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Pollinator visitation, stigmatic pollen loads, and among-population variation in seed set in Lythrum salicaria
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Department of Plant Ecology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden.
2004 (English)In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745, Vol. 92, no 3, 512-526 p.Article in journal (Refereed) Published
Abstract [en]
  1. Small populations of reward-producing plants are likely to be less attractive to animal pollinators than large populations. As a result, both the quantity and the proportion of compatible pollen deposited on receptive stigmas, and seed output per plant, may be lower in small than in large populations.
  2. We examined whether pollinator visitation, pollen deposition and seed set varied with population size in the self-incompatible, tristylous herb Lythrum salicaria, in the Skeppsvik archipelago, northern Sweden. We documented both the number of compatible and incompatible conspecific and heterospecific pollen grains received per flower, seed set and degree of pollen limitation of long-styled plants in 14 populations of different size in two consecutive years, and recorded the visitation rate to individual plants and the number of flowers visited per plant in eight of the populations.
  3. As predicted, the visitation rate tended to increase, while the number of flowers visited per plant tended to decrease with increasing population size. However, visitation rates were low overall and temporally highly variable, and these relationships only approached statistical significance. The proportion and absolute number of compatible pollen grains received increased with population size, while the total amounts of conspecific and heterospecific pollen grains received did not vary significantly with population size. The results of supplemental hand-pollinations indicated that the among-population variation in seed set was due to insufficient transfer of compatible pollen in small populations.
  4. Seed output increased with the receipt of compatible pollen grains up to about 200 compatible pollen grains received per flower. Between 73% and 98% of the L. salicaria pollen grains received were incompatible, and between 9% and 81% of the pollen grains deposited were heterospecific (population means). However, there was no evidence that the deposition of high numbers of incompatible conspecific and heterospecific pollen grains reduced seed set.
  5. In the study populations of L. salicaria, variation in seed output and pollen limitation are apparently governed primarily by factors influencing the transfer of compatible pollen. The results are consistent with the hypothesis that a reduction in the efficiency of pollen transfer among compatible mating types may markedly reduce the reproductive output in small populations of self-incompatible plants. In light of the current rapid transformation and fragmentation of habitats, there is a pressing need both to clarify how the pollination success of plants with different pollination systems is affected by large-scale changes in population size, density and isolation, and to determine the demographic consequences of differences in pollination intensity.
Place, publisher, year, edition, pages
Oxford: Blackwell , 2004. Vol. 92, no 3, 512-526 p.
Keyword [en]
Lythrum salicaria, pollen deposition, pollinator visitation, population size, seed production
Identifiers
URN: urn:nbn:se:umu:diva-4645DOI: 10.1111/j.0022-0477.2004.00893.xOAI: oai:DiVA.org:umu-4645DiVA: diva2:143836
Available from: 2005-08-30 Created: 2005-08-30 Last updated: 2011-03-25Bibliographically approved
In thesis
1. Plant-animal interactions and seed output of two insectpollinated herbs
Open this publication in new window or tab >>Plant-animal interactions and seed output of two insectpollinated herbs
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

I combined comparative and experimental studies in the field and in the greenhouse to examine factors influencing reproductive success in two insect-pollinated herbs, the tristylous, selfincompatible perennial Lythrum salicaria and the self-compatible, biennial Pedicularis palustris. More specifically, I explored (i) the effects of plant population characteristics on the intensity and outcome of interactions with pollinators and seed predators, (ii) whether flower morphology affects duration of stigma receptivity, and (iii) whether damage-induced reduction in floral display reduces pollinator visitation and increases pollen limitation.

As predicted, the rate of pollinator visitation tended to increase and the number of flowers probed per plant tended to decrease with increasing population size in L. salicaria, but these relationships only approached statistical significance. By taking advantage of the pollen size polymorphism that is typical of many heterostylous plants, I could show that the number of compatible pollen grains received increased with population size, and that this was associated with a reduction in pollen limitation and increased seed output per flower. The deposition of high numbers of incompatible conspecific and heterospecific pollen grains did not appear to reduce seed set.

In P. palustris, fruit set and seed predation varied markedly among populations and years, but this variation could only partly be explained by variation in population size. Fruit set was positively related to population size, and seed predation was negatively related to population size, in one of three years. Similarly, the level of pollen limitation, which was quantified in two years, varied among populations, but was not related to population size, density or isolation.

In L. salicaria, both the duration of stigma receptivity and the effect of prior self-pollination on seed output varied among style morphs. These differences may contribute to morph-specific differences in pollen limitation and seed production documented in the field.

The results of a field experiment demonstrated that damage to the shoot apex may markedly reduce fruit production in L. salicaria, and suggested that this is mainly because damage reduces flower number. I found no evidence that a reduced floral display increased the severity of pollen limitation.

Taken together, the results show that interactions with both pollinators and herbivores may markedly affect reproductive output in the two plant species, and that the intensity of both mutualistic and antagonistic interactions vary considerably in space and time. Moreover, they are consistent with the hypothesis that pollination success should depend less on population size in selfcompatible than in self-incompatible plants.

Place, publisher, year, edition, pages
Umeå: Ekologi, miljö och geovetenskap, 2005. 23 p.
Keyword
plant reproduction, pollen limitation, pollinator behaviour, heterostyly, seed predation, Lythrum salicaria, Pedicularis palustris, stigma receptivity, self-pollen
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-576 (URN)91-7305-911-0 (ISBN)
Public defence
2005-10-01
Available from: 2005-08-30 Created: 2005-08-30Bibliographically approved

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