Some years ago, it was shocking to learn that one of my favorite ornamental shrubs, burning bush (Euonymus alatus), is invasive. It took a while for this news to sink in, and after seeing it invade woodlands across southern Illinois, I finally came to terms with the bad news.
This time of year, I always think of burning bush because of its spectacular fall color. As it turns the burning, bright red shade we all know it for, it sticks out like a sore thumb in woodlands. This damaging invasive threatens native plant diversity by invading and crowding out spring ephemeral wildflowers, native shrubs, and native tree seedlings. Not only does it threaten understory diversity, but it also alters our future forest overstory by blocking out new seedlings, which must be established to bring about the future canopy.
Burning bush is part of a sad story that includes many other non-native, soft-fruited shrubs such as barberry (Berberis spp.), privet (Ligustrum spp.), autumn olive (Elaeagnus umbellate) and bush honeysuckle (Lonicera spp.). These plants have rapidly moved from landscape settings to natural areas on the wings of birds, which eat and redistribute fruits across habitats.
Many of these plants were introduced as ornamental landscape plants for their natural beauty, such as burning bush’s spectacular fall color. Others, like bush honeysuckle and autumn olive, were introduced as a wildlife food source to provide berries for foraging bird species and other species. No one expected the negative consequences we experience first-hand today.
What about birds?
Fall migrating birds especially use berries as a food source during their long journey. As they consume these vital calories, it presents the potential to spread seeds far and wide, which is good for native species and part of their reproductive strategy. If non-natives are consumed and spread, the problem is further impacting native plant communities.
Fruit from invasive species has been shown to have lower nutritional value for birds in some research. Other studies have shown that birds prefer berries from native species over those from non-natives, suggesting that our native birds may not alter feeding patterns even if non-native berries are widely available.
We know that climate change is already altering the timing of bird migration, pushing it later in the season as our climate warms. As expected, plant phenology has also shifted later in the season, with research showing later fruit ripening. As these factors combine to change the natural sequences at play, it’s important to understand the direct impact on bird populations.
The proliferation of invasive shrubs in natural ecosystems raises a question about the extent to which these non-natives support migrating birds. Do they provide a significant food source for birds? In the face of climate change, can invasives supply our native birds with a food source that might replace the crowed-out native species? With invasives predicted to increase as climate change intensifies, the pressure on native plant communities will increase as well.
What does research say?
A 2020 research effort conducted in New England attempted to look for answers to these and other questions about the interactions between birds and invasive shrub species. Researchers speculated that the changing timing of bird migration and fruit ripening may potentially enhance invasive shrub species dispersal if birds feed on the more abundant non-native fruits. They measured fruit availability and bird feeding preferences to see if birds consumed more non-natives.
Invasive shrubs in New England have been shown to mature an average of 26 days later than native shrubs and persist longer into winter than native fruit crops. However, the 2020 study confirmed that birds prefer native species over invasives, and despite measuring higher amounts of invasive shrub berries present, birds were still focused on native fruits.
The most abundant fruits measured were from invasives, including multiflora rose (Rosa multiflora), autumn olive, privet, burning bush, and barberry. Interestingly, the most consumed fruits in the study were two trees, black gum (Nyssa sylvatica) and wild black cherry (Prunus serotina). However, the number of fruits available during migration was much less than fruits from invasives. Across the fall season, as invasives became more widely available later in the year, birds continued to eat more native species.
Given the bird’s preference for natives, even when present in much lower numbers, this research suggests that the fruits of invasives will not sufficiently replace native fruits as a food source for migrating birds. This result drives home the point that we must limit the spread of these non-native invasives to ensure our native plant communities can thrive and continue to provide this vital food resource.
I’m shocked to see that burning bush is readily available at garden centers across the Midwest. As gardeners, we need to help spread the word about the dangers this seemingly harmful ornamental can pose. If you already have a burning bush, consider removing it. There are lots of great native alternatives, and the birds will thank you as they stop by for a snack on their fall journey.
Main photo caption: The red berries of burning bush are eaten by birds and spread from the landscape to natural areas where this non-native, invasive shrub can take over and crowd out native plants. Photo by Ryan Pankau, Illinois Extension.
References
Gallinat et al. 2018. Herbarium specimens show patterns of fruiting phenology in native and invasive plant species across New England. American Journal of Botany 105, 31-41.
Greenberg, CH, and ST Walter. 2010. Fleshy fruit removal and nutritional composition of winter-fruiting plants: a comparison of non-native invasive and native species. Natural Areas Journal 30, 312-321.
Ingold, JL and MJ Craycraft. 1983. Avian frugivory on honeysuckle in southwestern Ohio in fall. Ohio Journal of Science 83, 256-258.
LaFleur et al. 2007. Invasive fruits, novel foods, and choice: an investigation of European starling and American robin frugivory. Wilson Journal of Ornithology 119, 429-438.
Menzel et al. 2006. European phenological response to climate change matches the warming pattern. Global Change Biology 12, 1969-1976.
Smith et al. 2013. The value of native and invasive fruit-bearing shrubs for migrating songbirds. Northeastern Naturalist 20, 171-184.
Stiles, EW. 1980. Patterns of fruit presentation and seed dispersal in bird-disseminated woody plants in the eastern deciduous forest. American Naturalist 116, 670-688.
Whelan et al. 1991. Spatial and temporal patterns of post dispersal seed predation. Canada Journal of Botany 69, 428-436.
White, DW and EW Stiles. 1992. Bird dispersal of fruits of species introduced into eastern North America. Canada Journal of Botany 70, 1689-1696.