CURWOOD: Well, as Jim Barilla suggests, some plants that might not be native to an area may still be useful additions. But others manage to become total pests, think of kudzu and Japanese Knotweed. Some invaders are less problematic, like bittersweet nightshade, a vine with pretty purple flowers and inedible red berries. Now scientists are trying to work out exactly how and why invaders like this find success, as Ari Daniel reports from Cleveland Ohio. DANIEL: Have you ever wondered what makes an invasive species set up shop somewhere it doesnt belong? In other words... BURNS: Why are some species able to invade, and others are not? DANIEL: Jean Burns is an ecologist at Case Western Reserve University, and her research is offering a clue to that riddle, and also helping to determine which places are most vulnerable to species invasions. [OUTDOORS, RAIN SOUNDS] On a cold, rainy morning Burns offered to tell me more on a farm thats become her outdoor la-boratory outside of Cleveland, Ohio. She spends a lot of time here, examining what seems like a simple question. BURNS: Where do organisms live, and why this place and not that place? DANIEL: And although the question is simple, the answer, and the implications of that answer, are anything but. Lets start with where her organisms of choice plants and where they put down their roots the dirt. She kneels down, and rubs a moist handful between her fingers. BURNS: So what we see, as humans its brown, its muddy it looks to us like a nice tilled field thats pretty uniform. But from the perspective of, say, a seedling thats germinating in a little patch of that soil, what that seedling is experiencing is the bacteria and the fungi that are right next to it in the soil. DANIEL: Those bacteria and fungi arent spread out evenly. Drop one seed at your toes, and toss another one a few feet away, and the seeds may find themselves in vastly different worlds of dirt. To a seed, that can mean the difference between germination and decomposition. Thats because the bacteria and fungi interact with the plants, sometimes helping, sometimes hurting. And Burns wants to know how these tiny dynamics in the dirt add up to determine why invasive plants grow where they grow when they do. BURNS: Invasive species are species that have come from somewhere else, and they have be-come often really problematic in the places where they invade. DANIEL: Burns has a pretty clever experiment underway to figure out what makes certain patches of soil vulnerable to plant invasion or not. To see the set up, we head indoors, inside her lab on the other side of the farm. [SOUND OF TOOTHPICKS DUMPED ON BENCH] Ecologist Angela Brandt dumps out a box of plastic party toothpicks onto a table. Shes gluing a tiny seed to each one. Brandt: Yeah, its a good downtime rainy day activity. DANIEL: So what kind of seed is that? Brandt: This is Solanum dulcamara, commonly known as bittersweet nightshade. Its an orna-mental vine it has very beautiful purple flowers and red berries. And its not native to North America. DANIEL: In fact, none of the seeds in this study are from here. Theyre all invasive and by the end of the experiment, this team of ecologists will have glued over 100,000 seeds from these plants to individual party toothpicks. Thats step one. Step two happens back outside on the farm. [OUTDOORS, RAIN, BIRDS] Nestled in the dirt are a series of large plastic pots, each one containing a spray of those plastic toothpicks sticking into the ground. Burns crouches down beside one of the pots with a healthy horsenettle plant. BURNS: Ah, this is an excellent example where you see this plant is right next to that toothpick because that seed was glued on there. DANIEL: The horsenettle was planted first. Then Burns added seeds of a different species. In this pot, that second species the bittersweet nightshade is just emerging from the soil. So the second plant is invading the first one. And if the flip is true too if the first can invade the se-cond, then its a sign of coexistence. And this ties in to the whole notion of invasive species because when they first appear on the scene, there are, by definition, very few of them. Theyre arriving in a place where the odds are against them. But a successful invasive... BURNS: ...its able to germinate and to grow and to persist. Thats basically what invaders have to do when theyre introduced to a native community thats already established, and they have to be able to overcome that competitive disadvantage. DANIEL: Burns research may help us get smarter about how to prevent, or at least slow down, species invasions. Down the line, shes hoping to ID the specific bacteria species that promote certain plants to establish themselves in the soil. But thats not all. Burns says her work can also inform how we restore places transformed by people. Take a nearby patch of land thats just been turned over to the Cleveland Metroparks. BURNS: Its a golf course. Its been a golf course for a hundred years. And what they would like to do is restore it into a native maple hardwood forest. And one of their questions, then, is how do they need to treat that soil in order to make their restoration plan really work? DANIEL: Its the kind of riddle that Burns research might just solve. By exploring how a bunch of invasive plants are duking it out each with their own microscopic army in the dirt shes coming to understand how to put back the plants that used to stand guard here in the first place. DANIEL: For Living on Earth, Im Ari Daniel. CURWOOD: Our story on invasive bittersweet is part of the series, One Species at a Time, pro-duced by Atlantic Public Media, with help from the Encyclopedia of Life.