Community Impacts of Prosopis juliflora Invasion: Biogeographic and Congeneric Comparisons Rajwant Kaur1, Wilfredo L. Gonza´ les2, Luis Daniel Llambi3, Pascual J. Soriano 3 , Ragan M. Callaway4, Marnie E. Rout4, Timothy J. Gallaher5, Inderjit1* 1 Department of Environmental Studies, Centre for Environmental Management of Degraded Ecosystems (CEMDE), University of Delhi, Delhi, India, 2 Laboratorio de Ecolog´ıa Evolutiva, LID, Facultad de Ciencias y Filosof´ıa, Universidad Peruana Cayetano Heredia, Lima, Peru´ , 3 Instituto de Ciencias Ambientales y Ecolo´ gicas, Universidad de Los Andes, La Hechicera, Me´rida, Venezuela, 4 Division of Biological Sciences, University of Montana, Missoula, Montana, United States of America, 5 Botany Department, University of Hawaii at Manoa, Honolulu, Hawaii, United States of America Abstract We coordinated biogeographical comparisons of the impacts of an exotic invasive tree in its native and non-native ranges with a congeneric comparison in the non-native range. Prosopis juliflora is taxonomically complicated and with P. pallida forms the P. juliflora complex. Thus we sampled P. juliflora in its native Venezuela, and also located two field sites in Peru, the native range of Prosopis pallida. Canopies of Prosopis juliflora, a native of the New World but an invader in many other regions, had facilitative effects on the diversity of other species in its native Venezuela, and P. pallida had both negative and positive effects depending on the year, (overall neutral effects) in its native Peru. However, in India and Hawaii, USA, where P. juliflora is an aggressive invader, canopy effects were consistently and strongly negative on species richness. Prosopis cineraria, a native to India, had much weaker effects on species richness in India than P. juliflora. We carried out multiple congeneric comparisons between P. juliflora and P. cineraria, and found that soil from the rhizosphere of P. juliflora had higher extractable phosphorus, soluble salts and total phenolics than P. cineraria rhizosphere soils. Experimentally applied P. juliflora litter caused far greater mortality of native Indian species than litter from P. cineraria. Prosopis juliflora leaf leachate had neutral to negative effects on root growth of three common crop species of north-west India whereas P. cineraria leaf leachate had positive effects. Prosopis juliflora leaf leachate also had higher concentrations of total phenolics and L- tryptophan than P. cineraria, suggesting a potential allelopathic mechanism for the congeneric differences. Our results also suggest the possibility of regional evolutionary trajectories among competitors and that recent mixing of species from different trajectories has the potential to disrupt evolved interactions among native species. Citation: Kaur R, Gonza´les WL, Llambi LD, Soriano PJ, Callaway RM, et al. (2012) Community Impacts of Prosopis juliflora Invasion: Biogeographic and Congeneric Comparisons. PLoS ONE 7(9): e44966. doi:10.1371/journal.pone.0044966 Editor: Francesco de Bello, Institute of Botany, Czech Academy of Sciences, Czech Republic Received March 1, 2012; Accepted August 16, 2012; Published September 12, 2012 Copyright: 2012 Kaur et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The research was funded by the University of Delhi. RMC gratefully acknowledges funding from the National Science Foundation, DEB 0614406, and the Rocky Mountain Research Station, United States Department of Agriculture Forest Service. WGL gratefully acknowledges funding from the Programa de Ciencia y Tecnolog´ıa (FINCyT), Contrato Nu011-FINCyT-PIBAP-2007. The funders had no role in study, design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: inderjitdu@gmail.com Introduction Why some exotic plants, when introduced to a new part of the world, become far more abundant and have greater impact than in their native range is one of the most puzzling questions in ecology [1,2]. Prosopis juliflora (Swartz) DC appears to be one of these species. Several species of Prosopis have been introduced to different parts of the world and four – P. glandulosa, P. velutina, P. juliflora and P. pallida – have become invasive [3]. Prosopis juliflora and P. pallida are tropical species and have become serious invasives in many parts of Africa, Middle East and Indian subcontinent. Prosopis juliflora is native to Central America, northern South America and the Caribbean islands [4] (Figure 1A), and P. pallida is native to northern South America. In their native ranges both species appear to coexist with large numbers of other native species. A significant amount of confusion exists in the identification of P. juliflora and P. pallida [3]. For example, a species previously identified as P. juliflora in Peruvian- Ecuadorian coast (one of the native ranges) is now identified as P. pallida or P. limensis [5]. We used morphological characters to confirm identity of P. juliflora [3] and treated both P. juliflora and P. pallida as a complex, ‘‘P. juliflora s. lat. (incl. P. pallida)’’ and recognize this as the P. juliflora complex. P. juliflora is a major invasive species in India, and has also invaded other regions throughout the world including Saharan and southern Africa, the Middle East, Pakistan, India, and Hawaii (USA) [3] where it appears to strongly suppress species native to those regions. Prosopis juliflora forms pure stands in its invaded range in India, and occurs in forests, wastelands and at the boundaries of crop fields (Figure 1B, C, D). Prosopis juliflora also occurs in saline habitats in Hawaii USA (Figure 1E). In its native range, densities of P. juliflora can be high relative to other leguminous shrubs and trees, but its canopies can have much stronger facilitative effects on neighbors than other leguminous tree species [6]. Many other Prosopis species, in their native ranges, create ‘‘resource islands’’ with higher concentrations of organic matter, nitrogen, phosphorus and potassium beneath their canopies and behave as strong facilitators of other species [7– PLOS ONE | www.plosone.org 1 September 2012 | Volume 7 | Issue 9 | e44966