Nonlinear effects of an aquatic consumer: causes and consequences

Area of Science:

• Ecology
• Aquatic Ecology
• Consumer-Prey Dynamics

Background:

• Per capita consumer effects on prey are often assumed to be independent of consumer and prey densities.
• Understanding these interactions is crucial for ecological modeling and predicting community dynamics.

Purpose of the Study:

• To investigate the assumption of density-independent per capita consumer effects.
• To examine the nonlinearities in the interaction between an aquatic herbivore (Daphnia) and its prey (phytoplankton).

Main Methods:

• A large-scale manipulation of Daphnia (consumer) density was conducted in an aquatic system.
• Consumer removal gradients were maintained to allow equilibration of consumer and prey abundances.
• Phytoplankton abundance and species composition were analyzed across different Daphnia densities.

Main Results:

• Strong nonlinear effects of Daphnia on total phytoplankton abundance and most common phytoplankton species were observed.
• Daphnia's suppression of phytoplankton was significant at low densities but diminished at higher densities.
• The observed deceleration was attributed to reduced Daphnia filtering at low prey densities, indicative of a Type III functional response.

Conclusions:

• The assumption of density-independent consumer effects is often violated in aquatic systems.
• Nonlinear consumer effects, like the Type III functional response, are prevalent and complicate ecological modeling.
• Field experiments should prioritize testing ecological models rather than solely parameterizing them.