Spillover effects from invasive Acacia alter the plant-pollinator networks and seed production of native plants

Invasive flowering plants can disrupt plant-pollinator networks. This is
well documented where invasives occur amongst native plants, however, the
potential for ‘spillover’ effects of invasives that form stands in
adjacent habitats is less well understood. Here we quantify the impact of
two invasive Australian species, Acacia saligna and Acacia longifolia, on
the plant-pollinator networks in Fynbos habitats in South Africa. We
compared networks from replicate 1ha plots of native vegetation (n=21)
which were subjected to three treatments: 1) at least 400m from flowering
Acacia; 2) were adjacent to flowering Acacia or 3) were adjacent to
flowering Acacia where all Acacia flowers were manually removed. We found
that native flowers adjacent to stands of flowering Acacia received
significantly more insect visits, especially from beetles and Apis
mellifera capensis, and that visitation was more generalised. We also
recorded visitation to, and the seed set of, three native flowering
species and found that two received more insect visits, but produced fewer
seeds, when adjacent to flowering Acacia. Our research shows that
‘spillover’ effects of invasive Acacia can lead to significant changes in
visitation and seed production of native co-flowering species in
neighbouring habitats; a factor to be considered when managing invaded
landscapes.