Understanding differences in behavioral characteristics between invasive non-native species and native species is an important step in preventing, managing, and mitigating environmental impacts. This study examined the differences between adult life-stage native Pomacea paludosa (Florida Applesnail) and adult non-native Pomacea maculata (Giant Applesnail) grazing behavior and rates on Vallisneria americana (Tapegrass), a plant of restoration importance, to assess the potential ecological impact. We used an experimental design with entire intact specimens of Tapegrass placed in 8 tanks of each of 3 treatment groups: (1) grazed by Giant Applesnail, (2) grazed by Florida Applesnail, and (3) control with no snails. Rates of herbivory on, and physical and total biomass damage to, Tapegrass by Giant Applesnail were 1.8 cm/hr, 2.5cm/hr, and 4.2 cm/hr, respectively. Rates of herbivory on, and physical and total biomass damage to, Tapegrass by Florida Applesnail were 0.2 cm/hr, 1.2 cm/hr, and 1.4 cm/hr, respectively. The mean growth rate of Tapegrass in tanks containing no snails was 0.13 cm/hr. We used one-way ANOVAs and Tukey posthoc tests to examine statistically significant differences between the 2 gastropod species for rates of herbivory on (P = 0.006) and total biomass damage to (P = 0.024) Tapegrass. No statistical difference between the 2 species was found for the physical damage rate (P = 0.18). Statistical differences were found between controls without snails and Giant Applesnail and Florida Applesnail, respectively, for herbivory (P = 0.001, P = 0.05), physical damage (P = 0.007, P = 0.001) and total damage rates (P = 0.001, P = 0.002). The observed grazing behavior of adult life-stage specimens of the 2 species differed substantially, with Florida Applesnail grazing along blade edges and Giant Applesnail completely cutting off blades from their bases. These results also show that Giant Applesnail consumed and removed more Tapegrass biomass at a faster rate than the native Florida Applesnail. The introduction of Giant Applesnail, with its greater herbivory and total biomass damage rates over the native apple snail and behavior that removes leaf blades, may shift competitive interactions in Tapegrass communities under pressure from non-native plant invaders such as Hydrilla verticillata (Waterthyme).
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Vol. 15 • No. 4