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.

Methods
Data were collected from June to September 2019, in Fynbos habitats in the Overstrand region, Western Cape, South Africa.

'allvisits.csv':

1ha habitat plots were surveyed for 30min periods, during which all flower-insect interactions were recorded.

Plant and insect species were identified both in the field and had specimens taken to be identified by Taxonomists (see paper).

'focals_allvisits.csv':

Three focal plants were observed for 20-30min periods at sites, during which all insect visits to the flowers were recorded.

Insect species were identified both in the field and had specimens taken to be identified by Taxonomists (see paper).

'focals_seedcounts.csv':

Flowers from three focal species were either bagged to exclude pollinators, or left unbagged. Seedheads were allowed to mature, and seeds were counted from each flower head.