Buddleia and rhododendrons offer beautiful vibrant blossoms, but these non-native plant species may have a dark side - causing an adverse effect on pollinators, writes Dick Ahlstrom
Gardeners love the bright displays delivered by summer flowering impatiens, or busy lizzy, and the sweet-smelling buddleia or butterfly bush. The beautiful blooms hide the fact that these non-native plant species may pose a threat to plant and insect life here.
The alien plant names read like a who's who list of the most popular garden varieties including fuchsia and rhododendron. Yet these plants are very invasive and have spread so rapidly that they may be having an unforeseen impact on important native pollinators including the bumblebee and both social and non-social bee species.
Caroline Nienhuis and Anke Dietzsch are trying to get a handle on the impact of alien invasive plant species. They are working on doctoral degrees at Trinity College Dublin and will be displaying their work at the Postgraduate Ecology Forum at Trinity on March 11-12th. Funding for their research comes via the Environmental Protection Agency and from Science Foundation Ireland.
"We are looking at alien plants in Ireland that have spread rapidly, looking at the impact they may have on native pollinators," explains Nienhuis. "Pollinators are in decline over the past few years because of changes in agricultural practices and lots of pollinator habitats have been transformed."
She is looking for evidence that the rapid growth of non-native plant species is contributing to pollinator losses. "The problem is that non-native plants have spread quite rapidly. When they flower they produce lots of nectar and pollen compared to native plants," she says. This is fine while the nectar runs, but there is a dramatic slump in food supplies when flowering ends. The foraging bees and bumblebees must then turn back to less productive native species.
The non-natives that have taken hold greatly enjoy the Irish climate and this has helped their spread. Nienhuis is looking at Impatiens glandulisera, an annual species originally from the Himalayas that is an effective self-seeder. It has now spread across the country. They may be beautiful plants, but there can be hidden dangers for pollinators that have evolved over the centuries based on access to a different set of plants.
Do the plants have the right mix of nutrients to keep the pollinators healthy, she asks. "We have collected pollen from impatiens, and nectar and we will look at the proteins in the pollen." This will be compared with pollen and nectar from native species to see if the aliens offer the right nutritional mix.
Dietzsch is focusing on the impact of Rhododendron ponticum on the native pollinators. Rhododendrons put on a beautiful display with their abundant blooms, but they too exert hidden effects on the bees, she says.
The plants grow for 10 or 12 years before they are mature enough to flower. "They give a massive flower display, but can produce a million seeds per bush," says Dietzsch. This helps them to spread like weeds if the habitat is right, as it is in Killarney National Park. For years the park has hired students to help pull up the growth of rhododendrons which are choking out what remains of the ancient oak forests there.
"If you have an area where you have rhododendrons then you have a problem. You get an imbalance, they get just too abundant," Dietzsch says. "Another down side is it produces toxic substances and releases these into the ground," she adds. This stunts growth in nearby plants.
These substances are also found in the pollen and nectar. While bumblebee species don't seem to be affected, it is less clear whether the toxins affect honey bees or solitary bee species.
Her study includes bumblebees that forage rhododendrons. The team is also sampling DNA from the bumblebees to ascertain the number of colonies in an area. "Bumblebees are social and all the bumblebees in a nest are sisters and very closely related," explains Dietzsch. DNA studies show the unrelatedness that indicates a different colony.
The data is preliminary, but there are already indications that the presence of non-natives can exert an influence, she adds. "In the areas where there are invasive species there are more bumblebee colonies."
Unfortunately the picture for solitary bees is less clear, and "most of the bee species in Ireland are solitary bees", she says.