Alien Invasion

The term invasive species conjures up some pretty graphic images of what exactly an invasive species might look like. It’s probably huge, armored and equipped with giant teeth. Chances are it’s highly adaptive and overpowers its native competition without any natural enemies—predators, herbivores, parasites, diseases, parasitoids etc.—to keep it in check.

In no time, biodiversity is plummeting, populations are destabilizing and ecosystem functioning is at risk. The image reminds me of those awful sci-fi invasion movies (e.g., Battlefield Earth and Aliens vs. Predator: Requiem) in which the aliens invade earth and wreck havoc on its inhabitants. But the alien invasion image is not quite an accurate representation of invasive species in real life.

Image Credit: Blog post, “How to Survive Alien Invasion Novels”.

In sci-fi alien invasion movies, the “good” side (us) and “bad” side (them) tend to be clearly defined. Extraterrestrial life invades Earth with the inevitable goals of exterminating human life, taking over the planet, harvesting humans for food or labor and stealing natural resources. The lines are well defined: the extraterrestrials are the bad guys and the humans are the good guys.

Invasions in Earth’s natural ecosystems are less straightforward. Invasive speciess leave evidence of their passing, but the term invasive means nothing more than that the species is spreading and outcompeting other species on a wide scale. A native species can do that just as a non-native species can. Therefore, most definitions of an invasive species focus on its origin—exotic, alien, non-native, non-indigenous and introduced to name a few. Invasive species tends to have a negative connotations hinging on the implication that its introduciton will lead to a harmful or undesirable outcome. For the purposes of this discussion, I will discuss invasive species as non-native species that outcompete others with undesirable results.

For example, one of the “hot” invasive species of the day is the quagga mussel (Dreissena rostriformis bugensis), a cousin of the better-known zebra mussel (Dreissena polymorpha). Where quagga and zebra mussels are both established, however, quagga outcompete the zebra mussels because they can colonize at greater depth and at a wider temperature range. This suggests quagga may pose an even more serious threat than zebra mussels.

Image Credit: U.S. Geological Survey. Quagga Gallery.

Both quagga and zebra mussels are native to Eastern Europe and tend to be detrimental to ecosystems in which they are introduced. They clog infrastructure and spoil recreational opportunities. They outcompete natives for food and resources. Cascading effects up the food chain can follow as the amphipod populations fish rely on for food collapse. Other ecosystem effects tied to Quagga muscle invasions include avian botulism outbreaks; elevated bioaccumulation of contaminants; and increased water acidity coupled with decreased dissolved oxygen concentrations.

Image Credit: New York Sea Grant. Aquatic Invasive Species, Zebra and Quagga Mussels.

Mussel outbreaks damage human infrastructure, as well. They congest water structures (e.g. pipes, screens) and increase maintenance costs. They accumulate on docks, buoys, anchors, beaches and boat hulls, disrupting recreation. Sharp mussel shells can cut people, necessitating shoes while walking on beaches and rocks. Finally, mussels attached to boat hulls can increase drag, clog engines, distress boat steering, and ultimately lead to engine malfunctions and overheating.

Quagga were initially found in the US Great Lakes in 1989 and are now established in Nevada (2007) and California (2008). They spread mainly on the hulls of improperly cleaned boats. Inspection efforts at and near both infected and non-infected water bodies attempt to slow the invasive mussel’s spread, but it can be difficult to detect infested boats as the mussels can be nearly invisible to the human eye. A recent discovery of a common soil bacterium—Pseudomonas fluorescens—that kills quagga and zebra mussels but harms no other organisms offers one potential source of control. EPA has approved certain uses of the biocontrol agent, but its full release is contingent on state approval.

Image Credit: U.S. Geological Survey. Nonindigenous Aquatic Species Database, April 2011.

Quagga mussels are no doubt a striking example of invasive species in earth’s natural system. But the invasion is not like those depicted in sci-fi movies. We can’t simply band together as the “good” side against the “bad” mussels. It’s more complicated because the mussels are just doing what they can to survive and thrive to the best of their ability. They did not maliciously invade the Great Lakes, nor did the human boaters who acted as vectors in the spread do so intentionally. So how can we respond to invasive species?

The obvious answer is to slow their spread through education and regulation. Once established, we can try to eradicate. There are countless techniques around to address these, with varying degrees of success. But it all comes back to awareness and education—the starting point for action.

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