Final answer:
Invasive species are non-native organisms that can threaten and alter the ecosystems they invade by outcompeting native species, changing the habitat structure, and spreading diseases. They may cause serious ecological damage, economic losses, and health concerns.
Step-by-step explanation:
Invasive species are non-native organisms that are introduced to a new environment where they did not originally evolve. Once there, they can sometimes spread rapidly, outcompeting native species for resources and altering the ecosystem. An example is the introduction of the zebra mussel (Dreissena polymorpha) in the United States, which has significantly changed the aquatic ecosystems it invaded.
Invasive species can modify the physical structure of their new habitat, as seen with the North American beaver in Tierra del Fuego, which transformed forests into meadows, and the Australian paperbark trees in the Florida Everglades, which changed prairies to forests. In some cases, these species also bring along parasites and diseases that can affect both native wildlife and human health. For instance, European crayfish have been decimated by diseases carried by imported North American crayfish.
In terms of ecological effects, invasive species can out-compete, predate upon, or introduce diseases to native species, which can lead to severe declines or extinctions in native populations. Moreover, they can induce significant economic losses in agriculture and other sectors.