Time of Introduction of the Alien Component of the Spontaneous Illinois Vascular Flora

Abstract

The current spontaneous vascular flora of Illinois consists of 811 (nearly 29%) alien species. Of the 811 spp., 708 (87.3%) are herbs and 103 (12.7%) are woody plants. Four hundred and forty alien species were reported before 1922; 163 additional species between 1922-1955; and 208 more between 1956-1978. The angiosperms contain a higher percentage of alien species (29.5%) than either the gymnosperms (25%) or fern and fern allies (1.35%). In the pteridophytes there is only 1 family that consists totally of alien species; in the gymnosperms there are none; and in the angiosperms there are 10 such families. The Poaceae and Compositae have had the greatest increase in the number of alien species between 1922-1978. The average number of alien species introduced into the spontaneous Illinois vascular flora per year between 1803-1978 is 4.6. The average number of introductions between 1956-1978 is 9.4, indicating the increased rate of alien species introductions in recent years.