Indeed, C. Furthermore, C. Finally, co-flowering other plant species surrounding the single plants of C. For less attractive plant species, effective pollen dispersal may be more spatially restricted. An open question of this and most other experimental studies on effective pollen dispersal is how exactly the observed increase in seed set affects the long-term persistence of isolated populations.
The role of ecological factors affecting long-distance effective pollen dispersal is still poorly understood [10] , [12] , [21] , [47]. The results of this experiment indicated that both flower visitor and habitat variables significantly affected long-distance effective pollen dispersal.
Bees and flies were the most abundant flower visitors of potted C. Our analysis suggested that bees, although less numerous visitors than flies, may have been relatively more important in effective pollen transfer from source to isolated populations of C.
Bees, particularly bumblebees and honeybees, are known to forage at spatial scales of several hundreds of meters and generally show a high flower fidelity [e. Large flower visitors usually forage over larger distances than small ones [49] and are therefore expected to be relatively more important for pollen dispersal over large distances.
Flower visitor body size did not appear to greatly affect the overall level of pollen dispersal to C. However, we found some evidence that the slope in the decline of effectively transferred pollen with distance from source populations was less steep if plants were visited by higher numbers of individuals and species of large flower visitors. Such an effect was not found for small flower visitors. This finding is an indication that large flower visitors were probably relatively more involved in pollen dispersal over larger distances than small ones.
These conclusions, however, are correlative and we did not quantify the amount of pollen insects carried on their bodies when arriving at the single-plant isolates. Moreover, our findings suggested that pollinators were more likely to visit single-plant isolates of C. It is conceivable that larger or more diverse collective floral displays of diverse and flower-rich intensively managed meadows attracted more foraging pollinators from restored meadows, thereby facilitating seed set of isolated C.
However, such potentially facilitative effects of the floral neighborhood might be limited to a constrained set of circumstances, e. However, Kwak et al. The question under what circumstances a diverse, flower-rich environment can promote or reduce long-distance pollen dispersal requires further investigation. Finally, we found that larger source populations resulted in a less steep slope of the pollen dispersal function and accordingly to a more effective pollen dispersal to remote single plant isolates of C.
This is in accordance with the findings of Richards and co-workers [10] studying effective pollen dispersal in the dioecious Silene alba. Hence, levels of gene flow by pollen to remote isolated plants should be higher when source populations of insect-pollinated plant species restricted to restored habitats are large but more restricted when source populations are relatively small.
This implies genetic connectedness of most populations of C. Using this species as a model plant, we suggest that other species restricted to restored areas and similar with respect to attractiveness to insect pollinators and pollinator community composition may show similar patterns of effective pollen dispersal. Thus, in the agricultural landscape of the Swiss lowlands, consisting of a very small-scaled mosaic of restored and surrounding intensively managed areas separated on average by less than m M.
Albrecht, unpublished data , most plant populations of neighboring restored areas may be connected by gene flow by pollen. However, our results also indicate a relatively high spatio-temporal variability in effective pollen dispersal, which should also be considered by conservation management.
Our findings indicate that a high diversity and abundance of co-flowering plants in the vicinity can increase the chances of isolated plants or small populations to receive pollinators and pollen from source populations, at least for our model species, and probably for similar plant species, indicating that plant diversity can have positive consequences for ecosystem functioning in intensively managed agricultural landscapes with an embedded network of restored meadows.
Moreover, we found evidence that effective pollen dispersal occurred over larger distances if source populations in these restored meadows were relatively large, emphasizing the importance of restoring areas sufficiently large to bear large plant populations.
Thus, this study emphasizes that incentives aimed at enhancing the connectivity of insect-pollinated plant populations restricted to restored or protected habitats should consider not only distance but also other factors such as population sizes and the diversity and abundance of co-flowering species.
The study greatly benefited from helpful comments by Jane Stout, Nick Waser and an anonymous reviewer. Conceived and designed the experiments: MA.
Performed the experiments: MA. Analyzed the data: MA. Wrote the paper: MA. Substantial contribution to conception and design of the study, critical revision of the manuscript for important intellectual content, final approval of the version to be published: PD MKO DK BS.
Browse Subject Areas? Click through the PLOS taxonomy to find articles in your field. Abstract Background Agri-environment schemes play an increasingly important role for the conservation of rare plants in intensively managed agricultural landscapes. Conclusions We conclude that insect pollinators can effectively transfer pollen from source populations of C.
Introduction Habitat fragmentation has been recognized as a major threat to the survival of local populations of plants and animals [1] , [2]. Materials and Methods Study species Centaurea jacea L. Study sites and experimental design The experiment was conducted from June to September in and in the south-eastern part of canton Aargau near Zurich. Seed set Single-plant isolates of C. Flower visiting insects On each potted plant of C. Vegetation survey In , species richness and flower abundance of the flowering plants surrounding the potted C.
Statistical analysis To test for a significant effect of distance on the seed set per capitulum, a generalized linear model with quasi-Poisson errors was used. Results Seed set as a function of distance from pollen donors The mean number of seeds per capitulum seed set of C. Download: PPT. Figure 1. Seed set of Centaurea jacea in source populations and single-plant isolates. Ecological factors affecting effective pollen dispersal to isolates In 48 hours we collected flower-visiting insects on flower heads of C.
Table 1. Insects visiting potted plants of Centaurea jacea. Figure 2. Relationship between covariables and seed set of Centaurea jacea isolates. Table 2.
Covariables influencing effective pollen dispersal from source populations to single-plant isolates of Centaurea jacea. Discussion Effective pollen dispersal as a function of distance from the source population The proportion of capitula of Centaurea jacea plants with access to pollinators that set seeds at a distance of m was significantly higher than that produced only by selfing.
Ecological factors affecting effective pollen dispersal The role of ecological factors affecting long-distance effective pollen dispersal is still poorly understood [10] , [12] , [21] , [47]. Author Contributions Conceived and designed the experiments: MA. References 1. Conserv Biol 5: 18— View Article Google Scholar 2. Harrison S, Bruna E Habitat fragmentation and large-scale conservation: what do we know for sure?
Ecography — View Article Google Scholar 3. Lienert J, Diemer M, Schmid B Effects of habitat fragmentation on population structure and fitness components of the wetland specialist Swertia perennis L. Basic Appl Ecol 3: — View Article Google Scholar 4. Paschke M, Abs C, Schmid B Effects of population size and pollen diversity on reproductive success and offspring size in the narrow endemic Cochlearia bavarica Brassicaceae.
Am J Bot — View Article Google Scholar 5. Conserv Biol — View Article Google Scholar 6. Kolb A Reduced reproductive success and offspring survival in fragmented populations of the forest herb Phyteuma spicatum. J Ecol — View Article Google Scholar 7. Wagenius S Scale dependence of reproductive failure in fragmented Echinacea populations.
Ecology — View Article Google Scholar 8. Gene dispersal. Evolution — View Article Google Scholar 9. Ennos RA Estimating the relative rates of pollen and seed migration among plant populations. Heredity — The pollen of animal-pollinated plants has a rough surface to help it stick to a pollinator.
Many flowers use colours to attract insects, sometimes helped by coloured guiding marks. Some have ultraviolet marks that can be seen by insects but are invisible to human eyes. Flowers are often shaped to provide a landing platform for visiting insects or to force them to brush against anthers and stigmas. It only has very small petals but big bright red clusters of stamens. Some flowers have scent to attract insects. Many of these scents are pleasing to humans too, but not all — some flowers attract flies with a smell of rotting meat.
Bird-pollinated flowers tend to be large and colourful so birds can see them easily against a background of leaves. Some flowers even change colour to tell birds when to visit. Most bird-pollinated flowers have lots of nectar, often at the bottom of a tube of petals. Birds need to brush against anthers and stigmas when reaching for the sugary reward with their long beaks. In any case, specialization by the animal is important in pollination, because interspecies transfers of pollen are disadvantageous, whereas with fruit and seed it is necessary only that they are dispersed away from the parent plant.
Most animal-pollinated flowers offer nectar, pollen or both as a reward to their visitors. Floral nectar seems to have no value to the plant other than as an attractant to animals and it has a cost to the plant, because the nectar carbohydrates might have been used in growth or some other activity. Presumably, the evolution of specialized flowers and the involvement of animal pollinators have been favored because an animal may be able to recognize and discriminate between different flowers and so move pollen between different flowers of the same species but not to flowers of other species.
Passive transfer of pollen, for example by wind or water, does not discriminate in this way and is therefore much more wasteful. Indeed, where the vectors and flowers are highly specialized, as is the case in many orchids , virtually no pollen is wasted even on the flowers of other species.
There are, though, costs that arise from adopting animals as mutualists in flower pollination. The fungal pathogen Microbotryum violaceum, for example, is transmitted by pollinating visitors to the. Figure Courtesy of Heather Angel. Many different kinds of animals have entered into pollination liaisons with flowering plants, including hummingbirds, bats and even small rodents and marsupials Figure However, the pollinators par excellence are, without doubt, the insects.
Pollen is a nutritionally rich food resource, and in the simplest insect-pollinated flowers, pollen is offered in abundance and freely exposed to all and sundry. The plants rely for pollination on the insects being less than wholly efficient in their pollen consumption, carrying their spilt food with them from plant to plant.
In more complex flowers, nectar a solution of sugars is produced as an additional or alternative reward. In the simplest of these, the nectaries are unprotected, but with increasing specialization the nectaries are enclosed in structures that restrict access to the nectar to just a few species of visitor.
This range can be seen within the family Ranunculaceae. In the simple flower of Ranunculus ficaria the nectaries are exposed to all visitors, but in the more specialized flower of R.
In the related Aconitum the whole flower is structured so that the nectaries are accessible only to insects of the right shape and size that are forced to brush against the anthers and pick up pollen. Unprotected nectaries have the advantage of a ready supply of pollinators, but because these pollinators are unspecialized they transfer much of the pollen to the flowers of other species though in practice, many general-ists are actually 'sequential specialists', foraging preferentially on one plant species for hours or days.
Protected nectaries have the advantage of efficient transfer of pollen by specialists to other flowers of the same species, but are reliant on there being sufficient numbers of these specialists. Charles Darwin recognized that a long nectary, as in Aquilegia, forced a pollinating insect into close contact with the pollen at the nectary's mouth. Natural selection may then favor even longer nectaries, and as an evolutionary reaction, the tongues of the pollinator would be selected for increasing length - a reciprocal and escalating process of specialization.
Nilsson deliberately shortened the nectary tubes of the long-tubed orchid Platanthera and showed that the flowers then produced many fewer seeds - presumably because the pollinator was not forced into a position that maximized the efficiency of pollination.
Flowering is a seasonal event in most plants, and this places strict seasonality limits on the degree to which a pollinator can become an obligate specialist.
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