Abstract
Bee communities in New Zealand are composed of native and introduced bees, including honey bees (Apis mellifera), which have been in the country for over 100 years. While there is evidence from other countries that honey bees compete with native bees, the interactions within New Zealand are not well understood. In addition, the arrival in New Zealand of the parasitic mite, Varroa destructor in 2000, and the subsequent loss of feral honey bees across the country has had unknown effects on the native pollinator community. How the loss of honey bees has affected other bee species can provide insight on the potential impact of honey bees prior to introduction to New Zealand and the potential impact prior to the introduction of varroa.
To determine the impact of introduced bees on native bees, resource utilisation and overlap between bee species was estimated. The potential impact of honey bees on native bees was also examined with the addition of honey bees hives at a field site on The Remarkables mountain range, Otago, New Zealand. Lastly, the potential competitive impact of honey bees on bumblebees was examined experimentally in a glasshouse.
Floral resources, composed of pollen and nectar, are the main food source for adult and larval bees. Different aspects of resource overlap can be compared to estimate the potential competition between bee species. However, quantifying the amount of floral resources available is difficult in heterogeneous landscapes and so to estimate this an index of floral density per plot was created for each plant species and extrapolated by the abundance within the study sites. The flowering status of each species was then determined for each survey period and analysed with respect to the floral choices made by each bee taxon.
In the field, all bee species collectively made up over 80% of floral visitors observed. Overall bees showed floral preferences that did not always correlate with floral availability. Introduced bees (honey bees and bumblebees) showed a preference for introduced plant species, Fabaceae in particular, which native bees did not generally utilise. Native bees showed widespread generalisation on both native and introduced plant species.
Native and introduced bees showed minimal resource overlap and clear preferences for different plant taxa. Where there was greatest floral overlap, resources were not likely to be limiting. Competition was more likely to occur between native Lasioglossum and Leioproctus bees, and between introduced honey bees and bumblebees. The widespread utilisation for introduced plant species with longer flowering times than native plants also suggest that introduced plants may be elevating local native bee populations above historic levels. Native bee populations may have been previously limited by seasonal resource availability rather than by resource abundance.
In an experimental glasshouse, where bumblebees and honey bees foraged together on artificial flowers, honey bees did not have a displacing effect on bumblebees. Honey bees and bumblebees also showed different behavioural responses to varying resource treatments, which may reflect how they respond to changes in resource availability. In response to reductions in floral quantities, honey bees showed behavioural responses consistent with a lack of recruitment of foragers. Bumblebees did not show any differences in foraging behaviour in the presence of honey bees but they did respond quickly to changes in resource quality. In contrast, honey bees were not able to differentiate between floral resource quality.
The reduction in feral honey bee populations due to varroa may have resulted in an increase in resource availability throughout New Zealand. However, as native and introduced bees have different preferences for flowers, floral resources may not have been limiting and so the impact of varroa may have little impact on native bees except where local honey bee and native bee densities were particularly high. Further research should examine the respective floral choices and nest site availability of native bees in other habitats for a more complete understanding of the impact of varroa and competitive potential of introduced bees on New Zealand pollinator communities.