Richness and abundance of invasive and expansive plant species and their associated effects on native vegetation in a semi-arid rangeland of the Rift Valley

Plant invasion studies in Ethiopia are primarily restricted to a few target species only. This study explored the richness and abundance of invasive and expanding plants (native but spreading species) and their effects on rangeland vegetation in the Middle Awash Valley, eastern Ethiopia. We systematically sampled forty-eight 20 × 20 m plots and recorded plant richness, abundance and herbaceous composition. A total of 22, including three of the most prolific species globally, Lantana camara, Parthenium hysterophorus and Prosopis juliflora, were recorded, distributing throughout the rangeland. The contribution of expansive species to total species richness was particularly high (17.21 %) compared with the invasive species (9.90 %). Overall, only eight (five invasive and three expansive) out of 22 were encountered in ≥ 21 % of plots. These species were analysed further for their individual as well as collective effects on native richness and diversity. Spearman’s correlation identified relationships between invasive/expansive plants and species richness and diversity. Two out of five invasive species were negatively correlated with herbaceous richness, while two out of five invasive and one out of three expansive species were negatively correlated with tree richness. Co-occurring invasive and expansive plants had negative relationships with herbaceous richness. Only one expansive and one invasive species had negative relationships with herbaceous species diversity, while negative relationships of native tree diversity occurred with two invasive species and one expansive species. Co-occurring invasive species had negative association with native tree diversity. Taken together, these results suggest possible ecological consequences to the rangeland vegetation related to expansive and invasive species, offering a possibility for informing decisions to assess subsequent priorities for both research and management acts. Importantly, given their higher contribution to total richness, the effects of expanding plants on native vegetation are potentially profound highlighting the need for shifts in focus, instead of solely on invasive plants, toward these species.