Density of invasive western honey bee (Apis mellifera) colonies in fragmented woodlands indicates potential for large impacts on native species

Abstract

Feral Apis mellifera colonies are widespread globally and cause ecological impacts as pollinators and competitors for food and nesting opportunities. The magnitude of impact depends on their population density, but knowledge of this density is poor. We document feral A. mellifera colonies at 69 per km2 in fragmented Eucalyptus woodlands in Australia, exceeding estimates from elsewhere in the world, and matched only by one other Australian study. We surveyed 52.5 ha of woodland patches with 357 nest boxes installed to provide nesting opportunities for threatened vertebrates. Our sites covered a region of more than 140 km across with repeated surveys over 3 to 6 years. We show that nest box use by feral A. mellifera colonies is influenced by box design (p = 0.042), with weak evidence for an interactive effect of type of vegetation at a site (woodland remnants vs. replanting) and woody cover within 500 m (p = 0.091). At 69 colonies per km2, this density is equivalent to the recommended stocking of hives for pollination of some crops and is therefore likely to influence pollination and lead to competition with other flower visitors. Apis mellifera is also likely to be competing for hollows with cavity dependent native fauna, especially in landscapes where there has been extensive tree removal.

Population demography of feral honeybee colonies in central European forests

“…In the few cases where colony densities were estimated, the numbers were comparable to those reported here (rural avenues in Poland [14]: 0.1 km −2 ; Hainich National Park, Germany [7]: 0.13 km −2 ; agricultural landscape in northwest Spain [36]: 0.17-0.22 km −2 ). In turn, all known feral populations which are evidently self-sustaining exhibit significantly higher colony densities: at least around 1 colony km −2 in temperate regions [19], and often greater than 5 colonies km −2 in (sub)tropical regions [12,72,73]. Therefore, it seems likely that in many of the European cases wild-living colonies might merely represent recent escapees from apiaries.…”

Bee pollination services and the burden of biogeography

“…Australia houses some of the world's highest population densities of feral honey bees [65] and while the incidental pollination service provided by unmanaged colonies is likely to be considerable, supplementation from managed hives may still be required. However, we were unable to distinguish between feral and apiary colony visitations.…”

The conservation of urban flower visitors Down Under

“…The introduction of both these species has resulted in large numbers of feral colonies. Honey bees are present throughout temperate and Mediterranean climates in Australia, where they may compete with hollow nesting birds and mammals for nesting hollows and with flower visitors for floral resources (Paini, 2004;Cunningham et al, 2022). Bumblebees are present throughout New Zealand and on Tasmania, where they are significant pollinators of weeds and fruit trees.…”

“…While supplemental nesting habitat (nest boxes) cannot completely compensate for a lack of nesting hollows for vertebrate flower visitors, they can support bat, marsupial, and bird biodiversity in Australian cities (Le Roux et al, 2016;Macak, 2020). However, nest boxes can also provide habitat for invasive species, in particular European honey bees (Cunningham et al, 2022), and their placement can therefore be counterproductive to the conservation of native flower visitors (Macak, 2020). For bees, placement of bee hotels is very popular worldwide.…”