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The hyperseasonal savanna experiences regular flooding, drought stresses and is a neotropical vegetation type threatened by global change including Acacia spp. invasions. To deepen the understanding of hyperseasonal savannas in southern Bahia State, Brazil, after the massive invasion of Acacia mangium, we aimed to answer whether (i) the plants can be separated into different metabolic groups of plants; (ii) Acacia invasion can change the functioning for different metabolic groups of plants; (iii) invasive Acacia uptake water differently from native species; (iv) there are different threats of Acacia invasion on different metabolic groups of plants. For the answers, we sampled two series of plots arrangements: Marcetia centered plots and Acacia centered plots. Then, we analyzed stable isotopes of C, N and O to ensure the proportions that are expected in each metabolic group of plants. We detected both C3 and C4 metabolic groups of plants. Two C3 species are possibly CAM facultative. The functioning of C3 plants as a functional group was not affected by the Acacia invasion, but this result does not exclude an expected species turnover between C3 herbs and C3 trees. The C4 plants of invaded Mussununga lost their response of increasing water use efficiency to the increasing Leaf N%. Plants depend on the same water source as the soil water from recent rains. There are differences in δ18O among species possibly because some grow mostly during the rainy season with the 18O-enriched water meanwhile the invader Acacia mangium grows throughout the year whenever it rains. C3 plants largely dominate these ecosystems. There is a possible threat to C4 plants due to shading. Hyperseasonal savannas should be considered critically endangered by global change, especially after Acacia invasions. Initiatives for conservation of hyperseasonal savannas should be considered for effective conservation of these ecosystems. [ABSTRACT FROM AUTHOR]