Several years ago I stopped in upon my neighbor Nathan, a dairy farmer, to deliver a stack of accumulated newspapers. Nathan heated his dairy barn (and a good portion of the biosphere) with an outdoor wood burning furnace. While Nathan was perhaps not in compliance with the Kyoto protocols, he welcomed fuel sources from his neighbors. In the interests of community cohesion, I felt it was my civic duty to assist him. This visit, however, almost induced a cardiac event, as I drove up to the rear of the farmhouse and discovered a massive collection of bound volumes awaiting incineration.

“What the #%!  are you doing?” I asked, in a polite and respectful fashion.

“Library dropped off a truckload of unwanted books – great fuel,” Nathan responded.

Appalled by what I considered an oxymoron – “unwanted books,” I set about systematically weeding through this mountain of discarded knowledge to search for candidates worthy of sanctuary in my personal library. Amongst the treasures rescued were several late 19th century and early 20th century volumes that provided windows into the past. Some were only collections of agricultural statistics yet they reflected a culture and lifestyle that has been paved over and lost in our quest to homogenize and commercialize the landscape.

While Nathan was perhaps not responsible for the destruction of the Library at Alexandria, I am certain that, as intelligent people (well you ARE reading Leaf Litter); we can all appreciate the haunting specter of knowledge being lost. Imagine logging on to Wikipedia and finding that it only covers A to M and -­ in violation of a fundamental axiom of every IT person I have ever had the pleasure to work with — no one backed up the data!

A similar scenario is unfolding across the continent as invasive organisms disrupt and destroy millennia worth of accumulated genetic, species, and ecosystem diversity. Before we have a chance to fully read the stories behind individual species, probe the molecular structure of genotypic variation or understand the complex evolutionary relationships, this knowledge is being fed into the furnace. Just as Nathan was inadvertently liquidating value in his quest to keep his cows productive, our economic globalization is creating ancillary damage on a tremendous scale.

What are invasive organisms? The nomenclature itself can be confusing and can raise the ire of those with a vested interest in raising and importing foreign species.  Terms such as “exotic,” “alien” and “non-native” may be construed as too broad in scope. At Biohabitats, we favor the definition adopted by the National Invasive Species Council:

An “invasive species” is “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.”

Regardless of the terminology applied, the damage is real and overwhelming. The most comprehensive cost analysis of the impact of invasive species yet performed was developed out of Cornell University and estimated yearly damages in the United States of approximately $120 billion dollars (1). This figure can be considered as conservative, as economic data is not available for many invasive organisms.

While the geographic isolation of populations over millennia has blessed us with our current biotic diversity, our accelerating global level of trade and the associated movement of goods and material is breaking down these barriers and mixing biota in the fashion of an uncontrolled experiment. Our knowledge of the kind of disruptive, system-level changes that are occurring in native communities is limited and focused primarily upon the most dramatic examples of invasion. Pages are being ripped out and scribbled on before we even have a chance to read and fully understand an ancient text. I find myself empathizing with archeologists who locate an important cultural site of the Mayan civilization only to find it has been looted a few weeks before.

No ecosystem is immune to invasive infection and the Great Lakes bioregion provides an excellent example. As a major point of entry into the United States and Canada, the region is on the front lines for many invasive aquatic threats. Organisms such as the zebra mussel, one of many nasty critters introduced via ship ballast, are now established species that have changed the nature of these ecosystems. Within the last 25 years, the zebra mussel has spread to the point where it is now estimated to cause as much as one billion dollars in damage each year to Great Lakes fisheries. The tendency of this mollusk to act as a bio-accumulator of toxins and pathogens has resulted in its being implicated in the large-scale mortality of ducks and other waterfowl that feed upon it.

The terrestrial environment in the region offers little refuge as the forest systems are also currently threatened with catastrophic loss. Just last August, Hemlock Woolly Adelgid (Adelges tsugae) (HWA) was identified in Emmet County Michigan. HWA is a particularly nasty invasive insect from Japan. Originally discovered feeding upon hemlock in Virginia during the 1950s, it has spread at the rate of approximately 10 to 20 miles per year and now occupies the eastern half of the native range for this important forest tree. Transported by wind, birds, and human activity this critter feeds by sucking on the xylem ray parenchyma cells of infested plants. Both Eastern Hemlock (Tsuga Canadensis) and Carolina Hemlock (Tsuga caroliniana) experience heavy mortality after several years of adelgid activity. The insect is very easy to identify in the field during the early winter to early summer as it produces a cottony white protective covering at the base of the needles. The waxy coating resembles the head of Q-tip stuck to the underside of the fine branches.

Biohabitats personnel have worked with this insect since the 1980s and have first hand experience with the ecological devastation that it can cause. Our staff witnessed this wildfire as the leading edge hit Northern Maryland and South-central Pennsylvania twenty years ago. Tree mortality was high. Chemical control, while effective on individual plants, was not feasible for large scale forest populations. Currently, the front line of the war on HWA is moving North and West. It lies from the Great Smoky Mountains up into Central Pennsylvania and is now knocking at the southern door of Vermont, New Hampshire and Maine. Spot infestations have been experienced outside of this range, typically as a result of the transport of infected nursery stock. What makes the discovery in Emmet County so chilling is that it is the first detection of the pest in wild hemlocks in the state.

As anyone who has ever been privileged to venture into the cathedral-like atmosphere of a mature stand of stream-side hemlocks can attest to, hemlock forests fill a unique niche in the East. Studies have found distinct differences between the macroinvertebrate  composition and brook trout abundance in streams draining hemlock forests as opposed to hardwood systems (2). HWA has the potential to seriously alter many local ecosystem processes, from the stand microenvironment, to nitrogen availability, soil mycorrhizal associations, litter quality, soil microbiology and canopy throughfall chemistry (3). The cascade of forest level impacts from this one exotic introduction rivals those resulting from the loss of the American Chestnut in the early twentieth century. However, while the oaks were able to fill some of the chestnut void, no native alternative comes close to approximating the functions of the Eastern Hemlock.

What can be done to address the loss of Hemlock? In Michigan, eradication is still a possibility. However, in the states from Georgia to Maine, options are limited. Individual high-value plants and stands can easily be treated and protected through chemical intervention. But again, this methodology is not economicly feasible on a forest-level scale. Sivicultural pre-emptive salvage logging? Hardly a preservation strategy and, although Eastern Hemlock has been found to have a low level of genetic variation (4), what if you inadvertently cut down the one example of a genotype that has resistance? Biological control? Host-specific beetles imported from Japan, Sasajiscyumnus tsugae and Laricobius nigrinus, hold some promise but are still under study and evaluation. Hybridization with resistant Western Hemlock (Tsuga heterophylla) or Asian hemlock species? So far that approach has been unsuccessful. What can have a dramatic impact on the adelgid spread are extreme cold winter temperatures (below -50 F or -200C) (5). So perhaps our best strategy would be to pray for a nice artic cold front to pass by every January. I actually find myself appreciating the ecological value of deep cold as I load my woodstove. And no, I do not use hemlock as fuel. I would sooner burn books. Of course, given current global climate trends, even this may ultimately be in vain.

What’s left besides a nihilistic acceptance of this loss? Perhaps a similar approach as was applied in the rescuing of burning books – move them away from the fire. Ex-situ plantings are being attempted in an effort to preserve the genetic record of Tsuga canadensis (6) and Tsuga caroliniana (7). While stocking an ark may be a last ditch effort for Giant pandas and California condors we still have a lot of uninfected hemlock territory out there. If, as a country, we recognized the loss of biodiversity as a national security threat on a par with petroleum sources, it is likely HWA (along with the other invasive nightmares) would be stopped cold. In the meantime, get out and experience a mature hemlock stand while you have a chance. Just bring along a camcorder for your grandchildren.


(1)  Pimentel, D., R. Zuniga, D. Morrison. 2005. Update on the environmental and economic costs associated with alien-invasive species in the United States. Ecological Economics 52:273-288.

(2)  Syder, C.D., et al. 2005. Long–term effects of hemlock forest decline on headwater stream communities, pp. 42-55. In Proceedings: Third Symposium on Hemlock Woolly Adelgid in the Eastern United States. USDA Forest Service publication FHTET-2005-01.

(3)  Jefts, S., D. Orwig. 2005. The effects of HWA outbreaks on ecosystem level changes in southern New England, pp. 305-306. In Proceedings: Third Symposium on Hemlock Woolly Adelgid in the Eastern United States. USDA Forest Service publication FHTET-2005-01.

(4)  Zabinski, C. 1992. Isozyme variation in eastern hemlock. Can. J.for.Res. 22:1838-1842.

(5)  Skinner, M.,B.L. Parker, S. Gouli, T. Ahikaga, and H.B. Teillon. 1999. Low lethal temperature for hemlock woolly Adelgid. Environmental Entomology 28:1085-1091.

(6)  Esham, B.D., et al. 2005. Preserving eastern hemlock gene pools through ex situ plantings., pp. 191-198. In Proceedings: Third Symposium on Hemlock Woolly Adelgid in the Eastern United States. USDA Forest Service publication FHTET-2005-01.

(7)  Tighe, M.E., W.S. Dvorak, W.A. Whittier, J.L. Romero, and J.R. Rhea. 2005. The ex situ conservation of Carolina hemlock. pp. 180-190. In Proceedings: Third Symposium on Hemlock Woolly Adelgid in the Eastern United States. USDA Forest Service publication FHTET-2005-01.

Got an idea?

Contact The Editor

Sign up for Leaf Litter

Browse by topic

Browse by year