Effects of the availability of floral resources on plant-pollinator interactions and the implications for the long-term survival of plant populations
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Date
29/11/2018Author
Evans, Tracie Marie
Metadata
Abstract
1. Insect pollinators have been shown to alter their foraging patterns in
response to habitat and landscape composition, particularly in relation
to changes in the availability of floral resources which provide essential
pollen and nectar provisions. Changes to pollinator behaviour and
community composition, may alter the distance, directness and
frequency of pollen movement and thus, the compatibility and genetic
relatedness of pollen transferred between plants. We still lack good
understanding of how variation in the spatial and temporal availability of
floral resources drives pollinator responses and in turn, affects the
fitness of outcrossing plants. Knowledge in this area could contribute to
improved management interventions to enhance pollination services for
plant conservation.
2. Through a combination of habitat and landscape scale field
experiments, I explored how the availability of floral resources at
different spatial scales affected plant-pollinator interactions, pollen
transfer and mating success in plant populations, particularly those
isolated from conspecifics. This involved introducing different species
of plants in experimental arrays across a range of study systems that
varied in structure and floral availability. Over the course of the thesis, I
measured the community composition and behaviour of pollinators
visiting experimental arrays; focusing on traits considered important for
pollen transfer (e.g. Inter-tegular (‘IT’) span). Pollen movement was
quantified within and between populations (5-150m) and the resulting
plant outcrossing rates were measured using different methods
including paternity analysis and the use of a dominance inheritance
system. In addition, the implications of variations in pollinator foraging
and pollination services can be attributed to pollen and gene flow and
subsequently the reproduction and fitness of plants were assessed as
a means of predicting the impacts on longer-term plant survival.
3. Findings from this thesis demonstrate reductions in the activity density
(the abundance of actively foraging pollinators) and richness of
pollinators and thus, the potential for plant visitation in response to a
high abundance of floral resources within a habitat. This led to
disruptions in pollen transfer, illustrated through a lower incidence of
intra and inter-population pollen movement, and ultimately, reduced
plant outcrossing rates. In parallel, plant seed set and germination rates
were also reduced in habitats with high resource availability. Changes
to pollinator communities and pollination services varied with the spatial
scale at which floral resources were measured. Pollinator communities
(activity density, richness and IT span) were most affected by floral
resource abundance at a local scale (1-50m), particularly within a 20m
radius of a plant population. Intra-population pollen movement was
similarly affected by floral resources at a local spatial scale (within a 1m
radius of a plant population). In contrast, no effect was observed on
pollinator communities, intra-population pollen movement or plant
reproduction when floral resources were measured at a landscape
scale (within a 100-1500m radius of a plant population). However,
findings were variable across different experiments at the same scale
of measurement. For instance, the availability of floral resources at a
local scale did not always elicit an effect on plant reproduction. This
reflects differences in plant species identity and the effects of breeding
system and floral traits, illustrated through variations in visitation rates
between plant species. Inconsistencies were further observed with
pollinator activity density and richness, which were not related to floral
resources at a habitat scale in one chapter.
4. This thesis highlights the importance of the availability of floral
resources at a local scale on plant-pollinator interactions and pollination
services to plants. Co-flowering plants within florally rich habitats
compete for pollinators and subsequently, visitation and pollen transfer
between individuals of low density plant populations is diluted rather
than facilitated. This suggests that although pollinator abundance and
diversity may be enhanced through florally rich habitats (e.g. habitats
implemented under the agri-environment scheme), pollination services
are not automatically improved for plants which are present at low
frequency in the landscape. This needs to be considered when
designing and implementing management for threatened or isolated
plants where plants may instead benefit from focused interventions. For
instance, pollination services may be increased by efforts to maximise
the facilitative effect of surrounding habitats, while increasing the ability
of threatened or isolated plants to withstand competition from co-flowering
plants.