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Andrew M. Liebhold
Andrew (“Sandy”) Liebhold is a research entomologist with the USDA Forest Service
Northern Research Station in Morgantown, WV. His research focuses on invasion
biology and various aspects of the population biology and landscape ecology of forest
insects. Liebhold received his B.S. in Biology from Allegheny College in 1978 and his
Ph.D. in Entomology from the University of California at Berkeley in 1984. He was a
postdoctoral at the University of Massachusetts from 1984-1988 and has been with the
US Forest Service since 1988. He is also adjunct faculty with Penn State and West
Virginia Universities. Liebhold is a member of the Editorial Board for the journals
Population Ecology, Biological Invasions, and Ecology Letters. He also serves as
coordinator of the International Union of Forestry Research Organizations Research
Group 7.03, “Entomology”. He currently is leading a new IUFRO task force on
biological invasions in forests (http://www.iufro.org/science/task-forces/biologicalinvasions ). Liebhold was presented the IUFRO Scientific Achievement Award in 2010
and elected as a fellow of the AAAS in 2015. He has authored over 200 journal papers,
largely on the topic of biological invasions, forest insect ecology and quantitative
ecology. (see http://www.sandyliebhold.com/pubs/pubs.htm)
Invasion Pathways and their Management
Andrew Liebhold, US Forest Service Northern Research Station, Morgantown, WV
26505
The term “pathway” is used to refer to the mechanisms by which non-native organisms
move from one part of the world to another. While the concept of a pathway also applies
to the means by which species locally expand their range following establishment, here I
will limit my discussion mainly to pathways responsible for inter-continental movement
of invading organisms.
In many cases, the identity of invasion pathways may be unknown, since invading
species are typically discovered well after the point in time that they were initially
transported to a new area. In these and other cases, there is great value in seeking to
deduce the identity of pathways. Identification of pathways is a necessary precedent to
their management. Prevention of invasions via pathway management is perhaps the most
effective approach to managing invasions; in many, cases a relatively small amount of
effort toward prevention can avert a multitude of future impacts and costs associated with
new invasions.
For many groups of non-native species in forests, the principle invasion pathway may be
intentional introduction. This includes a large number of non-native predators and
parasites that were introduced for the purpose of classical biological control of
undesirable non-native species. Though there may be a very few examples where such
biological control agents have had adverse impacts on non-target organisms, the vast
majority of these species are innocuous in their effects on the native biota and many have
greatly alleviated the impacts of serious alien invasive species. The vast majority of
biocontrol agents having adverse impacts were introduced many years ago and modern
regulations effectively prevent the importation of agents with generalized effects on nontarget organisms.
The vast majority of invasive plants were introduced intentionally. Mostly, these were
introduced for use in ornamental settings but a few were also introduced for use in
agriculture or other applied uses. Again, there has been progress made in regulating plant
imports and at least some species are currently prohibited. A similar situation exists for
vertebrates, many of which were introduced for purposes of hunting, fishing, pets or fur
production.
The vast majority of invertebrates and plant pathogens have been accidentally introduced.
For some groups, there is definitive information regarding the history of how they were
introduced. For example, a large fraction of ground beetles, earthworms and other soildwelling organism were introduced either with soil that was transported as maritime
ballast material or with soil associated with imported plants. Both practices are no longer
permitted; shipment of plants in soil was prohibited beginning with the plant quarantine
act of 1912.
Most species of bark and woodboring insects have been historically introduced with
wood, including round wood and wood packing material (WPM). Most cargo is
associated with WPM (e.g. pallets, crating and dunnage) and so associated with increases
in imports to the US over the last 5 decades has been a parallel increase in the
introduction of bark and wood boring insects, many of which have had huge impacts on
forests. Recognizing the need for a phytosanitary treatment to exclude insects and
pathogens associated with WPM that can be applied uniformly and not impede trade, the
International Plant Protection Convention (IPPC) recently identified a standard for heat
treatment of fumigation of wood, International Standard for Phytosanitary Measures No.
15 (ISPM-15). Studies show that ISPM-15 has reduced the arrival of potentially
damaging organisms by 30-50% but it has not been 100% successful. Nevertheless,
economic analyses identify a clear economic benefit of ISPM-15. Work is currently
underway to improve ISPM-15 treatments and there is some consideration of completely
eliminating WPM.
In addition to wood, the other dominant pathway by which most forest insects and
disease have entered the US is with live plants. The majority of these species arrived
prior to 1912 when the Plant Quarantine Act was implemented. Prior to this, plant
imports were essentially unregulated and a large number of damaging pests entered on
imported plants. From 1919 to the early 1950’s most plant imports were effectively
banned but with the move toward free trade, these regulations were relaxed and plant
imports have increased. All plant imports are required to be accompanied by a permit and
subjected to inspection by USDA inspectors. Historically, the USDA has utilized a “black
list” system by which imports of certain plant genera were prohibited. But recently, the
USDA implemented a “grey list” system, which added a third category of not permitted
pending risk assessment (NAPPRA). Several woody tree genera have been added to the
NAPPRA list and very few tree genera are currently imported to the US (except from
Canada).
With the passage of the Plant Quarantine Act of 1912, the USDA has organized a system
for inspecting imported cargo as well as baggage of arriving passengers to the US. State
employees initially performed inspection but in 1919 this was taken over by USDA
inspectors. In 2002, the responsibility to inspect incoming baggage and cargo (except live
plants) was transferred to the Customs and Border Patrol of the Department of Homeland
Security. There are three major purposes for conducting inspection. First, inspection of
imports can directly pre-empt arrival of damaging organisms. In most cases, this is not an
effective strategy since the volume of imports dwarfs the practical capacity of any
inspection system. Second, inspection and the threat of costly phytosanitary treatments,
fines or shipment rejection may provide deterrence and motivate passengers and
importers to not ship infested items. Third, and probably most important, inspection
provides information that can be used to characterize an invasion pathway. Analyses
demonstrate that the most frequently intercepted species are most likely to ultimately
establish due to the importance of propagule as a driver of establishment.
Perhaps the pathway that is most challenging to manage is illegal imports. This include
live plants transported by passengers in luggage, reptiles imported via e-commerce and
plants that are smuggled by individuals who find it easier to circumvent prohibitions or
import regulations. Evidence indicates that with steadily increasing passenger numbers
from other countries, such illegal imports are playing an increasing role as a dominant
invasion pathway. New strategies are needed to identify and close all types of illegal
import pathways.