NSW National Parks and Wildlife Service, 2002
ISBN 0 731 36496 1
4. Management Issues and Threats
The overarching conservation concern with all three of these species is the small total population size, the current low numbers of large, reproductively mature plants and the extremely small area occupied by each species. This makes each species very susceptible to extinction through stochastic events such as disturbance from wildfire, drought and severe browsing.
Drought appears to be a natural threat, particularly for Z. formosa, as tolerance of adults to extremely dry conditions, such as those experienced on the site in 1997/98, has been demonstrated to be low.
In more recent years browsing by Swamp Wallabies has probably become a more serious threat to Z. buxijugum and Z. parrisiae than it would have been prior to European settlement. This threat is suspected to have increased due to numbers of wallabies in the area having increased with the clearing and establishment of pasture on nearby farmland, coupled with a general rural community trend in recent years to less frequent shooting of these animals.
More than two thirds of the mature population recorded in 1987 (125 plants) died as a result of the 1997/8 drought. The large number of seedlings is encouraging, but survival rates of this recruitment may depend on continuing favourable seasonal conditions and continuing low levels of browsing by wallabies and rabbits, which are both present at the site.
In 1999 the land surrounding the site was the subject of a Rural Residential subdivision application. Without appropriate controls, there is potential for this subdivision to indirectly impact on the species, through increased nutrients and sediments being carried on to the site via a small creek draining through the site from the proposed adjacent subdivision. Weed infestation could subsequently become a problem for a section of the population that occurs along a drainage line. BVSC has thus imposed some specific controls on the development to minimize potential indirect impacts on Z. formosa. These controls are detailed in section 5 of this Recovery Plan.
The Z. buxijugum population is in an area not currently subject to domestic stock grazing, nor is there likely to be any in this area. Prior to winter 2000 there appeared to be no immediate specific threat to this species, however the severe browsing damage caused by wallabies at that time has had a major impact on most plants, and few are expected to flower in spring 2001. Providing such browsing events occur only every few years then the majority of plants could be expected to recover in the intervening periods and the population remain reasonably stable. Monitoring will be important to determine whether the frequency and impact of possible future browsing events is becoming a significant threat requiring management action.
Fires of low to moderate intensity are unlikely to impact on the entire population, as the site is very rocky and unlikely to carry a fire across the whole of the habitat.
The major ongoing threat appears to be browsing by wallabies. Both young plants and seedlings are still being severely effected by browsing. The most likely current cause is macropods (an automatically triggered camera placed at the site during 1999 has recorded a Swamp Wallaby adjacent to a browsed Zieria plant and goat dung is no longer evident). Because there are so few large, reproductively mature plants, this ongoing browsing is considered a significant threat, particularly as it appears to be slowing the rate of movement of seedlings and smaller plants into the larger size classes.
In 1999 leaf-eating insects were observed to be causing substantial leaf-area loss, particularly on some young plants. Such damage has been considerably less during 2001.
- Initial population counts for all three species were undertaken in 1986 and 1987 by Briggs and Leigh (1990) (see Table 1).
- In 1986 propagating material was collected from all three species in order to establish ex-situ collections at both the ANBG and the Mount Annan annex of the Royal Botanic Gardens (MABG). The surviving ex-situ collections currently held at each of these institutions are as follows (Stig Pedersen (ANBG), pers. comm.; Richard Johnston (MABG), pers. comm.).
34 plants of a single genotype at ANBG and 18 plants from a single genotype at MABG. The genotype at ANBG is from a different parent than that at MABG.
6 genotypes at ANBG, with the numbers of plants from the various genotypes being 74, 20, 18, 6, 3 & 1. 3 genotypes at MABG, one of 3 plants, the others of 2 plants each. The genotypes at ANBG are from different parents than those at MABG.
6 genotypes at ANBG , with the numbers of plants from the various genotypes being 26, 19, 17, 14, 6 & 5. 1 genotype of 4 plants at MABG. The genotype at MABG is from the same parent as one of the ANBG genotypes.
- The adult population of each species was recounted on February 4 and 5, 1999 by NSW NPWS .
- In March and April 2001, NPWS undertook detailed population counts for the three species, allocating plants to one of four height classes (see Table 2).
- In March and April 2001, NPWS established three monitoring plots in the populations of Z. formosa and Z. buxijugum. For each plot the heights of all plants were measured, and the plants tagged and mapped. In the case of Z. parrisiae, the entire population was measured, tagged and mapped.
- BVSC imposed several controls on a Development Application lodged in 1999 for a Rural Residential subdivision adjacent to the area supporting Z. formosa. These measures included (1) specifying the location of building envelopes so as to ensure drainage from those sites does not flow onto habitat of Z. formosa habitat, (2) changing the location of the access road to avoid sediment runoff from the gravel road onto the Zieria site, and (3) a requirement for BVSC consent to clear native vegetation.
- In April 2001 NPWS installed 10 wire mesh guards around selected Z. buxijugum plants to protect them from browsing by wallabies. The plants chosen for protection were scattered across the site, with the aim of ensuring a source of seed-producing plants would remain across the site in case there are further severe browsing events. If further severe browsing does occur, then it is expected that protection of some large seed-producing plants will assist future seedling recruitment and population recovery.
- Goats were eradicated from the site by the landowner in 1987/88. Goats were considered the main threat at that stage.
- The population was revisited a few times between February 6 and August 8, 1999, and searches made for the two sites recorded in 1987. Both the previously known sites were eventually found and the population size determined. Mr Chris Parramore also located a small colony of plants of Z. parrisiae at Box Range between February 8 and 18, 1999 that had not been found in 1986/87.
- Wire mesh guards were installed during 1999 to protect some of the seedlings and young plants from browsing by wallabies. Several other seedlings were protected by placing branches of dead shrubs over the Zieria seedlings to discourage wallabies from feeding on them.