Effects of the availability of floral resources on plant-pollinator interactions and the implications for the long-term survival of plant populations

Field: Agricultural and Biological Sciences (Life Sciences)

1. Insect pollinators have been shown to alter their foraging patterns in \nresponse to habitat and landscape composition, particularly in relation \nto changes in the availability of floral resources which provide essential \npollen and nectar provisions. Changes to pollinator behaviour and \ncommunity composition, may alter the distance, directness and \nfrequency of pollen movement and thus, the compatibility and genetic \nrelatedness of pollen transferred between plants. We still lack good \nunderstanding of how variation in the spatial and temporal availability of \nfloral resources drives pollinator responses and in turn, affects the \nfitness of outcrossing plants. Knowledge in this area could contribute to \nimproved management interventions to enhance pollination services for \nplant conservation. \n2. Through a combination of habitat and landscape scale field \nexperiments, I explored how the availability of floral resources at \ndifferent spatial scales affected plant-pollinator interactions, pollen \ntransfer and mating success in plant populations, particularly those \nisolated from conspecifics. This involved introducing different species \nof plants in experimental arrays across a range of study systems that \nvaried in structure and floral availability. Over the course of the thesis, I \nmeasured the community composition and behaviour of pollinators \nvisiting experimental arrays; focusing on traits considered important for \npollen transfer (e.g. Inter-tegular (‘IT’) span). Pollen movement was \nquantified within and between populations (5-150m) and the resulting \nplant outcrossing rates were measured using different methods \nincluding paternity analysis and the use of a dominance inheritance \nsystem. In addition, the implications of variations in pollinator foraging \nand pollination services can be attributed to pollen and gene flow and \nsubsequently the reproduction and fitness of plants were assessed as \na means of predicting the impacts on longer-term plant survival. \n3. Findings from this thesis demonstrate reductions in the activity density \n(the abundance of actively foraging pollinators) and richness of \npollinators and thus, the potential for plant visitation in response to a \nhigh abundance of floral resources within a habitat. This led to \ndisruptions in pollen transfer, illustrated through a lower incidence of \nintra and inter-population pollen movement, and ultimately, reduced \nplant outcrossing rates. In parallel, plant seed set and germination rates \nwere also reduced in habitats with high resource availability. Changes \nto pollinator communities and pollination services varied with the spatial \nscale at which floral resources were measured. Pollinator communities \n(activity density, richness and IT span) were most affected by floral \nresource abundance at a local scale (1-50m), particularly within a 20m \nradius of a plant population. Intra-population pollen movement was \nsimilarly affected by floral resources at a local spatial scale (within a 1m \nradius of a plant population). In contrast, no effect was observed on \npollinator communities, intra-population pollen movement or plant \nreproduction when floral resources were measured at a landscape \nscale (within a 100-1500m radius of a plant population). However, \nfindings were variable across different experiments at the same scale \nof measurement. For instance, the availability of floral resources at a \nlocal scale did not always elicit an effect on plant reproduction. This \nreflects differences in plant species identity and the effects of breeding \nsystem and floral traits, illustrated through variations in visitation rates \nbetween plant species. Inconsistencies were further observed with \npollinator activity density and richness, which were not related to floral \nresources at a habitat scale in one chapter. \n4. This thesis highlights the importance of the availability of floral \nresources at a local scale on plant-pollinator interactions and pollination \nservices to plants. Co-flowering plants within florally rich habitats \ncompete for pollinators and subsequently, visitation and pollen transfer \nbetween individuals of low density plant populations is diluted rather \nthan facilitated. This suggests that although pollinator abundance and \ndiversity may be enhanced through florally rich habitats (e.g. habitats \nimplemented under the agri-environment scheme), pollination services \nare not automatically improved for plants which are present at low \nfrequency in the landscape. This needs to be considered when \ndesigning and implementing management for threatened or isolated \nplants where plants may instead benefit from focused interventions. For \ninstance, pollination services may be increased by efforts to maximise \nthe facilitative effect of surrounding habitats, while increasing the ability \nof threatened or isolated plants to withstand competition from co-flowering \nplants.