There is considerable interest in determining the range of habitats an invasive alien species could possibly reach. Since its discovery in the Great Lakes, the zebra mussel (Dreissena polymorpha) has spread rapidly throughout waterways in the eastern US, negatively impacting ecosystems and infrastructure. A close relative of the zebra mussel and also of the Dreissena genus is the more slowly-spreading quagga mussel (D bugensis), found primarily in the Great Lakes
Based on published reports of the species' preferred habitats and needs for survival, Thomas Whittier (Oregon State University) Paul Ringold (US Environmental Protection Agency), Alan Herlily(Oregon State University) and Suzanne Pierson (Indus Corporation) created a map to better determine where the quagga and zebra mussel may appear next, in their paper "A calcium-based risk assessment for zebra mussel and quagga mussel (Dreissena spp) invasion." Their research appears in the online e-view version of Frontiers in Ecology and the Environment.
According to the authors, "The rate of zebra mussel expansion was rapid from its discovery in 1988. After 1994, the rate of expansion slowed considerably."
While expansion of the zebra mussel's range continues in the Great Lakes and other inland locations, there has been no invasion of New England, the mid-Atlantic Piedmont and Coastal Plains, the Southeast, or areas west of the 100th meridian, despite climates and other conditions favorable for the organisms.
"Another Dreissena species, the quagga mussel was discovered in 1989 in the Great Lakes. It received less attention, as it appeared to be confined to deeper waters. However, as the quagga mussel spread, it began to dominate shallower waters previously occupied by zebra mussels," said the researchers.
Originally limited to the Great Lakes and St Lawrence River, the picture of a slow replacement by quagga mussels changed suddenly with the discovery of well-established populations in Lake Meade, Nevada, and downstream areas. As of September 2007, these mussels have been discovered in California and Arizona as well.
"Our preliminary assessments suggested that current mussel distributions in North America appear to be associated with calcium concentrations in surface waters," said Whittier. Calcium is an important element used in the creation of the mussels' shell and basic metabolic functioning.
Given the recent Dreissna incursion into western states and continued uncertainty regarding non-invaded areas in the East the authors developed and evaluated a national-scale map of Dreissena species invasion risk, based on calcium concentrations in streams and rivers. Their primary data were taken from several large-scale surveys made by the US Environmental Protection Agency's (EPA) Environmental Monitoring and Assessment Program. They also utilized data from the EPA's Wadeable Streams Assessment. The scientists then classified ecoregions into mussel invasion risk categories, from very low to high, based on calcium concentrations.
For the 48 contiguous states, regions comprising nearly 21 percent of land area were classified as very low risk or low risk. New England, most of the Southeast, and western portions of the Pacific Northwest fall into these categories.
High risk ecoregions comprised almost sixty percent of land areas. The majority of reported occurrences of Dreissena were in high-risk ecoregions.
There were a few exceptions, mostly in highly variable regions such as the Northern Lakes and Forests ecoregion, the Mississippi Alluvial Plain region, and Appalachian ecoregion.
" One must take into account the entire ecology for the species. The case of the zebra mussels in Arkansas River in the very low risk areas, reflects the water source rather than the local conditions. The other key requirement for Dreissena in river systems is the presence of an invaded upstream lake or reservoir to maintain a supply of larvae," said Whittier.
According to the authors there is conflicting evidence of the quagga mussel's calcium requirements, but they are assumed to be similar to the zebra mussels.
The researcher's map, which depicts the relative risk of invasion by Dreissena, can help determine where management resources -- especially in western states -- should be targeted.
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