Communities that occupy similar habitats in different regions of the globe normally include different species. Evolution of a new type of organism occurs at a particular location, and the new species must spread out from this location over time. Physical barriers typically prevent the species from colonizing all habitats that could potentially satisfy the species' niche requirements. The diversity of barriers is as rich as that of organisms. The global distribution of a species is both a historical contingency, or dependent on the particular series of events that occurred in the past, and a geographic contingency, or dependent on the particular spatial arrangement of elements of a landscape.
Human activity has greatly increased the transport of materials around the globe. Earthworms from Europe were introduced to the Atlantic coast of North America and have been steadily colonizing new soils each year. The root pathogen Phytophthora infestans was introduced from Mexico to the United States and then was transferred to Europe (causing the Irish potato famine) and from there to the rest of the world (Goodwin et al., 1994). Transport of soil is now the subject of international law and regulations. The difference in effects of the introduced species in these two examples is interesting, given the questions raised above. P. infestans in agroecosystems has a substantial impact because it is involved in aggressive exploitation of the dominant plant (an important ecosystem engineer) and clearly causes system reorganization. On the other hand, earthworms play the role of a detritivore involved in comminution of plant tissue. This can be considered a weakly interacting mutualism with plants. Interest in elucidating the ecosystem impacts of introduced soil organisms is increasing as the deleterious effects of the introduction of aboveground species are documented. Interest has also been generated in the relative success rates and characteristics of successful invasive microorganisms as interest in the use of genetically modified organisms has progressed.
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