Bio-Control: Tying together our Woodpecker Journey
(Originally published March 11th, 2014 on http://scienceofbirds.blogspot.com)
Invasive species are…complicated. Well, actually, they aren’t necessarily complicated themselves; the situations they breed are what is complicated. Take the issue of the explosive Asian Carp population in the Mississippi, which consumes much of the food sources required by an enormous set of native fish species to survive at all. Ecological catastrophe will follow if their invasion succeeds in conquering the Great Lakes. House Sparrows, an all-too-familiar species introduced from England in the 19th-century, presents such bellicose competition for nest cavities that they will literally murder Bluebird chicks in a nest box and go on to build their nest on top of their corpses.
But one species in particular, friends, has gotten my attention, and the attention of all those with trees above their heads…trees that may be slowly but surely falling and dying out mysteriously. This invasive is the Emerald Ash Borer (EAB in short). To the chagrin of interstate firewood smugglers, Emerald Ash Borers have pervaded much of North America from their origin in Asia, and, at this point, human efforts have largely been unable to stop their silent incursion.
The problem with invasive species is what makes them invasive. Being
non-native to the area that they are invading, the often face no natural opposition at any level of the ecosystem, and are often able to out-compete native species. Being successful at invading an area means being coincidentally better adapted to exploiting that ecosystem than that ecosystem’s natural occupants. It’s a bad deal folks, and yes, we should be scared.
We should be scared because…well…take Chicago. After losing an estimated 30 million ash trees in the Eastern half of the continent alone, 17% of street trees in Chicago are the ash trees that the Borers target. This means that on the city-owned property along streetsalone, 85,000 trees are up for grabs to our untouchable insect invader. Not enough for you? It is estimated that 300,000 ash trees occupy private property in the city — we are facing significant holes in ecosystems and shade over our heads. Not to mention safety hazards resulting from decrepit, infested trees.
Before we move on to the science of birds, what is it that ash borers do to trees, you ask? Ash borers are problematic primarily in their larval stage. In this stage, they live just below the bark in the top layer of wood, burrowing serpentine ruts. These ruts, which I’m sure you’ve seen, look like tiny squiggled indentations into the wood. Why is this an issue for strong and stoic trees? This top layer of wood is extremely important to the trees and all plants; this is where the plant’s vascular system is. Similar to our system of blood vessels and veins, this layer of wood contains cataracts that transport water and nutrients around the plant. Damaging it would be like damaging our blood vessels. And a badly infested tree can be entirely cut through its vascular system.
How do birds play into this, then? Woodpeckers (here they are again) specialize in feeding on larvae within this layer of wood. By preying on larvae and other insects within the bark and top layer of wood, woodpeckers help maintain the trees’ health and ultimately the health of the forest. It’s a great deal, you guys.
But the question is this: do woodpeckers (and nuthatches, for that matter) prey upon Emerald Ash Borer larvae? Normally, part of the success of invasive species is their lack of natural predators in the area they’re invading. Asian Carp are a perfect example of this; they have no predators to control their populations. Can we look to woodpeckers as a potential bio-control of invasive EAB’s, one that we don’t need to interfere with anymore than by preserving healthy woodpecker populations?
Here’s the best part. A bunch of middle-schoolers in Ohio helped us find that the answer is a resounding yes. A group of researchers recently conducted a study to answer these questions, in which they established observation plots in Michigan and Ohio to follow the dynamics of the forest ecosystem in response to the Borers. One of these plots lay behind an Ohio middle school, where the researchers reached out to the students to be part of a multi-year citizen science project. Here’s how it worked according to an article on this study by the University of Illinois at Chicago:
“A section of trees in the stand behind the school was cut down for examination each year for two years. The students searched for and painted all the holes they found in the bark of each tree — a different color each for large round woodpecker holes, for the characteristic crescent-shaped holes mature emerald ash borers make exiting a tree and for holes made by other insects.
Paint seeped through to dye the stem beneath, and after the bark was stripped the students could identify woodpecker holes that penetrated into emerald ash borer galleries, or into holes made by other bugs. The students tracked the fate of each bug that had been in the tree. Instead of relying on a statistical estimate of the insect population and thus the food source available, every bug and its fate were accounted for.”
And what statistics did the middle school scientists attain? In accordance with the hopes of any fan of ash trees, woodpeckers chose to prey on 85% of the Emerald Ash Borers within an infested tree. This means that woodpeckers are actively altering their prey-selecting behaviors to take full advantage of the new and increasing invasive food source. In a way, the Borers are actively invading right into the mouths of hungry woodpeckers. On top of that, another citizen science project, this one Cornell’s Project Feederwatch, revealed that the populations of three woodpecker species and the White-breasted Nuthatch actually increased in areas where the Borers were increasing.
So, is it enough? After all, 85% isn’t 100%, and this means that there are still Emerald Ash Borers left over in the wake of woodpecker predation. In a way, it is enough. Woodpeckers will not instantaneously snuff out the population of Emerald Ash Borers…it would be unrealistic to expect them to do so in the first place. If they could have done that, it is certain that they already would have. But they certainly will slow the increase of the Borers, and eventually, slowly but surely, as woodpecker numbers continue to increase, Emerald Ash Borer populations may be expected to decline, which eventually would lead to their disappearance. Be mindful; this is an assumption. But what a sweet dream it is to imagine that nature will do the job of eliminating one of its most virulent pests completely on its own.
What fantastic balance this is.
Like any decent science blog, I’d like to leave you with some questions. This is where we will tie together my previous two woodpecker posts.
First and foremost, think back to the post on Syrian Woodpeckers and their ecological trap within polluted, urban forest tracts. Now think back to the post before where we found that woodpeckers tie the separate continents back together — part of this is their remarkable consistency in foraging method. Everywhere you go, woodpeckers do just that: peck wood. So…
- If woodpeckers show this consistency, might woodpeckers around cities in North America also be drawn to polluted areas because of higher concentrations of prey?
- If this is a yes, would woodpeckers in urban areas more rapidly eat away at Emerald Ash Borer populations because of greater densities of woodpeckers paired with increased tree infestation?
- Would the percentage of Emerald Ash Borers ultimately consumed be higher in urban forest tracts than the 85% found in Ohio?
- Finally, if woodpeckers can limit the populations of invasive wood-boring insects and possibly eliminate them, is it possible that this isn’t the first time in their evolutionary history that they’ve done so?
Ultimately, just food for thought. Cheers everybody. You’ve been great.
Here’s the article on which this post is based:
Galatzer-Levy, Jeanne. “Emerald Ash Borer May Have Met Its Match.” UIC News Center. University of Illinois at
Chicago, 16 Dec. 2013. Web. 28 Feb. 2014. http://news.uic.edu/emerald-ash-borer-may-have-met-its-match
