Acacia invasion is facilitated by landscape permeability: the role of habitat degradation and road networks

Context: The sandy-savanna ecosystem “Mussununga”, a natural ecosystem that occurs as patches throughout the Atlantic Forest domain, is threatened by anthro-pogenic factors and biological invasions of Australian Acacia species. Habitat degradation in the Atlantic Forest domain and extensive road networks could facilitateAcacia invasion into Mussununga. Objectives: We investigated whether: (a) landscape permeability (measured by effective conductance) facilitates Acacia invasion; (b) forest fragments are barriers, and roads and highways are corridors for invasive spread of Acacia; and (c) size and shape of Mussununga patches play a role in biological invasion. Methods: Acacia invasion was investigated in 32 Mussununga sites within the Atlantic Forest domain. We tested the effect of a set of landscape permeability scenarios based on circuit analysis and nine other metrics of landscape structure on Acacia occurrence using three buffer-zone sizes (0.5, 1, and 2 km). Results: The likelihood of Acacia invasion significantly increased with landscape permeability. The best-fitting landscape permeability scenario designated road networks as corridors, intact forests and water surfaces as barriers, and degraded habitats as non-barriers. We also found that Mussununga areas within a 0.5 km buffer negatively affected the biological invasion by Acacia. Conclusions: Extensive habitat degradation by deforestation and dense road networks facilitate Acacia invasion into sandy-savanna Mussununga ecosystems. Landscape permeability may be used as a risk-assessment tool for biological invasion by Acacia species. Mussununga patches can be protected from Acacia invasion through preserving or restoring forest belts around them and disconnecting them from the road network.