Displacement of riparian vegetation by exotic species is a global phenomenon with the potential to affect leaf breakdown rates by shredders. We predicted that exotic riparian vegetation would have a greater effect on leaf breakdown by temperate than by tropical shredders because temperate shredders usually feed on a limited range of generally palatable leaves, whereas tropical shredders are naturally exposed to a higher variety of leaves, many of them unpalatable. We tested 3 hypotheses with common shredder assemblages from tropical Queensland and temperate Tasmania (Australia): 1) tropical shredders are equally efficient at breaking down native and exotic vegetation, whereas temperate shredders are less efficient at breaking down exotic vegetation; 2) tropical shredders are more generalist in their leaf choices than temperate shredders; and 3) shredders are more generalist in their leaf choices among exotic than among native vegetation. Hypothesis 1 was not supported. Caddisflies (tropical or temperate) were able to consume both native and exotic leaves, whereas non-caddisfly shredders fed only on native leaves, a result suggesting that shredding capacity depends on the identity of the shredder species or their phylogenetic position rather than on their origin. Hypothesis 2 was supported. Tropical shredders fed on various leaf types, whereas most temperate shredders chose one leaf type and fed on it for the duration of the experiment. Hypothesis 3 was not supported. Specificity of shredder choices did not differ between native and exotic leaves, a result suggesting that shredding behavior is not flexible, regardless of the leaf litter available. Thus, invasive riparian plants may affect leaf breakdown by shredders, particularly in temperate streams, but effects may vary depending on assemblage composition, the nature and timing of litterfall, and interactions with climate.
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