What is an Invasive Species?

What is an invasive species and what makes a species invasive?  Most invasive organisms do not at all behave invasively in their natural environment, so what makes them invasive when they enter a new environment?  How do we remove invasive species once they become established? These are all good questions to ask.  

A decent definition of invasive species given by The National Wildlife Federation on their website is: “An invasive species can be any kind of living organism—an amphibian (like the cane toad), plant, insect, fish, fungus, bacteria, or even an organism’s seeds or eggs—that is not native to an ecosystem and causes harm. They can harm the environment, the economy, or even human health. Species that grow and reproduce quickly, and spread aggressively, with the potential to cause harm, are given the label ‘invasive.’”

I would also add to that definition the harm they can do to native species. However, most definitions focus on the economic impact on humans, and at least this one lists the environment first. I suppose the harm to native species is probably implied in the ‘environment’ term.

What Makes a Species Invasive?

So, what makes a species invasive?  There are a few factors that make them invasive.   Invasive organisms already tend to be opportunistic r-strategist species that can reproduce rapidly in large numbers.  Then you add that species to a new environment it is not native to then it typically has no predators or other checks on its population. This allows its population to rapidly expand beyond its already high reproductive rate. 

Another huge, often overlooked factor contributing to the invasiveness of different organisms is what gives them a foothold in the new environment anyway.  Invasive species tend to have a low level of ecological specialization and tend not to tolerate competition for resources. This means that disturbed habitats make an ideal environment for many of them. All they need is a few individuals to survive to reproductive maturity and they can invade the disturbed environment. Disturbed areas are ideal for opportunistic species because little else will thrive in that environment, so they have little or no competition.

Take by far the majority of invasive plants. For example, they tend to thrive in disturbed habitats but will not invade an old-growth forest or even a second-growth forest.  But provide a recently cleared road, a clear-cut, cleared lot for development, etc, and they will take over. 

And to further complicate matters, not all species treated as invasive are even introduced.  Take the common cattail, Typha latifolia, for example.  It is native throughout North America but has, in recent decades, been forming monocultures throughout its native range.  This has led some people to believe it is invasive and needs to be controlled. However, what is actually creating the monocultures is wetland disruption. Our wetlands are being degraded with high nutrient input, heavy metals, and other toxins, as well as an altered water regime due to human activities.  These are all conditions that the cattail, and little else, will thrive in, thus creating the monoculture. Check out my blog on whether cattails are really an invasive native species or do human perceptions need to change? 

Why Are Invasive Species a Problem?

So if habitat disturbance is one of the main reasons a species becomes invasive, then why should we worry about them at all? If the habitat is no longer suitable for other species, should we not leave it to those who can live there?

The short answer is no. First of all, we need to be far more diligent with ecological restoration following disturbances, not just so we can prevent invasive species. We need to restore the ecosystem values, including biodiversity and the clean water, soil, and air we benefit from with restoration.

And most importantly, invasive species further threaten our already threatened native species by displacing them and using their land and resources. Sometimes they may even eat the native species, or their young, as is the case of the American Bullfrog for example. The American Bullfrog will quite literally eat anything it can fit in its mouth, including native fish, mollusks, salamanders, our native frogs and their young, and they will even eat their own young.

We need to protect our native species and the gene pools they still carry. Genetic diversity is the key to survival through environmental change. We have now created environmental change and the resultant extinction rates on a scale this planet has only seen before on cataclysmic scales of disaster, such as the asteroid impacts that have caused at least a few of our mass extinctions in the past. If we hope to have healthy, thriving ecosystems at the end of this, then we need to save as many of our native species as possible. One of the many ways we can do that is by controlling and, where possible, eradicating invasive species.

The Zebra Mussel

Take the zebra mussel, for example, which grows prolifically in pristine and polluted water alike, threatening our native mussels, fish, and aquatic ecosystems.  Freshwater mussels also face pollution, river channelization, siltation, etc. But now that they are also facing invasion by the zebra mussel, they are rapidly being extirpated in many areas and many species now face extinction.  The zebra mussel is a biofouling organism that smothers other mollusks and competes with other suspension feeders for food.  Stopping the degradation of our environment cannot stop the invasive mussel. 

Prevention, of course, does help. We can be diligent about cleaning our boats and gear between waterways. You may have heard the whole “Don’t Move a Mussel” campaign and have seen the boat checkpoints on our major highways. But any waterway connected to an infected one, regardless of preventative measures or how clean and pristine the water is, is still at risk of the spread of the invasive mussel. 

That is why we must understand the ecology of the invasive organism and try to find other means of control once they have already become established in the environment. That is why we need to address each invasive species on an individual basis to try to find methods of control.

What to Do About Invasive Species and Their Removal

Prevention is Best

In order to manage invasive species, the best strategy is to prevent them in the first place.  We need to put way more effort into ecological restoration following any type of man-made disturbance.  Methods that can be used include successional advancement, modification of the disturbance to make it unsuitable for the invasives, and planting native species to encourage competition because many invasives do not tolerate competition.  

While habitat disturbance by far is the biggest reason many species become invasive, that does not mean we should ignore the invasive organisms and only focus on the disturbance.  We are human beings, we disturb habitats; that is the nature of humanity.  That is why we need to be responsible when we do, like replanting with native vegetation as quickly as possible. However, as with anything in life, the answer is not that simple. We cannot simply alter our disturbance of the environment and expect that this will solve the invasive species issue. In many cases, the environment is already altered and cannot be restored to its original condition. In other cases, the invasive organism is already there and will not just ‘go away’ even if we can restore the ecosystem. We need instead to find a way to remove invasive species once they have become established.

Physical Control for Invasive Species Removal

Physical removal is almost always the best way to deal with invasive species removal. Physical control methods involve cutting, digging, burning, etc. The methods will vary for each species we are trying to remove. In future blogs, I will explain in detail for each species the best means of physical control.

Chemical Control for Invasive Species Removal

The common practice of using herbicides or pesticides is not at all effective in the long term. The use of chemicals to control invasive species is a form of disturbance in itself through the chemical alteration of the ecosystem. Chemical control also opens up gaps that allow invasive organisms to get a foothold. The best way to manage invasive species is to understand their ecology and make the environment more suitable for native than invasive species. 

Biological Control for Invasive Species Removal

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. Biological control involves using a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment.

The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the control agent is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800’s to help control the rat. To this day, there are still rats in Hawaii, but the mongoose has helped to completely decimate many native bird populations.

Conclusion

This introduces my new series of nature blogs on invasive species.  I will, of course, mostly focus on plants as that is my area of expertise, but I will discuss other organisms as well.  Each week I will describe an invasive organism, its distribution, and habitat preferences. Then I will discuss the various methods of prevention and different methods of control and eradication we can implement if they are already established in an area. Every organism is unique, so every approach also has a unique perspective.  

If you want to learn more about invasive organisms, check out the various invasive species organizations I listed in the resources below. Your local region likely also has its own local chapter where you can get involved in education, organized biological removal, and more. And if you have any questions or suggestions for a topic, I should cover, please Contact Me.

References and Resources

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Economic Cost of Invasive Species in Mexico In: Zenni RD, McDermott S, García-Berthou E, Essl F (Eds) The economic costs of biological invasions around the world. NeoBiota 67: 459-483. https://doi.org/10.3897/neobiota.67.63846 https://neobiota.pensoft.net/article/63846/

Invasive Alien Species of Mexico Official Checklist https://trilat.org/test-docman/annual-meetings/2013-annual-meeting-1/281-2013-invasive-alien-species-official-list-for-mexico-1/file

Invasive Species Council of BC https://bcinvasives.ca/

Invasive Species Council of Canada https://canadainvasives.ca/

National Invasive Species Council (United States) https://www.doi.gov/invasivespecies

North America Invasive Species Management Association https://naisma.org/

Polster, D.F. 2004. Restoration Encyclopedia: Invasive Species in Ecological Restoration. Paper presented at the 16th International Conference of the Society for Ecological Restoration International. August 24 – 26, 2004. Victoria, B.C.

Ricciardi, Anthony, Richard J Neves and Joseph B Rasmussen, 1998.  Impending extinctions of North American freshwater mussels (Unionoida) following the zebra mussel (Dreissena polymorpha) invasion.  Journal of Animal Ecology (1998), 67, 613-619.  

The National Wildlife Federation https://www.nwf.org/Educational-Resources/Wildlife-Guide/Threats-to-Wildlife/Invasive-Species

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