Monday, January 28, 2013

The Hybrid Conundrum by Moderator Anders Halverson

One of the most vexing problems posed by the widespread introduction of rainbow trout in the American West has been their propensity to hybridize with the natives. Take for example, the westslope cutthroat of the northern Rockies. Once the most widespread cutthroat subspecies, their range has been dramatically reduced in the last century due to the introduction of rainbows. Just how far it has shrunk, though, is a matter of some contention. According to the criteria applied by the U.S. Fish and Wildlife Service, a fish can still be considered a Westslope cutthroat even if it has a small amount of rainbow trout DNA (say from a great-grandparent). Such fish still occupy about 20 percent of their original range.

Westslope cutthroat native range
But if only those fish that have absolutely no rainbows in their family tree can be counted, then that number drops to less than 2 percent. What’s more, most of these purebred fish are restricted to high, isolated, headwater streams. They’re threatened by inbreeding depression and, since all the pure mainstem populations have been compromised, many of the genes that might have adapted the species to larger rivers can probably only be found today in hybrids.

Interbreeding between slightly hybridized westslopes and their unpolluted cousins might alleviate these problems. But it would also create new ones. There are the biological issues, of course—some studies for example suggest hybrid fish are less fit. But maybe more importantly, it raises a host of legal issues.

The Endangered Species Act passed by Congress in 1973 did not include any guidelines on hybrids. Thus, when the issue arose in the 1980s, Department of Interior lawyers concluded that hybrids could not be protected under the act. It seemed a reasonable decision, at least for a few years. But when new technology gave us the ability to sequence DNA, it became apparent that hybridization was far more common than anybody had previously thought. Certain wolves (which had a little coyote DNA) and other listed species were thrown into legal limbo.
A likely hybrid westslope-rainbow trout

After a brief period of struggle, the lawyers threw in the towel. Hybridization would have to be handled on a case-by-case basis, they concluded, since hybridization was “more properly a biological issue than a legal one.”

I wonder whether it isn’t also an issue of human values and psychology. Are we really so concerned about purebred westslope cutthroats because hybrids may be less fit or because biodiversity may be associated with ecosystem stability? Or do untainted westslopes somehow appeal to our thirst for the prelapsarian, the rare, or some other innate urge?


Mark LaRoux said...

It seems to me that a repeating theme is our human (or European?) need to 'draw lines' in whatever we are studying...states with their imaginary boundries, species with the 'legal' degrees of hybridization, cladists, etc. Heck, substitute Native American for westslope (just for fun) in the aforementioned story...More hybridization. The same issue was brought up for tiger salamanders...who's pure anymore and does it really matter? In all these examples it only matters because a lawyer or biologist says it does. Just like with the trout, does it matter if my grandparent was a 'pure' indian or half? If so, to whom?

Julie Dunlap said...

The situation you describe with westslope cutthroat is similar to American bison, which mostly have domestic cattle dna except for a few small groups (including Yellowstone's). There is evidence that hybrid bison are not as fit as nonhybrid animals (under the conditions studied). The authors conclude, ". . . genomic integrity is important for the conservation of the American plains bison."

Anders said...

Good points both.
It seems the more we know, the more we realize how ignorant we are, and the more arbitrary traditional lines become. I guess that's a good thing.
I find the species problem especially interesting. There are currently more than 20 different definitions for species out there, most of them developed in the last few decades. And since so much of conservation is based on protection of individual species or biodiversity (which is almost always quantified in terms of species), it's getting harder and harder to know what to do.