When predators unexpectedly decrease instead of increase plant production

Area of Science:

• Ecology
• Ecosystem Services
• Predator-Prey Dynamics

Background:

• Predators are often conserved for ecosystem services, such as regulating herbivore populations and enhancing plant production.
• In grasslands, birds are key predators of grasshoppers, a dominant herbivore influencing plant communities.

Purpose of the Study:

• To investigate the conditions under which bird predation on grasshoppers leads to increased plant production in grasslands.
• To determine if grasshopper consumption of specific plant species affects nitrogen availability and subsequent plant growth.

Main Methods:

• A multi-site, multi-year field experiment was conducted, excluding birds to assess their impact on grasshopper and plant dynamics.
• The study analyzed the relationship between grasshopper abundance, selective feeding on plant species, decomposition rates, and nitrogen availability.

Main Results:

• Bird exclusion led to increased grasshopper abundance in some cases.
• Grasshopper feeding habits influenced plant litter composition and decomposition rates, thereby affecting nitrogen availability.
• The ecosystem service of increased plant production due to bird predation was not consistently observed across all sites and years.

Conclusions:

• The ability of predators to enhance plant production through herbivore control is contingent on the herbivore's impact on nutrient cycling, specifically nitrogen availability.
• Grasshopper herbivory can increase nitrogen availability, but this mechanism does not consistently translate to increased plant production, limiting the reliability of this ecosystem service.