Journal of Tropical Ecology (2010) 26:127–137. Copyright © Cambridge University Press 2010 doi:10.1017/S0266467409990551 Networks of epiphytic orchids and host trees in Brazilian gallery forests Igor A. Silva 1 , Alessandro W. C. Ferreira, Maria I. S. Lima and Jo ˜ ao J. Soares Departamento de Bot ˆ anica, Universidade Federal de S ˜ ao Carlos, PO Box 676, S ˜ ao Carlos, SP, 13565-905, Brazil (Accepted 22 October 2009) Abstract: Species interactions have been recently depicted as networks, in which each species is connected to one or more other species in binary interaction matrices. Forty networks of epiphytic orchid and host tree species were assessed in Brazilian gallery forests. The nestedness of the networks was estimated with the NODF index and the significance was tested with null models. The phylogenetic structure of the network was also assessed, by searching for phylogenetic signals in the number of interactions and in the similarity of interacting species. In total, 105 orchid species and 132 host tree species were sampled. A nested pattern in all orchid–host tree networks was found. However, phylogenetic signals were not observed. The results support that the host specificity of orchids is small and most of the interactions occur among generalist orchids and generalist host trees. While the concept of species-specificity can thus be rejected, the extreme alternative – that interacting orchids and host trees are not a random subset of the regional species pool – can be dismissed as well. However, factors other than phylogenetic history may structure interaction networks of epiphytic orchids and host trees. Key Words: Brazil, commensalism, complex networks, host specificity, Orchidaceae, phylogenetic signal, phylogenetic structure INTRODUCTION Epiphytic vascular plants comprise a major proportion of the tropical flora (Kreft et al. 2004, Kr¨ omer et al. 2005), contributing up to one-third of vascular plant species in wet tropical forests (Gentry & Dodson 1987). Among plant families, Orchidaceae have a high number of epiphytes, with approximately 20 000 species in 800 genera (Dressler 1993). Some hypotheses have been proposed to explain the higher local richness of epiphytic orchids in tropical plant communities, such as recurrent environmental constraints preventing competitive exclusion (e.g. bark defoliation and tree falls, Benzing 1981) and vertical niche diversification (Gentry & Dodson 1987). A hypothesis that has recently received much attention is that of host-tree specificity (Blick & Burns 2009, Callaway et al. 2002, Laube & Zotz 2006). Although the preference of epiphytes for host trees has been broadly reported in the literature (Laube & Zotz 2006), few studies have tried to link the interactions of entire epiphyte assemblages with entire host-tree assemblages in a quantitative manner, searching for a 1 Corresponding author. Email: igor6cordas@yahoo.com.br general pattern at the community level (Blick & Burns 2009, Burns 2007). Species interactions may be depicted as networks (e.g. mutualistic, Bascompte et al. 2003; commensalistic, Burns 2007; parasitic, V ´ azquez et al. 2005). In interaction networks, each species is connected to one or more other species in binary interaction matrices, in which one group of species is listed as rows and the other group is listed as columns (Proulx et al. 2005). Interacting pairs of species receive a ‘1’ in their corresponding cell, whereas non-interacting pairs of species receive a ‘0’ in the matrix (Proulx et al. 2005). Recently, general patterns in networks are emerging from many types of species interaction (Bascompte & Jordano 2007). For example, networks of mutualistic interactions have nested patterns (Bascompte et al. 2003), whereas networks of antagonistic interactions generally have non- nested ones (Guimar˜ aes et al. 2006). Thus, networks of species interactions are a unifying framework to compare patterns of specialization within and among different types of ecological interaction (Bascompte & Jordano 2007, Proulx et al. 2005). Nestedness is a specific type of asymmetric interaction among species. This pattern is characterized by: (1) species with many interactions forming a core of interacting species; (2) species with few interactions interacting