. . . all the way to the “Red Zone” (7B, USDA Plant Hardiness Map)?
As the great philosopher and baseball “Hall-of-Famer,” Yogi Berra put it, “Making predictions is very difficult, especially about the future.” That is particularly true regarding the ultimate distribution of an invasive, exotic species. For instance, who would have predicted what would happen when three, small bundles aquatic plants from the Indian Subcontinent were placed into a canal near the Tampa airport around 1960? Unfortunately, those bundles contained one of the world’s worst weeds, Hydrilla verticillata. By the early 1970’s, hydrilla was established in almost all major water bodies in the state. At that time, the prevailing wisdom at Florida’s Bureau of Invasive Plants was that hydrilla might expand elsewhere in the Southern Coastal Plain, but none of us predicted that it would eventually reach Washington State and Maine. Certainly, the winter temperatures would stop this tropical plant, we all agreed. Hydrilla didn’t read the memo!
Now, in spite of Yogi’s admonition, scientists are trying to predict the ultimate range of the invasive, exotic apple snails (Pomacea canaliculata and P. insularum) in the Continental U.S. In the abstract of her exhaustive PhD dissertation on Pomacea insularum, Veena Ramakrishnan states that “the [laboratory] research results indicated that the main factor likely to limit the . . . distribution of P. insularum in North America was temperature, based on its incipient tolerated temperature range of 15.2°-36.6°C. Its 15°C lower lethal limit will restrict its distribution to southern and western coastal regions and Florida in the United States” (see Recent Publications, May, 2007)
The distribution predicted by Ramakrishnan would encompass the southern regions of Texas, Alabama, South Carolina, New Mexico and the southern and middle Pacific coastal regions and Sacramento Valley of California. “Particularly at risk to invasion by P. insularum is the state of Florida.” Her predicted range, though impressive, may be optimistically small. Further digging into the dissertation reveals that it took almost 27 straight days of exposure to 15oC to kill 99% of Pomacea insularum. Half of the snails survived for 5 days at 2oC. What will happen in the real world, where the snails aestivate in the bottom sediments during the winter? Worse yet, Pomacea canaliculata appears to more cold tolerant than P. insularum. Mochida (1991) reported that P. canaliculata can survive freezing for 15-20 days.
In its native Argentina, we know that “the southernmost record for [Pomacea] is Paso de las Piedras Reservoir (38o24’S), south of the Buenos Aires Province” (Martín et al., 2001). However, the local scientists have concluded that “hydrological discontinuity has been the barrier impeding the southward spread” into colder latitudes, not temperature (see Recent Publications for Martin & Francesco, 2006). In the Northern Hemisphere, the northernmost boundary of P. canaliculata is Kasumigaura Lake (36°06’N), Ibaraki Prefecture, Japan. In that area, “places of hibernation . . . were characterized by shallowness, high DO, and low pH, not by water temperature variations” (See Resent Publications for Ito, 2002).
In the U.S., an invasion of the exotic apple snails to Lat 36o would encompass most of North Carolina, Tennessee, and Arkansas, including most of that state’s vast rice producing areas. That may not be farfetched. The invasive snails have already been documented in Stoneville, N.C. (36o28’N). Will a broad range expansion to Lat 36o eventually happen? Who knows, but my guess is that that Pomacea insularum and P. canaliculata will ignore all of our speculative range maps and surprise us by heading north. Posted by Jess Van Dyke