Structure and dynamics of pollination networks: the role of alien plants Fernanda S. Valdovinos, Rodrigo Ramos-Jiliberto, Jose ´ D. Flores, Claudia Espinoza and Gioconda Lo ´pez F.S. Valdovinos (fevaldovinos@gmail.com), R. Ramos-Jiliberto, C.Espinoza and G.Lo´pez, Depto de Ciencias Ecolo´gicas, Facultad de Ciencias, Univ. de Chile, Las Palmeras 3425 N ˜ un ˜ oa, Santiago, Chile.  J. D. Flores, Dept of Mathematics, The Univ. of South Dakota, 414 E. Clark Street, Vermillion, SD 57069, USA. Research on ecological communities, and plantpollinator mutualistic networks in particular, has increasingly benefited from the theory and tools of complexity science. Nevertheless, up to now there have been few attempts to investigate the interplay between the structure of real pollination networks and their dynamics. This study is one of the first contributions to explore this issue. Biological invasions, of major concern for conservation, are also poorly understood from the perspective of complex ecological networks. In this paper we assess the role that established alien species play within a host community by analyzing the temporal changes in structural network properties driven by the removal of non-native plants. Three topological measures have been used to represent the most relevant structural properties for the stability of ecological networks: degree distribution, nestedness, and modularity. Therefore, we investigate for a detailed pollination network, 1) how its dynamics, represented as changes in species abundances, affect the evolution of its structure, 2) how topology relates to dynamics focusing on long-term species persistence; and 3) how both structure and dynamics are affected by the removal of alien plant species. Network dynamics were simulated by means of a stochastic metacommunity model. Our results showed that established alien plants are important for the persistence of the pollination network and for the maintenance of its structure. Removal of alien plants decreased the likelihood of species persistence. On the other hand, both the full network and the subset native network tended to lose their structure through time. Nevertheless, the structure of the full network was better preserved than the structure of the network without alien plants. Temporal topological shifts were evident in terms of degree distribution, nestedness, and modularity. However the effects of removing alien plants were more pronounced for degree distribution and modularity of the network. Therefore, elimination of alien plants affected the evolution of the architecture of the interaction web, which was closely related to the higher species loss found in the network where alien plants were removed. The introduction of alien species into native ecosystems, together with habitat deterioration and climatic change, constitute the major anthropogenic threats for the persis- tence and integrity of pollination systems (Memmott and Waser 2002). The effects of alien plant invasions on native pollination systems may be competitive, neutral, or facil- itative (Bjerknes et al. 2007, Sargent and Ackerly 2008). Alien plants may compete with native ones either through reducing visitation rates of pollinators or through increasing heterospecific pollen transfer on native flowers (Chittka and Schu ¨rkens 2001). However, an alien’s effects may be neutral if native plants are not pollen limited, or if they compensate for the loss of pollinators (Totland et al. 2006). At the landscape level, whenever alien plants represent valuable food resources for many pollinators, their effect on native biota could be more facilitative than competitive by means of increasing pollinator densities (Bjerknes et al. 2007). Recently, the study of anthropogenic impacts on pollination systems has switched from a ‘single species’ approach, which focuses on one or a few plantanimal species interactions, to the ‘pollination web’ approach, which focuses on entire communities (Memmott and Waser 2002, Olesen et al. 2002, Morales and Aizen 2006, Memmot et al. 2007, Aizen et al. 2008). Here we use the pollination web approach to evaluate the roles played by established alien plants in the preservation of the structure and the persistence of the member species in a real plant pollinator network. The dynamics of pollination networks, here represented as changes in species abundances, may be affected by the availability of mutualistic partners and by the pattern of interactions between plants and animals. That is, the dynamics within the network depends on its topology. Conversely, the evolution of the topology of a network is influenced by the dynamics of its member species, given that species and interactions are removed when populations go extinct following severe decreases in their abundances. Such systems of interconnected parts in which there is a Oikos 118: 11901200, 2009 doi: 10.1111/j.1600-0706.2009.17364.x, # 2009 The Authors. Journal compilation # 2009 Oikos Subject Editor: Jordi Bascompte. Accepted 6 February 2009 1190