Nutrient limitation and disturbance as tipping points affecting the strength of top-down control of stream algae
Angus R. McIntosh1, Barbara L. Peckarsky2, Jennifer M. Moslemi3, Steven Horn4, and Helen Warburton1. (1) School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, 8140, New Zealand, (2) Department of Zoology, University of Wisconsin, Madison, WI 53706, (3) Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853-2701, (4) Rocky Mountain Biological Laboratory, PO Box 512, Crested Butte, CO 81224
The importance of cascading trophic interactions varies among streams. We examined factors affecting the strength of top-down control of algae in Colorado streams. 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.