Mangrove forests are reported to be invaded by invasive alien species (IAS). This study was therefore aimed at studying the level of distribution of the IAS, Acacia auriculiformis A. Cunn. ex Benth. in mangrove ecosystems in the southern coast of Sri Lanka and assessing the risk to periphery of mangrove forest by considering the Rekawa mangrove forest as a model site. Growth performances of two mangrove species; Rhizophora mucronata and Avicennia marina in the presence of Acacia plants were also tested under three different competition levels; low, moderate and high. According to the results, infestation of Acacia plants was significant in the southern coast of Sri Lanka, particularly in Matara and Hambantota districts (p < 0.05). Species diversity determined as the Simpson diversity index was high (0.77) in the periphery of the Rekawa mangrove forest. Four true mangroves and two associates co-occurring with A. auriculiformis in the periphery could be observed during the field validation experiment. The highest seedling (15.4 ± 2.2 m–2) and sapling (11.2 ± 2.8 m–2) densities were reported for A. auriculiformis plants. Dominance, calculated as the importance value index of different species in the mangrove periphery varied from 18.0–120.6 and the latter highest was recorded for Acacia which has the highest relative density (42.1%) and the relative dominance (52.5%). The total leaf area of the Rhizophora plants grown in the high-competition level was significantly lower than that of the control plants, while the dry weights at three different competition levels were significantly higher (p < 0.05) than the control. This could be due to the higher root biomass allocation. In Avicennia plants, cumulative shoot height, total leaf area and dry weight of the plants grown at the high-competition level were significantly lower than that of the control plants (p < 0.05). A. auriculiformis plants grown with these true mangrove species better performed and did not show any significant deviation from the respective control plants. The level of survival of Acacia was significantly reduced at 25 psu (p < 0.05). Early intervention and serious scrutiny are much needed to reverse the possible impacts of IAS on mangrove forests and the need for forest conservation is emphasized.
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