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Caroline Nienhuis

PhD: The interactions between native bees and alien plants (2005-2008, EPA funding)

PhD Thesis Abstract:
This thesis examines the interactions between native bees, in particular bumblebees (Bombus
spp.), and the alien plants they visit. Most alien plants are well integrated into the resident
plant-pollinator network. They are thus likely to directly and indirectly affect the foraging
behaviour, resource use, and the abundance and diversity of individuals, populations and
communities of native generalist pollinators as they may augment but also alter the availability
and composition of forage resources, by attaining large floral densities and possibly displacing
native plants. To date only a limited number of studies have studied the indirect and direct
impacts of alien plants on native bees. I investigated the interactions between native bees and
alien plants through observational and experimental field studies that took place in sites along
the River Liffey and on Howth, Co. Dublin (Eastern Ireland) and in Spiddal, Co. Galway
(Western Ireland).
Bumblebee usage of alien and native plant resources (nectar and pollen) was examined
throughout the foraging season (April to September) and over two consecutive years in
invaded habitats. Although alien resources were collected by native bumblebees, they rarely
dominated usage as bumblebees relied greatly on native floral rewards. The quantity (nectar
and pollen production) and quality (concentration of essential amino acids in nectar and
pollen) of floral rewards were compared among a suite of alien (Buddleja davidii, Fuchsia
magellanica, Impatiens glandulifera, Rhododendron ponticum and Symphytum x uplandicum)
and native (Digitalis purpurea, Epilobium hirsutum, Rubus fruticosus agg, Vicia sepium)
plants that are pollinated by native insects. Although nectar and pollen rewards from alien
species were not generally higher in quality, they may quantitatively surpass those of native
species at a plant and population level.
The alien I. glandulifera was also used as a model species to quantify the effects of its
presence and removal on native insects. No impact of invasion and removal on standardised
insect abundance, B. pascuorum abundance, nor functional insect diversity was detected,
suggesting that localised small-scale removal of I. glandulifera may be feasible to prevent the
alien from setting seed. Additionally, the impacts of I. glandulifera on bumblebee (B.
hortorum and B. pascuorum) foraging behaviour and the effect of local alien plant density on
bumblebee visitation, stigmatic pollen deposition and seed set of two native self-incompatible
plant species (Lythrum salicaria and Stachys palustris) were examined. B. hortorum was more
flower constant than B. pascuorum. Increased B. pascuorum visitation to native plants at high
alien density resulted in increased alien pollen and reduced conspecific pollen deposition on
stigmas, and reduced seed set in L. salicaria, but not S. palustris.
Lastly, the effectiveness of native bumblebees (B. pascuorum) as pollinators of alien I.
glandulifera and their function in aiding its spread was explored by quantifying pollen
deposition and removal, and seed production after a single visit. The morphological fit
between flowers and bumblebee body parts was measured to assess their influence on pollen
deposition and removal. B. pascuorum effectively pollinated the alien due to its high visitation
frequency, the morphological fit with flowers, and individuals removing large pollen
quantities and inducing maximum seed set after a single visit.
The results of this thesis reveal that native bees interact greatly with alien plants, which do not
have a negative impact on native bees. Alien plant resources (nectar and pollen) may thus
supplement native resources, especially in disturbed, fragmented and suburban habitats.
Native bumblebees in turn are effectively pollinating alien I. glandulifera and may therefore
be aiding its spread.

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


photo of impatiens glandulifera

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Last updated 5 March 2013