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Invasive Species is a Loaded Term

Updated: Aug 27, 2021

If you are reading this page, there is pretty much a certainty that you are familiar with the term invasive species. You may have helped clear buckthorn, taken down dead ash trees after Emerald Ash Borers have killed them, dealt with oak trees affected by oak wilt, or you are at least familiar with some of the invasive plants such as Wild (poison) Parsnip that we have learned to avoid. You are almost certainly familiar with a number of invasive species that are altering our aquatic environments such as Zebra and Quagga Mussels, New Zealand Mud Snails, Spiny Water Flea, and a host of invasive salmonids such as the Great Lakes region Coho, Chinook, steelhead, and of course the omnipresent Brown Trout. Yeah, I went there and you will see why.

You see, when you look at the definition of invasive species, I will demonstrate that they all fit the bill.

How are invasive species defined? As per Executive Order 13112 (Section 1. Definitions) an "invasive species" is a species that is:
1) non-native (or alien) to the ecosystem under consideration and
2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health.

Seemingly, a more accurate definition of invasive species excludes species that us humans like. Certainly we do not consider any number of crops to be invasive - and nobody can argue that corn and wheat have not caused environmental harm. Is there anything that has had more impact on our environment than agriculture has? And yes, before you ask, I do like to eat and understand the necessity of growing food. The larger point is that if we are measuring environmental harm - and only environmental harm, agriculture leads that list. To be even more contentious, let's look at our pets. House cats are responsible for killing incomprehensible numbers of birds and small mammals, it is an undeniable fact. In Canada, house cats are estimated to kill 200 million birds - the second most deadly source are windows at 25 million birds (source).

"If we even step back from Canada and we look globally, we know for a fact that cats have contributed directly to the extinction of 34 species of birds. Next to rats and humans, of course, that's the biggest factor," said Ted Cheskey, a conservationist with Nature Canada.

Source: Cats, the No. 1 killer of birds in Canada


Now, obviously we do not consider cats to be invasive despite the fact that they are not native and they cause significant environmental harm. Nor do we consider Ring-necked Pheasants to be invasive. In fact, it is the state bird of South Dakota and who the hell would have an invasive species as their state bird? But like Brown Trout and Pacific salmon, they also have some negative impacts on the communities which they have been naturalized.

The discussion of what is invasive and what is not gets even more strained when we start talking about specific strains of a species becoming invasive, even when they are native. The classic example of this are Phragmites australis (Common Reed) and Reed-canary Grass (Phalaris arudinacea) which are both native plants but there are particular genotypes of these plants that are considered to be invasive. They are both species that have ranges than include both Europe and North America. Cultivars unique to Europe were brought to America and have become the dominant genotype of those species in North America. We often see these species referred to as invasive when it really is a particular genotype of the species that is being referenced. Talk about confusing.


Much of this discussion comes down to terminology - but words are important! Invasive species is sort of a term without a widely accepted definition mostly because there is always this "wiggle room" because we humans like some species more than others. Largely, these species we like more than others have some economic or emotional value for us. Less loaded words are "non-native" or non-indigenous which do not consider what we think of these species. My preference is for the term non-indigenous as it seems more elastic to fit species as well as genotypes.


Brook and Brown Trout Species Interactions


Here in Wisconsin, the Brook Trout is native species and the Brown Trout is non-indigenous. We see the impacts that Brown Trout have on our native Brook Trout as it is pretty significant. Travel around much of Wisconsin and Brown Trout are dominant in most of our trout streams. But if you travel out West, it is often Brook Trout that are negatively impacting native Cutthroat Trout - along with the non-native Brown and Rainbow Trout. And in the Great Smoky Mountains, Rainbow Trout are responsible for declines in native Brook Trout. It is quite evident that location seems to matter a lot in how native species are impacted by non-indidgenous species.

Source: Wild Trout Trust


Though one can argue that Brown Trout are filling a niche that has been altered or created by our degradation of streams - and there is some truth to argument - that they negatively impact Brook Trout is really not debatable. The argument is much the same as for Ring-necked Pheasants and some of the prairie species that they have replaced (Greater and Lesser Prairie Chickens, Sharp-tailed Grouse). Though, much the same in the Galliformes birds, the non-indigenous pheasant has negatively impacted the native bird species (Google Search results). Brown Trout negatively impact Brook Trout over much of the area where they overlap within the Brook Trout's native range.

Native Brook Trout
A Brook Trout from one of the Coulee streams - a rarity compared to their Brown Trout cousins.

Want some evidence that Brown Trout negatively impact Brook Trout? A project by Kirk Olson, Kristina Pechacek, the Wisconsin DNR, and the University of Wisconsin - La Crosse on Maple Dale Creek, a Bishop's Branch tributary in the West Fork Kickapoo watershed similar to the Seas Branch Brook Trout restoration project demonstrates the effect that Brown Trout have on Brook Trout. Kirk shared with Coulee Region TU that in 2020,

We captured and released 3,661 catchable size brook trout in the watershed, a major increase from last year, when we only captured 570. We didn’t see that level of brook trout increase in Cook Creek, which we also sampled the past two years, but did not remove brown trout. This indicates that the brown trout removal was responsible for the increase in brook trout in Maple Dale, as we expected based on research completed in other parts of the Driftless.

For more results from Kirk and the La Crosse WDNR office about 2020 activities, look back to the Coulee Region TU Facebook page in December of 2020 through February of 2021 for a number of posts with more information. Or better yet, watch Kristina Pechacek's Wisconsin Chapter of the American Fisheries Society video (linked here and below in the image).

The results are pretty staggering. And, yeah, I get it, you like Brown Trout. I do too. They get larger, they are more wary, and they seem to fit most of our streams better. This is why invasive is a loaded term. Brown Trout are non-native and they have a negative effect on native species but we like them so almost nobody is calling them invasive. But they have changed the communities in which they live. They have not only pushed Brook Trout to the coldest of streams but Brown Trout have also had an effect on other native fishes through their increased piscivory. There is evidence from the Grand Canyon, California, New Zealand, and closer to home - in Wisconsin and Minnesota. Brown Trout have decreased species richness - though as you may remember, species richness in coldwater streams should be low. They are just not the trout that is supposed to be here.


So What Do We Do?


It is pretty much impossible to put the genie back in the bottle. And to be honest, I do not think it would be very popular to remove Brown Trout if it were possible, which it is not. Brown Trout have been naturalized - the word we give non-indigenous species that have become an established part of the community - over a huge geographic area. We often use this word to differentiate wild, naturalized species from hatchery fishes planted in streams. We could not remove Brown Trout from the environment if we had wanted to - except in a few relatively small places.

LUNKER structures in a Driftless Area Stream
Overhead cover, such as these LUNKER structures, tend to promote Brown Trout over Brook Trout.

What we can do is help support projects that give Brook Trout a leg up on their Brown Trout competition. There have been two ways we have attempted to do this. First, is that there is some evidence that Brown Trout prefer more overhead and bank cover whereas Brook Trout will respond better to mid-stream current breaks. We have attempted to design habitat projects that give Brook Trout an advantage over Brown Trout but the results have been mixed, at best. Our understanding of how the two species use habitat is imperfect and conditions other than habitat may be what is driving Brown Trout to outcompete Brook Trout in most Midwestern streams.

Seas Branch Brook and Brown Trout data
WDNR data from before and after the Seas Branch Brown Trout removal project indicated by the dashed line.

Source: Wisconsin DNR; Kirk Olson, Kevin Mauel, and Kristina Pechacek, Evaluation of trout population trends and fisheries management in the West Fork Kickapoo River Watershed (2021)


The second and generally more successful strategy has been to use barriers to dispersal to isolate Brook Trout. A number of successful Brook Trout restorations have removed Brown Trout from above barriers and allowed Brook Trout to live without the competition and predation from Brown Trout. Not surprisingly, Brook Trout populations have thrived under these conditions. However, this management strategy is not without problems. First of all, it requires a significant amount of effort over several years to remove Brown Trout. One common issue with removal projects above barriers can be limited by habitat area. Relatively small, unconnected stream reaches such as those created above barriers are less likely to weather droughts, floods, fish kills, and other impacts. Small, isolated reaches will have smaller populations and there are genetic and ecological impacts unique to small, isolated populations. It is part of why there has been such a push to restore stream connectivity by removing barriers. Fortunately in my neck of the woods, trout densities are quite high and do not suffer from some of the genetic issues associated with small, isolated populations (inbreeding, genetic drift). But all isolated populations have the potential to suffer due to catastrophic events such as flood or drought.


And of course, we have to hope that some bait bucket biologist does not ruin it for everyone.


What You Can Do


There is no silver bullet or easy solution. The most successful management strategy has been to eliminated competition between and predation of Brook Trout by Brown Trout by removing Brown Trout. Maybe this is not as clear as I think it is - why we need to do something is that Brook Trout are our native species and we should do things to promote their long-term existence, particularly in the face of climate change.


What you can do is help support and promote Brook Trout restoration projects. Reach out to the DNR fisheries biologist in your area with project ideas - and support. Trout Unlimited chapters and other conservation organizations can prioritize Brook Trout over Brown Trout where feasible and likely to be successful. I would like to see more streams with regulations that promote the catch and release of Brook Trout and the harvest of Brown Trout. And, where we do have streams that support Brook Trout, keep those Brown Trout that the WDNR wants you to keep and give Brook Trout a foot up on their invasive competitors.


Yeah, invasive is a rather loaded term but there is no question that Brown Trout have negatively affected our native Brook Trout.

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