Many weeds that never previously existed in Canada were brought over by early European settlers who started wheat farming in the prairies. This is an example of

Introduced species or exotic species

Explanation:

The introduced species is also called as an exotic species. The exotic species is a species which is not native to a particular location or area but is introduced by humans either accidentally or with the desired species. Such species can be invasive means, it can compete with the native species for resources and can reduce the population of native species.

The given situation is an example of introduction of new species. The weeds are introduced exotic species which could have come along with the desired variety of wheat required for farming in the prairies.

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