Birds Loving Honeysuckle Invaders

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The reputation of one invasive species is getting a public image overhaul this Valentine’s Day. From backyard gardens to city rooftops, now is the time to embrace your non-native honeysuckle.

Scientists at Penn State University found evidence that bird populations are both more numerous and more diverse if honeysuckle invaders have sweetened the air. The findings champion the need to embrace this invasive species rather than weed it out of sight.

“Among conservation biologists, ecologists and managers, the default approach is to try to eliminate and root out non-native, invasive shrubs — anything that seems to change an ecosystem,” said biologist Tomás Carlo of Penn State University and co-author of the study, in a press release.

“The problem is that most native communities already have been changed beyond recognition by humans, and many native species are now rare,” Carlo said.

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Because of the long history some invasive species have in their new homes, the ecosystems may have adapted to the presence of the newcomers. Some species may have even become dependent on the invasive plants.

Carlo and co-author Jason Gleditsch, a biology graduate student at Penn State, wanted to know: “Are we sometimes doing more harm than good when we eradicate plants that, despite being introduced recently, have formed positive relationships with native animals?”

Invasive honeysuckle is very common in a region of Pennsylvania known as Happy Valley. Carlo and Gleditsch studied the number and diversity of birds in Happy Valley and compared those numbers to areas with lower amounts of honeysuckle.

The researchers found that the amount of honeysuckle present predicted the number and diversity of birds.

“The abundance of fruit-eating birds in the Happy Valley region is linked to the abundance of honeysuckle,” said Carlo. “Honeysuckle comprises more than half of all the fruits available in the landscape, and it benefits birds by providing them with a source of food in the fall.”

The birds and the honeysuckle had a, “you scratch my back, I’ll scratch yours” deal. The birds got more food, while the honeysuckle’s seeds were distributed by the birds. In nature, that’s called mutualism.

Pennsylvania now has three to four times more birds, such as robins and catbirds, than it did 30 years ago, especially in areas with a strong human presence, Carlo said. Some of this increase may well be explained by the introduction of fruit-producing non-native plants, like honeysuckle.

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“Invasive species could fill niches in degraded ecosystems and help restore native biodiversity in an inexpensive and self-organized way that requires little or no human intervention,” Carlo said.

The benefits weren’t just for the birds. The fruit-bearing American nightshade showed signs of benefiting from honeysuckle neighbors, too.

To measure the effects of growing near honeysuckle, the researchers first grew American nightshade in pots in a greenhouse. They let the nightshade’s fruit’s ripen, then placed then in areas with honeysuckle and in other areas with no honeysuckle but with other fruiting plants, both native and introduced.

The nightshade growing near the honeysuckle had 30 percent more of its fruit eaten and seeds dispersed.

“The newly introduced plants piggybacked on the success of the honeysuckle, which is a common phenomenon because fruit-eating birds usually feed on a variety of fruit, whatever happens to be available to them,” Carlo said.

“The same birds that ate the honeysuckle also ate the American nightshade, dispersing the seeds of both plants. It’s a win-win-win for all three: the birds, the honeysuckle, and the nightshades,” Carlo said.

After observing the benefits that non-native species can have on an ecosystem, eradicating them without considering the consequences could be a serious mistake.

“Nature is in a constant state of flux, always shifting and readjusting as new relationships form between species, and not all of these relationships are bad just because they are novel or created by humans,” Carlo said.

“We need to be more careful about shooting first and asking questions later, assuming that introduced species are inherently harmful. We should be asking: Are we responding to real threats to nature or to our cultural perception and scientific bias?” Carlo said.

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The research will be published in the journal Diversity and Distributions.

IMAGE 1: Anna’s Hummingbird male (Calypte anna) feeding from Mexican Honeysuckle flowers (Justicia spicigera), Arizona, USA. Credit: Charles Melton/Visuals Unlimited/Corbis

IMAGE 2: Waxwing eating honeysuckle fruit (Tomás Carlo, Penn State)

IMAGE 3: Waxwing eating honeysuckle fruit (Tomás Carlo, Penn State)

IMAGE 4: Phytolacca americana, also called pokeweed (H. Zell, Wikimedia Commons)