An Investigation of the Factors Influencing the Abundance and Species Richness of Introduced Species on Roadsides in the Illawarra

Negative impacts associated with the invasion of introduced species necessitates that their establishment and spread into new areas is minimised. Road verges facilitate the spread of introduced species along this disturbed corridor and into adjacent habitats. Revegetation of road verges by the Roads and Traffic Authority (RTA) aims to inhibit the germination of introduced species by the planting and/or seeding of native species and use of herbicide. The presence and spread of introduced species on verges has been widely studied; however there are still gaps in our knowledge of the success of particular treatments (i.e. seeding and planting) in their ability to withstand invasion. This study aimed to: i) determine if, and to what extent, the planting protocols of the RTA influence the abundance and cover of introduced species along road verges; and ii) identify if additional site specific factors and management practices impact the abundance and cover of introduced road verges. Flora surveys, site condition analysis and soil investigations were conducted on 40 road verge sites in the Illawarra region. Road verges had twice the percentage cover and three times more introduced than native species, with the most abundant natives being those that were planted by the RTA. The percentage cover and species richness of native plants did not vary with the use of either revegetation treatment, whilst the use of seeding decreased introduced species abundance. The age of the road verge revegetation was the strongest indicator of native species richness, with species richness decreasing with increasing age. Introduced and native species composition was significantly related to the age of revegetation. Modification of RTA species selection and revegetation techniques is believed to be responsible for the shift in native species composition, as minimal natural dispersal was observed. Introduced species composition was greatly influenced by the dominance of noxious Lantana camara on older road verges. The use of imported topsoil increased native species richness and decreased introduced species richness, highlighting the importance of proper management of in ‐ situ topsoil. The dispersal limitations of native plants necessitates that initial revegetation of road verges must include high diversity and richness of native species to increase road verge ecosystems resistance to invasion. To increase ability of current road verge ecosystems to resist invasion it is recommended that weed infestations within the road network are actively managed and species richness of road verges be manually increased