The research was conducted by doctoral student Stephanie Rivest (now graduated) from Dr. Heather Kharouba’s lab at the University of Ottawa, in collaboration with Dr. Elizabeth M. Wolkovich, an Associate Professor of forest & conservation sciences at the University of British Columbia. The study, conducted in the endangered Garry oak savanna ecosystem on Vancouver Island in British Columbia, shows that in the last few weeks of the summer, most flowers available are non-native and butterflies adjust their nectar foraging habits by visiting more non-native plants during this time.
“We observed 1143 nectar foraging visits by 14 native butterfly species across the entire season,” explained Dr. Stephanie Rivest an ecologist and conservation biologist. “83% of these foraging visits were to non-native plants, which was astounding. We also found that 64% of the butterfly species we observed visited non-native flowers most often”.
In the savannas where the research took place, the seasonal timing of flowering had a big influence on how butterflies foraged for nectar. “Later in the summer when conditions were drier, native flowers were really sparse,” described Dr. Heather Kharouba, Associate Professor of Biology in the Faculty of Science at the University of Ottawa. “So non-native plants, by continuing to flower later in the summer, could be extending the availability of nectar for butterflies.”
“83% of these foraging visits were to non-native plants, which was astounding. We also found that 64% of the butterfly species we observed visited non-native flowers most often”
Dr. Stephanie Rivest
— Ecologist and conservation biologist
A preference for non-native flowers?
The researchers also found that native butterflies might even prefer non-native flowers.
“When we considered times in the season when both native and non-native flowers were available, we found that butterflies chose to visit non-native flowers more often than expected based on non-native flower availability,” said Dr. Rivest. “What this means is that even when non-native flowers were less available, butterflies still chose to visit them.”
Just because non-native flowers might be the preferred nectar source for native butterflies does not mean that all non-native plants should be left to invade native ecosystems. The researchers caution against such a conclusion and instead advocate for a more balanced approach to invasive species management. “We should definitely remain wary of non-native plants, but our research shows that some non-native species can play positive roles in native ecosystems too,” explained Dr. Rivest.
“So non-native plants, by continuing to flower later in the summer, could be extending the availability of nectar for butterflies”
Dr. Heather Kharouba
— Associate Professor of Biology in the Faculty of Science at the University of Ottawa
Challenging the paradigm on non-native species
The research took place from May to August in 2019 in the Garry oak savanna ecosystem on Vancouver Island. These savannas, composed of a mix of Garry oak trees, rocky outcroppings, and grasslands were chosen because they are highly biodiverse and are home to over 100 species at-risk, including seven at-risk native butterflies. These savannas were also chosen because they are heavily invaded by non-native plants. Today, only remnant patches of Garry oak savannas remain after a 90% decline in total area since 1840.
“Invasion biology research has mostly focused on the negative impacts of non-native species. That’s why we wanted to investigate a potentially positive role of non-native plants as sources of food for native species. Also, few studies have evaluated the degree to which non-native nectar is being used by entire butterfly communities or has considered the role of seasonal changes in flower availability in governing this key interaction between butterflies and plants,” adds Dr. Rivest.
Many people still consider non-native plants to be detrimental for native ecosystems. But the reality is that their role in ecological communities is complex. Some non-native plants have the potential to positively interact with native species. For example, by providing food, creating habitat, or playing a role in ecosystem restoration.
“We believe it is worthwhile to evaluate the overall role of these species because it provides us with a more complete picture of the range of possible effects of non-native species on native species,” concluded Dr. Rivest.
The study Flowering phenology influences butterfly nectar foraging on non-native plants in an oak savanna was published in the journal Ecology on February 17th 2023.