Punching above their weight: low-biomass non-native plant species alter soil properties during primary succession Duane A. Peltzer, Peter J. Bellingham, Hiroko Kurokawa, Lawrence R. Walker, David A. Wardle and Gregor W. Yeates D. A. Peltzer (peltzerd@landcareresearch.co.nz), P. J. Bellingham, H. Kurokawa, D. A. Wardle, Landcare Research, PO Box 40, Lincoln 7640, New Zealand.  HK also at: Graduate School of Environmental and Information Sciences, Yokohoma National Univ., 79-7, Tokiwadai, Hodogaya, JP240-8501 Yokohama, Japan.  L. R. Walker, School of Life Sciences, Univ. of Nevada, 4505 Maryland Parkway, Las Vegas, Nevada, USA.  DAW also at: Dept of Forest Ecology and Management, Swedish Univ. of Agricultural Sciences, SE901 83 Umea˚, Sweden.  G. W. Yeates, Landcare Research, Private Bag 11-052, Palmerston North 4442 New Zealand. Non-native invasive plants can greatly alter community and ecosystem properties, but efforts to predict which invasive species have the greatest impacts on these properties have been generally unsuccessful. An hypothesis that has considerable promise for predicting the effects of invasive non-native plant species is the mass ratio hypothesis (i.e. that dominant species exert the strongest effects). We tested this hypothesis using data from a four year removal experiment in which the presence of two dominant shrub species (one native and the other not), and subordinate plant species, were manipulated in factorial combinations over four years in a primary successional floodplain system. We measured the effects of these manipulations on the plant community, soil nutrient status and soil biota in different trophic levels of the soil food web. Our experiment showed that after four years, low-biomass non-native plant species exerted disproportionate belowground effects relative to their contribution to total biomass in the plant community, most notably by increasing soil C, soil microbial biomass, altering soil microbial community structure and increasing the abundance of microbial-feeding and predatory nematodes. Low-biomass, non-native plant species had distinct life history strategies and foliar traits (higher foliar N concentrations and higher leaf area per unit mass) compared with the two dominant shrub species (97% of total plant mass). Our results have several implications for understanding species’ effects in communities and on soil properties. First, high-biomass species do not necessarily exert the largest impacts on community or soil properties. Second, low-biomass, inconspicuous non-native species can influence community composition and have important trophic consequences belowground through effects on soil nutrient status or resource availability to soil biota. Our finding that low-biomass non-native species influence belowground community structure and soil properties more profoundly than dominant species demonstrates that the mass ratio hypothesis does not accurately predict the relative effects of different coexisting species on community- and ecosystem-level properties. Non-native invasive plants can cause landscape-scale altera- tions by changing disturbance regimes (D’Antonio and Vitousek 1992), and affecting organisms and ecological processes both aboveground (Callaway and Maron 2006, Mitchell et al. 2006) and belowground (Ehrenfeld 2003, van der Putten et al. 2007). Although the impacts of invasive non-native plant species on soil nutrients are well documented (Vitousek and Walker 1989, Ehrenfeld 2003, Levine et al. 2003, Liao et al. 2008), the belowground impacts on soil biota are less well understood (Wolfe and Klironomos 2005, van der Putten et al. 2007). However, impacts of invasive plants on soil microbial diversity, the abundance of invertebrate biota and food web responses may be predictable because of cascading effects through various trophic levels (Mikola and Seta ¨la ¨ 1998, Wardle et al. 2003). Recent studies show that non-native plants can cause shifts in the composition of soil and litter-dwelling biota (Standish 2004, Belnap et al. 2005), which in turn has consequences for plant performance (Batten et al. 2008, Kulmatiski et al. 2008). Most studies investigating this issue have been conducted in experimental grassland plots, and do not consider the effects of both native and non-native plant species, or of multiple invasive species in the same community. Efforts to predict which invasive species have major impacts on ecological properties, either above or below- ground, have been generally unsuccessful (Parker et al. 1999, Sutherland 2004). Grime’s (1998) ‘mass ratio hypothesis’ predicts that a few, dominant species will drive ecosystem processes because they are the major drivers of ecological properties. This theory has been developed to show that the relative abundances of all the plant species in a community, along with values of their key functional traits, can be used to predict both aboveground and belowground community and ecosystem properties (Vile et al. 2006). In this light, dominance has recently been Oikos 118: 10011014, 2009 doi: 10.1111/j.1600-0706.2009.17244.x, # 2009 The Authors. Journal compilation # 2009 Oikos Subject Editor: Wim van der Putten. Accepted 15 January 2009 1001