James Alexander
Associate Professor (Biology)
"Organisms have conflicting reasons for dispersal and migration. There are reasons to stay in place; for example, many dispersing organisms die while dispersing. In some situations, staying with relatives increases survival and inclusive fitness. Other factors favor dispersing. Dispersal reduces competition with kin; for some species, parents chase away grown young, so that they are not future competitors."
Dispersal is the movement or transport of individuals away from their natal populations, which can have important micro-evolutionary consequences. In contrast, migration is the mass directional movement of entire populations from one location to another. Herds of bison, caribou, zebras and wildebeests move and return to a given location every year, following seasonal changes in food abundance. Salmon hatch in freshwater streams and then migrate to the sea, only to return once more to spawn in their natal streams. For Monarch butterflies, several generations pass before the offspring of an individual that started the migration makes it back.
Organisms have conflicting reasons for dispersal and migration. There are reasons to stay in place; for example, many dispersing organisms die while dispersing. In some situations, staying with relatives increases survival and inclusive fitness (helping relatives rear additional kin, who also carry your genes). Other factors favor dispersing. Seasonal variations in insect abundances strongly affect migratory birds. Dispersal reduces competition with kin; for some species, parents chase away grown young, so that they are not future competitors. In many birds and mammals, one sex tends to disperse more than the other, reducing inbreeding. Many ‘weedy’ species, such as dandelions, live in highly disturbed, relatively ephemeral habitats, due to their high dispersal abilities.
Dreissenid mussels (zebra and quagga mussels) successfully colonized the Great Lakes in the late 1980s, presumably due to increased international trade and the development of fast transoceanic ships shortening transit times. Twenty five years ago, I came to Louisville to study the potential of dreissenid mussels colonizing the Ohio River. Before I could start, mussels had already established populations here. My research shifted to determine if dreissenid mussels would thrive in both the Ohio and other North American rivers, and if expected climate changes would enhance their spread. My conclusions proved correct; dreissenid mussels have since expanded into major river systems throughout our continent. The costs associated with monitoring, prevention and maintenance of raw water systems are not cheap: dreissenid mussels cost cities and businesses millions of dollars annually.
The dispersal of individuals is important for metapopulations fragmented into smaller subpopulations, typically by habitat loss/alteration. Immigrants from other subpopulations prevent the extinction of a subpopulation.
However, the movement of organisms (or the lack thereof) also can have negative effects. I currently am interested in how a changed climate will augment the introduction of exotic molluscs (snails and bivalves) as well as the movement of native species. The most endangered taxa on Earth are freshwater molluscs, due to altered habitats, pollution, introduced species and small endemic ranges. Many currently threatened molluscs simply cannot move fast enough and are at risk from additional environmental stresses arising from a warmer climate. In contrast, introduced snails have established viable populations here in North America and are spreading. Many snails, native and introduced, are intermediate hosts to numerous parasitic worms (helminths) of human and veterinary importance. New interactions are occurring between snails and helminths, increasing the risks of helminthiases for not only us, but for our pets, livestock and wildlife as well.
