Among the top priorities of land management in the Cowling Arboretum is preventing the spread of harmful species which threaten the integrity of the Arb’s ecosystems. This is a difficult process, given that some invasives are more harmful than others and the removal process requires extensive resources.

Ninety-nine non-native species have been recorded in the Arb, though most of them are not a significant threat to native plants. Some, however, require aggressive management because of their unique abilities to outcompete native species, forming monocultures that reduce overall biodiversity and alter soil quality.

Among plants in the Arb, some of the most damaging species to forest biodiversity are two species of buckthorn and the Tatarian honeysuckle. These shade-loving species form massive understories and are directly responsible for soil erosion.

In prairie areas, the most damaging are thistles, the common parsnip, and certain clovers. Because these plants are partial to disturbed and open areas, they tend to aggressively overrun native prairies, outcompeting natives such as Indian grass, goldenrod, wild rye, and asters.

Non-native animals can also wreak havoc: for instance, native western bluebirds are often outcompeted for nesting sites by more aggressive European starlings; green ash forests along the Cannon River floodplain are threatened by the parasitic emerald ash borer, an Asian beetle.

Are all non-native species damaging, and all native species victims? The answer is not simple. Practically all local invasives are native to northern Eurasia, so they are able to tolerate Minnesota’s climate while possessing advantages, such as high growth rates and effective seed dispersal, for which many native species are unprepared. However, not all non-native species are necessarily invasive—that is, some foreign species grow in the Arb but do not necessarily invade. A prime example is the common dandelion, which is abundant and not a threat to biodiversity. Often, such species have close native relatives in Minnesota and, as a result, other species are adapted to competing with their forms.

Surprisingly, indigenous species can themselves become invasive when previous factors keeping populations in check disappear. Most notable is the white-tailed deer, which is abundant in the Arb. Without its primary natural predator, the wolf, it now has the potential to clear forest undergrowth by overgrazing.

The lines between which species are invasive, merely foreign, or native and damaging are important to consider in issues of habitat management. Thankfully, in the case of the Arb, decades of ongoing restoration have brought us closer to fully restoring a patchwork of healthy forest and prairie.