Consider this scenario: The tropical island of Hawaii is virtually snake-free. Smugglers, seeking to make money, brought several crates of snakes to the island. Unfortunately, while traveling across the island, the truck carrying the snakes was in an accident. The crates were thrown from the truck and broke apart. Most of the snakes were never recovered. Would this have any affect on the ecosystem here?
Those snakes affect the ecoystem because they need food so they go down and become a predator and hunt for food. If they bite people depending if its venomous then they'd seek medical help