The complex relationship between native and exotic plants may present a simple lesson for us.
A recent study by Winthrop University assistant biology professor Matthew Heard concluded that differences between native species and invasive species, such as kudzu, Japanese honeysuckle and Chinese privet, aren’t always a negative. In fact, the differences can help them coexist.
“The neatest part about the study (published in the journal Ecology Letters) is ... there’s a group of plants that behave one way and a group that behaves another way,” Heard said. “This difference in how they act – whether they’re susceptible to insects, or whether they grow a lot, whether they’re good at being competitors – this balance is played out because they’re different from each other.
“Just being different is what allows you to coexist in some ways. It’s sort of like you have room to act one way, and another species has room to act another way.”
Heard wrote his Ph.D. dissertation at Brown University. He and his Ph.D. adviser, Dov Sax, studied about 150 different species of native and exotic plants along the coasts of Rhode Island and Massachusetts, gathering data from beach or stand-line communities.
In 2008, they found that the number of native and exotic plants counted by a Brown student 10 years earlier remained essentially unchanged. This suggested evidence of the plants’ coexistence and the probability that their coexistence had gone on for hundreds of years.
They discovered that groups of exotic plants were competitively superior to natives, which may not surprise gardeners who struggle to keep invading plants under control. But contrary to popular assumption, that doesn’t necessarily mean that exotics always push out the natives. Herbivores, mainly insects, seem to eat more exotics – which keeps them under control and lets the native plants grow.
“We don’t know how long this balance will last,” Heard said, “but we know that it’s lasted for a while in these communities because they don’t have records of things dying and going extinct.”
Sax added that in addition to being able to coexist with natives, the exotics could even help them.
“Research published in the past two years is beginning to explore the very real and positive impacts that exotics have in some cases for native species,” he said. “In some cases, rare or endangered natives actually thrive when they live together with certain exotic species.”
Data for Carolinas lacking
Heard said his findings can be applicable in similar communities throughout the United States. “Beach plant communities are everywhere. We have them all along the East Coast, all the way from Maine through the Carolinas and down to Florida. ...
“The reason we might expect there would be native and exotic plants coexisting along the coast is, they’re some of the most likely places to have exotic species. They’ve got seeds and things that can come from two directions. They can come from the water itself, from being dispersed through that, and they can come from land. They’re very likely to have more species present in them than other places.”
Still, he said, there hasn’t been enough documentation to conclusively determine whether native and exotic plants can coexist in the Carolinas.
“That’s a good question,” he said. “The biggest problem with answering that is, we don’t have a lot of long-term records in places. We know that exotic plants and even invasive plants are here, but we don’t necessarily know what’s happening – what drops in, what drops out, what stays there.
“My opinion is that there are likely places where native and exotic plants do coexist, and that there are places where they don’t.”
Though the terms “exotic plants” and “invasive plants” are often used interchangeably, Heard said they aren’t always the same thing: “This is what confuses a lot of people. ‘Exotic’ tends to just mean that it’s from somewhere else and potentially that it’s become established here and can actually survive on its own.”
He said exotics often have a negative connotation because “there have been a lot of species that have caused serious problems, like kudzu and English ivy. Additionally, you’ve got plants in North Carolina wetlands like alligator weed that are taking over, and new mosquitoes that can establish and transmit diseases such as West Nile virus. And together all of these examples make people think that if it’s not from here, it must be terrible.”
Invasive plants are “pretty much everywhere now,” Heard said. “Plants are constantly being moved all over the world. There are even exotic plants that have become established in Antarctica that have been accidentally brought in by scientists and people traveling there.
“The idea that’s widespread is that places that are heavily disturbed, such as roadsides or where soil is constantly dug up – these are places where invasive species might come in. But there’s not a single habitat or place. They can really be anywhere.”
Be proactive in gardens
In places where exotics and natives don’t peacefully coexist – in your garden, for example – Heard says taking precautions is the best way to prevent invasive plants from taking over.
“Try to figure out before you get started what kinds of plants are likely to take over,” he said. “Things like English ivy and other plants that are common garden plants, people often don’t know can actually very easily take over and push out other species. ... Try to figure out what types of species show certain aggressive characteristics.”
Heard said that a lot of people don’t know where to find information on this. “The North Carolina Native Plant Society ( www.ncwildflower.org) has a link at the top of the page that has a list of invasive, exotic species. But there’s not a lot of great advice out there.”
A prime challenge is killing invasive plants without killing the natives.
“It’s not an easy thing,” he said. “Almost for each individual species you’re trying to deal with, you have a different response for each one.”
Insect species that thrive on invasive plants are too numerous to list, Heard said.
“We’ve spent a lot of time and money in the United States introducing insects to try and control exotic species, and it’s been a problem to a certain degree. They don’t always eat the plants we want them to control, and they often find other things to eat instead.”
Still, his perspective is less about controlling invasive plants and more about the balance in ecosystems and nature.
“It’s all about how things coexist in the world,” he said. “It’s not just about native and exotic plants.”