Everything’s fine until the invasives move in.
I’ve preached this before. Invasion by nonnative trout results in declines in native trout abundance. On the east coast, I’m talking specifically about invasion of nonnative brown and rainbow trout causing declines to native brook trout. But, what is the mechanism of decline?
Is it competition? Sure. Nonnative trout can outcompete native trout for food, habitat, and sometimes even mates (enter tiger trout).
Is it habitat preference? Yep, that too. Brown and rainbow trout tend to have higher thermal tolerances, and so they can live in a wider range of habitats. They can also occupy streams with altered flow regimes, higher sedimentation, and lower water quality.
What about growth? We have a trifecta- nonnative trout tend to grow faster than natives. This makes nonnatives better competitors, but bigger fish also tend to produce more offspring. So, populations of nonnative trout tend to grow fast and can quickly outnumber native trout (this usually isn’t the case of rainbows in Pennsylvania, but down south rainbow trout populations are taking off and outnumbering brook trout).
But, you know what else it could be? Maybe nonnative trout act as a strong selection pressure. This could cause native trout to become maladapted to their local environments because interactions with nonnative fish are acting as a stronger, more acute selection pressure than the environment. Huh?
Let’s break this idea down a little. We often think about the environment as the strongest selection pressure that shapes the genetics of populations. And, that’s not wrong. Through hundreds of years of natural selection and adaptation, trout populations have accumulated the genes and outward characteristics that make them best at surviving in coldwater stream habitats. At this point in the evolutionary time scale, the amount of variation in those characteristics is really quite small. Yes, brook trout show a lot of variability, but you can still identify a brook trout from, say, a bass that has spent millions of years evolving for life in a different type of habitat. Almost every brook trout is now well-equipped for life in the typical stream environment.
So, now we’re at the stage of fine-tuning the genes in populations. There’s a lot of genes that are good for life in a stream, but only a subset of those are also good for surviving a catastrophic flood. And, only another subset for devastating droughts, or unseasonably hot summers. So, natural selection is still at work. But, it has to wait for these very rare events to occur before there is large shift in the genes in a population. Until then, populations just maintain the characteristics that make them good at life in their streams.
But, then life in the stream changes. A nonnative fish invades, and starts imposing a new selection pressure. Suddenly brook trout, which are often the top predator in a small stream, need to compete with another species for food and habitat. And, because presence of the nonnative species is a constant pressure that can act on native species every day and in multiple ways, it starts acting as a stronger selection pressure than rare environmental events.
Think of the red line as the genetics in trout populations. Historically, back when fish were new to the animal kingdom, trout and bass probably looked very similar to one another. As evolution occurred, trout genes started becoming more adapted to stream life until there was very little variation in the genes of trout populations (relatively speaking). That was, until the nonnatives moved in....
It may sound a bit far-fetched, but a team of researchers recently completed a study to see if invasive trout could be acting as a selection pressure that overrides selection from the environment. Their work was conducted in Sweden, so in this case the invasive fish was our beloved brook trout, and the native was brown trout. What they found was that, in the presence of nonnative brook trout, brown trout developed stouter bodies, had a smaller home range, and even shifted their diets to consume more terrestrial prey. When brown trout weren’t in the presence of brook trout, they had short daily movements, high metabolic rates, and high activity.
How did brook trout cause this change? It seems to be related to a change in how brown trout live their daily life. When the only top predator, native brown trout can afford to live a high risk, high reward lifestyle. They are free to swim around, eat a lot of the best food (which are often bugs living on the stream bottom), live in the best environments, and defend quality territories from subordinate individuals. To sustain this lifestyle, fish need to have high metabolisms (to keep up with energy needs for swimming and fighting) and body shapes that are more slender, which are better for sustained swimming and foraging.
Now, add nonnative brook trout to the mix and brown trout are no longer standing at the top alone. There’s less freedom to move around and find insects on the stream bottom, and so trout switch to a “sit and wait” feeding strategy. Instead of actively foraging, they become drift feeders and wait for terrestrial insects to fall into the stream near them. The addition of brook trout also means there’s generally less food available for each individual, and so slower metabolisms (which require less food to sustain basic biological function) are favored over faster metabolisms. But, slow metabolisms are associated with reduced growth, reproduction, and movement, and so body shape changes and fish develop smaller home ranges.
So, the addition of a nonnative trout species results in more than just competition. It can also induce evolutionary change and alter the native species’ behavior, morphology, and physiology. Do these changes then make native species maladapted for everyday stream life? Or, could it reduce survival when there are catastrophic events? How does the presence of a nonnative change the adaptive potential of a native species? I think we need more study to really answer those questions.
*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
I know you’ve done it. You’ve gone fishing in two different streams, or maybe even just looked at photos of brook trout from different areas, and thought “why are these fish so different from one another?” And, I’m not just talking about length. You may have noticed that brook trout can show a very large range in color, patterning, body depth, fin size, etc. Even fish within the same stream can sometimes look completely different. What gives?
It’s a great question. And the answers potentially have big implications for our understanding of how trout respond to fragmentation. I’ve already told you that fragmentation results in population isolation, which can lead to loss of genetic diversity and eventually lead to local extirpation. But, there are other changes that can occur before an isolated population collapses- the population size might decline, the ratio of males: females might become skewed, fish may start behaving differently, and, that’s right, individuals may start taking on different physical appearances.
The question is whether fragmentation leads to predictable changes in morphology across populations. If so, it indicates that fragmentation could be a factor that influences the evolutionary trajectory of populations in a predictable way. For example, if fragmented populations show a tendency to have reduced body coloration, then we would know that fragmentation somehow operates as a selection pressure, and that bright coloration is somehow not advantageous in fragmented populations (this scenario is purely hypothetical, by the way).
So, scientists being scientists, someone went out and tried to determine if fragmentation is an evolutionary selection pressure that acts on brook trout morphology. In a recent paper, researchers from Canada sampled individuals from 14 brook trout populations in Newfoundland, Canada. These populations are all genetically distinct, and upstream of barrier waterfalls (begs the question as to how the brook trout got there. Let’s leave that story for another day, but it was all natural, promise). At each site, the team took measures of body size, weight, and sex. They also took very detailed pictures so that they could later digitally measure things like body color, number of spots, body depth, fin length, hump size, jaw length, etc.
Is fragmentation a significant selection pressure on brook trout morphology? Yes and no. The researchers definitely found that populations had very different morphologies. Isolation has prevented gene flow, which has put each population on it’s own evolutionary trajectory. But, population size and standing genetic diversity, which act as a proxy for the strength of the effect of fragmentation, didn’t predict morphology that well. This indicates that fragmentation, itself, isn’t a factor that influences morphological change. Rather, current habitat conditions seemed to have a stronger influence of morphology. Trout in warm, slow streams tended to grow larger and develop a larger hump on their backs (reminder: warm is relative, these populations are in Canada), fish from faster streams tended to have longer pectoral and pelvic fins, and sites with more acidic water had fish with redder color tones.
Interesting, the environmental associations tended to be stronger in females than in males, and females also had more morphological traits that were correlated to habitat. For example, females developed redder tones in deep, fast, warm water, but the association was weaker for males. Why this is the case isn’t entirely clear. But, it likely has something to do with the fact that sexual selection acts much more strongly on males. Sexual selection is a form of natural selection that is specific to traits that increase reproductive success. Think about brook trout spawning behavior, and how sexual selection may act differently on each sex. Females build redds and then wait for eligible bachelors to arrive. Males have to compete for access to females, and subordinate males aren’t going to produce many offspring. This means that natural selection (via sexual selection) is going to strongly favor males that have traits associated with fighting ability during spawning, even if perhaps they are a little less adapted for the environment outside of spawning. Sexual selection acts less strongly on females, and so natural selection is going to favor females that are generally adapted to their local environment. As a result, female morphology is more strongly correlated to habitat than male morphology.
So, why do we care about these results as fish managers and conservationists? While many organizations are making strides to increase movement corridors and reconnect populations, streams are still becoming fragmented by loss of thermal habitat, road crossing, dams, etc. The results of this study suggest that fragmentation, itself, doesn’t seem to pose a strong selection pressure. But, the habitat that the fish become isolated in does. By building a road crossing, we could be effectively deciding the morphological fate for brook trout populations. How this could influence population survival remains unclear, but changes in morphology don’t seem, at least right now, to result in rapid loss of population survivability.
The more you know…
*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.