VISITATION, FLIGHT DISTANCES AND SEED SET IN POPULATIONS OF THE RARE SPECIES PHYTEUMA NIGRUM (CAMPANULACEAE)

Effective long-distance pollen dispersal in Centaurea jacea

Background

Agri-environment schemes play an increasingly important role for the conservation of rare plants in intensively managed agricultural landscapes. However, little is known about their effects on gene flow via pollen dispersal between populations of these species.

Methodology/Principal Findings

In a 2-year experiment, we observed effective pollen dispersal from source populations of Centaurea jacea in restored meadows, the most widespread Swiss agri-environment scheme, to potted plants in adjacent intensively managed meadows without other individuals of this species. Potted plants were put in replicated source populations at 25, 50, 100 m and where possible 200 m distance from these source populations. Pollen transfer among isolated plants was prevented by temporary bagging, such that only one isolated plant was accessible for flower visitors at any one time. Because C. jacea is self-incompatible, seed set in single-plant isolates indicated insect mediated effective pollen dispersal from the source population. Seed set was higher in source populations (35.7±4.4) than in isolates (4.8±1.0). Seed set declined from 18.9% of that in source populations at a distance of 25 m to 7.4% at 200 m. At a distance of 200 m seed set was still significantly higher in selfed plants, indicating long-distance effective pollen dispersal up to 200 m. Analyses of covariance suggested that bees contributed more than flies to this long-distance pollen dispersal. We found evidence that pollen dispersal to single-plant isolates was positively affected by the diversity and flower abundance of neighboring plant species in the intensively managed meadow. Furthermore, the decline of the dispersal was less steep when the source population of C. jacea was large.

Conclusions

We conclude that insect pollinators can effectively transfer pollen from source populations of C. jacea over at least 200 m, even when “recipient populations” consisted of single-plant isolates, suggesting that gene flow by pollen over this distance is very likely. Source population size and flowering environment surrounding recipient plants appear to be important factors affecting pollen dispersal in C. jacea. It is conceivable that most insect-pollinated plants in a network of restored sites within intensively managed grassland can form metapopulations, if distances between sites are of similar magnitude as tested here.

Population Size, Genetic Variation, and Reproductive Success in a Rapidly Declining, Self-Incompatible Perennial (Arnica montana) in The Netherlands

Arnica montana is a rare and rapidly declining, self-incompatible plant species. In 26 populations in The Netherlands we investigated the relationship between population size and genetic variation using allozyme markers. Genetic variation was low in A. montana (  H_e = 0.088). There were positive correlations between population size and the proportion of polymorphic loci, the number of effective alleles, and expected heterozygosity, but not with observed heterozygosity. There was a significantly positive correlation between population size and the inbreeding coefficient. Generally, small populations showed heterozygote excess, which decreased with increasing population size. Possibly, the heterozygous individuals in small populations are survivors from the formerly larger populations with relatively high fitness. The F statistics showed a moderately high level of differentiation among populations (  F_ST = 0.140 ± 0.02), implying a low level of gene flow. For three out of four allozyme loci, we found significant inbreeding (  F_IS = 0.104 ± 0.03). Only 14 of 26 populations were in Hardy-Weinberg equilibrium at all four polymorphic loci. In a subset of 14 populations of various size, we investigated natural seed production and offspring fitness. Population size was positively correlated with seed set, seedling size, number of flowering stems and flowerheads, adult survival, and total relative fitness, but not with the number of florets per flowerhead, germination rate, or the proportion of germination. Offspring performance in the greenhouse was not associated with genetic diversity measured on their mothers in the field. We conclude that the fitness of small populations is significantly reduced, but that there is as yet no evidence that this was caused by inbreeding. Possibly, the self-incompatibility system of A. montana has been effective in reducing selfing rates and inbreeding depression.

Pollination ecology and patch-dependent reproductive success of the rare perennial Gentiana pneumonanthe L

The reproductive behaviour of Gentiana pneumonanthe L., a rare plant species in The Netherlands, was studied in a relatively large wet heathland population during summer 1989. The species co-occurred with the grass Molinia caerulea and co-flowered with Erica tetralix and Calluna vulgaris. The flowering period lasted from July to October, peaking in late August to late September. Flowers are protandrous. The species appeared to be self-compatible, but spontaneous self-pollination was strongly limited. Hand-crossing and hand-selfing resulted in the same amount of seed set as in natural pollination. Reproductive success was dramatically reduced late in the season. Pollination was achieved sternotribically by the species Bombus pascuorum, which visited the flowers for nectar. In the study area, three patch types were distinguished: co-dominated by Erica, by Calluna and Erica, and by Molinia, respectively. Frequency of visits to Gentiana was highest in the Erica-patch. However, this did not result in a higher seed set. In the Molinia-patch seed set was reduced. However, in the Molinia-patch the mean number of ovules was greater than in the others (as high as in plants raised indoors and in cross-pollinated plants). Fruits from the Calluna-patch had less ovules than those from the Erica-patch. We conclude that, in this remnant population, Gentiana is not pollination-limited. However, it is likely that in the Molinia-patch geitonogamy is frequent, which may lead to inbreeding depression. The greater number of ovules observed in Molinia-patches may reflect an excess of nutrients available there, relative to the Calluna-patch where there may be reduced water availability.