Do crayfish affect stream ecosystem response to riparian vegetation removal?

Abstract

Abstract Riparian vegetation management alters stream basal resources, but stream ecosystem responses partly depend on top‐down interactions with in‐stream consumers. Large‐bodied omnivores can exert particularly strong influences on stream benthic environments through consumption of food resources and physical disturbance of the benthos. Trophic dynamics studies conducted within the context of reach‐scale riparian vegetation manipulations can provide insights into the interactions and relative importance of top‐down and bottom‐up controls that determine ecosystem response to riparian change. Here, we examine how top‐down control by native crayfish omnivores (Cambarus bartonii) interacts with abiotic conditions created by reach‐scale removal of riparian rhododendron (Rhododendron maximum) in the southern Appalachian Mountains of the U.S.A. We conducted 32‐day trophic experiments by nesting five pairs of electrified (crayfish excluded) and non‐electrified (crayfish access) plots within each of two 300‐m stream reaches (one control and one rhododendron‐removed) for 1 year pre‐removal and 2 years post‐removal. Algal growth responded positively to the reduced canopy cover (post‐rhododendron removal) only under low flow conditions combined with the absence of top‐down control by crayfish. Leaf decomposition rates were reduced by c. 40% in the absence of crayfish, but higher inputs of rhododendron leaf litter during the summer following rhododendron removal reduced the effect of crayfish presence on decomposition. Riparian rhododendron removal also significantly increased benthic sediment and fine benthic organic matter, but crayfish exclusion did not affect these stream properties. Potential long‐term reductions in crayfish abundance could reduce the top‐down effects of crayfish and ultimately lead to higher algal growth and reduced leaf decomposition rates in streams where rhododendron is managed through removal.