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Weeds - Phase 1 - Research Highlights
Boneseed invasion on biodiversity
The impact of boneseed invasion on biodiversity
What the report is about
This project involved the surveying of 90 sites in Tasmania, South Australia and Victoria. There were 45 sites invaded by boneseed, Chrysanthemoides monilifera spp. monilifera, and 45 native (uninvaded) sites. Samples of the seed bank were collected and germinated in a glasshouse. The species richness of invaded sites and native sites and species richness in above-ground vegetation and the seed bank were compared.
Aims/objectives
The aim for this project had two facets:
- to establish whether there was a distinct range of species at risk because of boneseed invasion-that is, which species present at boneseed-uninvaded sites were missing from boneseed-invaded sites of the same vegetation type or location, or both
- to determine whether these species were present in the soil seed bank of these sites.
Methods used
A total of 90 sites were chosen in Victoria (28 sites), South Australia (32 sites) and Tasmania (30 sites); the sites offered equal amounts of boneseed-invaded and boneseed-uninvaded land in each state. Wherever possible, efforts were made to group invaded sites with uninvaded sites of the same vegetation type and similar location. Species richness was measured in 20 x 20-metre quadrants.
The sampling of standing vegetation was done between May and October 2009. Sites were chosen according to boneseed foliage cover. Some environmental variables - such as other groundcover, soils and disturbance history - were recorded, as were GPS points, the slope direction and current land use. Information about disturbance history and current land use was obtained from landowners or rangers in the parks and national parks.
Results/key findings
The project results confirm that boneseed reduces biodiversity. There is a 25 per cent reduction in native species richness in invaded sites. Furthermore, herbs, shrubs and trees are more deleteriously affected compared with graminoids, ferns and vines.
No change in species composition was evident between invaded and uninvaded sites, suggesting that the reduction in species richness has not entailed a significant replacement of species: rather, there was a loss of species present at sites. Only in the inland sites in Victoria was there a change in the species composition of the herb layer.
The seed bank is unlikely to constitute the dominant mechanism for restoration of sites. The seed bank's composition was vastly different from the above-ground vegetation and was similar across all sites in each state, despite being collected from different vegetation types and in invaded and non-invaded areas. Very few shrubs and trees germinated in the seed bank trial; herbs and grasses dominated in both richness and abundance. This could be the result of an inability to break the dormancy cues of some species, but it is more likely to be a deficit of seeds in the seed bank for the bulk of the species.
The species composition of the seed bank was affected by invasion in Victoria and South Australia but not in Tasmania, suggesting that invasion does influence the seed bank's capacity to restore vegetation. More importantly, however, the capacity of all sites to regenerate effectively from the seed bank is limited and, in order to ensure the conservation of biodiversity, monitoring of species richness at sites-particularly those where management of boneseed has occurred-will be needed so as to facilitate the establishment of missing species.
Recommendations
These results suggest a particular approach for the management of invaded sites for biodiversity conservation: rather than focus on specific species at risk, management should monitor species recovery following weed control and manage the return of all plant functional types.
In order that resource managers can establish long-term management plans it is important to gain information about which species might be missing from invaded sites - species that are present in uninvaded sites of the same plant community. Furthermore, an insight into the soil seed bank will provide further evidence of the presence and absence of these species: an abundance of native seeds in the seed bank might be able to reduce the impact of invasive species in the long term.