Trees, birds and bees in Mauritius: Exploitative competition between introduced honey bees and endemic nectarivorous birds?

Aims: To investigate effects of introduced honey bees, Apis mellifera L., on the nectar-feeding activity of two species of endemic nectarivorous birds, the Grey White-eye, Zosterops borbonicus mauritianus Gmelin, and the Olive White-eye, Z. chloronothos Viellot, on two endemic flowering trees, Sideroxylon cinereum Lam. and S. puberulum DC. (Sapotaceae), and to examine pollination efficiency of birds and honey bees. Location: An upland heath area on the island of Mauritius, Indian Ocean. Methods: We quantified visitation rates of endemic birds and introduced honey bees at two endemic species of flowering trees. Diurnal variation in nectar standing crop and nectar production was measured. Pollination efficiency of flower visitors was examined using bagging and caging experiments. Results: White-eyes were only nectar-feeding at the two Sideroxylon species early in the morning, stopping when honey bee foraging activity rapidly lowered nectar standing crops. White-eyes continued nectar-feeding at other flowering plant species, exploited less by honey bees, throughout the day. Honey bees were less efficient pollinators of the two Sideroxylon species than white-eyes. Main conclusions: Our results indicate that introduced honey bees could be interfering with endemic interactions between the two Sideroxylon species and the two white-eye species. However, because of lack of a neutral control site without honey bees, we cannot exclude other explanations. We do recommend, although, that honey bees need to be taken into consideration in the future conservation management of Mauritian ecosystems. We suggest that island ecosystems are especially vulnerable to introduced honey bees.