Expansive shrubs: Expansion factors and ecological impacts in northern Ethiopia

Abstract

Expansive native species have the same ecological and economic consequences as invasive alien species through the reduction of ecosystem goods and services. We assessed human disturbance and environmental factors that are contributing to the expansion of two expansive shrubs, Cadia purpurea and Tarchonanthus camphoratus in Desa’a dry Afromontane forest, northern Ethiopia. We also evaluated their expansion effect on plant species composition, diversity, structure, and regeneration, and herbaceous cover. The forest was stratified into “invaded” and “uninvaded” based on percentage coverage of the two shrubs. Vegetation data were collected from each stratum using 75 nested plots per strata of 20m × 20m, 5m × 5m, and 1m × 1m for adult tree/shrub, sapling, and seedling, respectively. Moreover, data on environmental and human disturbance variables were collected from the main plots. We found that higher cover of the expansive shrubs was related to both human disturbance and environmental variables while some were species-specific. Species richness and diversity of plant species decreased significantly with an increase in abundance of the expansive shrubs. However, evenness did not change significantly with an increase in the abundance of the expansive shrubs. The density, mean diameter, mean height, and mean basal area of the plants decreased significantly with an increase with the cover of the shrubs. The expansive shrubs had a significant negative impact on regeneration and herbaceous cover. This study shows a distinctive variation in floristic composition, diversity, and regeneration status between the invaded and uninvaded communities, which suggests the need to be counteracted by management measures. Converting the underutilized expansive species into use as part of the management option should be given urge attention before it loses its ecological resilience.