Plant animal and disease transmission

Species are limited in their ability to move to new con tinents by geographical isolation, and this limits the majority of the world's species from becoming invasive species on another continent. Dispersal overcomes the isolating mechanisms that prevent the movement of spe cies from one continent to another, and species have varying levels of abilities to disperse across the barrier and successfully open invasion windows. Species can move by natural dispersal vectors including wind, animal, and water transport.

After a plant species successfully disperses and estab lishes in a new continent, many species can vegetatively reproduce. One successful propagule can start a new population, and many plant species spread solely or mostly via asexual means (e.g., bulbils of Dioscorea opposi tifolia, plant fragments of Eichhornia crassipes (Figure 4), Salvinia molesta, Elodea canadensis, and H. verticillata). Wind dispersed invasive species with prolific seed

Figure 4 Eichhornia crassipes (water hyacinth) is a floating aquatic species native to South America, but invasive in tropical and subtropical wetlands around the world. Photo by B. Middleton.

production can also spread rapidly after their initial intro duction (e.g., Phragmites spp. and Typha spp.)

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