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The Role of Non-native Plants in Butterfly Communities

Updated: Feb 5


The Research

On September 13 I watched a presentation with the above title given by Dr. Heather Kharouba of the University of Ottawa, whose area of research is plant–insect interactions. The findings of her research come as a bit of a bad news bombshell to those of us who sing the praises of native plants and bad-mouth non-natives! Non-native plants apparently do indeed feature prominently in the diets of butterflies. (Note that this research was confined to butterflies, it did not include bees, the major pollinators.) 

Kharouba’s team investigated 10 sites each at a Garry Oak savanna ecosystem on Vancouver Island and in the Ottawa area. In both areas the sites were semi-natural, that is, with a mix of native and non-native species. In both, non-native species contributed the major supply of nectar, with 83% of foraging visits at the oak savanna sites being to non-natives (despite the fact that the latter were only 30% of the total flowers) and 63% of all the flowers visited in the Ottawa sites. Interestingly, in the oak savanna ecosystem, visits to non-natives were greater later in the season (in July) when natives had finished flowering. This was attributed to drought, with the natives being adapted to earlier season flowering during times of higher moisture. In the Ottawa sites it was the reverse, with non-native species being less available to supply nectar late in the season. (I suspect our local situation would match Ottawa’s somewhat, as we have abundant native Asteraceae -asters, fleabanes and goldenrods- flowering in late summer.) 

The oak savanna research has now been published in the journal Ecology (see reference) and I quote from their abstract: 

“We found that non-native plants were well integrated into butterfly nectar diets (83% of foraging observations) and that visitation to non-natives increased later in the season when native plants were no longer flowering. We also found that butterflies selected non-native flowers more often than expected based on their availability, suggesting that these plants represent a potentially valuable resource. Our study shows that non-native species have the potential to drive key species interactions in seasonal ecosystems. Management regimes focused on eradicating non-native species may need to reconsider their aims and evaluate resources that non-natives provide.”


Mormon Fritillary on arrow-leaved ragwort, Plateau Mountain, AB, 2021-07-27. Photo: M. Parseyan

The nutrient quality of the nectar (water, sugar and amino-acids) was investigated at the Ottawa sites, and no data has been published yet, but the researchers seem to think that it is equivalent or even better in the non-natives. The research also raises the fascinating question of whether the dearth of natives late in the season at the Vancouver Island sites and early in the season in Ottawa is due to anthropogenic changes in land use, such as agriculture. (This suggests to me that compiling species lists for given areas over long periods to document population changes and conducting phenology (timing of reproductive phases) could provide useful data, and might be a good subject for citizen science.) 

The Kharouba research team also investigated the use of plants for nectaring by Monarch butterflies (using adults artificially reared and released). They found that 58% of the foraging visits were to non-natives, and of the top five most-visited species four were non-natives. (Interestingly, the species were red clover, two thistle species, perennial sow-thistle and Canada goldenrod. I remove red clover, Canada thistle and perennial sow-thistle from semi-natural sites, and I never plant the native Canada goldenrod because it tends to form monocultures. What a complex web we weave, when first we practice to disturb!). Kharouba notes the astonishing rise over the last few centuries of the numbers of invasive species among various groups of organisms, and postulates that butterflies use non-native plants because many of the latter have generalized flower traits that make the nectar accessible and other floral characteristics in common with natives. 

What Does This Mean? 

This research has implications for us native plant practitioners, particularly if we extend that interest to reclamation and restoration. For a start, we should not be making the blanket statement that native plants are necessarily better for pollinators than non-natives on the assumption that pollinators are better adapted to using the long-extant resources. This may be true in some cases, but not all. Rather, our rationale for promoting native plants should be based more on the value of conserving their native genes, their intrinsic worth, as well as their total value in ecosystems, rather than their value to pollinators. 

Another important consideration relates to the removal of non-native species during weed control or restoration. If non-natives are significant contributors to the welfare of an ecosystem, should we be removing them? It’s a question that opponents of herbicides will no doubt be keen to ask. Kharouba raises the alarm (see the last sentence in the quotation from the Ecology paper) but also suggests that the benefit of nectar provision might be considered outweighed when invasive non-natives otherwise damage ecosystems. They recommend more planting with natives to counter native deficiencies, for example, gaps in flower availability or phenology.

Kharouba observes that non-natives are here to stay and will almost certainly increase. Modern ecological communities are in a state of flux, and non-natives appear to be more resilient to climate change than natives.  She would not recommend planting non-natives but advises caution before large-scale removal and provision of native substitutes. This is a problem I’ve certainly pondered myself when I’ve seen the high numbers of pollinators (not just butterflies) visiting Canada thistle or red clover, although I hate with a passion both these species and always attempt to remove them from the two grassland reserves I help manage. I am conscious of the fact that if I exterminate large patches of Canada thistle I should expect to plant native flowers to replace them. 

One member of the webinar audience asked whether it was better for native plant gardens to use red clover as a mulch rather than regular mulch. The answer was yes, this would be good for butterflies. The City of Edmonton is planning to plant red clover, but I cannot summon up any enthusiasm. (I do wish we could develop native legumes for naturalization instead.)

Although this research is obviously complex, I believe that studies of native/non-native plant interactions with pollinators could make excellent citizen science projects for our naturalists. We have people with the time and good photographic skills. Even knowing whic pollinators visit what plants in our area would be worthwhile and a possible contribution to both plant and butterfly conservation. 

The impetus for this kind of research comes in part by the precipitous decline of the Monarch butterfly, now rated a Species of Special Concern under the federal Species at Risk Act.


And Now for More Research….With Bees

Are native and non-native pollinator plants equally valuable for native wild bee communities?

Orange-belted Bumblebee on slender blue beardtongue, Fort Saskatchewan Prairie, 2019-06-14. Photo: P. Cotterill

This is the question asked by researchers Seitz et al. To answer it they established gardens of either 20 native plants or 20 non-natives at three sites in Maryland, USA, and observed visits by 120 species of bees over two seasons (2017 and 2018). 

From their findings the researchers concluded that non-native pollinator-friendly plants were frequented by a diverse bee community. Depending on the season, non-native plants attracted similar or higher numbers of individuals and species compared to native plants. Particularly in the early and late seasons, many bees chose non-native over native plants, while no differences were found in the middle of summer. Visits increased, as may be expected, with increasing floral cover (native plants were scarcer in spring and early summer).  Individual bee species varied in their preference for native vs non-native species and, as anticipated, specialist bees preferred native plants as compared to generalist bees which were more likely to favour non-natives. (This makes sense: old, long-established species have had more evolutionary time to develop specialist relationships.) Other complications are that specialist bees may use natives for pollen collection but non-natives for nectar collection. Also, some generalist bees alter their foraging behaviour to become more specialized.

The authors conclude that their “study suggests than non-native plants can complement native pollinator-friendly plantings because they are visited by a broad spectrum of bees and buffer gaps in grassland native flowering plant times, particularly in early spring. However, non-native plants also alter the composition of plant communities, may not support as many specialist bees, and appear to affect individual and network specialization of bee communities with unknown consequences for plants and bees.” 

They go on to suggest that selective non-native plants may be used in restoration, e.g., in flower strips alongside crop fields, to provide increased bee forage but that care should be taken that these non-natives do not spread into natural or semi-natural communities, and that non-natives are only planted as complementary to natives. 

Editorial comment: This too has consequences for how we do naturalization and restoration locally. Possibly we should be attempting to plant more spring-flowering plants. Since in our climate forbs get going slowly in our frozen ground, some of this may be directed towards our early-flowering shrubs – willows (planted from cuttings, decadent specimens in reserves pollarded to produce new flowering shoots, etc.), Canada buffaloberry, pin cherry and saskatoon. Violets, buttercups, and several woodland species are also early flowerers. Our volunteers could help with growing some of these species! 

Reference

Seitz, N., D. vanEngelsdorp, S.D. Leonhardt. Ecology and Evolution 13 Oct 2020. 10, 23, pp. 12838-12850.

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