Invasive carp are widely reported to harm ecosystems. In Australia, carp are a serious pest, and consequently investigations of biocontrol options are underway.
Best practice biocontrol requires cost/benefit evaluation. To assist this generic carp impacts on aquatic ecosystems have been summarised and general predictions were tested by comparing both dryland river ecosystems with and without carp, and ecosystem responses to a gradient in local carp density.
Expectations were that in the presence of carp, and with increasing carp density, there would be increasing turbidity, decreasing densities of macrophytes and macroinvertebrates and associated changes in assemblage composition, resulting in decreasing native fish density.
Not all expected responses were found, indicating that general carp impact understanding requires modification for dryland rivers. Notably, carp did not increase turbidity or impact macroinvertebrate assemblage density or composition, likely due to key attributes of these dryland rivers. In contrast, there were large impacts to native fish biomass, not from the mechanisms expected, but likely from food resource monopolisation by carp. Macrophyte occurrence was reduced, but macrophytes are naturally rare in these rivers. There was extirpation of an endangered river snail Notopala sublineata likely due to carp predation.
Impacts to native fish may be reversible by carp control but reversal of impacts to the snail may require carp elimination and snail reintroduction. Modelling is necessary to predict the probability of beneficial versus undesirable outcomes from carp control, and complementary measures to control other stressors may be needed.
Benefits of carp control to dryland river ecosystems are fewer than generally predicted. This reinforces that ecological understanding cannot always be transferred between diverse settings. It highlights the need to understand system characteristics relevant to causal impact pathways when applying generic carp impact models to specific settings. This has global relevance to future carp control efforts.