European (and Africanized) Honeybees (Apis mellifera)
Sources: Buchmann, Stephen & Gary Nabhan, 1996, The Forgotten Pollinators, Washington D.C.: Island Press (especially Chapter 10: "New Bee on the Block", pp. 169-83)
Some 385 years ago, English settlers at Jamestown, Virginia, brought European Honeybees into America. Since then, these bees have spread everywhere in North America where habitats are survivable, and they now number in the billions. Their historic impact on our native plants and bees remains to some extent incalculable because no environment free of European honeybees has ever been subjected to an experimental test case comparing before and after their introduction, but a number of scientists suggest that Apis mellifera can often outcompete native species such as bees, ants and wasps. The basis for this superior adaptability lies in its "superior ability to detect, to direct its peers toward rewards (via dance language), and to rapidly harvest floral resources." (p. 171)
In the 1970s and 80s, William Schaffer and others examined the pollination of our native shindagger Agave schottii. Our shindagger is pollinated by a variety of native bees, but the researchers found that, where they introduced honeybees to a shindagger field that had previously had only a few feral honeybees among a variety of natives, the honeybees soon predominated over the natives at those sites which had the most nectar per plant. While not overtly aggressive, they rapidly swamped out the other bees by their sheer numbers. The implication is that Honeybees have "immediate and far-reaching consequences on the guilds of nectar and pollen-feeding animals present -- and on the reproductive and fruiting success of native plants such as the shindagger Agave....(p.171-2)
While measuring the effect of European Honeybees on both native pollinators and the plants pollinated is a difficult task, and still at its early stages, enough evidence exists to convince many investigators that biodiversity is being significantly threatened by this invasive species.