Robbers vs. invaders: The effects of alien tree invasions on nectar robbing in bird-pollinated Erica species.

Abstract

Invasion by alien woody plants strongly affects plant-animal interactions, often reducing invertebrate and bird abundance, thereby altering pollination and ultimately influencing plant reproduction. However, it remains unclear whether invasive alien trees also affect interactions typically considered antagonistic, such as nectar robbing. This study investigates the effects of invasive alien trees on insect communities and how these, in turn, affect the occurrence and frequency of nectar robbing in bird-pollinated Erica discolor and Erica unicolor. Floral-visitor observations were recorded at 27 uninvaded and 31 invaded fynbos (a Mediterranean-type shrubland) sites along the southern Cape coastal mountains, South Africa, from April to June 2023. We assessed factors affecting (1) nectar-robbing insect abundance, (2) the rate of nectar robbing, and (3) the impact of nectar robbing on pollination rate. Although overall robbing rates did not differ between invaded and uninvaded sites, the species composition of nectar robbers shifted. The Cape honey bee (Apis mellifera capensis) dominated robbing in uninvaded sites, whereas small solitary bees dominated robbing in invaded sites. Robbing appeared to have no effect on bird pollination rates in either Erica species. This study is the first to directly compare the widely used cumulative measure of robbing rate with observed robbing rate, revealing a significant positive relationship between the two. Together, these findings offer insights into how invasive alien trees can alter fynbos ecosystems and emphasise the critical role of insect communities in shaping plant-animal interactions.