Summary 1.Alien invasive plants possessing attractive flowers can affect the interactions between native plants and their pollinators. The few studies conducted so far have reported positive, negative and neutral effects of the presence of an invasive species on the pollinator visitation rates and seed output of native species. However, the role played by the density of the alien species has been seldom explored. 2. While high densities of the invasive species can negatively effect the pollinator visitation rates and seed output of the native species, due to sequestration of pollinators by the invasive species, at lower densities the invasive can attract pollinators that otherwise would not visit patches of native species, positively affecting their reproduction. 3. Using observations and pollinator exclusions at a site at 2800 m altitude in the central Chilean Andes, we show that the alien herb Taraxacum officinale (Asteraceae), possessing showy floral capitula, shares pollinators to a high degree (> 90%) with two co-occurring native Asteraceae: Hypochaeris thrincioides and Perezia carthamoides . Pollinator exclusion indicated that both natives are highly dependent on insect visitation to produce seeds. 4. Using manipulative supplemental hand-pollination and experiments, where we varied the density of T. officinale , we determined that seed output in H. thrincioides and P. carthamoides is pollen-limited and that its magnitude is not affected by the presence of the exotic. Further, while the presence of one individual of T. officinale around focal native individuals showed neutral or facilitative effects on pollinator service or seed output in the two native species, the presence of five individuals of T. officinale negatively affected these reproductive variables. 5. Synthesis . Our results suggest that the effects of the presence of alien invasive plant species with attractive flowers vary with their density. This demonstrates the need to experimentally test for the potential impacts of introduced species at different densities before prematurely coming to conclusions regarding their assumed negative or positive effects in native ecosystems.

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Although ontogenetic changes in resource use within species are common in animals, these changes have not been widely considered in studies of guild structure within communities. The occurrence of one or more shifts in resource use in an individual of a given species during its life should mean that it would also belong to different guilds at different life stages. We specifically addressed this issue by describing the feeding habits of ten species of carnivorous fishes occurring in tidepools in rocky intertidal areas along the coast of central Chile. Most of these species undergo clear ontogenetic dietary shifts and a feeding guild structure of this group of fishes was established that takes these dietary shifts into account. Each species was divided into a number of size classes. Dietary overlap values between both intraspecific and interspecific size-class pairs in the entire group of ten species were used to construct a phenogram of dietary similarity through an UPGMA cluster analysis. Numbers of guilds and their memberships were established objectively by applying a bootstrapping procedure. Four "ontogenetic" feeding guilds (OFGs), each consisting of size-classes of species, were recognized. The majority of species belonged to more that one guild. Interestingly, when the bootstrapping procedure was applied to a phenogram based on the diets of "taxonomic" or complete species, only one significant guild was found. The implications of these ontogenetic dietary shifts for interspecific interactions are substantial because the identity of the species with which each fish species shares resources change through their lives. The usefulness of taxonomic species for investigating potential competitive interactions in this assemblage is greatly undermined.

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Soil nutrient availability directly enhances vegetative growth, flowering, and fruiting in alpine ecosystems. However, the impacts of nutrient addition on pollinator visitation, which could affect seed output indirectly, are unknown. In a nutrient addition experiment, we tested the hypothesis that seed output in the insect-pollinated, self-incompatible shrub, Chuquiraga oppositifolia (Asteraceae) of the Andes of central Chile, is enhanced by soil nitrogen (N) availability. We aimed to monitor total shrub floral display, size of flower heads (capitula), pollinator visitation patterns, and seed output during three growing seasons on control and N addition shrubs. N addition did not augment floral display, size of capitula, pollinator visitation, or seed output during the first growing season. Seed mass and viability were 25-40% lower in fertilised shrubs. During the second growing season only 33% of the N addition shrubs flowered compared to 71% of controls, and a significant (50%) enhancement in vegetative growth occurred in fertilised shrubs. During the third growing season, floral display in N addition shrubs was more than double that of controls, received more than twice the number of insect pollinator visits, and seed output was three- to four-fold higher compared to controls. A significant (50%) enhancement in vegetative growth again occurred in N addition shrubs. Results of this study strongly suggest that soil N availability produces strong positive bottom-up effects on the reproductive output of the alpine shrub C. oppositifolia. Despite taking considerably longer to be manifest in comparison to the previously reported top-down indirect negative effects of lizard predators in the same study system, our results suggest that both bottom-up and top-down forces are important in controlling the reproductive output of an alpine shrub.