Honeybees reduce fitness in the pioneer shrub Melastoma affine (Melastomataceae)

Abstract

The agistment of managed hives of the introduced honeybee, Apis mellifera, in or adjacent to conservation areas in Australia is controversial. The effects, if any, of honeybee-foraging on native plants and their native-bee pollinators is poorly understood as most studies to date have concentrated on bird-pollinated systems. Furthermore, such studies have been undertaken in temperate Australia where feral and managed hives have been present for more than 150 years. In tropical Australia the impact of honeybees on the native biota is not known—yet the information is needed to assist with planning for the management of the large areas now under control of conservation authorities. We undertook a comparative study of honeybee and native bee pollination of the pioneer species Melastoma affine in tropical north Queensland, Australia, at a site where honeybees were recently introduced as managed hives. Melastoma affine is utilised by many animals in this ecosystem and its pollination mechanism is representative of several other pioneer species of the rainforest margin. Melastoma affine obligately relies on bee pollination to effect seed-set. Native bees were the most abundant floral visitors to M. affine although significantly more honeybees than native bees were sometimes present at flowers at the end of the morning. Honeybees were poor pollinators of M. affine compared with native bees. Honeybees deposited significantly less pollen on stigmas than native bees and honeybees actively removed pollen from stigmas. Consequently, fruit-set was less likely and seed-set was significantly lower in flowers to which honeybees were the last visitor, compared with cases where native bees were the last visitor—and the last visitor to M. affine flowers was most often A. mellifera. In 91% of interactions between honeybees and native bees, native bees were disturbed from foraging at flowers by honeybees. Honeybees reduced fitness in M. affine in this study and we thus conclude that honeybees are an undesirable introduction in montane tropical-rainforest systems in Australia and based on our findings we strongly recommend that honeybees not be agisted in or adjacent to conservation areas in the wet tropics of Australia.