Introduction to Invasive Species (Plants & Animals)
Asian carp in the Mississippi River. Kudzu vine in Fiji. Burmese pythons in Florida. Lionfish in the Caribbean. Water hyacinth in China. Cane toads in Australia. All of these plants and animals are invasive species that cause catastrophic damage in their new ecosystems. In many cases, humans are responsible for their spread and reacting to their impact.
According to the United States Forest Service, an invasive species is both non-native to the ecosystem and capable of causing environmental, economic, or human harm. In other words, it’s a species that’s not supposed to be someplace that’s causing problems in that place.
While the term invasive species suggests that these species are crusading across the globe in a diabolical attempt to take over, that’s not quite right. Invasive species are not just the winners of “survival of the fittest.” They had human help to get to a new ecosystem that simply isn’t prepared for them.
For a bit more information on exactly what an invasive species is, check out the TED Ed video below.
Thankfully, not every garden plant or escaped pet leads to a new invasive species taking over the world.
What Makes a Non-Native Species into an Invasive Species?
You don’t see hordes of strawberry plants wreaking havoc in Nairobi or herds of alpaca demolishing the Scottish landscape. So why are red deer in Argentina and trumpet tree in Malaysia such big problems?
Before we get too deep into this discussion on invasive species, let’s look at a few terms:
Native Species: These are the plants, animals, fungi, and other life forms that evolved in a given area. For example, giraffes are native to Namibia and the Baikal seal is native to Russia.
Non-Native Species (also known as exotic species or alien species): These are plants, animals, fungi, and other life forms that did not evolve in a given environment but are not harmful. Most plants in your garden are likely non-native.
Invasive Species: As stated above, an invasive species is both non-native and harmful to the ecosystem, economy, or human population in its new environment. A species can be native and harmful, but is not invasive in that case.
In general, invasive species share several traits that make them poised for taking over an ecosystem:
- Fast growth. This allows the invasive species to outcompete slow-growing native plants or animals, especially if there’s a window for opportunity.
- Hardy. Plants or animals that are delicate and sensitive are likely to die off rather than become invasive.
- Rapid and/or varied reproduction. Many of the most aggressive invasive plants are able to reproduce from seeds, stumps, roots, and cuttings. Even invasive animals, such as rabbits in Australia, tend to reproduce quickly.
- Generalistic. Species that can eat different things and survive in a variety of ecosystems are more likely to become invasive than picky eaters!
Most invasive species are relatively harmless in their native environments because they have natural predators, competitors, and seasonal limitations that keep them in check. If the same species is introduced to an environment that lacks these natural checks on the population, it’s not surprising that it balloons out of control!
Some invasive species do not perfectly fit the outline above. If you scroll through the International Union for the Conservation of Nature list of the world’s 100 worst alien invasive species, you’ll notice that some of the species with relatively slow growth or reproduction (such as goats or red deer).
The Cost of Invasive Species
By definition, an invasive species causes damage to the ecosystem, the economy, or humans. There are no harmless invasive species, because a harmless non-native species is not considered truly invasive.
Invasive species cause damage in a variety of ways. They may:
- Damage crops. Invasive diseases, fungi, and pests pose significant threats to important human crops. A study published in Ecological Economics titled “Update on the environmental and economic costs associated with alien-invasive species in the United States” found that invasive insects and pathogens cause roughly U.S. $40 billion per year in damage to crop and forest production.
- Push out native species. Invasive species out-compete and over-eat, causing serious damage to native species in an ecosystem. An analysis of the International Union for the Conservation of Nature Red List (the most comprehensive list of the conservation status of the world’s species) found that invasive species are the second most common threat associated with species that have gone completely extinct, and are the most common threat associated with extinctions of amphibians, reptiles and mammals.
- Cause erosion. Invasive eucalyptus trees in South Africa have caused massive erosion damage by secreting chemicals that suppress growth of new plants. The lack of new greenery leads to loose soil that easily washes away in rain or wind. This is often made worse with fire damage, another risk in areas impacted by invasives.
- Damage to infrastructure. According to a white paper from the U.S. Department of the Interior Invasive Species Advisory Committee, Dreissenid mussels in the Great Lakes caused $3.1 billion dollars worth of damage in six years from 1993 to 1996 to the power industry alone.
- Contribute to wildfires. Along with climate change and ill-advised fire suppression efforts, invasive species contribute to the risk of increased wildfire frequency and intensity. In the western United States, invasive grasses provide readily available fuel that allow fires to grow quickly and spread widely. In South Africa, invasive species guzzle water and dry out the land, leaving the area vulnerable to fire.
- Degrade the experience of recreation, hunting, or fishing. When invasive species out-compete native species, that’s not just a problem for the ecosystem. Tourists and fishermen have already felt the negative impacts of invasive species in the world’s coral reefs: 84% of the world’s reefs are already impacted by invasive species.
- Eat native animals. Aside from outcompeting native plants or animals, invasive predators pose a unique problem: they might eat the locals. For example, foxes on an Australian island nearly wiped out a colony of adorable little penguins until a farmer came up with a creative solution (hint: it’s another canine). Burmese pythons in Florida and mongooses in Hawaii cause similar problems.
- Spread disease. Invasive insects and plants are particularly threatening to humans because of their ability to spread devastatic diseases around the globe. Read about how the invasive cassava mosaic fungus from Africa harms cassava in Asia, and how invasive species of mosquito carry diseases like Zika, Dengue and Yellow Fever.
According to the United States Forest Service, the economic cost of invasive species damage and control is estimated to be as much as $100 billion a year in the United States.
Humans Help Spread Invasive Species
The majority of invasive species didn’t just pack their bags and move to greener pastures of their own accord. Rather, they were transported by humans. There are four main ways that invasive species end up in an ecosystem that they can thrive in.
Escaped Garden Plants or Pets
Many invasive plants, such as Purple Loosestrife and Himalayan Blackberry, are beautiful or edible. Humans brought them along to plant in gardens when they moved to a new part of the world. Likewise, Burmese Pythons and Feral Pigs are likely escaped pets or livestock.
Other invasive species are adept hitchhikers. They use the hulls of boats, the cuffs of your hiking pants, or even the hair of your dog to hitch a ride across normally insurmountable distances. This is how Norwegian Rats, Zebra Mussels, and many other invasive species found their way.
In a few cases, predators such as mongooses in Hawaii or foxes in Australia, humans introduced an invasive species in the hopes that it would help control a different invader. This may help reduce the impact of the original invasive species (rats in Hawaii and rabbits in Australia), but the introduced predator may become invasive in its own right.
Learning from their mistakes, scientists are now extremely careful to vet a potential predator before attempting any control methods using another introduced species.
Milder winters or longer summers allow many invasive species to exploit habitats that simply weren’t available to them in the past. Additionally, changing climates may favor invasive species because they are more adaptable to change than native species. Climate change and invasive species can be a devastating combination for many ecosystems.
The Invasive Species Removal Controversy
Invasive species are by definition “fit” — they outcompete and outlast the other species around, taking over an area. But they’re more than just successful. Invasive species are harmful.
If survival was a game, invasive species were dealt a good hand, then they got an illegal assist from humans. Whether you believe this constitutes “survival of the fittest” or not is up for interpretation.
The fact is that native species can be incredibly harmful to the economy and ecology of an area. Many non-native species are actually able to slide into an ecosystem without damaging it.
A native species can still cause ecological, economic, or human harm — just look at the human damage caused by native Guinea worms in Africa or the economic harm done by native locusts in Australia and beyond.
Some scientists point out the fact that some native species are also harmful to ecosystems and economies. In a 2011 paper in “Trends in Ecology and Evolution,” Ken Thompson and Mark Davis point out that a more sound and effective approach to managing species may be to focus on building infrastructure to deal with harmful species in general.
Thompson and Davis point out that with global shifts in climate, nutrients, and disturbance regimes, almost every species on the planet is facing a shifting environment. Focusing on just the non-native species may ultimately be unwise and close-minded as we neglect to control potentially harmful native species and eradicate harmless non-native species.
More and more scientists are arguing that we should leave harmless non-native species alone. They argue we should focus on the worst invasive species (and even aggressive, harmful native species). These scientists urge pragmatism, rather than unilaterally removing all non-native species in the name of preserving the original ecosystem.
Of course, some preservationists disagree. They would rather strive for a pristine, untouched ecosystem wherever possible.
The situation is complicated by the fact that there’s almost nowhere left on earth that is truly “pristine.” Harmful native species may be detrimental because a natural predator went extinct (or was removed, as in the case of Yellowstone’s wolves). Some species of flower struggled to reproduce after their native pollinator went extinct — until a non-native pollinator stepped in.
You can learn more about the Yellowstone wolf reintroduction below — and how the original removal of this native predator led to destruction by an otherwise harmless native species, the elk.
Most scientists agree that building stable ecosystems as a whole is the best strategy going forward. This is likely to include a combination of reintroducing native species, artificially controlling the populations of some native species, removing invasive species, and allowing some non-native species to fill a nice.
Others argue that humans have meddled enough and should take more of a hands-off approach going forward. Let the species duke it out themselves.
Whether you agree with removing all non-native species, aiming to balance an ecosystem, or letting nature run its course, it’s likely we’ll see examples of all three approaches across the world.
Humans Can Help Control Invasive Species
Humans can also have a huge impact on controlling harmful invasive species. The best way to get involved is to learn about the threats to your area by checking out local Forest Service resources, nature centers, or other information centers related to public lands.
One of the best resources for learning about the problems in your area of the U.S. is the National Invasive Species Information Center. The site has a map of the U.S. where you can learn about threats specific to your area. For the International version of this information (everywhere except the U.S.), click here.
Once you know what the biggest problems are, you will be able to help prevent the spread of invaders by cleaning your hiking boots, avoiding areas that are riddled with invaders, or planting native species instead of exotics in your garden. There are many ways to avoid making the problem worse!
It’s not all about prevention, though. Invasive species are already a serious problem in many ecosystems around the world. Your local resources will also help you learn about volunteer opportunities for removing invasive species. Whether you’d rather help hunt feral pigs or spend a few hours pulling weeds, there generally are volunteer opportunities for everyone.
One of the coolest ways to get involved with invasive species removal is through food. Initiatives to eat the invaders are popular worldwide. This is a creative way to help curb invasive species populations — though it’s unlikely to succeed as an all-out eradication method.
Focusing our efforts on removing the most damaging invasive species while preventing their further spread can help make invasive species a less intense threat to our economy and ecosystems.