Shrubs of Wisconsin
cathartica can be identified by the small
thorns born at the tips of most branches. Leaves are toothed and may
be arranged both alternately and oppositely on the same branch. The fruits,
similar in appearance to a cherry (but with two or more seeds inside rather
than the single seed of cherries), are very dark colored and numerous.
European Buckthorn is a troublesome invasive exotic species. The fruits are eagerly eaten by birds and the seeds are spread widely. When the trees reach maturity they produce abundant fruit and within a few years there are commonly thousands of seedlings around the base of each mature tree. Cut stumps sprout vigorously and usually require treatment with an herbicide, if they are too large to pull out.
See other web sites for methods of control, for example:
The nature conservancy web site