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Summary Mountain ecosystems can exert different selection pressures on plant populations over small scales due to steep environmental gradients. Gene flow between lower- and higher-elevation plant populations could lead to the increased integration of potentially adaptive genes and thereby to outbreeding vigour under current rapid climatic changes. Alternatively, gene flow across elevation could disrupt gene complexes leading to outbreeding depression. We explored the responses of higher (1800 m a.s.l.) and lower (1200 m a.s.l.) Ranunculus bulbosusL. populations to intra- and interelevational and short- as well as long-distance gene flow by means of controlled crosses and assessment of the offspring's germination, growth and fitness. Outbreeding differed between elevations of origin: higher populations were negatively affected by pollen flow from lower populations at the earliest life stages, but tended to be positively affected in terms of growth later on while gene flow across small vs. large distances had no impact on growth traits. Lower populations were not significantly affected by the elevational origin of pollen per se. However, fitness of offspring of populations from both higher and lower elevations was jointly affected by elevational and regional origins of pollen, in which case the gene flow distance had a comparatively stronger positive impact on fitness (outbreeding vigour) than elevational origin. Synthesis: Our results indicate that compared to gene flow at regional scales, gene flow across elevational gradients has a minor effect on R. bulbosus. The lack of outbreeding depression suggests that potentially adaptive genes might well be integrated across populations resulting in an increased resilience of R. bulbosus and potentially similar montane plant species in changing climatic conditions.

Summary1. Invasive alien plant species pose a severe threat to native plant communities world-wide,especially on islands. While many studies focus on the direct impact of alien plants on nativesystems,indirecteffectsofplantinvadersonco-floweringnatives,forexamplethroughcompetitionforpollinationservices,arelesswellstudiedandtheresultsarevariable.2. We used six temporally and taxonomically highly resolved plant–pollinator networks from theisland of Mahe´, Seychelles, to investigate the indirect impact of invasive alien plant species onremnantnativeplantcommunitiesmediated by sharedpollinators. Weemployedfullyquantitativenetwork parameters and information on plant reproductive success, and pollinator diversity andbehaviour,todetectchangesinplant–pollinatornetworksalonganinvasiongradient.3. The number of visits to and fruit set of native plants did not change with invasion intensity.Weighted plant linkage and interaction evenness, however, was lower at invaded sites than at lessinvaded sites. These patterns were primarily driven by shifts in interactions of the most commonpollinator, the introduced honey bee Apis mellifera, while weak interactions and strong nativeinteractionsremainedunchanged.4. Synthesis. The implications of these findings are twofold: first, quantitative network parametersare important tools for detecting underlying biological patterns. Secondly, alien plants andpollinatorsmayplayagreaterroleinshapingnetworkstructureathighthanlowlevelsofinvasion.We could not show, however, whether the presence of invasive plants result in a simplification ofplant–pollinatornetworksthatisdetrimentaltonativeplantsandpollinatorsalike.Key-words: Indian Ocean, indirect interactions, inselbergs, interaction connectance, invasionecology, invasive alien species, plant communities, pollination webs, Seychelles IslandsIntroduction

Summary 1. Many plant species are currently experiencing negative consequences of habitat fragmentation as a result of reductions in population size and disruptions in pollination services. Plants in specialized pollination systems might be especially vulnerable to changes in plant population size and density resulting from land-use changes. 2. Representing such a system, we chose the globeflower Trollius europaeus L. with its pollinating fly Chiastocheta, which also acts as a seed predator at its larval stage, to investigate the effects of small plant population size on reproductive success and pollinator abundance. 3. Reproductive output of T. europaeus declined with increasing plant population size, while Chiastocheta abundance within T. europaeus flowers was independent of plant population size. However, at the local level, Chiastocheta numbers within flowers were inversely correlated to local T. europaeus flower density. We further found that increasing floral densities increased plant reproductive success at the population level. 4. Chiastocheta abundance was the main driver of reproductive output of T. europaeus through its dual role as an obligate pollinator and seed predator: at least some Chiastocheta flies were needed to secure pollination, but a continued increase in Chiastocheta flies within a flower incurred seed predation costs that greatly reduced reproductive success. Thus, high local flower density contributed positively to per capita reproductive output by diluting Chiastocheta abundance within flowers independently of overall population size. 5. Synthesis. Our findings highlight that plant population size is not always the main determinant of reproductive success for populations, but that other factors such as plant density and the specific ecology of a pollinator and its interplay with other population parameters can be more important in determining the fate of a population. Furthermore, the effects of plant population size and floral density on pollinator visitation in T. europaeus vary across scales, with implications for plant fitness. It is therefore important not to focus solely on pure plant population size in determining population viability. Thus, from a conservation perspective, even small and isolated T. europaeus populations may be viable and resistant to pollination-associated vulnerabilities depending on plant density at local (subpopulation) scales.

Summary 1 Multiple-species floral displays have been hypothesized to facilitate pollination by attracting a greater number and/or diversity of pollinators. Here I present experimental confirmation of pollination facilitation among coflowering plants that have morphologically distinct flowers. 2 Pollinator visits to Raphanus raphanistrum, a self-incompatible herbaceous plant, increased when it occurred with one or a combination of Cirsium arvense, Hypericum perforatum and Solidago canadensis than when it occurred alone. 3 Enhanced visitation to R. raphanistrum in mixed species plots was reflected by increased seed production. 4 Facilitative effects in pollination were conditional on the density and evenness of the floral mixture and graded into competition as the relative abundance of R. raphanistrum declined in a two-species mixture. 5 Previously proposed mechanisms for facilitative interactions cannot explain facilitation among florally distinct plant displays. An alternative mechanism of differential but complementary floral rewards is proposed to explain facilitative attraction of pollinators. 6 Facilitation of, and competition for, pollination has implications for regeneration by seed of rare or isolated plants, and of mitigating Allee effects that afflict such populations.

1 The reproductive output of Shorea siamensis, a widespread dipterocarp tree, was assessed in relation to disturbance and tree density at three sites in western Thailand during the 1996 and 1997 flowering seasons. The locations were similar except in disturbance history, which was reflected in decreasing tree density from undisturbed via moderately disturbed to disturbed sites. 2 Hand pollination experiments showed S. siamensis to be partially self-incompatible due to differential pollen tube growth and abortion of inbred fruit. Although more than 90% of flowers from trees at all sites were pollinated, pollen tubes developed in only a small proportion of these flowers. Both pollen tube development and initial fruit production were highest at the undisturbed site. Many fruit, presumably selfed, were aborted during development at all sites, but significantly more fruit were aborted at the disturbed site, resulting in lowered production of mature fruit. 3Shorea siamensis was pollinated by small Trigona bees, which exhibited significant declines in intertree movements with increasing distance between flowering trees. As resource availability did not differ between sites, differences in mature fruit set were considered to be mediated by changes in pollinator foraging behaviour at differing tree densities. Variation in seed set was found to be negatively correlated with distance to nearest conspecific both within and between sites. 4 At the two least disturbed sites observed seed set values corresponded with those expected by calculation. However, seed set at the disturbed site was significantly lower than expected. 5 Our results suggest that high reproductive success of S. siamensis is dependent upon cross-pollination which, through pollinator behaviour, is a function of tree isolation. This species may thus be subject to the Allee effect, where population viability is reduced disproportionately with a decline in population size or, as in this study, population density. The implications of these results for population recovery and genetic structure following disturbance are discussed.