Common milkweed, Asclepias syriaca (© B. Moisset) |
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) |
Geographic distribution of Asclepias syriaca. USDA map |
USDA maps illustrating the geographic distribution of species ofAsclepias. See the complete page |
Longhorn milkweed beetles (© B. Moisset) |
Lady beetle larva, Coleomegilla (© B. Moisset) |
Silver-spotted skipper (© B. Moisset) |
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.
Robins are among the birds that sometimes eat monarchs (© B. Moisset) |
© 2012, Beatriz Moisset. All rights reserved. This article is the property of Native Plants and Wildlife Gardens.
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Love this practical and reflective post-thankyou!
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