Abstract of PhD Thesis

The interactions between native bees and alien plants

Caroline Nienhuis, Trinity College Dublin (2009)

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.