Every species evolves in its home territory to have one to several ways to expand its range. They may be wind blown, rain splashed, carried by animals, or moved in soil or water. Almost all short-distance spread is through these natural dispersal mechanisms. In their home territory, short distance spread is rarely a problem because the resident plants and animals have evolved to coexist more or less peaceably.

On its own, emerald ash borer will generally move less than four miles a year. But with help from people, it can cover 55 miles per hour in firewood, nursery stock, or personal belongings.

On the flip side, long distance spread is almost always human assisted. Because long distance spread takes the species a long way from home, the resident plants and animals are not often prepared to cope with their new neighbor. Natural enemies are missing and host species often lack the natural defenses necessary to survive an attack by the introduced species. Once introduced, aggressive species are free to expand their range using their short distance dispersal mechanisms with a competitive advantage over native plant and animals due to the lack of natural enemies.

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