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

Prepared by Gustavo Heringer, Jan Thiele, Cibele H. Amaral, João A. A. Meira-Neto, Fabio A. R. Matos, Jan R. K. Lehmann, Tillmann K. Buttschardt & Andreza V. Neri

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Land degradation and biological invasions are listed among the main threats to biodiversity. The Brazilian Atlantic Forest, one of the Earth’s Biodiversity Hotspots, is strongly impacted by fragmentation through the conversion of natural ecosystems to human developed areas (e.g., agricultural land and timber forest plantations). Fragmentation causes a decrease in plant species richness, change in species community, and loss of carbon storage on the remaining natural fragments. At the same time, the conversion of natural ecosystems can increase habitat availability for disturbance-adapted invasive species, such as Acacia auriculiformis and Acacia mangium (hereafter, Acacia species) introduced worldwide outside their Australasian natural distribution. Further, roadsides provide habitat to invasive Acacia species and might act as dispersal corridors that facilitate invasion into natural ecosystems.

We, therefore, investigated if Atlantic Forest fragmentation and road networks increase the landscape permeability to biological invasion by Acacia species into Mussununga ecosystem. We investigated specifically (a) whether landscape permeability facilitates biological invasion by Acacia species; (b) whether indeed forest fragments are barriers to Acacia species, whereas road networks serve as corridors for Acacia species; and (c) whether Mussunungas themselves have any role in the biological invasion. Landscape permeability was measured as the effective conductance of the landscape to a random walker. In other words, effective conductance increases with the increase of suitable pathways for dispersal (e.g., road networks). Investigating how landscape affects the occurrence of Acacia species on Mussununga is relevant as this sandy-savanna ecosystem is characterized by nutrient-poor and acid soil within Atlantic Forest (picture above) and has been already suffering the impact of biological invasion. For instance, Acacia species invasion is associated with an increase in vegetation height, decrease in the abundance of native species, and death of trees on Mussununga ecosystem (Heringer et al. 2019).

We observed that the likelihood of invasion by Acacia species significantly increased with landscape permeability. Further, landscape permeability was better represented when we considered forest fragments and water surfaces as barriers and road network as corridors. These results reinforce the Atlantic Forest characterized by tall trees (about 30 m) act as a barrier, whereas the habitat degradation and disturbances associated with roads and highways favor the spread of Acacia species in the landscape. We also found that the area size of Mussununga had a negative effect on biological invasion. Therefore, we can conclude that extensive habitat degradation and dense road networks facilitate Acacia species invasion into Mussununga ecosystems, particularly in the smaller Mussunungas.

Landscape permeability can be used to predict which Mussununga patches are under a high risk of biological invasion by Acacia species. Furthermore, we propose that forest conservation and restoration around Mussununugas are effective actions to prevent the seeds of Acacia species from reaching the Mussununga patches. Such actions are highly relevant, considering that the Mussununga ecosystem is not explicitly protected by the main native vegetation laws of Brazil.

Reference:

Heringer, G., Thiele, J., Meira-Neto, J.A.A. and Neri, A.V. (2019) Biological invasion threatens the sandy-savanna mussununga ecosystem in the Brazilian Atlantic Forest. Biological Invasions, 21, 2045–2057. https://doi.org/10.1007/s10530-019-01955-5

This is a plain language summary for the paper of Heringer et al. published in Applied Vegetation Science (https://doi.org/10.1111/avsc.12520).