Displacement, Not Replacement

White-spotted charr...if you didn't know better you'd have sworn it was my beloved brook trout. Image courtesy of Tokyo Fly Fishing.

I have, and will continue to be, sparse with my updates.  I’m in one of the most dreaded times of a Ph.D. student’s tenure- the preparation for, and eventual taking of, my comprehensive exams (colloquially known as comps).
 
For those unfamiliar- comps are arguably the biggest hurdle that stand between a Ph.D. student and graduation. No two people have the same comps, but for me it will entail a 40-hour written test taken over the course of four days and a three-hour oral exam. But, it’s not the length that’s daunting.  It’s the topic.  Science.  Know anything and everything about science (and statistics, because good ecologists need to know what to do with their data after it’s collected). Just about the only way to prepare is to read as many textbooks, articles, and online guides as possible. So, read I must. Every day, all day, for the next month. Panic hasn’t quite set it, but I can tell it’s getting close.
 
While comps definitely add a bit of pressure, I can’t say the experience has been entirely miserable so far. It’s easy to get hyper-focused on one project and lose focus of how your work fits into broader ecological contexts. So, it’s been fun stepping back and thinking more broadly outside of hatchery introgression, which has been the object of all of my attention lately (but which we did finally submit for publication). And, I’ll write about all of that after comps. Promise.   
 
For now, in all my readings I stumbled across one very short article that seemed perfectly fit for this blog.  No long explanation required, but a research finding I suspect many of you will be interested in. The topic is one of my favorites: nonnative fish invasion on native populations. So often I hear people argue something to the effect of ‘nonnative fish are not causing declines in native populations, but simply taking over as native populations are dying off due to climate change, habitat loss, etc.’  To put another one, people often think that nonnatives are REPLACING, not DISPLACING native fish.
 
If this were true, if nonnative fish were simply replacing natives, then there wouldn’t be a lot we could (or probably should) do about it.  When a population of a species starts declining, it opens space in the ecological niche that usually needs to be filled in order to maintain ecosystem functioning. It’s like a job opening- if Debbie was going to quit, it’s better that someone fills the position rather than just leaving it vacant and hoping Debbie returns. Replacing Debbie would be like filling the empty ecological niche.  
 
But, what if Debbie had no intentions of leaving and is being forced out? In this case, Debbie would be displaced. The intruder will likely fill some of the same roles Debbie left behind, but may also neglect certain tasks. As we all know, two employees with the same job description rarely have comparable work effort and quality. The intruder is going to leave some of the niche open.
 
Bringing this back to fish, it’s really difficult to determine if a nonnative species are replacing or displacing a native species. If we were to track the number of fish of each species over time, we’d likely see that one species (usually the native) was increasing and one (usually the nonnative) was decreasing. But, that is not evidence for either replacement or displacement. If the nonnative species was absent, the native species may still decline due to habitat loss, genetic collapse, or overharvest. Or, it may thrive despite all the aforementioned stressors. The only way to know for sure would be to do several very controlled experiments where we artificially added or removed fish from streams and then monitored their populations for several generations.  But there are time constraints, and largescale changes to species communities are generally frowned upon in conservation. 
 
So…enter statistics, where we can model the relationship between the abundance of each species, time, and environmental variables to determine how each species effects one another. If that sounds vague, it is.  But, the details aren’t worth describing here. It’s just worth noting that a researcher from Japan recently used these models to investigate how invasion of nonnative brown and rainbow trout influenced the abundance of native white-spotted char (a close cousin to brook trout) using data collected over 15 years.
 
And his findings?  Nonnative trout clearly DISPLACE native trout. Moreover, rainbow trout also displace brown trout, so not all nonnatives are created equal. If you think about Pennsylvania, right now brown trout are outcompeting (whether it is replacement of displacement, we don’t know) brook trout.  In the future, could we see displacement of brown trout by rainbow trout?  Certainty possible.  
 
Perhaps more noteworthy, there was a significant time lag (8-13 years) between the initial invasion of nonnative trout and displacement of white-spotted charr. But, once displacement started, it was achieved rapidly (just a couple years).  This suggests that monitoring efforts following invasion may have to extend for several decades before the effect of invasion are realized. It’s not enough to make conclusions about invasion based on only a few years of data, and certainly not enough to make inferences on the cause (be in replacement or displacement) with such limited information.

*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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