Whether natural or cultural, every species takes advantage of opportunities to disperses/migrate to colonize and multiply. And, when they colonize/invade a new place (mostly already occupied), other species that have already there before (e.g., native species) would be affected. Some may adjust, thrive, and advance, while others may become extinct. Eventually, a new ecological community establishes, until another species invade/colonize or environmental condition changes. This is what every species does. Every species has some kind of dispersal mechanisms.
Ecological community species interactions/compositions and ecosystem processes are dynamic and ephemeral, while our some of ecological disciplines are based on static perspective where set of communities and species interactions are static, complete, and integral. Nothing wrong with this. Within a limited time frame, they really can be considered static. For instance, many textbook describes the US southeastern oak-hickory forest as "primary/virgin/old growth" forest, but in reality, the forest was originally chestnut forest before chestnut blight wiped them out in 1900-40s. In decadal timeframe this forest is a stable oak-hickory forest, while in centuries timeframe it is an altered dynamic forest. Now, should chestnut be considered "exotic" species in this altered forest community? I don't think there is no objective measures to define what is considered I think it is imperative that to clearly state "you own working (often subliminal) definition" of native/non-native community/species in terms of time/spatial scale. Toshihide "Hamachan" Hamazaki, PhD : 濱崎俊秀：浜ちゃん Alaska Department of Fish & Game Division of Commercial Fisheries 333 Raspberry Rd. Anchorage, Alaska 99518 Ph: 907-267-2158 Fax: 907-267-2442 Cell: 907-440-9934 E-mail: [email protected]
