Why are invasive species dangerous to the environment?
they pollute the environment.
they are always predators.
they threaten biodiversity.
they bring abiotic factors.

The correct answer is - They threaten biodiversity.

The invasive species are dangerous because they threaten the native species of the place where they are introduced. They are competing for the same niche in the food chain as some of the species, for the same food, or are predators from whom the local fauna can't defend or escape because they don't have the needed defense mechanism nor can develop it immediately.

The perfect example are the cats and the rabbits in Australia.  The cats became an apex predator that brought lot of native species on the verge of extinction, and even managed to totally destroy some species, while the rabbits compete with the local animals for the same food, but the rabbits have much faster reproductive system and are outnumbering their competition and thus pushing it slowly aside.

The answer is: they threaten biodiversity.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.

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