MSU Extension

Humans have been a major vector of plant transport for centuries, more so in recent times. One has to wonder a bit about who cultivated whom, when thinking about species such as apples, wheat or corn.

Most exotic species are largely benign to the environment and are often helpful to humans. Many species are so naturalized that many people assume that they’re native. Think dandelions. There are a few species, however, that have given the idea of “exotic” a bad name. These species should not be planted and existing occurrences should be prevented from spreading and eradicated, where possible. 

The most important factor regarding “exotic invasive” species is the likelihood for causing ecological or economic harm or harm human health. What constitutes “harm” has variable definitions.

In addition to the argument of where a species originated, there’s also the issue of how a plant species has been bred over time, often for various purposes. So many of our garden flowers are far-removed from their original genetics through decades, or centuries, of breeding. Tulips, for example, an icon of the Netherlands, originated from Turkey. This idea also holds true for ornamental shrubs and trees.

Furthermore, an individual species can have different genetics across its range. Sugar maple, for instance, is not quite the same from southern Michigan to the west end of the Upper Peninsula. It’s even more different in northern Minnesota, where the climate is drier and colder. Sometimes, a species in one place might be almost unrecognizable in another place.

Plant species have different degrees of genetic “plasticity”. For example, red pine is pretty much the same everywhere; while green ash and red maple have a wide genetic alphabet, thus, the creation of so many horticultural varieties.

Plant species with a set of genetics from, say, Iowa could be considered an exotic in northern Michigan. It might be able to grow, but not as well as the native version of the same species. Prairie plants, in particular, often have these sub-species variations.

The term “exotic species” simply means a plant (or animal) whose species or genetics are different and come from somewhere else. That “somewhere else” might be another continent, another region, or maybe something as local as another lake or pond.

The term “invasive” refers to the ability of a species to reproduce and expand its range in a place where it previously did not exist. Some organizations now define invasive as always an exotic. However, sometimes, native species can become “invasive” under altered ecological conditions, “invading” niches that were previously occupied by other species, or becoming “unnaturally” common. This leads to dysfunction in ecological systems. 

For homeowners and forest owners, these questions should be considered when deciding what to plant, and where the selected species geographically originated. If you’re going to plant sugar maple, then try to find a local nursery that uses local seeds.

If you’re considering exotic species (or subspecies), then learn first about its potential to become aggressively invasive. Poor choices include species such as Japanese barberry, privet, frog-bit, or garlic mustard. Some species are illegal to introduce to Michigan, even if they’re already here. Exotic species often do not fit well into the native food web. As an example, Doug Tallamy, University of Delaware, constructed a list of native and exotic plants and the number of insect larvae each species supports.

If the local garden center or tree nursery personnel don’t know the answers to these questions, consider finding another vendor or do some searching on the Internet. As the interest in plants grows, and the amount of choices expands, a responsible gardener or landowner will suit the species to the site, especially in wildland landscapes. Plants that are transported from other states (or countries) can also carry exotic pests. That’s how the hemlock woolly adelgid was introduced to the western Lower Peninsula.

However, non-invasive exotics can be good choices. Many plants have interesting characteristics that can enhance property values and make for more visually diverse ownerships. Sometimes, quite attractive genetic hybrids are sterile, resulting in less risk of spread.

North America still has many mostly native vegetation types, although exotic plants, animals, and diseases have changed, and will continue to change, these resources. Typically, the changes are detrimental to both ecological diversity and function, and sometimes for long-term human welfare. Impacts from Asian long-horned beetles and hemlock woolly adelgids are prime forest examples.

Nevertheless, other parts of the world, such as Europe and regions of Asia have become considerably homogenized. In some ways, we can see our future in those places. It’s not only about the appreciation of natural diversity, there is also a set of environmental services which are affected. But that’s another story. The bottom line is to learn about what you plant and try to make informed choices.