The first comprehensive study on the effect of invasive plants on indigenous wildlife is sobering.
Researchers at Virginia Tech have confirmed that when invasive plants take over an area, they actually alter the ecosystem and deplete the native animals’ natural food sources. It’s a major driver of wildlife extinction, the researchers say. And it’s even worse than they thought.
Invasive plants are on a tear. With increasing global trade and travel, nonnative species are showing up in new ecosystems all the time. They may look like they're rooted in place, but they have ways of traveling as stowaways on ocean going containers picked up at gardens doors or even just when the wind kicks up, scattering their seeds to the four winds,
And that's what's happened on one property in Blacksburg. “This is a heavily invaded place.” says Jacob Barney, associate professor of environmental science at Virginia Tech who led the two-year study. “Just about everything you can see here is not supposed to be here.”
We're walking on a lovely trail in Blacksburg’s Heritage Park on a late summer day. The place is bursting with flora but not a lot of fauna. “It’s an abandoned farm,” Barney explains. “It's been minimally managed but heavily disturbed and invasive species are ‘disturbance adapted’ species, so they love places that are torn up, walked on. Any sort of disturbance, they thrive in.”
Some of the invaders are just gorgeous, not all for sure, but that’s why they were brought here. It turns out they’re pretty poison for the existing ecosystem.
Vasiliy Lakoba is a grad student who worked on the study. He points to a spreading plant with light magenta, thistle-like flowers on the edge of the trail. “This is spotted knapweed.” In addition to all the flowers, it has a “lot of, kind of, small leaves and you can see it branching a lot.”
All that branching means lots of scaffold for spiderwebs, “and it's been found that there's a really interesting effect that you get, which is, more of these web weaving spiders on the spotted knapweed and then in turn, those spiders catch a lot of insects that otherwise would be suppressing the population of the knapweed."
It's like the invasive spotted knapweed is mounting a pre-emptive strike on would-be predators. This is just one way invasives literally remake the landscape over time, creating an ecosystem more favorable to them. Take the Multiflora Rosa from East Asia, prized for its dense growth that acts as a living fence. At first it also seemed perfect for birds to nest, but even though it's berries may look delicious to native birds, they're not nutritious.
“So, it's like the birds getting a candy bar versus getting something more substantial,” says Lakoba, “something with more protein in it, some more fat in it.” And those empty calories hurt birds’ ability to migrate or survive the winter. The effect is a slow but steady culling of native species. A new report by the Cornell Lab of Ornithology has found that North America lost 3 billion birds in the last 50 years.
Jacob Barney points out that only a tiny percentage of nonnative plants become invasives. But the ones that do play an outsized role in reducing native animal populations. “And what we also found is that, over time there's sort of a compounding effect. So, the longer an invasive plant is in the ecosystem, there's fewer and fewer animals in that ecosystem. So, we could look at the place we're standing in now count the number of animals. And in 30 or 40 years, there's likely to be even fewer here.”
What we know now is that ecosystems are incredibly interconnected and that they need to be actively protected. Barney sees this new information as more proof that it’s important to further activate people for conservation efforts. “You know, one of the really tricky things about invasive species is, once they're established, they’re here forever, so really getting at them early is your only real chance of minimizing or eliminating an invasion.”
This first ever comprehensive meta analytic review of the impacts of invasive plants on indigenous animals is published in the journal Gobal Change Biology.