The western honey bee, Apis mellifera, lives worldwide in approximately 94 million managed hives but also wild throughout much of its native and introduced range. Despite the global importance of A. mellifera as a crop pollinator, wild colonies have received comparatively little attention in the scientific literature and basic information regarding their density and abundance is scattered. Here we review 29 studies that quantified wild colony density directly and analyse a larger dataset including an additional 7 studies that quantified density indirectly using genetic markers. Densities varied from 0.1 to 24.2/km2 at 38 locations worldwide and were 24 times lower in Europe (0.35/km2) than Africa (8.4/km2) on average. Survey area varied from 1.2 to 924km2 and was negatively correlated with density. Survey areas were largest in Europe (average of 70.4km2) and were partly responsible for the low densities reported in this region. After controlling for survey area in a GLM, mean annual temperature and net primary productivity became important predictors of density. This model was used to estimate wild colony numbers at a regional scale, which varied from approximately 135 million in Latin America to 8 million in Europe and 250 million worldwide. Overall, wild colonies were estimated to outnumber managed hives in all regions except Europe and were estimated to be over twice as numerous worldwide. This is a significant result given that A. mellifera is often viewed as a domesticated species that primarily lives under human management.
Pollination is an important ecological process. However, the needs of plants and pollinators are not always met. Pollen limitation commonly reduces seed set and bees often experience nectar dearth. Using a theoretical cost-benefit optimization model we show that natural selection acting at the level of individual plants and pollinators will result in positive feedback that exacerbates pre-existing imbalances between nectar supply and demand. When pollinators are scarce plants will be selected to produce more nectar to outcompete other plants in attracting pollinators, and when pollinators are abundant plants will be selected to produce less nectar. We encourage the testing of this novel hypothesis and propose several ways of doing this via comparative study and experimental manipulation. We also suggest that evidence for seasonal variation in foraging conditions provides preliminary empirical support. If our hypothesis is correct it means that pollination faces a particular challenge in balancing nectar supply with demand.