(1) Lactuca serriola is a winter or spring annual weed confined in England to the south-east. Each summer from 1976 to 1981 counts were made of the numbers of L. serriola plants between posts 100 m apart along the whole length of the M5 motorway from Exeter to Birmingham (250 km). This motorway twice crosses the geographical distribution limit of L. serriola and its verges provide a set of habitable sites for the species in which local variation is minimized so that geographical trends in its abundance can be discerned. (2) Its distribution on the motorway was closely related to altitude; it seldom occurred above 85 m, but below 60 m it was common. Colonies of the plant did not become smaller or sparser towards its northern or southern distribution limits which were therefore abrupt both spatially and relative to the environmental gradients. These facts suggest that very small changes in the controlling factors may determine the position of the limit. (3) There was a large and unexpected increase in its abundance and range in 1977 as compared with 1976. This appears to have been caused by the exceptional hot dry weather of 1976. In subsequent years its population density slowly returned to that of 1976. (4) Abrupt geographical distribution limits of the sort described here have seldom been identified, but they may be frequent in fugitive species.
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