Jonathan M. Levine Æ Sally D. Hacker Christopher D.G. Harley Æ Mark D. Bertness Nitrogen effects on an interaction chain in a salt marsh community Received: 14 November 1997 / Accepted: 11 May 1998 Abstract Nutrients can structure communities by in- fluencing both plant interactions and plant herbivore interactions, though rarely do studies integrate these processes. In this study we examined how nitrogen fer- tilization influenced (1) the positive interaction between the marsh elder, Iva frutescens, and the black rush, Juncus gerardi, and (2) the quality of Iva as a host plant for the aphid, Uroleucon ambrosiae. Previous studies have shown that by mitigating soil salt accumulation and hypoxia, Juncus is essential to the survival of Iva and its aphid herbivore at mid-marsh elevations. To address the eects of nitrogen on this interaction, we compared fertilized and unfertilized Iva plants subject to Juncus removal and control treatments in the field. Additionally, we measured the monthly population growth rates of aphids transplanted onto these Iva plants. Iva leaf biomass and flower number results in- dicated that fertilizing Iva eliminated its dependence upon Juncus, such that fertilized plants grown without Juncus were not dierent from unmanipulated plants. Aphid monthly population growth rates through mid- summer revealed that fertilization also eliminated the indirect dependency of aphids on Juncus, so that aphid growth rates on fertilized Iva without Juncus neighbors were similar to rates on unmanipulated Iva. Results also indicated that fertilizing Iva grown with Juncus increased Iva size, potentially enabling these plants to support larger aphid populations. Our results suggest that only under conditions of nitrogen limitation are the positive eects of Juncus essential to the mid-marsh persistence of Iva and its aphid herbivore. Furthermore, we found that nitrogen eects on aphid populations may arise not only from a direct eect of nutrients on Iva size but also through the indirect eects of nitrogen on the interaction between Juncus and Iva. We argue that studies inte- grating processes occurring both within and between trophic levels, are important to fully understanding the community-wide eects of nutrients. Key words Nitrogen Æ Salt marsh Æ Positive interaction Æ Insect herbivory Æ Trophic interaction Introduction Nutrient availability has gained the attention of terres- trial community ecologists because of its tremendous potential to influence plants and higher trophic levels. Nutrients can aect the patterns of plant community diversity (Huston 1979, 1994; Goldberg and Miller 1990) and succession (Tilman 1988; Berendse and Elberse 1990) through the mediation of both competitive (Wil- son and Keddy 1986; Tilman 1988; McGraw and Cha- pin 1989; Levine et al. 1998) and facilitative (Chapin et al. 1994; Callaway 1995) interactions. In addition, studies have shown that by increasing the nutrient con- tent of host plants, nutrients can have important positive eects on the abundance and distribution of insect her- bivores (Southwood 1973; Onuf et al. 1977; Slansky and Feeny 1977; Dixon 1985; Bryant et al. 1987; Bowdish and Stiling 1998). Though ecologists agree that nutrients can dictate both plant interactions and plant-herbivore interactions, few experimental studies have made a link between nu- trients and these intra-trophic and inter-trophic level processes. Specifically, when nutrients mediate plant competitive and facilitative interactions, they influence the physiology, abundance, and diversity of plants (Tilman 1988), all of which are likely to have important Oecologia (1998) 117:266–272 Ó Springer-Verlag 1998 J.M. Levine 1 (&) Æ S.D. Hacker 2 Æ C.D.G. Harley 3 Æ M.D. Bertness Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, USA Present addresses: 1 Department of Integrative Biology, University of California, Berkeley, CA 94720, USA e-mail: levinejm@socrates.berkeley.edu 2 Department of Botany, Washington State University, 14204 NE Salmon Creek Avenue, Vancouver, WA 98686, USA 3 Department of Zoology, University of Washington, Seattle, WA 98195-1800, USA