Impact of Alien Plant Invaders on Pollination Networks in Two Archipelagos

Mutualistic interactions between plants and animals promote integration of invasive species into native communities. In turn, the integrated invaders may alter existing patterns of mutualistic interactions. Here we simultaneously map in detail effects of invaders on parameters describing the topology of both plant-pollinator (bi-modal) and plant-plant (uni-modal) networks. We focus on the invader Opuntia spp., a cosmopolitan alien cactus. We compare two island systems: Tenerife (Canary Islands) and Menorca (Balearic Islands). Opuntia was found to modify the number of links between plants and pollinators, and was integrated into the new communities via the most generalist pollinators, but did not affect the general network pattern. The plant uni-modal networks showed disassortative linkage, i.e. species with many links tended to connect to species with few links. Thus, by linking to generalist natives, Opuntia remained peripheral to network topology, and this is probably why native network properties were not affected at least in one of the islands. We conclude that the network analytical approach is indeed a valuable tool to evaluate the effect of invaders on native communities

Impact of Alien Plant Invaders on Pollination Networks in Two Archipelagos

Mutualistic interactions between plants and animals promote integration of invasive species into native communities. In turn, the integrated invaders may alter existing patterns of mutualistic interactions. Here we simultaneously map in detail effects of invaders on parameters describing the topology of both plant-pollinator (bi-modal) and plant-plant (uni-modal) networks. We focus on the invader Opuntia spp., a cosmopolitan alien cactus. We compare two island systems: Tenerife (Canary Islands) and Menorca (Balearic Islands). Opuntia was found to modify the number of links between plants and pollinators, and was integrated into the new communities via the most generalist pollinators, but did not affect the general network pattern. The plant uni-modal networks showed disassortative linkage, i.e. species with many links tended to connect to species with few links. Thus, by linking to generalist natives, Opuntia remained peripheral to network topology, and this is probably why native network properties were not affected at least in one of the islands. We conclude that the network analytical approach is indeed a valuable tool to evaluate the effect of invaders on native communities

Alternative approaches of transforming bimodal into unimodal mutualistic networks. The usefulness of preserving weighted information

Pollination and seed dispersal networks are by definition bimodal, linking two sets of species, plants and animals. Bimodal networks are often analysed after being transformed into unimodal ones, since most attributes in network theory are defined for the latter. Such unimodal projections (e.g. of plants sharing flower visitors or seed dispersers) map potential inter-specific competition or facilitation, and can thus be useful for instance when identifying native species potentially sensitive to aliens in the communities. In this work, we introduce procedures to project unweighted and weighted bimodal networks into unimodals, for animals or plants, and calculate two centrality measures that inform us about the species' role in the communities. By using 20 empirical weighted networks worldwide, we obtained 160 unimodal networks via four projection methods and evaluated correlations among centrality parameters across the different methodologies to assess how consistent the results are when including different link weights between species. Degree centralities obtained by projecting unweighted and weighted bimodal networks were not significantly correlated, suggesting that the role of the species differs when considering link weights in the original bimodal networks. By contrast, betweenness centralities were highly correlated, indicating the consistent importance of the species as connectors regardless of the projection method used. We conclude that preserving the weighted information when transforming bimodal into unimodal networks may allow us to make more realistic predictions on the potential competitive or facilitative interactions among species of one set (e.g. plants) that share species of the other (e.g. flower visitors or dispersers). © 2011 Gesellschaft für ökologie.The study is framed within projects CGL2004-04884-C02-01BOS, CGL2007-61165BOS and CGL2010-18759BOS.Peer Reviewe

Pollen transfer from invasive Carpobrotus spp. to natives - A study of pollinator behaviour and reproduction success

The influence of invasive plant species on native community composition is well-documented, but less is known about underlying mechanisms. Especially scarce is knowledge about effects on biotic interactions such as relationships between native plants and their pollinators. In this study we investigate if pollen transfer from the invasive and highly pollen productive Carpobrotus spp. affects seed production and/or seed quality in three native species. We monitored pollinator movements and pollen loads on pollinators and native stigmas, and in a field pollination experiment we investigated the effect of invasive pollen on reproduction. Invasive pollen adhered to pollinators, pollinators switched from Carpobrotus spp. to natives, invasive pollen was transferred to native stigmas, and it affected seed production in one species. Although all possible steps for interference with seed production were found to be qualitatively taken, invasive pollen has probably little impact on the native community because the frequency of invasive pollen transfer to natives was low. However, pollination interactions may change with plant abundance and our study provides evidence that pollen transfer from Carpobrotus spp. to natives does occur and have the potential to affect seed production. We found the species identity of shared pollinators to be of importance, higher flower constancy and lower capacity of pollen adherence are likely to result in less invasive pollen transfer. © 2007 Elsevier Ltd. All rights reserved.This research was supported by a post-doc grant to A. Jakobsson from the Swedish Council for Forestry and Agricultural ResearchPeer Reviewe

Competition for pollinators between invasive and native plants: Effects of spatial scale of investigation (note)

In this study we show that spatial scale of investigation affects the estimated strength of competition for pollinators between an invasive and a native plant species. The effect of the invasive herb Oxalis pes-caprae on pollinator visits to the native herb Diplotaxis erucoides was studied when the invader was (1) totally present, (2) present on a large scale (of hectares) but absent on a small scale (of square metres), and (3) totally absent. No difference in number of pollinator visits to D. erucoides was found between treatment 1 and 3, i.e., between total presence and total absence of the invader. However, when the invader was removed in the small scale while still remaining in the large scale, a higher number of visits to the native was recorded. Our study thus shows the importance of incorporating multiple spatial scales to allow for investigation of hierarchical effects on competition for pollinators, and it suggests that small-scale studies of effects of invasive plants on pollinator visitation might risk overstating negative effects of the invader.Peer Reviewe

Consequences of plant invasions on compartmentalization and species' roles in plant-pollinator networks

Compartmentalization-the organization of ecological interaction networks into subsets of species that do not interact with other subsets (true compartments) or interact more frequently among themselves than with other species (modules)-has been identified as a key property for the functioning, stability and evolution of ecological communities. Invasions by entomophilous invasive plants may profoundly alter the way interaction networks are compartmentalized. We analysed a comprehensive dataset of 40 paired plant-pollinator networks (invaded versus uninvaded) to test this hypothesis. We show that invasive plants have higher generalization levels with respect to their pollinators than natives. The consequences for network topology are that-rather than displacing native species fromthe network-plant invaders attracting pollinators into invaded modules tend to play new important topological roles (i.e. network hubs, module hubs and connectors) and cause role shifts in native species, creating larger modules that are more connected among each other.While the number of true compartmentswas lower in invaded compared with uninvaded networks, the effect of invasion on modularitywas contingent on the studysystem. Interestingly, the generalization level of the invasive plants partially explains this pattern, with more generalized invaders contributing to a lower modularity. Our findings indicate that the altered interaction structure of invaded networks makes them more robust against simulated random secondary species extinctions, but more vulnerable when the typically highly connected invasive plants go extinct first. The consequences and pathways by which biological invasions alter the interaction structure of plant-pollinator communities highlighted in this study may have important dynamical and functional implications, for example, by influencing multi-species reciprocal selection regimes and coevolutionary processes. © 2014 The Authors Published by the Royal Society. All rights reserved.Peer Reviewe

Data3 Species Level

Data3 Species Level contains data for the analysis of role shifts and effects of explanatory variables on dependent variables at the node (=species level). Detailed information about the compiled datasets and the statistical analysis is given in the Methods section of the article. For further information please contact: [email protected]

Data from: Consequences of plant invasions on compartmentalization and species’ roles in plant–pollinator networks

Compartmentalization—the organization of ecological interaction networks into subsets of species that do not interact with other subsets (true compartments) or interact more frequently among themselves than with other species (modules)—has been identified as a key property for the functioning, stability and evolution of ecological communities. Invasions by entomophilous invasive plants may profoundly alter the way interaction networks are compartmentalized. We analysed a comprehensive dataset of 40 paired plant–pollinator networks (invaded versus uninvaded) to test this hypothesis. We show that invasive plants have higher generalization levels with respect to their pollinators than natives. The consequences for network topology are that—rather than displacing native species from the network—plant invaders attracting pollinators into invaded modules tend to play new important topological roles (i.e. network hubs, module hubs and connectors) and cause role shifts in native species, creating larger modules that are more connected among each other. While the number of true compartments was lower in invaded compared with uninvaded networks, the effect of invasion on modularity was contingent on the study system. Interestingly, the generalization level of the invasive plants partially explains this pattern, with more generalized invaders contributing to a lower modularity. Our findings indicate that the altered interaction structure of invaded networks makes them more robust against simulated random secondary species extinctions, but more vulnerable when the typically highly connected invasive plants go extinct first. The consequences and pathways by which biological invasions alter the interaction structure of plant–pollinator communities highlighted in this study may have important dynamical and functional implications, for example, by influencing multi-species reciprocal selection regimes and coevolutionary processes

Aliens in the community: Consequences of plant invasions on compartmentalization and species' roles in plant pollinator networks

Comunicación presentada en el: 12th EEF (European Ecological Federation) Congress, celebrado del 25 al 29 de septiembre de 2011 en Ávila (España). Congreso en el que se celebraron conjuntamente: 10th Annual Conference of the Spanish Association for Terrestrial Ecology; 13th Annual Meeting of the Portuguese Ecological Society; 3rd Iberian Congress of EcologyCompartmentalization – the organization of ecological networks into subgroups of species that are not connected by interaction links with other groups (compartments) or have a higher probability of interacting with one another than with other species (modules) – has been identified as a key property for the stability, functioning and evolution of multitrophic communities. Invasions of often highly generalized alien species may lead to the fusion of compartments or modules and alter the functional architecture of networks through shifts in the distribution of distinct topological roles a species can play in the network. We tested these hypotheses for alien plant invasions of plant–pollinator networks using a dataset of 44 paired networks from seven published studies, each pair consisting of an invaded and a control network lacking alien plant invaders. The number of compartments was indeed lower in invaded compared to networks without alien plants, but not the number of modules detected by simulated annealing. The effect of invasion on modularity (estimating between-module differentiation) was contingent on the study system. However, module size, i.e. the mean number of species forming a module, increased following invasion, also after accounting for variation in network size. Moreover, plant invasions altered the composition of species’ topological roles; in particular, the average number of species acting as module hubs, i.e. species highly linked within but not among modules, almost doubled following invasion. We discuss the implications of our findings for the conservation and restoration of plant–pollinator communities in the face of biological invasionsPeer Reviewe

Germination patterns throughout an insular altitudinal gradient: The case of the Macaronesian endemic plant Rubia fruticosa Ait. (Rubiaceae) in El Hierro (Canary Islands)

6 pages, 2 figures, 1 table.-- Article in press.Seed germination percentage and rate of Rubia fruticosa, an endemic Macaronesian shrub was assessed with seed lots collected at three levels of an altitudinal gradient (200–800 m asl) located in El Hierro Island (Canarian Archipelago). Both seeds collected from control plants and those found in lizard droppings were studied. A significant decrease in the germination percentage was observed with control seeds and those seeds which are previously eaten by lizards if collected at 800 m asl. This indicates that R. fruticosa is a plant whose optimal germination environment is located between 200 and 500 m asl, occurring in higher altitudes only under sub-optimal conditions. The effect of lizard gut on R. fruticosa seeds was variable in the two study years: while germination percentage of seeds having passed through the lizards was not significantly different to that of control seeds in the first year, an increased germination was found for the animal-treated seeds in all three altitudinal zones in the second year. In general, germination rate of control seeds was rather variable between years. However, improvement of R. fruticosa seed germination caused by lizards may be important for its fitness; and a co-evolution may have occurred, since lizards and seeds of this endemic plant have been intensively interacting for millions of years in the lower zones of the Canary Islands.Benigno Padrón, David P. Padilla and Patricia Marrero received grants financed by the Ministerio de Educación y Ciencia of Spain, Gobierno de Canarias and Consejo Superior de Investigaciones Científicas (Proyectos Intramurales Especiales, 2004 3 0E 169), respectively. This contribution was financed by the projects CGL2004-04884-C02-01/BOS (Ministerio de Educación y Ciencia) and PI042004/037 (Consejería de Educación, Cultura y Deportes, Gobierno de Canarias). The two projects have been partially supported by Feder funds from the European Union