Document: MAR-3-83-5

Naturalized floras are largely the products of deliberate introductions: Assessment of temperate and subtropical floras.

MACK, R.N.* ¹ and M.ERNEBERG ²

Washington State University, Pullman, WA 99164 USA ¹
Natural Environmental Research Institute, 8600 Silkeborg, Denmark ²

Abstract:
Vast numbers of plant immigrants have long arrived in the U.S., but very few immigrations result in naturalization; the hazards of environmental stochasticity for immigrants are clearly acute and chronic. Deliberately introduced species and those non-indigenous species introduced as contaminants in imported crop seeds experience post-immigration cultivation that could greatly enhance their persistence in spite of such stochasticity. We assessed the proportion of naturalized taxa (species, subspecies, varieties) that had been deliberately introduced or introduced as seed contaminants in a group of U.S. regional and state floras that span much of the environmental amplitude of the U.S. The number of taxa in these naturalized floras vary substantially: northeastern U.S. (approx. 600), Hawaii (approx. 800), Florida (approx. 1150). The naturalized floras of New York, northcentral Texas, and Rhode Island each have about 400 taxa. We considered a naturalized species to have been introduced deliberately if there was an unambiguous historic record of its use in its native range, the U.S., or both. Despite the vast differences among these floras in their physical environments and the length and character of plant immigration history, the proportion of the naturalized flora that was likely introduced deliberately is consistently greater than 50%. In the eastern U.S. the largest fractions of these deliberately introduced species were introduced as seasoning or putative sources of medicine; in subtropical Hawaii and Florida, ornamentals are the largest single fraction of the naturalized flora. Among naturalized species that were apparently introduced deliberately are many threats both to conservation and agriculture, including Abutilon theophrasti, Berberis vulgaris, Cynoglossum officinale, Eleusine indica, and Senecio jacobaea. The remaining proportions consist of taxa that were historic seed contaminants (usually < 10%) and taxa with unknown modes of entry (approx. 30%). The link between deliberate introduction and subsequent post-immigration cultivation leading to naturalization, although still largely circumstantial, provides an explanation that transcends the taxonomic diversity of naturalized species and the environments in their new ranges.

Keywords: modes of introduction, deliberate introductions, seed contaminants

This abstract is being presented at: 8:45 AM in session:
Oral Session #2: Conservation Ecology.