What is invasion? Most of us probably have a very general idea of what this concept is: some form of intrusion, disruption, or interruption. The concept of an invasive species might be less familiar, though. So what does “invasive species” really mean? To answer the question of how a species is able to invade, scientists have come up with dozens and dozens of theories. To answer what is an invasive species, the answer is slightly simpler, though there are different variations of the definition depending on your source.
An invasive species is: 1) a non-native species that 2) is causing or is likely to cause ecological, economic, or human health problems in its new ecosystem . We commonly hear about invasive plants and insects, but invasive species can be any type of living organism such as a plant, fungus, animal, or microbe.
A non-native species (also called an “exotic species” or “alien species”) is any species that is not naturally occurring, or indigenous, in an area. When thinking about whether a species is native or non-native, it’s also important to think about spatial scale. Where did the non-native species come from? Country or continent of origin are often mentioned when talking about invasive and non-native species, but the spatial scale of where a species goes from being native to non-native can be within a country or an even smaller region. A native species is a species that has historically occurred in an area (and was not introduced by human actions) . A species would be considered non-native if it is found outside of its historical range due to accidental or deliberate human actions that introduced the species to a new area . For example, a non-native species in a river in Massachusetts might have come from a river in Montana, Brazil, Asia, or elsewhere. As long as that species was not historically found in that river and is now successfully surviving and breeding, it is an established non-native species!
Just because a species is non-native does not mean that it is invasive. To be considered as an invasive species it must meet that second qualification based on the above definition. That second part states that it is causing or is likely to cause some sort of negative impact. This part of the definition is a little bit more abstract since we - as humans - have to determine whether the species creates a problem in its introduced range. For example, how do we characterize an ecological problem? Is it when biodiversity is reduced? When a once common native species becomes endangered likely due to the invasive species? What “ecological damage” means might vary slightly from person to person, but characterizing other types of problems is much easier. For example, characterizing the economic cost of invasive species due to damage, management, and removal is more concrete. And the total is not a small amount either - approximately $138 billion per year in the United States alone . Invasive species cause crop loss and many other types of damage on top of the money needed to remediate and control invasions. The important thing to remember here is that invasive ≠ non-native. A non-native species has been introduced to a new area, but has not become a problem yet. In fact, it often takes years for an “exotic species” to become an “invasive species” .
Fig. 1 Garlic mustard (Alliaria petiolata) a highly invasive plant in New England and across the midwest United States. This species is not only costly to remove, but can alter biodiversity and decrease the quality of the soil it grows in with the effects lasting even after it has been removed. Photo credit: hspauldi on Flickr. Source
In the United States, there are approximately 4,000 known exotic plant species alone, but ‘only’ 400 of them are considered invasive . What has caused so many species to be introduced to the United States? Accidental or deliberate human actions are the leading cause of invasive species being introduced to an area . For example, a fish accidentally escaping an aquarium in Florida could cause an introduction of an exotic (or invasive) species. And this happens to be how the invasive lionfish were introduced into Biscayne Bay along the Florida coast (Fig. 2) . Another example of how an exotic species could get to a new location would be stow-away insects on or in wood packing materials. And you guessed it! This is how another invasive species got to the United States. The Asian long-horned beetle is believed to have gotten to the USA via wood pallets that were coming from Asia (Fig. 3) .
Fig. 2 Red lionfish (Pterois volitans) were introduced to waters off the coast of Florida by accident. But now they have very successfully invaded most of the Caribbean and a few juveniles have been found as far north as wasters off the coast of New York! They have been so successful as invaders partly because they have few predators in their introduced range and are resistant to most parasites . Photo credit: Alexander Vasenin on Wikipedia. Source
Fig. 3 An Asian long-horned beetle (Anoplophora glabripennis) next to a close-up of tree damage it caused (left). Even though the Asian long-horned beetle is a ‘small’ insect, it can do a lot of damage even to large trees! All of the holes and abrasions on and in the logs were caused by Asian long-horned beetles (right). Left picture photo credit: Wikipedia, source. Right picture photo credit: USDA on Flickr, source
So what can we do to stop the spread of invasive species? There are a few very easy precautions we can all take to help stop the spread of invasives [7, 8]. If you know there are invasive species on your property, try to properly remove as many as you can and if you want to landscape your yard, only use native plants or non-native species that have been well researched. When hiking or camping, tracking seeds on shoes and tires is an easy way to spread invasive plants. To prevent spreading species this way, clean any dirt off of yourself and car before traveling between areas. My last tip would be to buy local firewood and do not transport logs between states; many invasive forest insects move to new areas by ‘hitching a ride’ on your firewood! In fact, (not) transporting firewood is so critical to the management of invasive forest pests that many states have - or are trying to enact - laws regulating or banning movement of firewood across state lines [9, 10, 11].
So now that we’ve talked about what native, non-native, and invasive species are, check back soon for part 2 of Invasive Species and Invasion to learn about how invasive species are able to conquer a new ecosystem!
 “What is an Invasive Species?”, USDA (2016) [Accessed May 22 2016]
 “Frequently Asked Questions about Invasive Species”, Center for Invasive Species and Ecosystem Health (2009) [Accessed May 22 2016]
 “Non-native Species”, United States Park Service [Accessed June 4 2016]
 “Four Threats – Quick Facts”, US Forest Service (2006) [Accessed May 22 2016]
 “Lionfish Fact Sheet”, National Oceanic and Atmospheric Administration [Accessed May 22 2016]
 “Asian Longhorned Beetle”, New York Invasive Species Information [Accessed May 22 2016]
 “What You Can Do”, US Fish and Wildlife Service (2016) [Accessed June 4 2016]
 “Invasive Plants”, The Massachusetts Office of Energy and Environmental Affairs [Accessed June 4 2016] http://www.mass.gov/eea/agencies/dfg/dfw/natural-heritage/land-protection-and-management/invasive-species/invasive-plants.html
 “New Hampshire to Ban Out Of State Firewood”, New Hampshire Public Radio [Accessed June 14 2016]
 “Regulations on Movement of Firewood”, Connecticut Department of Energy and Environmental Protection (2016) [Accessed June 14 2016]
 “State by State Information”, Don’t Move Firewood [Accessed June 14 2016]
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