The complex nature of headland shrub encroachment: The case of Headland Zieria (Zieria prostrata)

Abstract

Headland Zieria (Zieria prostrata) is an endangered species restricted to four headlands with a potential population of 1000 individuals. The species also occurs within the endangered ecological community Themeda grassland on sea cliffs and coastal headlands in the New South Wales North Coast, Sydney Basin and South East Corner Bioregions. Shrub encroachment of native species is perceived as a threatening process within these grasslands, and also to the unique species within them, such as Headland Zieria. Suggested management actions for the community and Headland Zieria include the removal of other shrubs by frequent fire or mechanical means. We conducted a survey and correlative analyses to test the validity of these proposed actions. We provide evidence that Headland Zieria is facilitated by a higher density of nearby shrubs which may provide protection from the elements and decrease competition from other understorey species but is eventually out competed by them when the grassland fully transitions to a Banksia shrubland. We suggest that Headland Zieria is an ecotonal specialist that may require an invasion front of shrubs and/or isolated patches. The implementation of fire and/or overstorey shrub removal may be detrimental where populations of Headland Zieria occur. Our study highlights the need to look more closely at interactions before management actions changing vegetation structure and composition are implemented.