When is a fish not just a fish?

Hard to imagine, but this is one of the most threatened ecosystems in the world.

Imperiled
 
That’s one of those fancy words scientist love to throw around to make our sentences more sophisticated. And, for some reason, journals are more likely to publish “freshwater ecosystems are among the most imperiled worldwide”’ than “kiss ‘em goodbye, freshwater streams are tanking.” Beats me.
 
But, think about that (using whichever sentence you prefer).  Freshwater streams are among the most threatened ecosystems on this planet and, on average, freshwater fish are going extinct far faster than marine animals and all the air breathers. That’s no joke. We hype up the polar bears (for good reason….), while in plain sight is an ecosystem that is crashing before us. To put this into perspective, if we consider local extinctions, otherwise known as population extirpation, from climate change alone, the frequency of local extinctions in freshwater ecosystems is about 20% higher than marine or terrestrial environments.
 
20%!
 
But, let’s play devil’s advocate. A local extinction simply means a species is lost from a specific ecosystem. For example, brook trout might go extinct from a specific stream reach. When a species goes missing, it leaves a hole in the ecosystem that usually gets filled by another species that, in many ways, operates in the same way. The new species will usually eat the same stuff, have the same population sizes, and seem to be a 1:1 replacement.  But, is it really the same?
 
Think about it from a business perspective. If all the burger restaurants leave town, it opens up the market for another burger restaurant to move in. But, not all burgers are the same. Will the new place have the same menu?  Will the food taste as good?  Will they generate as many jobs as the old?
 
Will new species function just like their locally extinct predecessors? A study recently found that the answer is probably not. And, a species doesn’t necessarily have to go extinct for the ecosystem to fundamentally change after a new fish species move into down. Invasion fundamentally changes how an ecosystem operates.
 
Looking across continents, a group of researchers aimed to answer the question “how much does functional diversity change when a species invades a freshwater ecosystem.”  You can think of functional diversity as the “menu” of the new burger restaurant.  It’s basically how a species operates within an ecosystem- how far does it move, how much does it reproduce, what does it eat, how does it interact with other species, etc. Big changes in functional diversity can mean big changes to other fish species present in a stream, as well as the insect community, plant community, and the flow of nutrients through the environment. 
 
What the study found was that species invasions increased average functional diversity by 150%. If you want to continue with the burger analogy- the menu of the new place is 150% larger. But, bigger is not always better. A pristine ecosystem evolved with a certain amount of functional diversity, and an increase by 150% means that the ecosystem is probably getting stressed in new ways. For example, they found a general pattern that invading species having larger, deeper body shapes. Species with this body patterns tend to live in slow-moving waters, and really excel in life in deep pools and impoundments. If streams and rivers are dominated by those species, and there are fewer fish living in swifter currents, then it could reduce predation on certain insect species which, big picture, will disrupt the food web.
 
And, 150% just represents the AVERAGE change in functional diversity. They also found that changes were higher than average when the invading species was truly nonnative (like, maybe from a different country, as opposed to from a neighboring watershed), and when the original ecosystem only contained a few species.
 
So, why bring this study up on a trout blog?  I frequently like to imagine what stream ecosystems are going to look like in 200 years.  Right now, we are already seeing rapid changes in the species diversity in stream ecosystems.  We’ve stocked a lot of nonnative fish, to the point that it is sometimes difficult to know when a species is truly native anymore. Of course we know the history with brown trout. But, smallmouth bass?  Nonnative. Channel catfish? Mostly nonnative. Bluegill?  Guess what- mostly nonnative. A lot of these species mix with native species in cool and warmwater rivers and, as climate change advances, we continue to see these species creep further into the headwaters in search of cooler water. It’s now not that uncommon to find bluegill and brook trout together. These two species aren’t direct competitors, but how are those invasions going to change the stream ecosystem as a whole?
 
I guess only time will tell….

*Note: Content in this post is my own and may not reflect the opinion of the manuscripts' authors or the agencies they represent.  I encourage you to read the manuscript, found here, so you can contribute to the discussion
 
 

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