Stage-specific interactions can have large effects on food webs and ecosystem processes. We investigated stage-specific interactions between signal crayfish (Pacifastacus leniusculus) and cutthroat trout (Oncorhynchus clarkii), the 2 dominant consumers coexisting in a small stream ecosystem. We used enclosure experiments to assess effects of young-of-the-year (YOY) or adult crayfish on individual growth of YOY or adult fish and vice versa. Adult crayfish had adverse effects on the individual growth of YOY fish. Adult fish had no significant effect on YOY crayfish. There were reciprocal negative effects of YOY crayfish and YOY trout on each other's growth rates. Adult crayfish had the largest effect on leaf-litter loss rate, whereas fish and YOY crayfish had no significant effect. However, both adult and YOY crayfish reduced benthic invertebrate abundances in leaf packs to <⅓ those of the controls, whereas fish had no significant effects. Neither consumer species had significant effects on the invertebrates in gravel baskets in the central portion of the enclosures. These results show that ontogenetic-stage-specific effects complicate the characterization of foodweb interactions between species. Ontogenetic stage is important in determining magnitude of the interactions between these 2 consumers.
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