Potential Effects of an Invasive Nitrogen-Fixing Tree on a Hawaiian Stream Food Web

Falcataria moluccana (albizia) is an exotic nitrogen (N)-fixing tree currently invading riparian forests in Hawai‘i, U.S.A. This study examined how this invasion is impacting stream ecosystems by using naturally occurring stable isotopes of carbon (C) and N to compare food web structure between a noninvaded and an albizia-invaded stream reach on the island of Hawai‘i. Isotopic signatures of particulate organic matter (POM), macroalgae, invertebrates, and fishes were collected and compared between the two stream reaches. Stable C isotopic signatures of organic matter sources (POM and macroalgae) and consumers (amphipods, caddisflies, crayfish, and fishes) from the invaded site were depleted in C compared with the noninvaded site. In contrast, all samples from the invaded site were enriched in N compared with the noninvaded site. Results from IsoSource and two-source mixing models suggested that albizia was a major contributor to diets of lower-level consumers within the invaded site, displacing POM and macroalgae as their major food sources. Albizia was also an indirect C and N source for higher-level consumers within the invaded site because albizia was the major dietary constituent of their prey. In addition, N enrichment of the macroalgae at the invaded site suggests that albizia may be an important N source to benthic primary producers and could be further altering the food web from bottom up. Our study provides some of the first evidence that invasive riparian N-fixing trees can potentially alter the structure of stream food webs. The presence of nitrogen (N)-fixing plants in forests can greatly impact nutrient cycling and affect the export of N from watersheds through shedding and decomposition of their N-rich leaves on the forest floor (Binkley et al. 1992, Compton et al. 2003, Scott et al. 2008). In some instances, N inputs from Nfixing plants can be substantial and can exceed industrial N inputs (Wright and Rasmussen 1998). Excess N inputs to soils can result in N-saturation, a condition where excess N in soils can leach into nearby streams, lakes, and groundwater (Aber et al. 1989). Thus, the invasion of N-fixing trees in watersheds could substantially alter N dynamics and ecosystem functions of both the forest and stream. For example, colonization of logged regions by the native N-fixing Alnus rubra (red alder), an early successional species, in the Pacific Northwest of the United States resulted in increased concentrations of nitrate (NO 3 ) and dissolved organic N (DON) in streams from leaching of N from soils (Compton et al. 2003). N from alder has also been shown to stimulate primary production, fungal activity, and rates of leaf litter breakdown in streams (Goldman 1961, Volk et al. 2003, Ferreira et al. 2006, Gulis et al. 2006), Pacific Science (2010), vol. 64, no. 3:367–379 doi: 10.2984/64.3.367 : 2010 by University of Hawai‘i Press All rights reserved 1 Funding was provided by a Seed Grant awarded to J.P.T. by the University of Hawai‘i at Hilo Research Council. Manuscript accepted 18 August 2009. 2 Department of Forest Sciences, University of British Columbia, 3041-2424 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada. 3 Marine Science Department, University of Hawai‘i at Hilo, 200 West Kāwili Street, Hilo, Hawai‘i 96720. 4 U.S. Department of Agriculture Forest Service, Institute of Pacific Islands Forestry, 60 Nowelo Street, Hilo, Hawai‘i 96720. 5 Corresponding author (e-mail: trishaa@interchange