Predicting the spread of invaders

A broad scale approach to preventing the arrival of potential invaders is to identify major 'migration' pathways, such as hitchhiking in the mail or cargos and on aircraft or in ships, and to manage the risks associated with these (Wittenberg & Cock, 2001). The Great Lakes of North America have been invaded by more than 145 alien species, many arriving in the ballast water of ships. For example, a whole series of recent invaders (including fish, mussels, amphipods, cladocerans and snails) originated from the other end of an important trade route in the Black and Caspian Seas (Ricciardi & Maclsaac, 2000). A ballasted ocean freighter before taking on cargo in the Great Lakes may discharge 3 million liters of ballast water that contain various life stages of many plant and animal taxa (and even the cholera bacterium Vibrio cholerae) that originate where the ballast water was taken aboard. One solution is to make the dumping of ballast water in the open ocean compulsory rather than voluntary (this is now the case for the Great Lakes). Other possible methods involve filter systems when loading ballast water, and on-board treatment by ultraviolet irradiation or waste heat from the ship's engines.

The most damaging invaders are not simply those that arrive in a new part of the world; the subsequent pattern and speed of their spread is also significant to managers. Zebra mussels (Dreissena polymorpha) have had a devastating effect (see Section 7.2.2) since arriving in North America via the Caspian Sea/ Great Lakes trade route. Range expansion quickly occurred throughout commercially navigable waters, but overland dispersal into inland lakes, mainly attached to recreational boats, has been much slower (Kraft & Johnson, 2000). Geographers have developed so-called 'gravity' models to predict human dispersal patterns based on distance to and attractiveness of destination points, and Bossenbroek et al. (2001) adopted the technique to predict the spread of zebra mussels through the inland lakes of Illinois, Indiana,

... and to predict the spread of invaders

Figure 7.13 The spatial arrangement of patches (dark) of breeding habitat (left hand panels) and breeding plus dispersal habitat (right hand panels) in a typical landscape containing flying squirrels (Pteromys) (top panels) and a random forest location (bottom panels). This flying squirrel landscape contains 4% breeding habitat and 52.4% breeding plus dispersal habitat, compared with 1.5 and 41.5% for the random landscape. Dispersal habitat in the squirrel landscape is much more highly connected (fewer fragments per unit area) than in the random landscape. (After Reunanen et al., 2000.)

Figure 7.13 The spatial arrangement of patches (dark) of breeding habitat (left hand panels) and breeding plus dispersal habitat (right hand panels) in a typical landscape containing flying squirrels (Pteromys) (top panels) and a random forest location (bottom panels). This flying squirrel landscape contains 4% breeding habitat and 52.4% breeding plus dispersal habitat, compared with 1.5 and 41.5% for the random landscape. Dispersal habitat in the squirrel landscape is much more highly connected (fewer fragments per unit area) than in the random landscape. (After Reunanen et al., 2000.)

Michigan and Wisconsin (364 counties in all). The model has three steps involving (i) the probability of a boat traveling to a zebra mussel source; (ii) the probability of the same boat making a subsequent outing to an uncolonized lake; and (iii) the probability of zebra mussels becoming established in the uncolonized lake.

summed for each county over the total number of zebra mussel sources. Titi, then, is the number of infested boats that travel from county i to an uncolonized lake u:

1 Uninfested boats travel to an already colonized lake or boat ramp and inadvertently pick up mussels. The number of boats, T, that travel from county i to a lake or boat ramp, j, is estimated as:

Tj = AiOiWjC- a where Ai is a correction factor that ensures all boats from county i reach some lake, Ot is the number of boats in county i, Wj is the attractiveness of location j, ctj is the distance from county i to location j and a is a distance coefficient.

2 Infested boats travel to an uncolonized lake and release mussels. The number of infested boats Pi consists of those boaters that travel from county i to a source of zebra mussels,

The total number of infested boats that arrive at a given uncolonized lake is summed over all the counties (Qu). 3 The probability that transported individuals will establish a new colony depends on lake physicochemistry (i.e. key elements of the mussel's fundamental niche) and stochastic elements. In the model, a new colony is recruited if Qu is greater than a colonization threshold of f

To generate a probabilistic distribution of zebra mussel-colonized lakes, 2000 trials were run for 7 years and the number of colonized lakes for each county was estimated by summing the individual colonization probabilities for each lake in the county. The results, shown in Figure 7.14, are highly correlated with the pattern of colonization that actually occurred up to 1997, giving confidence

Figure 7.14 (a) The predicted distribution (based on 2000 iterations of a stochastic 'gravity' model of dispersal) of inland lakes colonized by zebra mussels in 364 counties in the USA; the large lake in the middle is Lake Michigan, one of the Great Lakes of North America. (b) The actual distribution of colonized lakes as of 1997. (After Bossenbroek et al., 2001.)

Figure 7.14 (a) The predicted distribution (based on 2000 iterations of a stochastic 'gravity' model of dispersal) of inland lakes colonized by zebra mussels in 364 counties in the USA; the large lake in the middle is Lake Michigan, one of the Great Lakes of North America. (b) The actual distribution of colonized lakes as of 1997. (After Bossenbroek et al., 2001.)

in the predictions of the model. However, areas of central Wisconsin and western Michigan were predicted to be colonized, but no colonies have so far been documented. Bossenbroek et al. (2001) suggest that invasion may be imminent in these locations, which should therefore be the focus of biosecurity efforts and education campaigns.

Of course invaders do not all rely on human agency; many disperse by their own devices. The red fire ant (Solenopsis invicta) has spread rapidly through much of southern USA with dramatic economic consequences (see Section 7.2.2). The species, which originated in Argentina, occurs in two distinct social forms. The single-queened (monogyne) form and the multiple-queened (polygyne) form differ in their patterns of reproduction and modes of dispersal. The queens from monogyne colonies take part in mating flights and found colonies independently, whereas the queens from polygyne colonies are adopted into established nests after mating. As a result, the monogyne populations spread three orders of magnitude more quickly than their polygyne counterparts (Holway & Suarez, 1999). The ability of managers to prioritize potentially problematic invaders and to devise strategies to counter their spread can be expected to be improved by a thorough understanding of the invaders' behavior.

Was this article helpful?

0 0
Lawn Care

Lawn Care

The Secret of A Great Lawn Without Needing a Professional You Can Do It And I Can Show You How! A Great Looking Lawn Doesnt Have To Cost Hundreds Of Dollars Or Require The Use Of A Professional Lawn Care Service. All You Need Is This Incredible Book!

Get My Free Ebook


Post a comment