Invasion Theory

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In the immediate area surrounding our homes and offices, we are experiencing an outbreak by a highly destructive species – the emerald ash borer. This beetle invaded the Windsor-Detroit area and is spreading via both diffusive population growth, and jump (or long-distance) dispersal. This beetle kills native ash trees and has the potential to become as great (or greater) a problem as Dutch elm disease (e.g. 1,000,000,000 and 900,000,000 ash trees are at risk in Ontario and Michigan, respectively). We are interested in a number of questions regarding this beetle including:

1) How was it vectored to North America (from China)?

2) How quickly is the species spreading, and by what mechanisms?

3) Can we reduce or stop its outward spread, and if so, by what means?

To answer these questions, as well as those pertinent to other recent outbreaks by pathogenic diseases (west Nile, SARS, Monkey Pox, Mad Cow), we think it necessary to proactively target vectors that transfer these species from their native areas to North America. By focusing on vector biology, we may help address issues of human disease, and plant and animal disease or pest transmission.

Invasion biology really took off after publication of Charles Elton’s excellent volume in the 1950s. This book focused ecologists’ attention on the importance of issues including habitat disturbance, insularity (continents vs. islands) and native species diversity in affecting invasion success. While papers continue to be published on these topics, often supporting tenets of Elton’s work, many other studies have revealed that invasion success is a very complex area of study. For example, we contend that invasion hypotheses ought to be tested in a logical sequence, beginning with the question of propagule pressure (no invasion is possible unless propagules are transferred to the new ecosystem. If we can satisfy ourselves that sufficient propagule pressure exists (appropriate ratio and number of reproductive males and females, for example), then we can ask questions regarding the physical and chemical nature of the habitat. Some species may fail to succeed despite sufficient propagule pressure because they are physiologically intolerant of conditions in the habitats (UV light and acidity, for example affect zooplankton species, soil nutrients/toxins affect plants). If the propagule supply is not limiting, and the species appears capable of surviving ambient conditions, then we can ask questions regarding species interactions. Predators, competitors, or pathogens in the new habitat, all of which may reduce invasion success, may affect invading species. So by asking the appropriate questions, at the appropriate times, we may help determine the factors that affect invasion success.

We are also interested in the growing popularity of the Enemy Release Hypothesis. This hypothesis suggests that invaders perform better in their introduced range than their native range because they lose their enemies (often but not always parasites) during the colonization process. Essentially, the colonizing population carries only a subset of the complement of natural enemies from the home range because only a small (usually) number of colonists manage to invade the new habitat. This causes a founder effect for both the colonizing species and its enemies. We think this hypothesis should be subjected to critical examination because alternative hypotheses may account for the loss of enemies and because, paradoxically, the colonizing population may not want to lose its associated flora/fauna. Imagine an invader carrying a pathogen that reduces its viability or reproductive output. Loss of this species during colonization would ordinarily be viewed as a positive event (for the colonist), but what happens if the pathogen also has the potential to affect the viability of species in the introduced range with which the colonizing species will compete for resources? North American squirrels that invaded Great Britain provide a good example of this. These squirrels carried a virus that affected them adversely; however, when the North American squirrel came into contact with the native British squirrel, they transmitted the virus to the native, causing massive losses to the native population. Thus, carrying the pathogen – even though harmful to itself - ultimately proved beneficial to the invader because competition with native squirrels was dramatically reduced. Moreover, if an invader can lose it enemies during colonization, it could also lose its facilitators. Losing symbiotic fungi could adversely affect colonizing plants, so loss of associated species does not always translate into a benefit for the colonizing species. We are interested in looking at all of the possible permutations of the Enemy Release Hypothesis to determine when the loss of beneficial versus when it is harmful. Another popular hypothesis that may suffer from confounding is the Invasional Meltdown Hypothesis.

Most of the work in our lab focuses on invasion vectors into the Great Lakes and Chesapeake Bay. Once species establish in the Great Lakes via long-distance movement by, for example, commercial ships, then other human-mediated vectors may allow them to spread to inland lakes. These vectors, including contaminated fishing lines and live-well water in pleasure boats, cannot spread the species from the native region to North America, but they may effectively disperse the species once it is already here. By measuring the human vectors, we are developing model to predict where aquatic invaders will spread.