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 non-invaded 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 ¹³C compared with the noninvaded site. In contrast, all samples from the invaded site were enriched in ¹⁵N 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, ¹⁵N 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.