As a biologist who works on pollination, two of the top questions I receive on a regular basis are what to plant to support pollinators, and whether non-natives are bad for pollinators. Although I can state a number of relatively easy-to-grow plants that are both good for supporting pollinators in our area (past blog articles: Want flowers next year? Pollinator plants to plant this fall, What should I plant to help pollinators? and Helping pollinators in small green spaces are some ideas), it is really hard to give a hard answer based on research knowledge to the second question. In today’s article, I decided to try to explain why this is so, and still try to answer that question. Spoiler alert: my answer is basically an “it depends” with some big caveats. Keep reading to see what I mean.
Concepts meet reality
One of the characteristics of scientific knowledge is that it accumulates, and that knowledge we collect can eventually be used to make predictions about what one may expect to observe under new conditions. These predictions are of course not necessarily correct all the time, but their accuracy is expected to increase with the amount of knowledge that we have about the processes we are seeking to predict. As a scientific field, biology has indeed accumulated a lot of knowledge, which allows us to make some pretty logical predictions about the potential effect of non-native plants on pollinators. Let’s go over some of those predictions.
As I mentioned in a previous post, “What are local ecotype plants and why do they matter to pollinators?“, the interactions we observe today between plants and their pollinators are the result of millions of years of (co)evolution, which has modulated the “matching” of needs of the two parts of the interaction. This means that evolution led to species that interact to be generally well-adapted to each other. For example, species that have been interacting for a long time at a given place (as is the case of species that are native to a place) tend to temporally match better with each other; bees that use certain plants tend to be active when those plants flower, and plants that need certain bees tend to flower when those bees are active. This means that it is plausible that if those plants or bees are not available at a certain time point, they will not be able to survive properly because their needed partner is not around. In the context of exotic plants, it is possible that the fact that they are exotic makes them poor pollinator plants, since they are more likely to not be “attuned” to the temporal needs of the local species.
From a different perspective, plants and pollinators have an interaction that is based on the consumption of floral rewards (pollen, nectar, etc.) by the pollinator (and in some cases, such as that of moths and butterflies, the consumption of other plant tissue in their larval stages). Those rewards, however, are not chemically identical across plants, meaning that each plant species, genus or family will tend to have rewards that contain chemical compounds specific to them, and to which the pollinators that consume them are adapted, leading to different levels of specialization. The practical consequence of this is that not all pollinators visit all plants, but rather that certain pollinators will only visit the plants that offer rewards they can physiologically consume and digest. These adaptations to deal with certain chemical compounds evolve over a long time, meaning that it is more likely that species native to the same area will be able to effectively display them, simply because those species have been exposed to those compounds for longer and are thus more likely to be adapted to them.
Given all of this knowledge, it is expected that species that have been evolving with each other over long periods of time – such as species native to the same region – would also be more beneficial to the pollination partners than species that are not native to the same region. And indeed, there are some studies that show that some non-native plant species are not chemically or temporally accessible to some bee species, leading to certain bee species avoiding them altogether because they cannot manage the chemical compounds they carry. That said, those studies are rarely counted, not because people do not want to take them on, but because testing such ideas on many species across the world requires a lot of funding, time, access to appropriate sites, and technical skill… all of which makes the studies extremely hard to run.
So, are non-natives good or bad for pollinators?
Despite the large technical, financial and logistical complexities of testing the points I noted above specifically, some studies have tried to evaluate the effect of non-natives on pollination interactions and the parts involved through the study of spaces that already carry different levels of natives and non-natives. These studies confirm some of the predictions but also bring a more nuanced light to the topic… which points to the need for more studies and my “it depends” type of answer to the question. So, let me tell you what these studies tend to show.
A relevant study on this is one in which gardens that had different levels of ground cover by native and non-native species were studied in terms of their floral visitation. In that study, they measured A LOT of gardens and followed floral visitation of all the plants present throughout a whole season. What they saw was that generally pollinators tended to be more abundant when most natives were flowering, but that later in the season non-natives could provide resources to pollinators that were still around but had no native plants to visit, because those were not flowering anymore (or not yet). Based on that study, one would suggest that planting natives is indeed good (that leads to the most visits by pollinators) but that displaying non-natives that flower later in the season (especially when natives that flower at that time of the season are not available) can help support some pollinators during those times because they compensate for the lack of native floral resources available to pollinators. This said, that study was geographically restricted (a small region of Germany), included only urban gardens, could not manipulate the composition of the plant choice (this was on gardens that were already established based on the homeowners’ choices) and was done through one season only. The authors also recognized that many times the non-natives were of the same family of a native plant, meaning that perhaps the pollinators were already slightly pre-adapted to their chemistries, and that the results would thus be different if no native related plants had been present. Also, they warned about the use of non-natives that could have potential for invasiveness. (Exotic garden plants partly substitute for native plants as resources for pollinators when native plants become seasonally scarce)
Only a few studies were done after this one, but they found slightly different results. For example, a study done on flower plantings in Paris over two seasons found that native plants indeed support more diverse, abundant, and specialized pollination interactions, but that the non-natives used in those sites appeared to support more generalist interactions than the native species. This would suggest that native plants appear to be positioned particularly well to increase pollinator diversity and to support *specialized* pollinators that would otherwise not be supported at all by the non-native plants.
Finally, another study (from the USA) that looked not so much at the plants but at the pollinators, found that non-native non-honeybee bees tended to clearly prefer non-native plants over natives, meaning that those plants would tend to provide support especially to non-native bees, and thus appear to assist in some ways the survival of non-native bees that are in some cases competing for resources and displacing native bee species. (Native and exotic plants play different roles in urban pollination networks across seasons)
This is complicated! What do I do with this information?
Generally, I would interpret all of this as pointing to:
- Native plants should be preferred when possible because they do indeed support larger and more complex sets of pollination interactions.
- If they are non-invasive, one can maybe consider planting non-natives, especially if plants of the same family or genus are already native to the region. However, it is always important to ask oneself why one wants to absolutely do this.
- Non-natives can help support pollinators during the “summer dearth”, when few natives are available… although one can also try to increase the frequency of natives that do flower during those times.
By Anahí Espíndola, Associate Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.
Anahí also writes an Extension Blog in Spanish! Check it out here, extensionesp.umd.edu, and please share and spread the word to your Spanish-speaking friends and colleagues in Maryland. ¡Bienvenidos a Extensión en Español!