Alien Invasion: Understanding invasive species

Above: A swarm of European starlings, one of the world’s top invasive species.
Image ©

It may sound like science fiction, but alien invaders are real!

Invasive alien species are living organisms introduced to an area where they didn’t originally live. In this new area, they cause problems for native species, biodiversity, ecosystem services, or human wellbeing. But what are these problems? How many invasive species are there? How did these invaders get to where they are today? To find the answers – read on!

Glossary of alien invasion

Before we can understand invasive species, we must first understand native and exotic species.

Native species are species that are found naturally in a given area and habitat. They have long been known to live and thrive in that area. For example, the white trillium is Ontario’s provincial flower. It grows in eastern North America.

Above: The white trillium is native to eastern parts of North America.
Image ©Mykola Swarnykm, Wikimedia Commons

Meanwhile, exotic species are species that are not native to a habitat. Instead, they were introduced to that area. However, they do not necessarily harm their new ecosystem or the native species in it. For example, many of the flowers people grow in their hobby gardens are exotic. Ornamental roses or hibiscus are examples.

Above: Roses that are not native to your garden are called exotic species.
Image © jorgeantonio,

But when a new species harms native species, it becomes invasive.

How do invasive species affect native species?

Invasive species have these impacts on native species because they can outcompete them. Invasive species spread and reproduce quickly, taking over the food and habitat that native species need to survive. When native species disappear, the ecosystem can’t always function as normally or as healthily as it should.

One of the major problems with invasive species is that they can lead to the extirpation or extinction of native species. Extirpation is the loss of a species from an area. Extinction is the loss of a species from the entire planet.

For example, an invasive plant could outcompete a native plant that a small mammal relies on for food. If the small mammal cannot eat the invasive plant, and all of its native plant food disappears, that mammal might also disappear. Both the native plant and small mammal would be extirpated from that area.

Then, a larger predator that feeds on those smaller mammals might have less food. Take owls, for example. If owls have fewer small mammals to feed on, the number of owls in that area might go down. What do you think would happen if these owls lived only in the area where the invasive species was introduced? That owl could become at risk of extinction.

Above: An example of a food chain
Image © Siyavula Education, CC BY-ND

Did you know? European starlings were introduced into New York City’s famous Central Park in 1890 by a man who wanted to see every bird mentioned by William Shakespeare in the park. They are one of the 10 most invasive species in the world! You can read about them and the other 9 most invasive species in the world at this link.

How does all this affect humans?

Invasive species can also impact us as humans. Some of them can physically hurt us. For example, poison ivy is a native species in many parts of North America. It has chemicals that can give you itchy, painful rashes when you touch the leaves. If it’s a dangerous native species in your area, you might have learned to avoid it. You might have even heard the rhyme, “Leaves of three, let it be”.

Above: Poison ivy is native to parts of North America.
Image © Stilfehler, Wikimedia Commons

However, the giant hogweed (Heracleummantegazzianum) is an invasive plant in North America. It is a member of the carrot family and is native to southwest Asia. Giant hogweed has toxic sap that can irritate your skin and cause serious burns! It looks similar to many of our harmless native plant species. But because it’s not a native plant species, we haven’t had as much time to learn how to recognize or avoid it. It also looks similar to many of our harmless native plant species.

Above: Giant hogweed is invasive to North America.
Image © Appaloosa, Wikimedia Commons

Did you know? Giant hogweed looks similar to other, harmless plants such as cow parsnip, queen anne’s lace (wild carrot), woodland angelica, valerian, lovage, and Purplestem angelica. Live in Ontario? Click here to learn how to recognize giant hogweed and stay safe!

Invasive species can have other impacts on human health. For example, the Asian tiger mosquito carries over 20 dangerous human diseases like yellow fever and chikungunya fever.

Invasive pest species can also destroy economically valuable trees and crops. This causes humans to lose revenue, building materials, and food. The emerald ash-borer in Canada, the Spanish slug in Europe, or the red palm weevil in the Mediterranean are examples of crop-destroying invasive pest species.

How many invasive species are there?

There are over 16, 000 invasive species worldwide – and their numbers are growing. Researchers found that the numbers of invasive species recorded worldwide have been increasing over the last 200 years. Almost 40% of invasive species have been recorded in recent history, between 1970 and 2014. In other words, this problem shows no signs of going away.

Most invasive species have been introduced by us humans, both accidentally and on purpose. Invasive species got to their new homes as Europeans sailed around the world in the 1800s. They also moved around as worldwide trade by ships, trains, automobiles, and even airplanes increased throughout the 1900s.

For example, Europeans in the 1800s travelled on large ships to explore and colonize new countries and continents. Norway rats snuck onto these ships and were transported along with humans to these new areas. On arrival, the rats spread quickly and caused countless wildlife deaths. For example, they killed seabirds on remote islands. These seabirds had never encountered mammals before, and had no defenses against them.

Did you know? You can search for invasive species on the Global Invasive Species Database.

Above: Norway rats, which you may see around town, are actually an invasive species. They often arrive in new locations by ship.
Image © Reg McKenna, Wikimedia Commons

How can we stop invasive species?

Invasive species are good at spreading and surviving in new areas. This makes it very difficult to control and remove them! Two common methods often used are physical removal and biocontrol. Digging up invasive plants or poisoning invasive animals is an example of physical removal. Introducing another species that might kill the target invasive species is an example of biocontrol. In future articles, I will discuss specific species and the methods that are being used to control them in more detail.

What can we learn from the fact that the numbers of invasive species are continuing to grow worldwide?

This tells us that our efforts to manage and control the spread of invasive aliens have not been effective enough. Conservationists need to develop new and better strategies to stop invasive species and repair the damage they cause.

Throughout my series on Alien Invaders, and other Let’s Talk Science resources, we will explore invasive species in more detail. Who are they? What do they do? How did they get to where they are today? What can we do to stop them?

I hope you will join me on this exploration of Alien Invaders – and learn how you can stop them!

Learn More

Species Extinction: Do Humans Play a Role? (2018)
CurioCity by Let’s Talk Sciece

Extinct and Extirpated (Accessed 2018)
Canadian Biodiversity

How You Can Help (2014)
Canadian Council on Invasive Council

Unwelcome Guest: the High Cost of Invasive Species (2015)
CurioCity by Let’s Talk Science


IUCN, International Union for Conservation of Nature (2017)
Invasive alien species on the rise worldwide

U.S. Fish & Wildlife Service Invasive Species (2012)
Frequently asked questions about invasive species

Ontario Wildflowers, Walter Muma (Accessed 2018)
White Trillium

Ontario’s Invading Species Awareness Program (2018)
Zebra and Quagga Mussels

Invasive Species Centre (2016)
Learn about invasive species

Ontario’s Invading Species Awareness Program (2018)
Giant Hogweed Heracleum mantegazzianum

European Environment Agency (2013)
Invasive Alien Species

Forest Invasives (2015)
Emerald Ash Borer

Seebens, et al., Nature Communications (2017)
No saturation in the accumulation of alien species worldwide

Alaska Department of Fish and Game (Accessed 2018)
Invasive Species - Norway Rats (Rattus norvegicus)

Leanne Grieves

I was born and raised on the Canadian prairies in the province of Manitoba. As a child, I spent countless hours outdoors and exploring nature. In fact, I spend countless hours as an adult doing the same! In 2012, I earned my Bachelor of Science degree in biology from the University of Winnipeg, Manitoba. There, I gained my first research experience working with marsh-dwelling red-winged blackbirds. I moved to Hamilton Ontario and completed my Master of Science degree in biology at McMaster University. I spent about a year in total in Puerto Rico studying acoustic communication in a unique bird species, the smooth-billed ani. Currently, I am a 3rd year PhD candidate studying chemical communication, avian malaria, and mate choice in song sparrows at Western University in London Ontario. While I am interested in all sorts of creatures, I have been working with birds since 2008 and I wouldn’t have it any other way! (That photo is me catching my final song sparrow of the year.) I love sharing knowledge and encouraging others to appreciate nature, which is why I am thrilled to be a volunteer science writer with Let’s Talk Science.

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