The effects of invasive Acacias on local flower visitation networks and plant reproductive success in Fynbos habitats of the Walker Bay area, South Africa

Abstract

Floral provision by invasive plant species has the potential to disrupt local flowervisitation
networks. This can affect the abundance, diversity and reproduction of
native plant and insect species. Here I investigate the impact of two highly
invasive Australian legumes, Acacia saligna and Acacia longifolia, in South
African Fynbos habitats. Twenty-one one-hectare sites were selected, and grouped
into seven triplets based on habitat traits. Seven pristine sites acted as control
plots, seven sites were adjacent to Acacia invasion, and seven were adjacent to
Acacia invasion but Acacia flowers were removed. Flower removal enabled the
effects of the Acacia vegetation and their floral resources to be separated. The 21
sites were sampled throughout the Acacia flowering season from August to
October, using three methods: 1) flower visitation to Acacias and local flora was
observed at all 21 sites and plant-pollinator network indices compared; 2) pan
traps were used to compare insect biomass and assemblages between pristine and
invaded sites, and 3) flower visitation and seed production were compared for
three focal plant species that shared floral traits with Acacias, at pristine and
invaded sites. While insect and floral species richness remained unaffected by
Acacia, the flower-visitation networks at invaded sites had increased links per
species and increased insect visitation per flower. Coleoptera abundance in pan
traps was significantly greater at invaded sites, though overall insect biomass was
unaffected. Two of three focal plant species produced fewer seeds and had
reduced visitor species richness at invaded sites. The effects on flower-visitation
networks were observed at least 100m from Acacias themselves i.e. there is an
overspill effect. Acacia invasion is a major threat to Fynbos habitats; this research
expands upon the poorly understood impacts of these species on local pollination
ecology, and presents the first comprehensive flower-visitation networks for three
endangered Fynbos habitats.

Date of Award	26 Nov 2020
Original language	English
Awarding Institution	University of Bristol
Supervisor	Daniel Robert (Supervisor) & Jane Memmott (Supervisor)