Algal resource depression by macro-invertebrate herbivores in a chalk stream: An empirical approach

Abstract

Herbivory is a globally important ecosystem function, occurring in all major biome types; including benthic freshwater habitats. Algal biofilms and their herbivore consumers are therefore important components of stream food webs. However there is relatively little empirical data quantifying the strength of these algal-herbivore interactions, or how these vary with herbivore identity, size, and biofilm physiognomy. Interactions across a diverse herbivore guild were investigated in a chalk stream, using mesocosms to determine the distribution of algal-herbivore interaction strengths. A series of experiments were used to assess: herbivore link strength distribution; context-dependency of interaction strength; the relationship of body size with interaction strength; and the effects of competing grazer species on algal resources. The algal-herbivore sub-web was dominated by weak interactions which concurred with empirical and theoretical evidence, and further supporting web stability theory. Interactions were highly context-dependent, with interaction magnitude and species identity both affected by algal biofilm type. Grazer species identity was important for determining body size relationships. Although competitive effects were apparent, they were not statistically detectable. This research builds on previous investigations of algal grazer interactions and food web structure by emphasizing; the importance of grazing as an ecosystem function, and the diversity of interactions occurring in model systems. The use of experimental mesocosms may be limited in terms of ‘real’ systems, but does provide a valid response of model systems that are both useful and valid tools for assessing community ecology