But a scientific research paper published in the Journal of Applied Ecology last year (Salisbury et al. 2015) is challenging the assumption that natives are always the way to go.
These scientists made sure this hypothesis was supported through a garden experiment. They planted 3m x 3m garden plots with either native (to the UK, where the study was conducted), near-native (close, but not native to the British Isles themselves), or exotic (Southern Hemisphere) plants that are commonly planted in gardens. Then for 4 years, they kept track of how many flowers and pollinators there were every 4 weeks. Each garden type was replicated 18 times at each of two sites. After 4 years of study, they had counted 7,979 pollinators in their garden plots!
They found that native and near-native garden plots had a greater abundance of pollinators than the exotic plots (yay—native gardening for pollinators validated!) However, the number of visits of pollinators to flowers directly corresponded to how many flowers there were at a given time. Because the exotic garden included species that bloomed later in the summer compared to the natives, at those certain times of year, the pollinators highly preferred the exotic species.
In short, the pollinators preferred the natives, except when the exotics were the only thing blooming. They’ll take what they can get.
Of course, there are additional questions that need to be tested. When pollinators get more to eat later in the year than usual, does this really help them? Do their populations increase? Are they healthier? Which pollinator-friendly exotic plant species are safe to plant in gardens, and which are (or will become) invasive? This latter question is the focus of a huge amount of research in ecology today. Much progress has been made, but there is still much to learn about the causes and consequences of invasive species.
If you’re going to take these scientists up on their idea to add late-blooming exotics to your pollinator garden, please do your research. Some species are known to be highly favored by pollinators. For instance, in this experiment, the authors found that solitary bees absolutely loved Eryngium agavifolium (33% of all visits by that particular type of bee over the 4 years were to this particular exotic species)! Also make sure the exotic plant you choose isn’t good at escaping from gardens, and most importantly, make sure it’s not already a known invasive species (checking to see if it's listed as "noxious" in any states is a good place to start.) A plethora of internet resources will tell you what’s safest and what’s not for your native ecosystem.
Citation: Salisbury, A., J. Armitage, H. Bostock, J. Perry, M. Tatchell, and K. Thompson. 2015. Enhancing gardens as habitats for flower-visiting aerial insects (pollinators): Should we plant native or exotic species? Journal of Applied Ecology 52:1156–1164.
About the authors:
Andrew Salisbury, James Armitage, and Helen Bostock are from the Royal Horticultural Society, UK
Joe Perry is from Oaklands Barn, UK
Mark Tatchell is from Laurels Farm, UK
Ken Thompson is from the University of Sheffield