Streams, Bugs, and the Future of Species: An Interview with Chris Funk

On an unseasonably hot day last week in Fort Collins, Colorado, I sat down with Chris Funk, assistant professor of biology at Colorado State University, to learn about a new research project aimed at predicting species’ vulnerability to climate change. Called Evotrac, the project, led by eight investigators from three universities, is focused on insects—and to a lesser extent fish and frogs—that live in headwater streams in the Colorado Rockies and the Ecuadorian Andes.

(WARNING: For ecology wonks only!)

HR: What’s Evotrac all about?

Chris Funk: A lot of the limitations on predicting the effects of climate change have to do with our limited basic ecological and evolutionary understanding. So you have to step back and ask some bigger, basic questions in the field. Our premise is that the historical selection pressures that these species have experienced wherever they live are going to influence how vulnerable they are to climate change.

Our idea is that the type of variability in temperature and stream flow that an organism experiences is going have an important effect on its traits, and those traits will determine how vulnerable it is to changes in temperature and changes in disturbance. So, for example, if a species experiences lots of variability in temperature throughout the year, you’d predict it has a high tolerance to changes in temperature. In Colorado, a species that lives here all year round has to be pre-adapted to a lot of variation in temperature. So if temperature rises, or becomes more variable with climate change, then those organisms would be less vulnerable.

We’re also looking in Ecuador. An organism at a given altitude in cloud forest in the Andes throughout the year experiences a relatively small range of temperatures. At the same elevation in Ecuador as in Colorado, it’s a relatively constant temperature all year round. And because those organisms in the tropics don’t experience much variability in temperature, we predict that they have relatively narrow thermal tolerances.

HR: What would that mean for tropical species?

CF: It suggests some tropical species might be more vulnerable to climate change. Even if the absolute temperature changes very little in the tropics compared to the temperate zone, because those species are very sensitive to changes of temperature, just a small amount can throw them over the edge.

This idea is known as the climactic variability hypothesis. It’s been attributed to Dan Janzen, from a paper titled, “Why mountain passes are higher in the tropics.” The idea is, say you’re in the middle of a mountain range in the tropics. There’s a greater cost to going over it than there is in the temperate zone, because you have to go into a climate that you’re not adapted to.

HR: Has this hypothesis been tested?

CF: There’s been some testing of it, but not so much in freshwater systems. And there are all sorts of predictions based on this hypothesis. One is narrower thermal tolerance in the tropics. Another is that you’re going to have narrow elevational ranges in the tropics—so that if you looked at the species’ distributions, you’d predict that they’d have narrow bands corresponding to elevation, kind of like those Jell-o cakes where you’ve got different flavors at different levels, to use a friend’s analogy. Whereas in the temperate zone, it’s all one flavor of Jell-o.

You can also make predictions about dispersal. Because it’s costly to move in the tropics, species will have lower dispersal abilities. This is something else that will determine vulnerability to climate change, because it’s important to be able to move and track suitable habitat when that habitat is changing in space. You want to be able to move from your currently suitable habitat to one that becomes suitable in the future.

HR: What else are you hoping to accomplish, besides testing that hypothesis?

CF: Another thing we’re trying to do is link the changes that occur because of climate change to predicted effects on ecosystem processes. Evolutionary history affects the traits of species that you find in a given spot. Now we’ve got this new rapid climate change, and the traits an organism has are going to affect whether they persist in the system. And then you have a new set of species that persist, and those will affect the ecosystem processes—say, nutrient cycling. And those changes will in turn feed back on the kind of organisms that can live there.

HR: You’re planning to compare related species from each area. So you would take, say, a mayfly in both these places and look at how it’s physiologically different? How do you measure the differences?

CF: In general, we’re finding one mayfly species in a given family in Colorado and a similar one in the same family in Ecuador, and then we’ll look at their thermal tolerances. The most basic standard way of doing it is stick them in a little container and heat up the water and see at what point they have a hard time functioning. That’s the physiological measurement. For dispersal, we’re using population genetic techniques to look at gene flow between populations. If there’s more gene flow—if the populations are more homogenous—there’s more dispersal. And if they have greater dispersal ability, you’d predict that species would be more resistant, less vulnerable to climate change.

HR: What’s the ultimate goal? Just because we can make predictions doesn’t mean we’re going to do something about it. Does this change the way we might think about conservation?

CF: In the broadest sense, our job is to help predict where there will be certain areas that are hotspots of vulnerability, so we can focus our management and policy in those areas, because obviously there are limited resources. Up until recently, people have been more worried about the temperate zone and the Arctic/Antarctic than the tropics. But the problem is that you don’t know anything about the sensitivity of organisms to a given unit change in temperature, or a given change to the flow regime of a stream.

Potentially we might be able to imagine some sort of vulnerability map of the western hemisphere, and red means you’re very vulnerable to climate change and blue means you’re not. And maybe, if these hypotheses are right, then the tropics would show up very red.

There also might be differences between groups of organisms. Some family of stonefly might have a really low dispersal ability. There are some stoneflies that have no wings. So we might say not just that these regions are vulnerable, but these organisms in these regions. And, say a particular organism has a very important functional role in ecosystem processes—say it’s very important in nitrogen cycling, which is an important process in these headwater streams. We could say, then, that it’s gonna really change how these streams work at a bigger level if the organism disappears.

So you could argue whether you care about one individual species of stonefly or not. But if you say that this whole functional group is really vulnerable, and that its absence changes the ecosystem processes, and all these headwater streams connect to rivers, and these rivers are used extensively by people… There’s an easy argument about the importance of headwater streams, and freshwater in general, for human welfare.

Photo credit: Jeanne Robertson

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