Blurred lines: Can climate change-induced range expansion qualify a species as invasive?

By Abigail Lynch

Management of fish and fisheries is a tough job.  But, management of fish and fisheries with climate change will pose new quandaries that we have yet to encounter.  This week’s Fisheries Blog poses a question as food for thought: Does range expansion from climate change qualify a species as invasive if it impacts resident native species in its new habitat?

I do not know the “right” answer to this dilemma, or if there even is one.  But I do know that many fisheries managers will be asked to make decisions on how to manage the impact of these range expansions in the not-too-distant future, whether or not there is a simple solution.  So, this post will be comprised of many questions and, really, no answers.  Please feel free to weigh in with your recommendations in the comments section at the bottom of the post.

Sea Lamprey are invasive species in the Great Lakes.
Sea Lamprey are invasive species in the Great Lakes.

What is an invasive species?

According to the U.S. National Invasive Species Council, “invasive species are non-native (alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm, or harm to human health.”  At first glance, this definition seems straight forward: Native or non-native?  Harmful or not?  Pretty simple standards, right?  For example, Sea Lamprey (Petromyzon marinus) is a parasitic, jawless fish that attaches itself to a host fish and feeds off its blood.  In the Great Lakes, where they are not native, a single Sea Lamprey can kill upwards of 40 lbs of fish in its lifetime.  Without any natural predators in the Great Lakes system, the species has had devastating consequences on the fisheries – one of the major reasons many populations of native species collapsed in the 20th century.  Lionfish (Pterois volitans) have had a similar impact on reef communities in the Atlantic, Caribbean, and Gulf of Mexico.  The predatory fish has reduced native populations by up to 80% in some locations.  (For more information on lionfish removal, please read The Fisheries Blog’s post on fishing derbies

Lionfish are invasive species in the Atlantic, Caribbean, and Gulf of Mexico.
Lionfish are invasive species in the Atlantic, Caribbean, and Gulf of Mexico.

Native or non-native?

So, while it is easy to pin-point that Sea Lamprey and Lionfish entered the ecosystem via anthropogenic means (construction of canal systems and release of aquarium fish, respectively), what happens when that anthropogenic influence is less direct?  What happens if a species expands its range to a new habitat of its own accord?  Is it a non-native species?

There are many interesting examples of founder effects in evolutionary history where species have colonized new locations.  But, what happens if the reason that a species can colonize a new location is anthropogenic?  Smallmouth Bass (Micropterus dolomieu), for instance, are expanding their range northward into lakes across the northern U.S. and southern Canada.  As piscivores, Vander Zanden et al. (2004) suggest they have the potential to dramatically reduce local, native minnow abundances.  Sharma et al. (2009) further postulate that the reduced prey fish populations will impact resident Lake Trout (Salvelinus namaycush) which may be forced to switch to less desirable food resources.

Will Smallmouth Bass be invasive if their range expands because of climate change?
Will Smallmouth Bass be invasive if its range expands because of climate change?

Harmful or not?

In the Smallmouth Bass example, managers will have to prioritize Lake Trout or Smallmouth Bass when making management decisions on these ecosystems.  While Smallmouth Bass are likely to impact minnows and Lake Trout, managers will have to decide if Smallmouth Bass are causing economic or environmental harm or harm to human health.  If they are causing harm, they could be considered an introduced species (non-native but non-harmful).  But, if they are causing harm, that could be considered invasive.  Then, should “nature be left to its own devices” or does the situation require human intervention?  This question becomes further complicated when you add stakeholders into the equation.  Smallmouth bass are considered a very popular freshwater sport fish and anglers will likely be excited to fish for them in these new habitats.

Anthropogenic climate change is leading to their range expansion and they will likely impact the fish communities in their new habitats.  Keeping in mind that these scenarios are very complex, consider this simplified hypothetical: Imagine you manage a lake with a trophy Lake Trout population threatened by Smallmouth Bass (through anthropogenic expansion). Your management budget only contains enough funds to 1) ignore smallmouth and continue necessary lake trout stocking, or 2) don’t stock lake trout and use funds to reduce smallmouth (but not eliminate them). There are a lot of blurred lines with climate change and no simple black-and-white answers.  Do Smallmouth Bass qualify as an invasive species? 


Vander Zanden, M.J., J.D. Olden, J.H. Thorne, and N.E. Mandrak.  2004.  Predicting occurrences and impacts of smallmouth bass introductions in northern temperate lakes.  Ecological Applications 14(1): 132-148

Sharma, S., D.A. Jackson, C.K. Minns.  2009.  Quantifying the potential effects of climate change and the invasion of smallmouth bass on native lake trout populations across Canadian lakes.  Ecogeography 32: 517-525.

2 Comments Add yours

  1. pcooney2013 says:

    You raise some important issues Dr. Lynch.
    Assisted migration and invasive species are tremendously interesting and important topics.

    Dr. Dana Sackett mentions how climate change coupled with human driven fish movements is a contentious and difficult issue in the UK (

    Yellowstone Park is also faced with difficult issues to save native species from a non-native invader using a massive amount of resources (

  2. That was a very interesting post, Dr. Lynch. As I research colonizations to the Arctic, perhaps facilitated by climate change, I can’t help but weigh in.

    The commonality between the terms colonizing, range expanding, and invading is “movement”; the difference is the perception of “harm” or potential impacts to novel ecosystems and their organisms. There are undisputed movements occurring, both northward and into higher elevations, which are likely facilitated by climate change. However, if we change the timescale from the immediate present and recent past to a more historical perspective, the current distribution of most fish species in areas that are currently being “invaded” is the result of post-glaciation colonizations. Perhaps it is not the colonizations themselves but rather the rapid rate of these colonizations that is most alarming.

    But is it the rate of change that makes a colonizing species “invasive”? Rather, it is the association of these movements with “harm” that creates those negative connotations. Shifting focus from an individual species to a broader ecosystem-level perspective may also change that perception of harm. Of course, the environmental conditions that are facilitating successful range expansions by opportunistic species are also likely negatively impacting native species and their prey. The northern extent of species’ ranges is moving northward; is the southern extent of those ranges also moving northward? After all, in a dynamic global ecosystem, species have three choices: move, adapt or die. Colonizations are inherently risky due to the potential impacts on novel systems, but they may also create opportunities.

    While this is a challenging issue for managers and resource-users alike, perspective is an important factor moving forward. As distributions often extend beyond management (and political) boundaries, and those distributions are changing, perhaps the broader question is: do we focus management on an area or a species? There are no easy answers.

    Karen Dunmall

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