Contrasting seasonal patterns in the acceptability of a naturalised and an introduced macrophyte for a generalist shredder

Abstract

Given the increasing problems with invasive aquatic plants in many freshwaters, more research on macrophyte acceptability by herbivores is warranted. Our study deals with the comparison of the acceptability in different seasons of a naturalised (Elodea canadensis) and a recently introduced species (Elodea nuttallii) originating from North America, for an omnivorous shredder, the amphipod Gammarus roeseli. It also compares acceptability trends of various plant parts (apex, defoliated stem and foliated stem) in the case of E. nuttallii. Two determinants of plant palatability (the plant dry matter content and the carbon/nitrogen ratio) were measured at each season for the two macrophytes. Our results show contrasting seasonal patterns for the two Elodea species. The main result is the highly contrasting acceptability of the two Elodea species in summer. Moreover, the consumption of the macrophytes tends to increase from spring to autumn. The lower acceptability of E. canadensis could be explained by a higher dry matter content, reflecting higher physical and chemical defences than those of E. nuttallii. The contrasting consumption patterns between the two macrophytes observed in summer could indicate different strategies against the herbivores that should be related to their introduction history. The lower carbon/nitrogen ratio in autumn could explain the increase of acceptability of both Elodea species in this season. The transition towards dormancy of both macrophytes and the subsequent decrease in defence mechanisms may induce colonisation by epiphytes and microbial decomposers and therefore the increase of nitrogen content.