False Emeralds: A Case Study on the Dangers of Invasive Species

When was the last time you thought about introduced species?  And I mean really thought about them.  How prevalent they are and how much differently the world and the Pacific Northwest in particular would look without them.  In order to think about that, take a second and picture what you had for dinner last night.  Unless your meal consisted entirely of blackberries, Camas, Huckleberries, or some other native plant then you have introduced species to thank for the side dishes.  Much the same can be said for any meat you may have enjoyed, with only a select few commonly eaten animals being native to this region. 

These introduced species sound pretty good so far, so then what’s all the fuss about?  It’s largely because of them that we’re able to enjoy the lifestyles we do now, and the world sure would be a lot different if species native to an area stayed within their original range.  On the food production front alone, that would mean we in the United States would have to pay a whole lot more to import our beef, wheat products, and the majority of our beloved fruits and vegetables just to name a few favorites.  So again the question must be raised, with all of these seeming benefits from the cultivation of different introduced species, why are they so often pointed to as being a bad thing?  The answer to that can be found through an understanding of what happens when an introduced species gets a little out of control and makes the transition to an invasive one.

Before delving too much further, I should spend some time to define my terms. In this context when I use the phrase introduced species I simply mean any species which has been moved outside its native range, the range in which it coevolved with the other organisms present in the environment, through human activity.  While similar to introduced species, invasives have the key distinction of just being bad news all around.  While introduced species are classified as such based only on their movement into an area, invasive species are given their particular title only once they prove to be really good at spreading rapidly throughout the new habitat and outcompeting anything else that gets in their way.  As part of this rapid growth invasives often cause extraordinary damage in terms of economic and ecological costs, a major portion of this damage in the form of biodiversity loss (Invasive, 2013).  Most often the brunt of this damage is born by native species, those that have in most instances not dispersed widely.  This means that the effects of invasives in an area are best seen through the subsequent loss of endemic species which when lost are gone forever.

A good overview of the ways in which invasive species work can be seen in the below video; though the music is a bit silly, it’s worth a look because it provides a nice concise look at invasives and even some common steps taken to slow their spread.

Due to this large risk of significant damage and loss, it is in the best interest of ecologists to predict if a new species being introduced to an area will make the jump from “introduced” to “invasive.”  Trouble is that this turns out to be a really hard thing to predict.  However, ecologists have been able to develop a kind of generalized rule about invasive species, at least for plants.  Deemed the “rule of 10s” this relationship states that in general 10% of plants introduced into an area will survive in the wild and of that number 10% will then go on and become invasive (Iowa, 2013).  A similar relationship is seen with animals, although they hold a higher predicted rate of invasiveness with about 25% of introduced vertebrate species becoming invasive (Vander, 2005).  This higher rate makes sense though, as vertebrate animals are generally more adaptable to new environments than plants due to motile nature.

So now we’ve discussed the basics of what constitutes introduced and invasive species, and how common they are.  It is time then to return to the specific dangers of invasives, and what better way to do so than through the use of an example? 

Adult Emerald ash borer with size reference
 and characteristic trunk damage (Freshfromflorida.com)

The Emerald Ash Borer (Agrilus planipennis) is an insect belonging to the wood-boring beetle family.  As its name implies, this beetle exclusively targets trees in the Ash (Fraxinus) genus.  Also as is clear from the image, the beetles are a striking emerald green.  What makes this insect so dangerous as a pest is that it targets every species of the genus, not just a select few ash varieties.  Originally introduced to the Midwest of the United States from Asia on imported lumber, this beetle was first spotted around Detroit, Michigan in 2002.  Recent dendrochronological (tree ring dating) data now shows that the Emerald Ash Borer had been established in Michigan at least 10 years prior to its detection in 2002 (EmeraldAshBorer.info).

Since adult EAB’s are capable of flying up to a mile at a time in search of new host trees, they are quite hard to contain and that mobility coupled with their fast reproductive cycle means that this pest has the potential to spread incredibly rapidly through a stand of trees.  After EAB eggs hatch on the bark of the ash trees, the newly emerged larva travel through the bark and into the phloem of the tree where they proceed to feed and grow for about 1000 days before emerging as adults.  This extended period of hidden growth means that it’s possible for a tree to show no outward sign of infestation for long periods of time even though it is filled with thousands of Emerald Ash Borers larva.  This allows the beetles to persist unseen and untreated until it is too late to help the infested tree.  Once a tree is infested with those larva, its survival rate drops to zero as the Ash Borers starve the tree by consuming its sugar transporting phloem cells (Poland, 2006).

Another video with great music.  This short clip provides a great overview and recap of the origin, spread, and mechanism of attack of the Emerald ash boring beetle.  Also valuable in the video is a short discussion of the current methods being used to slow the beetle’s spread and hopefully limit its future damage.

Prediction of future EAB spread (US Forest Service).

Emerald Ash Borers are thought to be one of the most serious threats currently facing North American forests. As is shown by the image to the left their suspected future impact is huge, and these pests have the potential to kill every one of the 7.5 billion ash trees currently growing unless they are stopped.  Not only would this vast destruction eliminate an entire genus from our forests, it would also have unpredictable effects on the overall forest structures we see today.  At this point in time, there hasn’t been much progress in the way of preventing infestation in trees, and officials are relying on early detection and containment efforts along with public awareness campaigns aimed at halting the spread of this pest (The Nature Conservancy).  The only other current research out there is centered on the natural "enemies" of the Emerald Ash Borer in its native Asian ranges, but implementing that kind of strategy to control one pest's population would require further unpredictable introductions (Liu, 2003).

Serious as it is, the Emerald Ash Borer is but one example of the very real danger posed to global biodiversity by invasive species.  There are countless other plant and animal species out there doing as much or even more damage to the environments they are introduced into.  And in our increasingly interconnected world, the rate of new species introductions is only going up.  So while it is true that only a small fraction of species introduced into an area ever become invasive (remember the rule of 10s from earlier?), this high level of introductions has led to alarming invasive species establishment rates estimated at 6 per year in California and up to 15 per year in Hawaii and Florida (Center, 2013).

By now I hope I have done a good job conveying just how big of a concern invasive species are.  Their movement into places where there are no barriers to their spread leads to the decimation of native populations which often turn out to be only found in that place.  So although the damage caused by invasive species is often discussed in terms of dollar amounts, the real damage is more tangible than that.  It is in the irrevocable loss of the species which give our favorite places their special character.  And with the increasing rate of species introductions we now run the very real risk of losing more of that which can never be replaced.

References

Center for Invasive Species Research. UC Riverside, n.d. Web. 25 May 2013.

"EmeraldAshBorer.info". EmeraldAshBorer.info. 2012-05-21. Retrieved 2013-05-21.

"Invasive Species - Forest Disturbance Processes - Northern Research Station - USDA Forest Service." Invasive Species - Forest Disturbance Processes - Northern Research Station - USDA Forest Service. N.p., 11 Apr. 2013. Web. 26 May 2013.

Iowa State University. "Invasive species widespread, but not more than at home range." ScienceDaily, 5 Mar. 2011. Web. 26 May 2013. 

Liu, Houping, et al. "Exploratory survey for the emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae), and its natural enemies in China." Great Lakes Entomol 36 (2003): 191-204.

Poland, Therese M., and Deborah G. McCullough. "Emerald ash borer: invasion of the urban forest and the threat to north Americas ash resource." Journal of Forestry 104.3 (2006): 118-124.

"The Nature Conservancy. Protecting Nature. Preserving Life.™." Emerald Ash Borer. N.p., n.d. Web. 26 May 2013.

Vander Zanden, M. Jake. "The success of animal invaders." Proceedings of the National Academy of Sciences of the United States of America 102.20 (2005): 7055-7056.