ranges has (i) greater negative impacts on species richness compared to its native range and compared to a congeneric species in its invaded range, (ii) more pronounced effects on soil chemical characteristics and allelochemical pools than a congener, and (iii) greater inhibition of species native to the invaded ranges in soil and leaf leachate experiments than a congener. We tackled this general issue by comparing (1) subcanopy species richness under P. pallida canopies with species richness in open areas, in its native range Peru and P. juliflora in its native range Venezuela, and two parts of its non-native range, India and Hawaii, (2) subcanopy species richness under P. juliflora and the Indian native P. cineraria in India, (3) the chemical characteristics (inorganic ions and total phenolics) of the soil from the rhizosphere of the two congeners, (4) the effects of litter from the two congeners on a suite of native Indian species, and (5) the effect of soil amended with leaf leachate of the two congeners on local crop species in India. Materials and Methods Prosopis juliflora and Species Richness in Native and Non- native Ranges Prosopis juliflora is part of the Prosopis juliflora-pallida complex, a taxonomically complicated, interrelated, and controversial group both in the native and non-native ranges of species in this complex [5]. Because of this pervasive taxonomic confusion, we took a conservative approach to sampling in Venezuela, native range of P. juliflora, and in Peru, the native range of what is most likely to be Prosopis pallida [5]. Our intent was to make a broader measurement of the impacts of the Prosopis juliflora complex in the native ranges by using both ends of the species that make up the P. juliflora-pallida complex. We located two field sites in Peru, (i) Chulucanas (S 05u119 49.40; W 080u169 53.30; 174 m) and (ii) Catacaos (S 05u279 06.50; W 80u149 37.20; 253 m). We located two field sites in Venezuela, in the native range of Prosopis juliflora [5], (i) Mucumi (N 08u309 10.00; W 71u219 53.00; 1000 m) and (ii) Puente Real (N 08u289 41.00; W 71u249 38.00; 650 m). Both sites are within the Lagunillas Semi-arid Enclave, which is an inter-Andean enclave, the largest in Venezuela with an area of 262 km2. The vegetation at both study sites is classified as thorn scrub. In these sites the dominant elements in the upper stratum were the thorny shrubs Prosopis juliflora and Acacia macracantha, which form an open and discontinuous canopy up to 3 m in height. The columnar cacti Stenocereus griseus, Pilosocereus tillianus and Cereus repandus are emergent elements (up to 5–7 m in height), which tend to be spatially associated with these leguminous shrubs. The intermedi- ate stratum (up to 50 cm) is formed by shrubs of Croton spp., Capparis odoratissima, herb Hibiscus phoeniceus, cacti Opuntia caribea, Opuntia depauperata, Opuntia aff. elatior and Solanum spp. A bromeliacea of the Pitcairnia genus, the herbs Jatropha gossypifolia, Cnidosculus urens and Evolvulus sp., and annual plant species were also present. Three field sites were located in India and two sites were located in Honolulu, Hawaii, non-native ranges of P. juliflora. The sites located in India were: (i) Delhi (N 28u359 58.60; E 077u109 15.60; 233 m), (ii) Kutch, Gujarat (N 23u559 58.20; E 069u489 50.40; 407 m), and (iii) Jaipur, Rajasthan (N 27u019 00.30; E 075u469 12.90; 484 m). The three Indian sites were chosen to represent a range of climates, with Delhi, Gujarat and Rajasthan having humid subtropical, tropical monsoon and tropical arid climates, respectively. The two sites in Hawaii were: (i) Sand Island (N 21u 189 12.50 W 157u 529 51.70 and (ii) Iroquois Point-Hau Bush (N 21u 199 24.90; W 157u 589 16.70 and N 21u 189 20.90; W 157u 19 52.50), both at sea level. At each site in Peru, we randomly placed 1 m2 quadrats in areas of large patches of P. pallida and where P. pallida was not present, Invasion of Prosopis juliflora what we refer to as ‘open areas’. We recorded the number of species in each plot. In 2008, we sampled 96 pairs of plots at Chulucanas and 92 pairs at Catacaos. In 2009, we sampled 72 pairs in Chulucanas and 120 pairs at Catacaos. At each site in Venezuela in July 2012, we randomly placed one 1 m2 quadrat under each of 15 different P. juliflora trees and in open grassland adjacent to each tree where P. juliflora was not present and recorded the number of species in each plot. At the Delhi site P. 2 juliflora primarily occurs in dense stands (3.6 trees per 25 m ). Here in May 2010 we established 10 randomly placed 565 m2 quadrats under P. juliflora, with each plot representing the understory of a different set of several trees, and 10 565 m2 quadrats in open area outside of the P. juliflora stands. At Gujarat, the site was a scrub forest of P. juliflora (1.4 trees per 25 m2) in which 7 randomly placed 565 m2 plots were located under P. juliflora and 7 in open areas away from P. juliflora canopy, during June 2008. At Rajasthan, P. juliflora tended to be smaller and occur in very large closed canopy patches precluding pairing local sub canopies and 2 open patches (4.4 trees per 25 m ). Thus 10 randomly placed 565 m2 quadrats were located under P. juliflora roughly in the centre of a typical stand that was several kilometres in diameter, with control quadrats (no P. juliflora) approximately 2 km away, from the quadrats with P. juliflora but in what appeared to be very similar environmental conditions. Sampling was in June 2010. At Delhi and Rajasthan, open areas were selected outside P. juliflora stands because these stands were closed-canopy woodlands rather than savanna-like vegetation. At Delhi, the open area was characterized by few far-separated trees of Albizzia lebbeck, Balanites roxburghii with interspersed patches of shrub species including Grewia sp., Capparis sepiaria, Capparis decidua and Zizyphus nummularia, and grasses including Cenchrus ciliaris, Chrysopogon fulvus and Heteropogon sp. At Rajasthan also, open area consisted of far- separated tree species (Dalbergia sissoo and Azadirachta indica ) with interspersed patches of shrubs (Zizyphus sp., Lantana camara, Morus sp.) and grasses (Saccharum sp. and Cenchrus ciliaris). However at Gujarat, open areas and P. juliflora subcanopy quadrats were interspersed within the P. juliflora stand due to non-overlapping canopies and open areas included tree species (Acacia senegal and Wrightia tinctoria), shrub species (Euphorbia caducifolia, Commiphora wightii and Grewia sp.) and herb species (Tephrosia sp., Rhynchosia sp. and Cenchrus sp.). Prosopis juliflora was first recorded at the Sand Island site in Hawaii in the early 1970’s and today this area holds the largest population of Prosopis juliflora and P. juliflora x P. pallida hybrids in Hawaii. The second site, Iroquois Point-Hau Bush, is within a small stretch of undeveloped coastal area. Its location is approximately 10 km to the west of the Sand Island site with a very similar climate to that of Sand Island. Within this area, there is a large established population of P. pallida with P. juliflora and hybrids found near the coast. In October-November 2011, 565 m2 quadrats were placed at each site in areas of dense infestation of P. juliflora and areas where it was not present. The number of plant species present in each quadrat was counted. Our initial plan was to use 1 m2 plots in the two non-native ranges, India and Hawaii, as we did in the native range, of P. juliflora, Venezuela and of P. pallida, Peru. However, the number of species present was much lower under canopies of P. juliflora in the non-native ranges, India and Hawaii. We therefore used larger plots to avoid large numbers of zeros in these plots. We did intra- site species richness comparisons under P. juliflora canopies and nearby open areas where we used identical sampling methods, at each site in native and invaded range and then compared the effect of P. juliflora on species richness observed in its invaded ranges to those observed in its native range. PLOS ONE | www.plosone.org 3 September 2012 | Volume 7 | Issue 9 | e44966