Invasive riparian plant litter alters aquatic insect growth

Abstract

In a laboratory study, we examined growth and survival of the caddisfly, Lepidostoma unicolor, feeding on two types of native leaf litter (Alnus rhombifolia [Alder] and a Salicaceae mix [Salix spp. and Populus fremontii]) and two invasive, non-native species (Tamarix ramosissima [saltcedar] and Arundo donax [giant reed]). Larval survival was high (>85%), and did not differ, among those fed Alnus, Salicaceae, or Tamarix litter, but was much lower (20%) for larvae fed Arundo litter. Mean dry biomass of larvae fed Tamarix was 45% greater than that of larvae fed Alnus, and both were significantly greater than the biomasses of insects fed Salicaceae or Arundo. Although both Alnus and Tamarix increased in percent nitrogen with conditioning, Alnus had a significantly higher nitrogen content (4.9 and 3.6%, respectively). Final C : N-values for Alnus and Tamarix were below 18, while C : N for Arundo and Salicaceae were 56 and 44, respectively. Greater growth of larvae fed Tamarix is likely due to the high nutritive value of the conditioned litter, whereas conditioning of Arundo litter did not result in improved nutritive values. Larvae in the Alnus and Salicaceae treatments fed on the entire surface of the leaves until only the skeletons remained. In contrast, larvae in the Arundo treatments focused feeding activity along the margins and the torn portions of the blades. The low nutritional quality of Arundo and the high quality, but ephemeral nature of Tamarix litter potentially have negative effects on stream invertebrate production owing to the quality and duration of availability of leaf litter, as compared with native riparian vegetation.