Regional and landscape-scale patterns of shrub invasion in tropical savannas

The shrubby vine Cryptostegia grandiflora and the shrub Ziziphus mauritiana were both introduced to northern Australia over 100 years ago and have become invasive in savanna woodland environments. Data from a land resource survey were used to examine regional- and landscape-scale distribution patterns of these species in the Dalrymple Shire, an area of over 6 1/2 million hectares in northeast Queensland. Each species was present at <10% of the 2362 sites examined and most frequent and abundant close to Charters Towers, the major settlement of the regions. C. grandiflora was recorded at 50% of sites within 20 km of the town and in 14 out of 21 of the region's major sub-catchments. Z. mauritiana was recorded at 32% of sites within 20 km of Charters Towers, but in only three sub-catchments. Little of the variation in frequency and abundance of C. grandiflora and Z. mauritiana was accounted for by landscape factors, including geology, soils, or vegetation. While survey results do not absolutely distinguish between history, habitat and disturbance in explaining the weed's current distributions within the region, a strong influence of historical factors is suggested. Both exotic species were much less abundant than Carissa spp., a native taxon that has purportedly increased in the region in recent decades. In spire of their current prominence as weeds, there is potential for further increase by both C. grandiflora and Z. mauritiana. This increase could include expansion from the zone of high abundance and proliferation within that zone. While the results of such surveys must be interpreted with caution, they can yield useful information about regional patterns of plant invasion.