Top-down control in a detritus-based food web: fish, shredders, and leaf breakdown

Area of Science:

• Ecology
• Aquatic Ecology
• Food Web Dynamics

Background:

• Detritus decomposition is crucial for aquatic ecosystem functioning.
• Trophic cascades, initiated by predators, can alter ecosystem processes.
• Understanding fish-predator effects on invertebrate shredders is key to aquatic food web research.

Purpose of the Study:

• To test if fish predation reduces shredder abundance and leaf breakdown rates.
• To investigate the occurrence of a trophic cascade in a detritus-based stream food web.
• To analyze how differential predator responses among shredder taxa influence leaf decomposition.

Main Methods:

• Leaf packs of willow leaves were placed in stream cages with and without fish access (exclosures vs. controls).
• Invertebrate and shredder abundance and biomass were quantified over 112 days.
• Leaf breakdown rates were measured, and shredder density effects were analyzed per riffle.

Main Results:

• Excluding fish significantly increased invertebrate and shredder abundance and biomass in leaf packs.
• Leaf breakdown rates differed significantly between fish exclosures and controls, with variable effects across riffles.
• Most shredder taxa decreased in abundance due to fish predation, but one taxon showed increased biomass in controls.

Conclusions:

• Fish predation reduces the abundance of most shredder invertebrates in leaf packs.
• Fish indirectly influence leaf breakdown rates, but effects can be complex and vary among stream locations.
• Differential vulnerability of shredder taxa to fish predation can lead to outcomes that deviate from simple trophic cascade predictions.