Fish business

The little Google weather icon is showing me snow showers on Friday. If that comes to fruition, it'll push our field work back - again. But, sometime soon, we'll be headed out to start looking at some ecological interactions involving freshwater fish in a few of our local streams. We're planning a series of microhabitat studies using darters, a group of small freshwater fish in the Family Percidae, to investigate how environmental factors influence community structure. That's a rock darter, Etheostoma rupestre, to the right, and we know that it prefers very specific microhabitats. Other species, like this blackbanded darter (Percina nigrofasciata) have very different affinities. It's ecology, but there are some evolutionary questions as well, i.e., how has selection shaped different darter species for different environments. Both of those images, by the way, come from the tremendous web site put together by the Alabama Department of Conservation and Natural Resources.

The overlap between ecology and evolution is par for the course. I find myself talking about many of the same topics in my ecology classes and my evolution classes. The two disciplines are often looking at similar questions, with only the time factor distinguishing one from the other.

Well, the line's blurring a bit more. A new study coming out in The Proceedings of the National Academy and referenced at Science Daily has used another group of fish (guppies) in streams in a different part of the world (Trinidad) to examine the way in which evolution can influence environment. It's usually the other way around, of course. We normally see the way in which environmental pressure leads to selection for certain traits, leading to evolutionary change. That's because, normally, we don't think that evolution can happen fast enough to bring about ecological effects. But that's obviously not always the case. This study, led by David Reznick of the University of California - Riverside, is a great illustration.

Reznick and his coworkers worked in two tropical streams with very different fish communities. One stream contained a fish assemblage including guppies and a diverse group of predators, some of which fed on the guppies. The other stream had a much simpler community structure - just a few guppies and a few other, non-predatory species. In previous work, Reznick and his team had demonstrated that predation could bring about fairly dramatic changes in the life histories of the guppies - guppy communities exposed to intense predation reached sexual maturity quicker, produced more young, and even developed different body shapes.

The current study goes a step further. The researchers collected guppies from the two streams and put them into identical artificial streams, designed in such a way as to mimic the natural streams from which they were collected. After a month, they found that the guppies from the two different types of communities had had dramatically different effects on their new streams. It started with feeding habits - guppies from the high-predation streams fed heavily on insect larvae, while guppies from predator-free streams had more of a taste for algae. This led, in turn, to differences in productivity and nutrient-cycling in the two artificial streams. The evolutionary impacts on the guppies was leading to environmental differences in the streams in which they were found. Furthermore, the results seem to correspond to conditions the researchers see in natural streams in the region.

Not surprising, maybe. But something else to think about when we're rock darters out of Alamuchee Creek.

Lots of interesting ideas coming out of studies of stream fish. Another one coming up later this week.