Some years ago, it was a shocking realization to learn 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.
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 moved from landscape settings to natural areas on the wings of birds, which eat and redistribute fruits across habitats. Once established, they threaten native plant diversity by invading and crowding out spring ephemeral wildflowers, native shrubs and native tree seedlings.
Many of these invasives 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 birds and other species. No one expected the negative consequences we experience first-hand today.
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 seed far and wide, which is good for native species and part their reproductive strategy. If non-natives are consumed and spread, it compounds the problem with further impacts to native plant communities.
Fruit from invasive species has been shown to have lower nutritional value for birds in some research. Other studies have shown birds prefer berries from native species over those from non-natives, suggesting our native birds may not alter feeding patterns even if non-native berries are widely available.
We know climate change is altering the timing of bird migration, pushing it later in the season as our climate warms. As would be expected, plant phenology has also shifted later in the season, with research showing later fruit ripening. As all of these factors combine to change the natural sequences at play, it’s important to understand the direct impact on bird populations.
The proliferations of invasive shrubs in natural ecosystems raises a question on 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.
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 both bird migration and fruit ripening may have the potential to 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 were consuming 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 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. Intestinally, two trees, black gum (Nyssa sylvatica) and wild black cherry (Prunus serotina), were the most consumed fruits in the study, although the number of fruits available during migration was less than fruits from invasives. Across the fall, 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 burning bush is readily available at garden centers across the Midwest. As gardeners, we need to spread the word about the dangers this seemly 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.