Functional group identity does not predict invader impacts: differential effects of nitrogen-fixing exotic plants on ecosystem function Stephanie G. Yelenik 1,2, *, William D. Stock 3,4 & David M. Richardson 1,5 1 Institute for Plant Conservation, University of Cape Town, Rondebosch, 7701, South Africa; 2 Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA, 93106, USA; 3 Botany Department, University of Cape Town, Rondebosch, 7701, South Africa; 4 Centre for Ecosystem Management, Edith Cowen University, Joondalup, 6027, WA, Australia; 5 Centre for Invasion Biology, University of Stel- lenbosch, Private Bag X1, Matieland, 7602, South Africa; *Author for correspondence (e-mail: yelenik@ lifesci.ucsb.edu) Received 18 October 2005; accepted in revised form 20 January 2006 Key words: Acacia saligna, available soil nitrogen, exotic plants, fynbos, litter decomposition, Lupinus luteus, organic soil nitrogen, South Africa Abstract The introduction of exotic plants can have large impacts on ecosystem functions such as soil nutrient cycling. Since these impacts result from differences in traits between the exotic and resident species, novel physiological traits such as N cycling may cause large alterations in ecosystem function. It is unclear, however, whether all members of a given functional group will have the same ecosystem effects. Here we look at a within functional group comparison to test whether an annual (Lupinus luteus) and a perennial (Acacia saligna) N-fixing exotic species cause the same effects on soil N cycling in the fynbos vegetation of South Africa. We measured litterfall quantity and quality, and soil total nitrogen and organic matter for each vegetation type as well. Available nitrogen was quantified using ion exchange resin bags monthly for 1 year. We used microcosms to evaluate litter decomposition. Although both exotic species increased the available nitrogen in the soil, only Acacia increased the total soil N and organic matter. This could be explained by the slow decomposition of Acacia litter in the microcosm study, despite the fact that Acacia and Lupinus litter contained equivalent N concentrations. Presumably, low carbon quality of Acacia litter slows its decomposition in soil, resulting in retention of organic nitrogen in Acacia stands after clearing for restoration purposes. The differences in long term impacts of these annual and perennial species highlight the fact that not all N-fixing exotic species exert equivalent impacts. Ecologists should consider multiple traits rather than broadly defined functional groups alone when predicting invader impacts. Introduction Given that exotic species can have severe impacts on the ecosystems they invade (Vitousek et al. 1997), but that only a small fraction actually exert these large impacts (Williamson and Fitter 1996), ecologists would benefit from a better understand- ing of which invaders exert significant effects and of the mechanisms by which they do so. A popu- lar approach to understanding plant impacts on ecosystems is to group species into discrete func- tional groups (Chapin 1993). Yet this approach is not without criticism (Eviner 2004). Our goal here is to see if the ecosystem impacts of plant species from within the same functional group are better understood from a multiple trait standpoint. Biological Invasions (2007) 9:117–125 Ó Springer 2006 DOI 10.1007/s10530-006-0008-3