Growth of an invertebrate shredder on native (Populus) and non-native (Tamarix, Elaeagnus) leaf litter

1. Large-scale invasions of riparian trees can alter the quantity and quality of allochthonous inputs of leaf litter to streams and thus have the potential to alter stream organic matter dynamics. Non-native saltcedar ( Tamarix sp.) and Russian olive ( Elaeagnus angustifolia ) are now among the most common trees in riparian zones in western North America, yet their impacts on energy flow in streams are virtually unknown.
2. We conducted a laboratory feeding experiment to compare the growth of the aquatic crane fly Tipula (Diptera: Tipulidae) on leaf litter from native cottonwood ( Populus ) and non-native Tamarix and Elaeagnus . Tipula showed positive growth on leaf litter of all three species; however, after 7 weeks, larvae fed Tamarix leaves averaged 1.7 and 2.5 times the mass of those fed Elaeagnus and Populus , respectively. Tipula survival was highest on Populus , intermediate on Tamarix and lowest on Elaeagnus .
3. High Tipula growth on Tamarix probably reflects a combination of leaf chemistry and morphology. Conditioned Tamarix leaf litter had intermediate carbon : nitrogen values (33 : 1) compared to Populus (40 : 1) and Elaeagnus (26 : 1), and it had intermediate proportions of structural carbon (42%) compared to Elaeagnus (57%) and Populus (35%). Tamarix leaves are also relatively small and possibly more easily ingested by Tipula than either Elaeagnus or Populus .
4. Field surveys of streams in the western U.S.A. revealed that Tamarix and Elaeagnus leaf packs were rare compared to native Populus , probably due to the elongate shape and small size of the non-native leaves. Thus we conclude that, in general, the impact of non-native riparian invasion on aquatic shredders will depend not only on leaf decomposition rate and palatability but also on rates of leaf litter input to the stream coupled with streambed retention and subsequent availability to consumers.