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house finch invasive species
The house finch, originally from California, has become an invasive species in Wisconsin, crowding out native birds.

Q: What makes a plant or animal "invasive" instead of just "non-native"?

A: Non-native plants and animals are those that come from somewhere else, usually another country. When they start to reproduce in a new location they're said to be "naturalized."

Only a few of the naturalized plants and animals will become invasive, said Don Waller, a professor of botany and conservation at UW-Madison.

"There are many different exotic, non-native species that can come into a new area, either because they are intentionally or accidentally introduced. Only a fraction of those actually get established, and only a subset of the naturalized species actually explode and become invasive."

Wisconsin is host to some invasive species that have roots right here in America. The house finch, for example, originated in California and is flourishing in its new range.

Originally, scientists thought invasives prospered mainly because they have left diseases and predators behind, but other processes may also be at work, says Waller, who studies invasive plants like buckthorn and garlic mustard.

"There seems to be an evolution of increasing invasiveness. There may be a switch in life history or an evolution away from defenses that they no longer need in their new home, so they are free to put more resources, energy, material into reproduction."

By crowding out native species, invasives are one of the biggest causes of declining biodiversity.

- Produced in cooperation with University Communications

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