The importance of crayfish in the breakdown of rhododendron leaf litter.

1. Rhododendron (Rhododendron maximum) is a common evergreen shrub in riparian areas of the southern Appalachians, where its leaves can comprise a large proportion of leaf litter in streams. However, they are relatively refractory 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 southern Appalachian stream in both summer (when leaves other than rhododendron are relatively scarce) and autumn (when other leaves are relatively abundant). We conducted two leaf decay experiments, one in summer and one in autumn, using pre-conditioned leaves. Macroconsumers were excluded from the benthos of a fourth-order stream using electric fences; we predicted that excluding 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. Macroconsumers accounted for approximately 33% and 54% of rhododendron decay in summer and autumn, respectively. We attribute this effect to direct shredding of rhododendron by crayfish. Biomass of insect shredders, insect predators and fungi did not differ between control and exclusion treatments, indicating that insectivorous sculpins (Cottus bairdi) had no effect on rhododendron 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, rhododendron 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 organisms eating leaves in this treatment and, as shredder biomass increased, remaining leaf pack mass decreased. In the control treatment, however, we did not see this relationship; here, the effect of insect shredders was presumably 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 influence an ecosystem process such as leaf decay in both summer and autumn. Given the threatened status of many crayfish species in the United States, this finding is especially relevant. Even small alterations in crayfish assemblages, whether via loss of native species and/or introduction of exotic species, may have significant repercussions for ecosystem function.We conducted two electric exclusion experiments, one from 16/7/99 - 29/8/99 (Summer experiment) and the other from 3/10/99 to 28/11/99 (Autumn experiment). Pre-conditioned rhododendron leaf packs (i.e., incubated in stream for ~ 2 mos. prior to experiment) were placed in 0.25 m^2 PVC frames lined with copper wire (total = 10 frames, arranged in 5 pairs, for each experiment). Electric fence chargers were connected to one frame in each pair; this electric current prevented macroconsumers (i.e., fishes and crayfishes) from entering the area. We then examined how macroconsumer access (i.e., control) and exclusion (i.e., electrified) treatments differed in terms of insects colonizing leaf packs, fungal biomass on leaf packs, and leaf breakdown throughout each experiment. At the beginning of each experiment we also collected leaves from 10 1 m wide transects across the stream, to assess availability of rhododendron versus non-rhododendron leaves