Biodiversity and trophic structure of soil nematode communities are altered following woody plant invasion of grassland Lori A. Biederman * , Thomas W. Boutton Department of Ecosystem Science and Management, Texas A&M University, MS-2138, College Station, TX 77845-2138, United States article info Article history: Received 31 March 2009 Received in revised form 17 June 2009 Accepted 28 June 2009 Available online 10 July 2009 Keywords: Faunal analysis Decomposition pathways Chronosequence Prosopis glandulosa Texas USA abstract Woody plant encroachment is an important land cover change in dryland ecosystems throughout the world, and frequently alters above and belowground primary productivity, hydrology, and soil microbial biomass and activity. However, there is little known regarding the impact of this geographically wide- spread vegetation change on the biodiversity and trophic structure of soil fauna. Nematodes represent a major component of the soil microfauna whose community composition and trophic structure could be strongly influenced by the changes in ecosystem structure and function that accompany woody encroachment. Our purpose was to characterize nematode community composition and trophic structure along a grassland to woodland chronosequence in the Rio Grande Plains of southern Texas. Research was conducted at the La Copita Research Area where woody encroachment has been documented previously. Soil cores (0–10 cm) were collected in fall 2006 and spring 2007 from remnant grasslands and woody plant stands ranging in age from 15 to 86 years, and nematodes were extracted by sugar centrifugation. Neither nematode densities (3200–13,800 individuals kg 1 soil) nor family richness (15–19 families 100 g 1 soil) were altered by woody encroachment. However, family evenness decreased dramatically in woody stands >30 years old. This change in evenness corresponded to modifications in the trophic structure of nema- tode communities following grassland to woodland conversion. Although root biomass was 2–5 greater in wooded areas, root-parasitic nematodes decreased from 40% of all nematodes in grasslands to <10% in the older wooded areas, suggesting the quality (C:N or biochemical defenses) of woody plant root tissue could be limiting root-parasites. In contrast, bacterivores increased from 30% of nematodes in grasslands to 70–80% in older woody patches. This large increase in bacterivores may be a response to the 1.5–2.5 increase in soil microbial biomass (bacteria þ fungi) following woody encroachment. Therefore, while energy flow through grassland nematode communities appears to be distributed nearly equally among herbivory, fungivory and bacterivory, the energy flow through nematode communities in wooded areas appears to be based primarily on bacterivory. We speculate that these shifts in nematode community composition and trophic structure could have important implications for ecosystem patterns and processes. First, the low abundance of root-parasitic nematodes (and presumably root herbivory) under woody plants may be one mechanism by which woody plants are able to establish and compete effectively with grasses during succession from grassland to woodland. Second, the large increase in bacterivores following woody encroachment likely accelerates microbial turnover and the mineralization of N, thereby providing a feedback that enables the persistence of N-rich woody plant communities. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Woody plant encroachment into grass-dominated ecosystems is a globally extensive shift in plant community structure that has strong potential to alter key processes (such as primary productivity, allocation patterns, biogeochemical cycles, and hydrology) at ecosystem, regional, and global scales (Hibbard et al., 2001; Pacala et al., 2001; Jackson et al., 2002; Zavaleta and Kettley, 2006). Although many studies have focused on the impacts of woody plant encroachment on ecosystem processes, little is known regarding the potential for this vegetation change to influence the biodiversity and trophic structure of belowground biological communities. Soil microfauna, such as protozoa and nematodes, are important constit- uents of soil food webs (Bonkowski, 2004). Their activities regulate the size and function of fungal and bacterial populations in the soil * Corresponding author. Present address: Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011, United States. Tel.: þ1 979 845 0283. E-mail address: lbied@lycos.com (L.A. Biederman). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2009.06.019 Soil Biology & Biochemistry 41 (2009) 1943–1950