Populations and communities

Nutrient limitation and disturbance as tipping points affecting the strength of top-down control of stream algae

The importance of cascading trophic interactions varies among streams. Angus McIntosh and colleagues examined factors affecting the strength of top-down control of algae in Colorado streams.

Rocky Mountain Biological Laboratory	The East River, near the Rocky Mountain Biological Laboratory

A grazing assay in stream-side channels at ten streams varying in biotic and abiotic conditions indicated invertebrate grazers were less effective at depressing algal abundance in disturbed streams. Removal of nutrient limitation by adding N and P increased net algal biomass independent of disturbance. A microcosm experiment further revealed an interaction between grazer identity, predatory fish and nutrient addition that affected grazer interaction strength. Baetis mayflies were better grazers per unit of biomass than Alloymia caddisflies, but Baetis were less effective grazers in the presence of fish cues. Consequently, addition of nutrients resulted in accumulations of algal biomass that were greatest with caddisflies, intermediate with mayflies constrained by predator cues, and least with unconstrained mayflies. Thus, the strength of cascading effects of predators on algal resources depended on grazer species and nutrient status. Furthermore, those variations in grazer interaction strength combined with patterns observed in the field assay indicate that the tipping point between algal accumulation and algal reduction depends not only on nutrient limitation, but also on grazer composition and disturbance regime.

Bobbi Peckarsky (left), Maruxa Alvarez (middle) and Marge Penton (right)

People involved in this project

University of Canterbury
Angus McIntosh
Helen Warburton

Cornell University
Barbara Peckarsky
Jennifer Moslemi

Rocky Mountain Biological Laboratory
Steven Horn