Invasive species can pose an additional threat to ecosystems already threatened by landscape change, with climate change likely to compound these pressures. Although invasion and landscape change are acknowledged to interact, there are few studies that assess their interactive effects on native communities. This study applied a hierarchical landscape approach and mixed effect modelling to investigate the influence of landscape change on the extent of exotic plant invasions and how they interact to affect native plant communities in fragmented Brigalow ecosystems of sub-tropical eastern Australia.
The outcomes of the study indicate that the landscape matrix had a strong influence on the extent of invasion in patches of Brigalow by the two dominant exotic pasture grasses in the region, buffel grass (Cenchrus ciliaris) and green panic (Panicum maximum). At the farm-scale, on a number of production landholdings, the direct effects of phosphorus enrichment in the patch and propagule availability from buffel grass pasture in the surrounding landscape were the most important predictors of buffel grass cover in the patch. However, phosphorus enrichment in patches and the availability of propagules are conditional to landscape fragmentation and landform, thereby influence invasion extent through complex interactions. There was also a change in understory plant species composition and cover along a gradient of intensifying buffel grass invasion and land-use intensity. However, this effect increased as the duration of pasture use and canopy height increased, and decreased along soil phosphorus enrichment and patch isolation gradients. These results indicate that if disturbance occurs prior to invasion establishment, then relatively degraded patches and landscapes may have fewer species to lose while remaining species are tolerant to invasion. Invasion effects may also be determined by the species duration in the landscape.
At the regional landscape-scale, the land-use matrix was the most important factor to predict invasion extent of buffel grass in forest fragments. Buffel grass invaded patches more severely in landscapes with higher minimum temperatures and potentially fewer frosts and is restricted from patches with high canopy cover. In contrast, green panic invaded patches more severely in landscapes with higher rainfall, although high canopy cover may not exclude invasion. This highlights that general conclusions on the impact of site and landscape scale environmental attributes are not necessarily transferable between invasive species. The effect size of buffel grass invasion on the native understorey also changed along landscape change and climatic gradients, indicating that invasion effects were more severe in cooler and relatively intact landscapes. This may be due to landscape change occurring prior to invasion, with disturbed landscapes having fewer species or may be resistant to invasion. Similarly, species resistant to invasion may be selected in warmer landscapes where invasion is higher.
The study demonstrates the utility of applying a hierarchical landscape approach to understanding invasive spread in fragmented ecosystems and its effect on native plant communities. The research findings have important implications for the management of exotic plant invasions, by providing evidence: that 1) site-scale management of invasion must focus on maintaining or building resistance to invasion by maintaining or restoring vegetation structure as well as minimising patch and landscape fragmentation; 2) fertiliser use should be minimised to restrict invasion establishment or at least minimise invasion extent; and 3) management must take into consideration the land-use history including invasion species duration in landscape in order to effectively prioritise management.