What are native plants?
Short: Plants are considered native in areas where they are fully incorporated into local wildlife communities. Native plants are utilized by local wildlife for food and shelter, just as local wildlife is utilized by native plants for services such as pollination and seed dispersal. In these ways, plants and wildlife depend on one another to survive. These interactions can exist only because the plants and wildlife have been influenced and molded by one another over lengthy time periods. Without these historic relationships, most plants and animals are disconnected and unable to fully utilize one another. In short, native species have developed relationships that permit them to use and be used by other native species.
Long: Many historical definitions of a native species, such as those listed below, often share a common theme:
“A plant is considered native if it has occurred naturally in a particular region, ecosystem, or habitat without human introduction.” – The National Wildlife Federation
“A native species is one that occurs naturally in a particular region, ecosystem and/or habitat and was present prior to European settlement” – Wild Ones
“A native species is a species that, other than as a result of an introduction, historically occurs/occurred in that particular habitat” – U.S. Fish & Wildlife Service
These definitions share a similar structure in that they mention the species’ occurrence in a particular region at a particular time (typically before human intervention). While at first these definitions may seem satisfying, further inspection reveals inherent flaws which prevent a species’ native status from escaping ambiguity.
Occurrence – While the current distributions of species are being mapped with increasing accuracy, the complete historic distributions of species are often difficult or impossible to determine with certainty. Despite an impressive archive of historical occurrence records, issues arise particularly when attempting to classify a species as historically absent in an area. It is impossible to differentiate between areas where the species is indeed absent and areas where the species was present but not detected in the historic record. As such, in most locations it is impossible to definitively state a species’ occurrence or lack thereof, and thus it is impossible to objectively define a species’ native status in these locations. A reliance on patchy historic occurrence records leaves the classical definitions of a native species in a cloud of ambiguity.
Region – The regional components of the historical definitions serve to further impede the objective classification of a native species. Regions themselves are conceptual constructs created by mankind and not necessarily adhered to by the rest of the natural world. The ways in which regions are delineated are numerous and can have a marked effect on the defined nativity of a species. For example, if one considers the United States as a region, then the Saguaro Cactus (Carnegiea gigantean) of the Sonoran Desert in Arizona would technically be “native” to Toledo, Ohio as well. While this is an extreme example, an equally extreme case can be seen in many nurseries across the country where Blue Spruces (Picea pungens) are commonly advertised as a “native” yard tree, when in fact the species is only native to some ecosystems of the Rocky Mountains.
Defining regions on a smaller scale and with attention to biologically relevant characteristics is a sensible improvement. The development of the term “ecoregion” describes just that, a geographical region identified by common characteristics such as geology, hydrology, climate, associate flora and fauna, etc. Various ecoregion maps have been developed such as those by the USFS, Bailey and Cushwa (1981), and The Nature Conservancy. While these ecoregional maps can be incredibly useful, they are ill suited to serve as the foundation for the universal definition of native species. The scales and environmental thresholds used to make ecoregional classifications are infinite, resulting in an infinite number of possible ecoregional maps. While these maps are invaluable in countless other applications, picking an arbitrary ecoregional scale to use as the standard for defining native species is just that – arbitrary.
Time – The temporal aspects of most historic definitions also raise several concerns. Most mention a time before human intervention, specifically a time prior to European settlement in the United States. Using a single snapshot in time as the standard for determining a species’ nativity entirely ignores the dynamic nature of the natural world. Species' distributions shift over time in response to various environmental factors, whether anthropogenic in origin or otherwise. For example, North American prairies (and the individual species that make them up) widely shifted their distributions following the last glacial retreat. Just as species distributions in the past have changed dramatically over time, they will continue to change into the future.
While one may correctly argue that species distributions in more recent periods (yet still prior to human intervention) would more accurately predict how species would likely be distributed today if not for human intervention, this still requires one to determine the exact time that humans began altering the environment and also makes the assumption that humans are not a part of the natural world. Overlooking the controversial assumption of humanity being unnatural, it is still difficult or impossible to determine when humans began altering the environment. Much attention is directed to the period prior to European settlement in America, however it is widely accepted that Native Americans were altering the landscape long before European arrival. The requirement of identifying a time before human intervention opens up another round of often unanswerable questions, further blurring the lines of the historic definitions.
A Modern Definition:
The dependency of the historic definitions upon the wildly subjective terms above results in an equally subjective definition of a native species. In order to bypass this confusion, Toledo Zoo embraces an ecologically functional definition of a native species in which a species is considered native if it fits into the complex network of ecological interactions that have developed within a given community throughout evolutionary time. In this way, nature itself will determine if a species is native by fully incorporating it into the complex network of relationships within a given ecosystem.
Take, for example, Swamp Milkweed (Asclepias incarnata). When planted in Toledo, Ohio, wildlife frequents its flowers, gathering nectar and pollinating the plant in return. Additionally, the leaf tissue is consumed by Monarch Butterfly (Danaus plexippus) caterpillars, and Milkweed Leaf Beetles (Labidomera clivicollis) which, along with many other species, have specialized over evolutionary time to tolerate the toxins produced by the plants in an attempt to deter herbivory. Additionally, these species sequester the toxins in their own tissues, making them unpalatable to potential predators. The root system of the plant penetrates several feet into the ground where they provide suitable habitat for a plethora of native microfauna. In the ways highlighted above (and countless others), Swamp Milkweed is deeply integrated into the ecosystem. As such, Swamp Milkweed should be considered native in Toledo, Ohio.
Next, let's contrast the native Swamp Milkweed with a popular ornamental plant, the Hybrid Tea Rose. While the large, showy blooms may catch the eye of passing pollinators, the structure of the flower is such that animals are unable to access what nectar may be inside. Interested pollinators not only end up empty handed, but also waste precious time and energy investigating these ecologically useless flowers. Additionally, the leaves are often left untouched by wildlife, ignored by all but the occasional invasive Japanese Beetle (Popillia japonica). Clearly, this plant is not integrated into the local ecological community, and as such cannot be considered native. While there are some examples of non-native species that may be utilized to some degree by local wildlife (such as an ornamental flower with accessible nectar), the key distinction between it and true native species lies within the degree to which it is utilized by the ecological community. Nectar is certainly an important resource for wildlife, but a true native species will be utilized to a far higher degree by the local community (specialist herbivores, complex pollination and seed dispersal relationships, etc.).
In conclusion, the historical definitions provide hints as to a species' native status, however ecological communities themselves are the best judges of nativity. Only species that are fully incorporated into the intricate relationships of an ecosystem can be considered truly native.