|Common milkweed, Asclepias syriaca (© B. Moisset)|
Definitions of native plants abound. None is entirely satisfactory in all circumstances but each may serve a specific function. Some strive for scientific accuracy; others serve practical purposes. Moreover, in some instances the nativity of a plant may not matter to the native-plant gardener. None of us is about to give up growing tomatoes regardless of their non-native status.
The Federal Native Plant Conservation Committee
proposes the following definition: “A native plant species is one that occurs naturally in a particular habitat, ecosystem, or region of the United States and its Territories or Possessions, without direct or indirect human actions.” Such definition may be useful for policy making.
The Lady Bird Johnson Wildflower Center
of the University of Texas at Austin provides a sampling of definitions. Here are two:
“Native plants should be defined as those that have evolved and adapted to a specific location and have remained genetically unaltered by humans” (Wasowski. The American Gardener, 1998).
“All indigenous, terrestrial, and aquatic plant species that evolved naturally in an ecosystem” (US Forest Service).
Some may prefer other definitions from that list. In my opinion, the best ones are those that recognize the significance of co-evolution, habitats and ecosystems.
In North America, 1492 is commonly used as the cut-off date. It signals the arrival of Europeans on this continent and also the subsequent era of exploration of the entire planet. We may regard it as an arbitrary date; but it is one with practical usefulness as well as with historical significance. The numbers of introductions of non-native species and the distances to which they are transported started growing dramatically in 1492 and continue to grow at accelerated rates.
Humans have been introducing species to other lands from the beginning of agriculture (and unintentionally even earlier). Polynesians took pigs to Hawaii more than a thousand years ago. In recent times, Europeans introduced a new breed on the island. Is one breed more native than the other? Does it matter? We need to remember that the recently introduced breed is larger in size, more invasive, and more destructive of habitats. Habitat restoration may justify eradication of the European introduction. But removing the earlier breed, which is embedded in Hawaiian tradition and culture, would not be wise. This thorny issue will have to be decided by Hawaiians. I don’t envy them the task or the heated conflicts that this issue creates.
The Three Sisters of Native American agriculture –corn, squash and beans– are not truly native to North America. They were first cultivated in a region of Mexico and Central America several thousand years ago and carried farther north and east as well as into South America in prehistoric times. It is tempting to regard these crops as native plants, but they are not really so. Once again, the question is: Does it matter? Would those devoted to conservation and restoration want to eliminate these non-invasive, economically significant, long-established crops? Of course, not! This is not the purpose of restoration.
Further back, all species of organisms have been on the move at a certain point. To grow and multiply is a mandate as old as life itself. Those that successfully multiply need to expand their territories and invade new areas. The present geographic distribution of any given species tells us only a small part of the story. To understand the concept of native organisms, we need some knowledge of the origin, evolution, and dispersal of species and taxonomic groups.
We can use as an example that quintessential native plant, the common milkweed (Asclepias syriaca
) and its relatives, the other milkweed members of the genus Asclepias
|Common milkweed flower and one of its visitors, a bumble bee (© B. Moisset)|
The family of milkweeds originated in Africa many millions of years ago. Its members have been spreading out ever since. They occupied Asia, evolving into a number of new species along the way. They crossed the Bering Strait around 30 million years ago and, once again, they diversified into a number of related species. Nowadays, there are more than a hundred species of the genusAsclepias
in North America. Some crossed the Isthmus of Panama and invaded South America, where they evolved into a handful of species. I find it very interesting that many millions of years later a mammalian species would originate in Africa and follow a similar itinerary all the way to the tip of Patagonia in South America. I am talking about us, Homo sapiens
|Geographic distribution of Asclepias syriaca. USDA map|
|USDA maps illustrating the geographic distribution of species ofAsclepias. See the complete page|
The USDA maps show the distribution of the common milkweed, Asclepias syriaca
, and of other members of the genus. The common milkweed is found in all the Eastern United States and Canada and also in parts of the West. Other milkweeds occupy more limited territories, some overlapping each other. We can be certain that their geographic ranges have not remained constant in the thousands of years of their existences. They responded to climatic changes that caused glaciers to expand and retreat by migrating north or south and by contracting and expanding their territories accordingly. Still, we regard them as native to the areas where they are naturally present.
|Adult monarch on common milkweed (© B. Moisset)|
Milkweeds didn’t come alone. They are part of ecological communities that include other species, some tightly, other loosely, linked. Milkweed butterflies, having tied their destinies to those of milkweeds, followed their host plants. The genusDanaus originated in Africa and spread out along with milkweeds covering similar territories. These were the ancestors of the monarch butterfly (Danaus plexippus) and its relatives.
|Longhorn milkweed beetles (© B. Moisset)|
In fact, the southern monarch (Danaus erippus
), a resident of southern South America, is so similar to the North American monarch that both were considered members of the same species until recently. Curiously, this southern monarch, like its northern sister, is migratory. Even more curiously, it migrates south to a colder climate, when winter comes. This is the kind of natural history mystery that keeps me awake at night. I hope that somebody uncovers the secrets of the southern monarch and lets me know.Monarch butterflies are not the only insects that take advantage of milkweeds. A whole menagerie has evolved to feed on these plants. There are milkweed longhorn beetles, large and small milkweed bugs, milkweed weevils, and milkweed tussock moths, just to name the most familiar ones. All of them are adapted to the strong milkweed toxins. These poisons remain in their tissues and give them some protection against predators, which find them inedible.
|Lady beetle larva, Coleomegilla (© B. Moisset)|
However, despite the protection that the milkweed toxins provide to its feeders, some predators and parasites are adapted to this inconvenience. Several species of birds and mice feed on monarchs, both in this country and in Mexico, where they spend the winter. A number of predatory and parasitic insects also depend on monarch butterflies for nourishment. Thus the milkweeds and their dependents and the other components of the food chain are all linked together. They have been co-evolving for millions of years and are functional parts of their ecosystems.
|Silver-spotted skipper (© B. Moisset)|
These relationships matter when we talk about native plants. A non-native organism, one newly arrived into an ecosystem, lacks this kind of co-evolved interactions with the other members of the community. It is not a functional component of the ecosystem. Eventually, new interactions will develop given enough time. This may take tens of thousands or millions of years. This is why the best definitions of native plants involve the words “co-evolution,” “habitat,” “community,” or “ecosystem.”
In some instances, it is hard to tell what is native. In certain cases, it may not be important or practical to consider the nativity of a plant, as in the example of the crops grown by Native Americans. That which really matters is the long established and complex relationships
present in ecosystems. A native plant is one that is ecologically linked to other components
of the ecosystem where it is found.
Some important concepts lack a perfect definition. We need to name them, nonetheless. Biologists would be lost if they couldn’t use the concept of “species” just because no definition fits all the circumstances. This word is not only important but absolutely necessary. One may need to use different definitions of “species” depending on the discipline; but that is no reason to give up the word. The concept of “native” organisms is equally necessary. It is here to stay, regardless of the difficulties that may arise at times.