Nitrogen enrichment enhances the negative top-down effect on plant functional traits

Area of Science:

• Ecology
• Environmental Science
• Plant Science

Background:

• Eutrophication from human activities alters ecosystem function.
• Nutrient enrichment can intensify top-down ecological effects, impacting plant growth.
• Coastal salt marshes are vulnerable to anthropogenic pressures and trophic interactions.

Purpose of the Study:

• To investigate the combined impacts of nitrogen addition and crab herbivory on Phragmites australis growth.
• To test the hypothesis that eutrophication exacerbates top-down effects on plant communities.
• To elucidate the mechanisms linking nutrient enrichment, herbivory, and plant functional traits.

Main Methods:

• A 3-year manipulative field experiment was conducted in the Yellow River Delta salt marsh.
• Nitrogen addition and controlled crab access (ambient, caged, excluded) were applied to Phragmites australis plots.
• Plant growth parameters, leaf nutrient content, and herbivory rates were measured. Structural equation modeling was used to analyze relationships.

Main Results:

• Nitrogen addition increased leaf nutrient content and palatability, leading to higher crab consumption.
• Combined nitrogen addition and crab herbivory significantly reduced Phragmites australis height, leaf length, and breadth.
• Crab exclusion treatments enhanced plant growth, indicated by increased leaf number and biomass.
• Structural equation modeling confirmed direct positive links between nitrogen addition, leaf nutrients, and herbivory, and a negative link between herbivory and plant traits.

Conclusions:

• Crab herbivory exerts a significant detrimental top-down effect on Phragmites australis growth.
• Nitrogen enrichment exacerbates this top-down effect, amplifying negative impacts on plant functional traits.
• Combined nutrient enrichment and herbivory pose a threat to coastal wetland vegetation, contributing to degradation.
• Findings provide a theoretical framework for wetland conservation and management under global environmental change.