The comparative demography of the pasture weed Echium plantagineum between its native and invaded ranges.
Abstract
Echium plantagineum is native to the western Mediterranean Basin, where it is a common, but not dominant, component of species-rich annual grasslands. Since its introduction in Australia, E. plantagineum has spread to infest vast areas of predominantly agricultural land in south-east and south-west Australia, where it can be the dominant pasture species. To unravel the ecological factors responsible for high population abundance of E. plantagineum in Australia, its demography was compared between sites in the invaded and native ranges. Demographic parameters of E. plantagineum populations were estimated at a site near Canberra, Australian Capital Territory, in south-eastern Australia during 1995-97, and at a site near Evora in southern Portugal during 1994-96. Identical factorial experiments were set up at each site with treatment combinations of the presence or absence of grazing and pasture competition. The recruitment, survival, fecundity and seed bank dynamics of E. plantagineum populations were measured for each of the treatment combinations over 2 years at each site. These data allowed the estimation of demographic parameters describing the proportion of E. plantagineum individuals moving from one life-cycle stage to the next. Seedling establishment fractions were two to five times greater at Canberra than at Evora, and seed bank incorporation rates were three times greater at Canberra than Evora. These demographic differences were those most likely to play an important role in greater abundance of E. plantagineum in Australia compared with Mediterranean Europe. Neither seed bank survival rates nor seed production differed between populations at Canberra and Evora, while seedling survival rates were always lower at Canberra than at Evora. Neither grazing nor pasture competition limited the seed production or seedling survival of E. plantagineum populations at Evora more than at Canberra. An effective approach for the control of E. plantagineum in Australia may thus be through the reduction of the seedling establishment fraction. This may be achieved by maintaining significant pasture vegetation cover and reducing the available space for E. plantagineum establishment during autumn.