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Anke Dietzsch

PhD: Impacts of the alien invasive Rhododendron ponticum on native plants, pollinators and their interaction (2004-2008, SFI funding)

PhD Thesis Abstract:
This thesis addresses the impacts of an alien invasive plant, Rhododendron ponticum L., on
pollination processes and population dynamics of native Irish plant and pollinator
communities. Field studies were conducted in habitats in the West (Co. Galway) and East
(Co. Wicklow and Co. Dublin).
For the plants, observational and manipulative field experiments were used to investigate
both plant community responses and individual species responses of a native focal plant,
Digitalis purpurea L., to R. ponticum invasion.
Flower visitation rates and nectar productivity of the entire flowering community were
measured in uninvaded and invaded sites at early, peak and late flowering of R. ponticum.
Although nectar productivity of R. ponticum was higher than in the majority of native plant
species, there was little evidence for strong alien impacts on flower visitation of the plant
community and of single species within the community. Individuals of some species
revealed higher visitation rates in presence of the alien, but these differences were highly
variable in space and time. Species that showed a response to alien invasion overlapped
with the alien more in their pollinator assemblage and resembled the alien in high nectar
production and large floral displays. They are considered to belong to the same functional
In a second investigation, pollinator visitation to D. purpurea was investigated in sites of
different invasion levels and uninvaded sites over a two years period. Conspecific and alien
pollen transfer and seed set were evaluated in the second year. Native D. purpurea showed
a decrease in pollinator visitation in both years and lower conspecific pollen deposition on
native stigmata at high R. ponticum densities. In contrast, fruit and seed set were not
affected by R. ponticum of different densities.
For a third study, potted D. purpurea plants and cut flowering branches of R. ponticum in
water-filled buckets were used to mimic small single-species and mixed-species
populations at 29 experimental field sites. Both alien and native abundances were
manipulated in a replicated design. Flower visitation, nectar standing crop and seed set
were measured at each field site. While native flower visitation was higher in initially
larger D. purpurea populations, a further increase in native or alien plant abundance only
marginally increased flower visitation. Conspecific individuals facilitated each others
flower visitation, while alien abundance did not affect visitation. Fruit and seed set did not
differ for any population size treatment. D. purpurea is therefore assumed to display strong
resilience towards R. ponticum invasion.
For the pollinators, two native Irish bumblebee species, Bombus lucorum L. and B.
pascuorum Scopoti, that visit R. ponticum frequently were investigated.
First, a replicated laboratory feeding experiment was conducted using alien, native, two
control and two differently concentrated aconitine intoxicated diets. Survival rate of
individual B. lucorum worker bees was measured over a period of 28 days. Bee survival
among the six feeding regimes did not significantly differ.
Second, mark-recapture data and DNA samples of workers of both bumblebee species
were collected at two uninvaded and two invaded sites in two consecutive years. Molecular
analyses were carried out using specific microsatellite primers and standard techniques.
Bumblebee abundance and colony numbers for each site and year were estimated. For both
bee species, abundance estimates were subject to high deviation due to few captured bee
individuals. Abundance did not significantly differ between invaded and uninvaded sites.
In both bee species, colony numbers could be reproduced between years and were higher at
invaded sites than at uninvaded sites. The invasion of R. ponticum seems to facilitate social
pollinator populations in Irish habitats.
The findings are discussed in the broader context of scale dependency of impacts,
community dynamics and impacts of alien plant invasion on pollination in general.

Contact details
Current position: Research assistant, National Botanic Gardens, Glasnevin, Dublin


photo of rhododendron ponticum


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