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Invasive species are bad, right? At least, that’s what you always hear.

Like, take the brown tree snakes that accidentally showed up on the island of Guam around 1950. With no natural predators, these snakes reproduced like crazy and ate everything they could get their imaginary hands on. After a few years, they had wiped out the majority of Guam’s native forest birds.

So, not good. These kinds of non-native species are generally referred to as “invasive”, and they’re usually considered some evil that needs to be eradicated from their new homes. But, not all non-native species are totally terrible!

Here are six of them can actually do more good than harm. One of the most surprising examples of a helpful invasive species is the horse. Many people don’t realize that horses are non-native to the United States, because they’ve been living in the U.

S. for so long. But they haven’t always been here. Although the genus Equus did evolve in North America, all of the horses’s ancestors were extinct from the continent around 10,000 years ago.

And we still don’t know exactly what happened. Horses only returned to the area in the 15th century, thanks to the Spanish colonizers. But even then, their comeback wasn’t all sunshine and roses.

Many of these horses escaped and became feral, animals that were once domesticated and have since run wild. And today, many groups consider them an invasive species. These horses compete for the land and resources used by other animals, and they have a tendency to smash vegetation and damage plants by overgrazing.

Still, when these feral horses are removed from the wild and trained, they become some of our most helpful partners. For centuries, domestic horses have been invaluable in moving people and supplies around the U. S., and parts of the country likely wouldn’t have been settled as quickly without them.

Even today, hundreds of feral horses are taken in and trained each year, and even though fewer people are using them to explore the country, they’re still doing important work on farms and ranches. And as a bonus, they’re not out there wrecking the plant life. Of course, lots of them are still causing trouble in the wild, but at least some of them have found more productive roles.

Of course, no matter how many plants they eat, people still love horses. The same can’t be said for another non-native species, called the Tamarisk shrub. In the nineteenth century, various species of this shrub were introduced to the southwestern U.

S. from Eurasia and Africa. And initially, most people were pretty cool with it. The shrubs prevented soil erosion, and they served as both ornaments and as sources of shade.

And since they were drought resistant, they certainly didn’t mind living in their new home. But then, their reputation kind of tanked. When the area’s water supplies began to run low in the 1930s, the shrubs were accused of being water thieves.

They were even called ‘alien invaders’ during World War II. People kicked off decades of eradication efforts to try and get rid of these plants, but as it turns out, they might have just gotten a bad rep. Several studies have since concluded that the tamarisk’s water use isn’t significantly different from that of native tree species.

And the shrubs are also doing some good in the world. Specifically, they’re beneficial to a type of native bird called the southwestern willow flycatcher. These birds live in the vegetation alongside rivers and streams, things like cottonwood and willow trees.

But they’ve become endangered as the water has been diverted for other uses, and the vegetation has disappeared. Thankfully, the tamarisk shrub unintentionally came to the rescue. According to recent research, up to 75% of southwestern willow flycatchers have found new homes in tamarisk shrubs, at least in some spots.

The studies also suggest that babies raised in tamarisks were just as successful in life as those from nests built in native trees. So it seems like the tamarisk isn’t such a villain after all. If you’ve spent any time exploring the outdoors, you might have come across a bunch of stringy flowers called honeysuckle.

There are a few different species, and they’re native plants in many parts of the world, but not central Pennsylvania. Honeysuckle probably wasn’t introduced there until the 1800s, but it’s now thriving in a region pleasantly called Happy Valley. In fact, its fruit makes up more than half of all the fruit found in the area.

Although this non-native plant does compete for resources with the locals, it doesn’t seem to be causing too much trouble. In fact, it’s really helping out the birds! A few years ago, researchers started some very dedicated note-taking, comparing bird and plant data from urban, agricultural, and forested areas.

And they found that the amount of honeysuckle predicted both the numbers and diversity of birds within their studied region. In other words, the more honeysuckle, the more birds and more types of them! The team actually determined that the honeysuckle and bird communities had formed a mutualistic relationship, where the birds would eat their fruit and poop out the seeds elsewhere.

One gets nutrients it needs to live, and the other gets to spread around its seeds. The honeysuckle has improved the lives of other native plants, too, like nightshade. In one experiment, birds removed 30% more nightshade fruit in areas full of honeysuckle, compared to areas without the invasive plants.

That’s because these birds aren’t picky eaters, so when they stop by for some honeysuckle, they nom on some nightshade fruit too. It’s more than a win-win. It’s a win-win-win, at least for these three.

You might not have heard of it, but one of the most hated invasive species around the world is the European green crab. It’s originally from the northeast Atlantic Ocean and Baltic Sea, but it’s now colonized many of the world’s coastlines, spreading on ships and ocean currents. And it’s a total troublemaker.

These crabs chase off and kill native species, and they eat basically everything. You might even say they’re acting a little shellfish. But there’s one area of the world where they might be doing a little bit of good: Cape Cod.

European green crabs arrived in New England over a century ago and began their usual terrorizing routine. But things in the region have changed a lot since then. Over the years, recreational fishing and crabbing have taken out a lot of the native species in the area, including the predator to the native purple marsh crab.

Without any enemies to munch on them, these crabs then over-ate the local cordgrass, and dug so many burrows that soil erosion dramatically worsened. But fortunately, the purple marsh crabs are no match for the little green invaders. Studies have found that in places where green crabs have made a home, the ecosystems are recovering from the damage done by the overpopulating native crabs.

Basically, this invasive species is a violent bully. Green crabs either outright kill or just scare the marsh crabs so they spend most of their time hiding. When researchers stuck both species in the same cage, the green crabs evicted the marsh crabs from their burrows, and over 85% of them died.

Another month-long test showed that the mere presence of a single green crab caused marsh crabs to spend basically the entire time hiding, even if the green crab was locked up. By the end of the month, the marsh crabs had eaten much less cordgrass than usual. It’s kind of like a crab cage match.

Two crabs enter. One crab leaves. Another coastal invader that’s helped improve damaged ecosystems is Gracilaria vermiculophylla, a seaweed native to waters near Japan.

Over the years, it’s spread around the North Atlantic, likely due to the export of oysters, and you can now find it scattered across beaches, looking like gross bunches of matted hair. Besides outcompeting native algae for resources, this seaweed can also form dense mats in places like shallow bays and estuaries, which get in the way of all kinds of stuff. These mats do things like blocking light from reaching lower photosynthetic life, decreasing the amount of oxygen, and shifting current flows, which affects how food settles for deposit feeders.

However, studies have shown that where native habitats have declined or disappeared for other reasons, Gracilaria provides some much needed vegetation. It can bring back the area from the brink of barrenness. Other algae and small immobile animals can attach to it, and it can provide shelter and food for other critters, like gastropods, crustaceans, and other small invertebrates.

Outside the water, the seaweed is also widely used in agar production, a gelatinous substance used as a food thickener and in petri dishes. So it’s a bit of a mixed bag, but we’re glad it’s doing some good out there. And finally, sometimes we intentionally take species and bring it to a new land to help.

This can totally backfire, but in the case of the Aldabra Giant Tortoise, everything seems to be working out okay. Giant tortoises are a trademark of the Galapagos, but they’re also found on islands in the western Indian Ocean. Initially, two main groups lived on these islands: the giant tortoises inhabiting the Seychelles, and those living on the Mascarene islands east of Madagascar.

And then, humans showed up. By the mid-1800s, nearly all of the tortoise species on the islands had been knocked out. Many were over-harvested by humans, and others had their young killed by invasive cats, rats, or dogs.

In the end, only the Aldabra Giant Tortoises survived. They live in a place appropriately called Aldabra in the Seychelles. It’s an island atoll, or land linked up by a ring-shaped coral reef.

It’s pretty isolated from the other islands in the area, and an atoll doesn’t have enough space for human settlement, so that’s probably why it currently has the largest population of giant tortoises in the world. And now, some of them have been purposefully transplanted to other islands where their tortoise cousins no longer exist. For example, islands in the Mascarene, like one called Mauritius, have seen a decline in certain fruiting plants, because their seeds are no longer being distributed by extinct native tortoises.

So in the year 2000, a group of Aldabra giant tortoises were brought to the island. And now, they’re chowing down on the local flora, pooping seeds out all over the place. In fact, their digestive process actually helps break down the protective outer coating of some of these seeds, which makes the chance of germination higher.

The tortoises are also successfully breeding on the island, and there have been similar success stories in other places in the Mascarene, too. All these species make you wonder if it’s time to retire the term “invasive” species. After all, none of them, whether they’re good or bad for their new environment, are actively choosing to invade.

There’s no green crab or seaweed generals ordering their troops around. They’re just plants and animals that wound up in a new land that’s way easier to live in. Either way, these non-native creatures shouldn’t all be painted with the same brush, and by studying them rather than immediately trying to remove them, we might be able to find ways to help protect struggling ecosystems.

Thanks for watching this episode of SciShow! If you want to learn more about invasive species, we have another episode about that, and if you just want a new SciShow in your subscription box every single day, hit that little subscribe button. [♪ OUTRO].