivores in grassland ecosystems.
• Nitrogen addition negatively impacts plant diversity by reducing herbivore abundance and effectiveness.
• Nitrogen deposition can shift ecosystem control from top-down (herbivore-driven) to bottom-up (resource-driven), harming plant diversity.