Do invasive species pollute the gene pools of other species?

"These chilies and avocados are fantastic -- let's take some back to Europe with us!"

The Age of Discovery, starting in the 15th century, was a time of phenomenal far-flung adventure. Transoceanic travelers risked their lives and attempted outrageous feats to chart a particularly devilish route for the first time or uncover a historically monumental find.

While on these daring quests, explorers carted around all manner of plants and animals to remote locations, taking specimens back with them on return voyages. And when the boats landed back home or at some stop along the way, they often simply let their specimens out. These actions, and most notably the delivery of alien species to newly discovered islands with their own isolated populations, probably didn't strike many of the sailors as particularly noteworthy. A couple of mice here, a wasp or two over there, a stray pig wandering off; what's the big deal? Well, as it turns out, through actions like these, world travelers unwittingly launched one of the greatest biological experiments of all time.


And what affect have these buccaneers/mad scientists had? For the most part, the alien species they delivered were fairly innocuous or unsuccessful in establishing permanent populations. But sometimes, they triumphantly put down roots. And in a small percentage of cases, they flourished with unprecedented and unsettling success that put severe pressure on native species.

There are many ways invasive and native species can react to an abrupt habitat mash-up of this sort. Sometimes one species can simply be more competitive and successful at acquiring food resources, either naturally or by shifting its behavioral patterns. But another, sneakier method of adapting involves genetics and some breakneck-pace evolution. More about that on the next page.


Evolution and Dynamite Genes

Look at him -- quite the ladies man!

Competitive evolution can be a two-way street. Sometimes a native species bucks up and wins, but the pressure from a beneficially adapted invasive species is often too intense. Some success stories for natives, however, include the soapberry bug which evolved different beak lengths in 50 years flat to take advantage of a new invasive host species' anatomy [source: Stanford University]. Another case happened in Hawaii, where the banana plant was introduced around a thousand years ago, a microsecond in evolutionary terms. Since then, five species of moths that we know of evolved to take advantage of the tasty new treat. Today, however, those same moths are slowly losing their battle against yet another invasive introduction -- wasps and flies that were imported to help with agricultural pest control.

In other cases, the clear winners are the invasives. Take mallard ducks, for instance, and we run into a case of hybridization. Mallards have been introduced into numerous habitats worldwide, and once there, they aren't overly picky about whom they mate with. So mallard hybrids crossed with the New Zealand gray duck, the Hawaiian duck and the Florida mottled duck have all started pushing around the native purebreds still in existence.


Same thing with Sitka deer from Japan. They've started mixing genetically with red deer in the U.K., a process called introgression, and they're impinging on the native species' genetic integrity. Finally, let's take a closer look at the California tiger salamander.

The California tiger salamander evolved separately from the barred tiger salamander for some 3-to-10 million years, yet when barred tiger salamanders were transported to California for bait breeding (where, presumably, a few slippery specimens successfully attempted a bid for freedom), they started breeding with the already threatened California natives.

The result was hybridized salamanders that can not only breed with one another, but with both of the parental species of salamander. The real kicker, however, is that when the hybrids were studied for telltale signs of genetic alteration from the original natives, it was found that while they were genetically very similar, there were three genetic markers that were significantly ramped up. And guess what? Those three genetic markers seem to be affecting competitive reproductive success. The hybrids not only produce larger larvae than the natives, their little ones develop faster as well, giving them a leg up on the competition.

So what does that mean for the native population? Should these hybrids be considered a subspecies of the California tiger salamander, or of the barred tiger salamander? Or maybe a new species all their own? Bottom line, species delineation is a finicky business, and since Mother Nature isn't allowed many chances to shake it out for herself, things are starting to get messy from all our meddling -- intentional or not. But answering the question of whether invasive species can tamper with the gene pools of other species is easy: The answer is a resounding yes.


Lots More Information

Related HowStuffWorks Articles

More Great Links

  • Global Invasive Species Database Web site. (8/10/2010)
  • "Glossary." Native Seed Network. (8/10/2010)
  • Invasive Species Web site. Wisconsin Department of Natural Resources. (8/1/2010)
  • Fitzpatrick, Benjamin et al. "Rapid spread of invasive genes into a threatened native species." Benaroya Research Institute at Virginia Mason and Department of Genome Sciences, University of Washington. Dec. 28, 2009. (8/2/2010)
  • Mooney, H.A. and Cleland, E.E. "The evolutionary impact of invasive species." Stanford University. 2001. (8/10/2010)
  • National Biological Information Infrastructure Web site. (8/10/2010)
  • National Invasive Species Council Web site. (8/10/2010)
  • National Invasive Species Information Center Web site. (8/10/2010)