The importance of crayfish in the breakdown of rhododendron leaf litter

1. Rhododendron (Rhododendron maximum) is a common evergreen shrub in ripar ian areas of the southern Appalachians, where its leaves can comprise a lar ge proportion of leaf litter in streams. However, they are relatively refra ctory and generally considered a low quality food resource for detritivores . 2. Our objective was to assess whether macroconsumers [primarily crayfish ( Cambarus bartonii)] influence rhododendron leaf breakdown in a forested sou thern Appalachian stream in both summer (when leaves other than rhododendro n are relatively scarce) and autumn (when other leaves are relatively abund ant). We conducted two leaf decay experiments, one in summer and one in aut umn, using pre-conditioned leaves. Macroconsumers were excluded from the be nthos of a fourth-order stream using electric 'fences'; we predicted that e xcluding macroconsumers would reduce the decay rate of rhododendron leaves in both summer and autumn. 3. In both experiments, breakdown rate was lower in exclusion treatments. M acroconsumers accounted for approximately 33 and 54% of rhododendron decay in summer and autumn, respectively. We attribute this effect to direct shre dding of rhododendron by crayfish. Biomass of insect shredders, insect pred ators and fungi did not differ between control and exclusion treatments, in dicating that insectivorous sculpins (Cottus bairdi) had no effect on rhodo dendron decay and that omnivorous crayfish did not exert an indirect effect via alteration of insect or fungal biomass. 4. The influence of shredding insects varied between summer and autumn. In summer, when other, more palatable leaf types were not available, rhododend ron leaf packs appeared to provide 'resource islands' for insect shredders. There was a significant inverse relationship between insect shredders and leaf pack mass in the summer exclusion treatment: insects were the only org anisms eating leaves in this treatment and, as shredder biomass increased, remaining leaf pack mass decreased. In the control treatment, however, we d id not see this relationship; here, the effect of insect shredders was pres umably swamped by the impact of crayfish. In autumn, when other leaves were abundant, insect shredder biomass in rhododendron leaf packs was less than one-third of summer values. 5. Even at low density (approximately 2 m(-2)) crayfish were able to influe nce an ecosystem process such as leaf decay in both summer and autumn. Give n the threatened status of many crayfish species in the United States, this finding is especially relevant. Even small alterations in crayfish assembl ages, whether via loss of native species and/or introduction of exotic spec ies, may have significant repercussions for ecosystem function.