Quantifying invader impact: applying functional response metrics to a rapidly spreading non-native species.
Risk assessment currently lacks pliable tools that can be reliably applied across a range of environmental contexts to predict the success and impact of non-native species (NNS). An approach adopting functional responses (FR) and the Relative Impact Potential (RIP) metric may offer considerable insight and predictive power regarding invader impacts, which may include losses in ecosystem function and biodiversity. We aim to evaluate the use of FRs and the RIP metric in predicting impact in non-native, data-deficient bivalves. Very little is known about the potential impacts of the Gulf wedge clam, Rangia cuneata outside of its native range, and as a rapidly spreading non-native species, it is highly suitable for this study. Comparisons of feeding activity and abundance between non-native species and functionally analogous native species may produce reliable impact predictions where significant disparity in these factors exists. We conducted field abundance surveys and laboratory feeding studies on R. cuneata, in order to quantify FRs and the RIP metric. Clearance rates and FRs were calculated for R. cuneata and the sympatric, native unionid duck mussel, Anodonta anatina, across a range of algal concentrations at 1.0 ppt salinity. Mean±SE clearance rates (ml g DW-1 hr-1) were found to be significantly greater for R. cuneata (294.31±65.17) than for A. anatina (125.99±29.59) with both species exhibiting a type I functional response. As a result of far greater field abundance of R. cuneata compared with A. anatina (37.7 m-2±6.1 versus 0.7 m-2±0.4 at sympatric sites) and a significantly higher feeding rate, the RIP score for R. cuneata was high (11.61) signifying likely ecological impact. Calculating the probability density function for the RIP includes uncertainty in the metric and also yielded a very high value (38.14), and these results provide the first evidence that R. cuneata should be considered a high-risk invasive species. The results of our analyses firmly support the assertion that R. cuneata will likely continue to have significant impact in its introduced range if management action is not taken. The rate of spread of R. cuneata, its established densities and RIP score are all comparable to severely invasive bivalves; therefore, R. cuneata may pose a significant threat to both freshwater and brackish water biodiversity and function. We conclude that FRs and the RIP metric may be extremely beneficial to making reliable impact predictions for non-native species and are particularly well suited for estimating the potential impacts of data-deficient novel and emerging non-natives.