Assessing impacts of a notorious invader (common carp Cyprinus carpio) on Australia's aquatic ecosystems: Coupling abundance-impact relationships with a spatial biomass model

Quantifying the ecological impacts of non-native species is essential for large scale conservation management. Estimating such impacts often require spatially explicit abundance(/biomass) models of the target non-native species and knowledge of abundance-impact relationships, both of which are rarely known. Common carp (Cyprinus carpio) is a major threat to aquatic systems globally and relatively well-studied but yet models quantifying landscape-scale impacts of carp invasions are lacking. Here, we undertake a nonlinear meta-analytic approach to describe biomass-impact relationships for eight impact metrics and then integrate these relationships with a recently published spatial carp biomass model for Australia in order to quantify the ecological impacts of the carp invasion on Australian aquatic ecosystems. For the meta-analysis we collated 286 abundance(biomass)impact estimates from 41 studies. Our meta-analysis identified both linear and nonlinear biomass-impact relationships across the impact metrics. Using a small, out-of-sample validation from carp removal experiments, the model had high prediction accuracy (within <20 % of estimate) for four metrics, but one metric (macrophyte recovery) was consistently underestimated by 10 to 35 %. Finally, we estimated that the carp invasion in Australia has resulted in decreases in macrophytes (median of -36 %) and macroinvertebrates (-31 %) and increases in nitrogen (2 %), plankton biomass (7 %), phosphorus (8 %) and turbidity (63 %). The invasive spread of carp has fundamentally altered Australia's aquatic environments and this study provides the first quantitative assessment of this impact and a tool to guide carp management decisions.