Question

Why is buckthorn an invasive species?

Answer 1

Buckthorn was imported to North America from Eurasia in the late 19th century as an ornamental bush.

Explanation:

Buckthorn produces a large number of berries (uneatable by humans) whose seeds are distributed by birds.
It can grow in a wide range of soil conditions (and in fact modifies the soil conditions to better suit its own needs), chocking out native species. Near where I live, buckthorn provides shelter through the winter for soybean aphids which cause significant damage to soybean crops.
The tips of branches usually end in thorns and tearing out buckthorn bushes can be an unpleasant task.