Consequences of plant population size for pollinator visitation and plant reproductive success
Creators: name: Vivarelli, Daniele <1978>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2007
Subjects: BIO/02 Botanica sistematica

description: Habitat loss and fragmentation have a prominent role in determining the size of plant populations,
and can affect plant-pollinator interactions. It is hypothesized that in small plant populations the
ability to set seeds can be reduced due to limited pollination services, since individuals in small
populations can receive less quantity or quality of visits. In this study, I investigated the effect of
population size on plant reproductive success and insect visitation in 8 populations of two common
species in the island of Lesvos, Greece (Mediterranean Sea), Echium plantagineum and Ballota
acetabulosa, and of a rare perennial shrub endemic to north-central Italy, Ononis masquillierii.
All the three species depended on insect pollinators for sexual reproduction. For each species,
pollen limitation was present in all or nearly all populations, but the relationship between pollen
limitation and population size was only present in Ononis masquillierii. However, in Echium
plantagineum, significant relationships between both open-pollinated and handcrossed-pollinated
seed sets and population size were found, being small populations comparatively less productive
than large ones. Additionally, for this species, livestock grazing intensity was greater for small
populations and for sparse patches, and had a negative influence on productivity of the remnant
plants. Both Echium plantagineum and Ballota acetabulosa attracted a great number of insects,
representing a wide spectrum of pollinators, thereby can be considered as generalist species. For
Ballota acetabulosa, the most important pollinators were megachilid female bees, and insect
diversity didn’t decrease with decreasing plant population size. By contrast, Ononis masquillierii
plants generally received few visits, with flowers specialized on small bees (Lasioglossum spp.),
representing the most important insect guild. In Echium plantagineum and Ballota acetabulosa,
plants in small and large populations received the same amount of visits per flower, and no
differences in the number of intraplant visited flowers were detected. On the contrary, large Ononis
populations supported higher amounts of pollinators than small ones. At patch level, high Echium
flower density was associated with more and higher quality pollinators.
My results indicate that small populations were not subject to reduced pollination services than
large ones in Echium plantagineum and Ballota acetabulosa, and suggest that grazing and resource
limitation could have a major impact on population fitness in Echium plantagineum. The absence of any size effects in these two species can be explained in the light of their high local abundance,
wide habitat specificity, and ability to compete with other co-flowering species for pollinators. By
contrast, size represents a key characteristic for both pollination and reproduction in Ononis
masquillierii populations, as an increase in size could mitigate the negative effects coming from the
disadvantageous reproductive traits of the species. Finally, the widespread occurrence of pollen
limitation in the three species may be the result of 1) an ongoing weakening or disruption of plantpollinator
interactions derived from ecological perturbations, 2) an adaptive equilibrium in response
to stochastic processes, and 3) the presence of unfavourable reproductive traits (for Ononis
masquillierii).