Human activities can decrease the abundances of vulnerable native species, yet increase abundances of other species (i.e., invasive species). Numerous previous studies have documented how changes in population abundances of one species can have cascading consequences for the other members of the ecosystem. However, our understanding of the concordant consequences of population invasions and population depletions is relatively limited. For example, in many impacted California streams, there has been an increase in nonnative species such as signal crayfish (Pacifastacus leniusculus) while decreasing abundances of native salmonids such as steelhead trout (Oncorhynchus mykiss) with unknown community and ecosystem-level consequences. Here we focused on understanding the functional roles of these two dominant species in a small coastal Californian stream. We performed a large-scale experimental manipulation of densities of these two dominant species, using stream pools that were isolated by low summer flows as large-scale and biologically-meaningful units of replication (average 70 m²). This experiment consisted of high and low density treatments of juvenile steelhead and crayfish, with four replicates of each treatment, for a total of 16 pools. The study was fully factorial, with pools being randomly assigned to a treatment--high and low steelhead and crayfish densities. Response variables that we quantified over the experiment included leaf litter breakdown, primary production, benthic invertebrate abundance, fish growth, diets, and abundance, as well as background information on light, temperature, and nutrients

Results/Conclusions

This experiment revealed that these two species, through both trophic and non-trophic pathways, differently impacted stream communities and ecosystem processes. For example, densities of crayfish strongly controlled rates of leaf-litter breakdown, with biomass of crayfish explaining 78% of the observed variation in leaf-litter breakdown rate. Leaf-litter breakdown was 4X faster with high densities of crayfish vs. low densities of crayfish. However, juvenile steelhead density did not significantly alter leaf-litter breakdown rates. Neither crayfish nor steelhead density was significantly related to growth rates of native sculpins. Not surprisingly, these two species have drastically different functional roles, but these results highlight the complexities involved with understanding the how communities and ecosystems respond to simultaneous processes of invasion and extinction.