Conserving weedy natives: two Tasmanian endangered herbs in the Brassicaceae

Abstract

Two species of endangered Brassicaceae, Barbarea australis and Lepidium hyssopifolium, occur in a few small populations in Tasmania. The former species is associated with streams where it occurs in vegetation with numerous exotics. The latter species is usually found in the root zone of exotic large trees, usually on roadsides, and often in the absence of many other native species. Populations of both species have disappeared since European settlement, some in the last two decades. Both species are rapid and prolific producers of easily germinated seed. Both species are absent from places grazed moderately or intensively by sheep or cattle. The establishment of new individuals of Lepidium occurs only on relatively bare ground. The species is tolerant of root competition and intolerant of above ground competition. It will also establish from soil-stored seed after mechanical disturbance. Its future is linked to the survival of grazing-free locations where above ground competition from herbs and grasses is subdued. Barbarea is a ruderal that requires freedom from stock grazing for its persistence in Tasmanian riparian habitats. These results reinforce the importance of some degraded ecosystems for biodiversity conservation, and the critical role of disturbance regimes in influencing the survival or extinction of a subset of native plant species. In the fragmented and variegated landscapes of today, weedy natives cannot necessarily be expected to survive in non-weedy environments.