Selective herbivory by an invasive cyprinid, the rudd Scardinius erythrophthalmus

Herbivory by non-native animals is a problem of growing concern globally, especially for ecosystems where there were few native herbivores or where they have been replaced by non-natives. The rudd (Scardinius erythrophthalmus) is an omnivorous cyprinid (native to Europe) now very widespread due to human introductions, yet it is unknown whether the invasive rudd feeds selectively among aquatic macrophyte species common in North America. We tested feeding selectivity by rudd using five species of aquatic macrophytes: Ceratophyllum demersum, Elodea canadensis, Najas flexilis, Stuckenia pectinata and Vallisneria americana. Fish were presented with known quantities of each macrophyte species in a randomized complete block design, and we measured the mean per cent mass remaining in each case. We also quantified differences in the chemical attributes of the five macrophyte species and determined whether feeding by rudd was related to dry matter content (DMC), per cent C by dry mass (%C), per cent N by dry mass (%N), and the concentrations of total soluble proteins, two organic acids (aconitic and oxalic acid), total soluble phenolic compounds (<1000 Da), nine soluble phenolic metabolites and total phenolic compounds. Rudd fed selectively, with consumption declining in the order N. flexilis > E. canadensis > S. pectinata > V. americana > C. demersum. Selection was positively related to %C and atomic C : N, but not DMC, %N or concentration of total soluble proteins, contrary to the expectation that rudd would select the most nutritious plants available. The concentration of aconitic acid was greatest in N. flexilis, a preferred macrophyte, contrary to the expectation that this compound provides resistance to herbivory. The concentration of oxalic acid, which negatively affects palatability, was highest in C. demersum, the least preferred macrophyte. Selection was also positively related to the concentration of total (and some specific) soluble phenolic compounds. The concentrations of caffeic acid, trans-caftaric acid and quercetin were positively related to macrophyte preference by rudd, whereas concentrations of cis-4-O- and trans-4-O-ferulic acid glucoside were negatively related. Selective feeding by rudd (which can be very numerous in North American fresh waters) could evidently alter macrophyte assemblages and hinder attempts to restore plant communities.