Defining “invasive species”— not as easy and probably more important than you think!

To solve any problem that impacts society, folks must agree on what the problem is and define it clearly. Unfortunately, the definition of an invasive species used by Minnesota’s (and most states’) government is neither precise nor the one accepted by many research scientists.

Examples of the problems resulting from lack of a precise definition abound, such as the passionate debate about whether or not to restrict stocking of muskellunge (muskies), Esox masquinongy, into Minnesota lakes. A favorite of some fishers, the muskellunge has been present in a relatively small number of lakes in the Upper Mississippi River watershed since the last glaciation ended about 12,000 years ago but now folks want to introduce them to more. But should introduced muskies be considered a native or an invasive species? There is a problem is with the legal definition and how it is followed. The Minnesota DNR (echoing a presidential executive order) defines invasive species as “species that are not native to Minnesota and cause economic or environmental harm or harm to human health.”

Although that may seem straightforward, there are two major issues. The first is that the relevance of “native” is unclear.  In determining what makes a fish native, what really matters (from a scientific perspective) is not the state a species is found in (fish are apolitical), but rather what area of the watershed that that species of fish swam into on its own, is now naturally evolving in, and presumably in balance with.  The issue should be ecological balance not politics. The second issue is related to the term “damage.” It is  unclear what this means or how it should be evaluated. In particular, the term “environmental damage” lacks scientific relevance. Ecosystems simply are what they are; they cannot be damaged, although they can change (especially because of things folks add to them), often in ways that people dislike.  Additionally, and perhaps most importantly, agencies and research organizations dealing with invasive species are not actually assessing economic damage. They aren’t routinely collecting data on the economic value of the resources they are managing and protecting or on how those values are being harmed or changed (or could be). Lacking objective metrics and data about ecosystem value, assessments of damage become largely a matter of conjecture. So administrators and managers decide what is native (and presumably “innocuous”) or invasive in our state, and what sort of effort is needed to address them, largely based on personal experience, which is by definition subjective and limited by who happens to be in the room. The end result? Unfocused, inefficient, scientifically indefensible, and unsatisfying policy.

Where does this leave us? We seem to have two options: we can either adopt a new definition or correct and then follow the present one. Most basic scientists, including me, use the definition coined by Professor Dan Simberloff (considered the father of the scientific discipline of invasion biology*): “species that arrive with human assistance, establish populations and spread.” Simberloff’s definition is objective, scientific, and quantifiable, focusing on the specific role of humans (vs. natural processes) in determining the presence a species. Using this more scientific definition, stocking muskies (or any species) into lakes and rivers where they were not naturally found is tantamount to introducing a non-native, invasive species if these species eventually breed and become established.

However, as appealing as Simberloff’s definition and its focus on natural processes is, it does not address “damage,” something many feel passionately about, especially if they are being asked to fund invasive species research and control. This is understandable and can easily be alleviated in a scientifically-rigorous manner simply by specifying that invasive species (as defined by scientific criteria) must by scored (or assessed) by how damaging they are (or are likely to be) in economic terms. For this, a rigorous socioeconomic evaluation of individual ecosystems would be conducted by applied economists to evaluate all aspects of ecosystems and roles of species in them and their combined economic values to all facets of society.

In this way, both the value of and damages to ecosystems could be assessed in quantitative and repeatable manners, and our efforts to target invasive species and systems could then be focused objectively and productively. Companies perform fact-based assessments of resource values all the time—why shouldn’t resource managers and scientists?

In summary, if politicians will not or cannot adopt the scientific definition of an invasive species, the present legal definition should be modified (its easy) and followed. The economic data that would be produced should then be used to determine exactly how damaging introduced (non-native) species actually are (or are likely to be) to all segments of society (i.e fishers, lake shore owners, resort owners, nature-lovers, canoeists, water-skiers, etc.) so reasonable decisions can be made at both the local (e.g. a biologically relevant scale) about whether and how particular species should be studied, regulated and / or controlled.  Either way, it is high time that we used applied economics to inform invasive species research, management and control. I suggest that at least a third of our efforts be directed in this way, with another portion devoted to policy evaluation. Invasive species are as much a social problem as a biological one: we must recognize this if we want to succeed in managing and controlling them in ways that we can benefit from.

P.S. Whether introduced muskies should be considered invasive or not according to the legal definition, is in my opinion, presently unanswerable because there seemingly are no socioeconomic data on their overarching ecological effects.  Perhaps we should get some?

*Simberloff, D. 2013. Invasive species: what everyone needs to know, Oxford University Press.

A story about carp Invasive species are a very big problem but only one of six faced by our lakes and fisheries.

A story about carp

Invasive species are a very big problem but only one of six faced by our lakes and fisheries.

February 19 2018


Hi again,

Our lives are more complex than we typically recognize. Only when we get sick or old does this complex balance become apparent.  The same is true of lakes and fisheries.  This was brought home to me by a 10-year study of common carp, Minnesota’s first, perhaps most damaging aquatic invasive species.

In 2005, my research team started work on the common carp in Lake Susan in the West Metro. I was told that this species was responsible for most water quality problems because of their habit of eating from  the bottom of the lake, but that the problem was impossible to control because they had millions of young.

A literature search showed there had been no systematic study so we got to work.  Our first step was to determine how many common carp are actually in our lakes (no one knew!); astoundingly, we found that half the fish biomass of many lakes was carp!  Next, we followed adult carp with radio-tags to learn where they moved and bred; amazingly, most bred not in the lakes where they were a problem but in adjoining degraded wetlands that lacked oxygen and predatory fishes1. We concluded that water flow and quality were the issue, so we aerated wetlands close to Lake Susan. Next, we located adult carp using radio-tagging as they aggregated and removed 90% them in just a few days!

The carp are now largely gone – invasive carp problem solved – what happened?

Water clarity in Lake Susan immediately and rapidly improved as predicted2 but then disappointingly collapsed late summer. High levels of nutrients released by the dairy industry in the 1930’s still reside in the sediments and appear late each summer. Next, pan-fishing boomed but within a year returned to basal levels as fishers removed up to 80% of all large fishes. Finally, invasive plants blossomed as they were no longer held in check by carp. These two issues are now the focus of another 10-year program.

I am extremely proud of this work – we controlled an invasive species for the first time without poisons, illuminated ecological functions that had been previously unknown—functions that the watershed district can, and is, addressing. However, we also learned that the health of Lake Susan, like all of our lakes, is driven not just by the presence of aquatic invasive species, but by a set of 6 intimately related factors, all of which must be addressed.

For great waters and lakes, we must address: 1) aquatic invasive species, 2) water quality; 3) water flows and volume; 4) fishing; 5) habitat. Of course, water temperature (#6 climate) must be considered too.

It would be far better and cheaper if we had taken proper care of Lake Susan in the first place.  Protecting key zones within watersheds can do this. It is not too late for most lakes. I will discuss these 6 threats next.

1.Bajer and Sorensen 2010. Journal of Biological Invasions 12:1101-1112.

2.Bajer and Sorensen 2014. Hydrobiologia. 2014: 1-9

Lake Lanao: A painful lesson about how quickly invasive species can bring a fishery to extinction

Jan 24 2018 (from FWCF site).

Title: Lake Lanao: A painful lesson about how quickly invasive species can bring a fishery to extinction


Greetings from the Philippines where I am visiting Fishbase (, the world’s leading research center dedicated to describing the biology of our planet’s fishes. This group provided much of the data which proved that the world’s ocean fisheries are being overfished and in decline.  I’d like to see if the same might be true for fresh water, including lakes and rivers in Minnesota, as understanding typically provides direction.

While at Fishbase, I met a Ph.D. student, Armi Torres (, who told me about her Ph.D. work on fisheries of Lake Lanao, one of the Philippines largest and most valuable lakes.  It is riveting.

Lake Lanao is a large (130 sq miles) and beautiful lake formed over a million years ago (  It is also deep (average depth of 200 feet) and clear, giving it considerable resiliency and allowing it to develop a highly productive fishery that has been stable for many millennia since man appeared on the scene. Until recently, as is the case in Minnesota, no one questioned that…

Indeed, an initial study in 19331 found over two dozen fish species including 18 endemic species found nowhere else and deemed it a global treasure.  This was confirmed in a 1963 study which found nearly 2 million pounds of native fishes being caught each year (;jsessionid=4BCCD1AC4163AD2BEA1113F504E41ADB.jvm1?sequence=1).

– But unfortunately, this was not to last because during the next ten years about half a dozen invasive fishes had been introduced and by the early 1990s, a 98% decline in native species biomass was evident. Further, by the early 2000s, many native species were no longer even being caught while inedible invasives had come to dominate captures2!

Tragically, when Armi investigated this lake again in 2017 she was shocked to find only one of the original 18 endemic species- the rest were extinct!  In less than 30 years one of the world’s greatest freshwater fisheries dating back many millennia, had collapsed with no hope of recovery because of new species introduced with unforeseen consequences.

Although Lake Lanao is located at the other side of the planet, fish are still fish, and lakes are still lakes – the lesson of invasive species is repeating itself all across the globe – all lakes must be protected from invasive species or, in blink of an eye, we risk losing everything.  There is no reason to repeat this mistake; I think we would be smart to work together to identify (and solve) this problem, and the FWCF is now partnered with Armi’s group.


  1. Herre, A.W. (1933) American Society of Naturalists 67: 154.
  2. Ismail, G.B., Sampson, D.B. & Noakes, D.L.G. (2014) Environmental Biology of Fishes 97: 425.

On old problem, a new solution

January 1 2018 (from FWCF site)

Title: An old problem, a new direction


I’m on sabbatical leave this year pursuing a new—and promising—line of research at the University of British Columbia’s Institute of Oceans and Fisheries. After decades spent studying control and removal of invasive species, I’m focusing on the best way to preserve and protect our waters to prevent us from having to deal with such problems.

How can we do this? That question has led me to the concept of “protected areas.” This is the notion that sparing key portions of ecosystems from the most damaging abuses may enable us to save the whole. Pretty basic—and no advanced science required, at least not initially.

The concept dates back to Franklin Roosevelt and it led to creation of our national parks and refuges. It was first applied to the oceans in 2002, when the term “marine protected areas” (MPA) was coined ( MPAs have been hugely successful. In the past 15 years, over 14,000 of them have been created worldwide, and there are now MPAs protecting about 5% of the surface of the oceans (

MPAs come in many different shapes and sizes – they fit local challenges. Most allow some type of fishing, which they actively seek to improve, and most also seek to improve local economies by giving control to local users of the resources. MPAs are seen as the best hope to save the world’s saltwater fisheries.  I believe the concept (in some form) should work in lakes and river too where we desperately seek new solutions.

Dr. Daniel Pauly ( and his team at the University of British Columbia have played key roles in developing MPAs for the past two decades. Dr. Pauly was the first to point out that we don’t even know how many fishes there are in the oceans and to then develop techniques to address this. He also discovered that removing large fish as fishers do leads to “fishing down of the foodweb”—resulting in populations of smaller and smaller fish ( And two decades ago, his colleague at UBC, Dr. Rashid Sumalia ( developed economic models to justify new types of fisheries management as well as MPA creation and  design.

Water is water and fish are fish—so the basic concepts underlying MPAs can be applied to freshwater ecosystems and ultimately provide the same benefits.  And yet they haven’t been! How and where to start?

We do not know how many fish there are (and were) in Minnesota lakes, and thus how to improve them – and this is a critical first step in figuring out freshwater protected areas. I’m working now with Dr. Pauly and his team to develop this expertise. We also don’t understand the economic value of our freshwater fish and waters. I’m working with Dr. Sumalia to figure out how to calculate that value for Minnesota.  Finally, I am looking at how multiple factors (water quality, habitat, climate, etc) interact to create great waters and fisheries- and how a new holistic approach is now needed to save them.

We know our freshwater legacy is invaluable and worth saving. But we need to be able to put some numbers to it so we—and our policy makers—can move forward and make well-informed choices.

I look forward to keeping you updated as this compelling project moves ahead and to hearing from you about what you’d like to know more about. In the meantime, 2018 is off to a promising start.

Hi again!

May 14 2018


I apologize for the long break  but I’ve been busy and posting at another site, that of the Fish and Waters Conservation Fund.  However, for the sake of simplicity, I have decided to blog at this site instead.  I’ll bring a few blog postings over and then start here again.  I hope this approach will enable us to communicate more frequently and readily about the value of  saving our freshwater ecosystems and their fisheries – and the need for new approaches to accomplish this!  Talk with you soon!!

Thanks for your patience


Hi, Thanks for joining me. Please let me introduce myself

Why am I writing a blog when I could—and probably should—be writing articles for scientific journals? Minnesota is my home, and I am extremely concerned about the environmental decline of our state’s lakes and rivers. I also love scientific discovery, but have become frustrated about science’s inability to keep up with the growing number of new problems and an inability/ reluctance to deploy approaches. I believe that, while Minnesotans care deeply about our lakes and rivers, their understanding of how they function and what is needed to keep them healthy is limited, and so we are unable to act. Many folks think the answer lies in new science, and many scientists think the answer lies in social change. Both avenues need to be pursued, and in a coordinated manner together and aggressively.

This is no one person’s or group’s fault. The causes of environmental degradation are many and complex —from climate change and pollution to aquatic invasive species to overfishing and overdevelopment. Life is changing rapidly and we need to adjust. Even things as simple as taking a boat from one lake to another can now cause irreversible damage. Mille Lacs is a good example and what is happening there should be our wake up call. With declines occurring so rapidly, it’s not surprising that there is a lack of understanding and knowledgeable leadership at all levels. We live in a democracy and it is not too late to bring about change and make things better. However, the longer we wait the more difficult the challenge.

I am convinced that even small changes can accomplish great good and remain hopeful that we can change conditions for the better if we understand the problems, work together to solve them including making some minor changes in how we live. Future generations of Minnesotans depend on us

In figuring out how to help our lakes and rivers, I think in terms of the biological sciences because that is what I know best. I will try to describe new and important scientific facts you may not be aware of. But policy and public action are equally important mechanisms for change and improvement, and I hope that readers can provide insights in those areas. In return for being honest and as objective as possible in this forum, I ask for your thoughts and invite your criticism, suggestions and feedback now and for future entries, which I hope to post monthly. Some of the topics I plan to address are below—please let me know if there are others you’d like to hear about.

What makes Minnesota’s aquatic resources so special and what threatens them?

What is a “novel ecosystem”?

What is an “invasive species”? Why should we care about them and can anything be done?

What have we learned from invasive species like the common carp?

What is an “invasive meltdown”?

What is a “Marine Protected Area” and how might this idea be applied in Minnesota?

Why care about climate change and what can we do?

Why care about nutrients in our lakes and rivers and what can we do about them?

Your ideas

This blog reflects my personal views; it is not endorsed by nor is it intended to represent the views of the University of Minnesota, my funders, or my colleagues and students.