The abundance of alien species is associated with the impact of Alternanthera brasiliana (L.) Kuntze on aboveground vegetation and soil seed bank in Ile-Ife, (Nigeria)

The introduction of invasive species is known to cause changes in the structure and composition of native communities. These changes are not only restricted to the aboveground vegetation, but may be equally expressed in corresponding soil seed banks, which are legacy effects of the vegetation. This study assessed the impact of the invasive plant, Alternanthera brasiliana, on the vegetation community and its soil seed bank in Ile-Ife, Nigeria. Sample plots were established in invaded plant communities, with high density of Alternanthera brasiliana, and uninvaded plant communities with low density or no presence of Alternanthera brasiliana. Subplots were placed in each large sample plot and in each subplot all rooted plant species were identified and counted. Samples from the post-dispersal soil seed bank were collected to estimate the soil seed bank density and floristics. The species composition of soil seed bank was compared with that of the above-ground vegetation to assess the invader's impact on the vegetation and soil seed bank. The results showed that the presence of Alternanthera brasiliana significantly impacted on the species diversity (t = 5.27; df = 18; p = 0.0003) and species evenness (t = 4.50; df = 18; p = 0.00005) in the aboveground vegetation, and the species diversity (t = 5.37; df = 18; p = 0.00004) and species evenness (t = 6.19; df = 18; p < 0.0001) in the soil seed bank. This study showed that Alternanthera brasiliana has caused ecological alterations in the aboveground vegetation as well as in the soil seed bank. It is likely that with increasing resident time, the impacts on native species will increase as Alternanthera brasiliana spreads.