How often are ecosystems top-down controlled? Experiments in grassland, grasshopper, and bird systems over time and...

Area of Science:

• Ecology
• Ecosystem Dynamics
• Predator-Prey Interactions

Background:

• Ecosystems are often assumed to be regulated by predation (top-down control).
• This top-down control hypothesis suggests predators limit prey populations, which in turn affects lower trophic levels.
• However, the complexity of ecological interactions may lead to varied responses.

Purpose of the Study:

• To experimentally test the prevalence and consistency of top-down control in terrestrial ecosystems.
• To investigate how bird exclusion affects plant and grasshopper populations over long time scales.
• To understand the variability of ecosystem responses to predation pressure.

Main Methods:

• Conducted 34-year experiments in four prairie habitats with bird exclosures and controls.
• Measured plant, grasshopper, and spider abundances in 100 m² plots.
• Utilized decision tree/discriminant analysis to identify key ecological drivers.

Main Results:

• Top-down control was observed in only 13.2% of cases.
• Ecosystem responses varied significantly, with plants and grasshoppers increasing/decreasing in various combinations.
• Plant and grasshopper responses were predictable using specific ecological indicators (e.g., ANPP, edible plant biomass).

Conclusions:

• Top-down control is not the predominant force in these prairie ecosystems.
• Ecosystem responses are highly variable across space and time, influenced by factors like climate change.
• Trophic interactions in ecosystems are more complex and variable than often modeled by simple top-down or bottom-up frameworks.