Seed limitation and the dynamics of feral oilseed rape on the M25 motorway

Abstract

Feral oilseed rape populations in 3658 quadrats (each of 100 m) on the verges of the M25 motorway were studied for two years. There was substantial turnover in site occupancy; 55% of quadrats had a different population density in 1994 than in 1993; 53% of the quadrats occupied in 1993 were locally extinct in 1994; and 20% of the empty quadrats in 1993 were occupied by oilseed rape in 1994. Verges next to the carriageway carrying traffic towards the main rape seed crushing plant at Erith in Kent had significantly more plants than the opposite verge carrying traffic away from Erith. Mean rape densities were also higher in the vicinity of exit and entry slip roads than on sections of verge between motorway junctions, but densities were not affected by the presence of rape crops or rape volunteers in adjacent fields. The apparent permanence of oilseed rape populations on motorway verges clearly belies substantial turnover in patch occupancy. In the absence of soil disturbance, rapid secondary succession (principally the growth of perennial grasses) tends to lead to local extinction within three years. Given sufficient soil disturbance, however, rape population density appears to be seed limited, and seed spillage can cause a two- to five-fold increase in mean population density.