Prevalently, most invasive species are moved from place to place via three primary methods: intentional, unintentional and natural. Intentional introduction, generally, is caused by human intervention. Cows, sheep, and horses as livestock and even the red fox were purposely introduced into new environments for game hunting. One major example of damage done by livestock is the Dust Bowl. The Dust Bowl happened when too many cattle fed on the grass around Kansas and never gave it a chance to grow back. This caused the once green plains to turn to dust and become unable to grow much. Humans have also introduced a species into an environment in order to eradicate another species in the area whether it is native or not. Some plants are considered invasive and were transported for human purposes, such as ivy. The unintentional method also includes human intervention, resources traded between people of different ecosystems have the potential of housing organisms, which can thrive and cause issues in its new home. Some pets have also accidentally become feral (undomesticated) because of their owners losing them, or setting them loose in the local environment, it finds others of its species and creates a local feral population, the most average example of this are generally small toad, frogs, and lizards. Last, but certainly not any less devastating, is the natural approach. Organisms migrate due to changing climates, a natural instinct that most animals possess. This process works especially well for plants however, because when an animal eats a plant, seeds and all, the seeds are in its excrement and it has the chance to grow wherever it was dropped. These methods are the most common and the most effective at spreading a species around the world. |  Just imagine is all the animals in the zoo got out and created feral populations in an urban area... |
