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This chapter considers movement of species over time. Many land managers and ecologists consider invasive species to be the most important factor in disrupting "natural" ecosystems today, at least in North America. This chapter takes a historical approach to the process of species range extensions beginning with changes in the absence of human vectors, then considering the specifically human role in disseminating species world-wide. Human-mediated species' range extensions are ancient in many parts of the world so that what we might think of as "natural" may also have a human aspect. Using examples of range extensions of plants, animals, and disease-causing organisms, the discussion offers cautionary tales of species that have been introduced on purpose or inadvertently and have later caused severe disruptions to native ecosystems, as well as more positive examples of cultural landscapes in which non-native species are fully integrated into diverse and functional ecosystems. Comparing change over time in species diversity and importance may show unexpected patterns, such as the increase in both native and non-native plant species at the same sites in England. Historical studies also indicate that removal of the exotics may not lead to reestablishment of the native flora. Integration of some non-native species into a new ecosystem may eventually be more or less complete. |
