Disturbance and herbivory

The interaction between herbivores and periphyton is generally considered to be strongest under favorable environmental conditions and diminished when environmental conditionals are extreme or highly variable. Most field studies of grazing have been conducted under low flow conditions and thus do not adequately represent interactions during the more physically stressful conditions associated with environmental extremes that can occur seasonally or episodically (Feminella and Hawkins 1995). Such extremes are commonly referred to as disturbances, and are often thought of as abiotic but can also include biotic factors such as invasive species. Many of the best documented examples involve extremes of current.

Grazing may result in an algal assemblage that is less vulnerable to scouring by floods, presumably by reducing mat buildup. When the periphyton in laboratory streams subjected to different grazing pressure by the snail Elimia clavaefor-mis experienced a common scour disturbance, most structural characteristics of the periphyton displayed significantly higher resistance to the disturbance in streams with snails than in streams without snails (Mulholland et al. 1991). Experimental reduction of grazers at locations of differing current velocity revealed an interaction between grazing and current in a Colorado stream (Opsahl et al. 2003). After 45 days, electrified tiles had significantly fewer grazers and more than twice the algal biomass compared with controls, but greater algal abundance on tiles in slow currents suggested that grazers differed in their ability to regulate algae across the current velocity gradient.

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