Ecological research,2014; 29: 969-979The negative effects of alien plant species on ecosystem structure and functions are increasingly recognised, and efforts to control these species are vital to restore degraded ecosystems and preserve biodiver- sity. However, we lack a full understanding of factors that determine alien species invasions along spatial gradients in herbaceous vegetation of tropical systems. We therefore examined the effects of community prop- erties, environmental variables and human-related dis- turbance factors on the invasion of the alien grass Echinochloa colona (L.) Link at small- and large scales in the Kilombero Valley wetland, Tanzania. Generalized additive mixed models showed that E. colona abundance on a small scale was negatively related to above-ground biomass and evenness of resident species, whereas E. colona abundance was positively related to grazing intensity. On a large scale, biomass (negatively related to E. colona abundance) and distance to river (positive) were important in explaining E. colona abundance. These findings support the assertion that different fac- tors may contribute to the invasion of alien plant species at different spatial scales, as also reported in many temperate systems. Overall, our results show that suc- cessful invasion of alien species is a function of plant community properties, human-related disturbance and favourable environmental conditions. Effective man- agement strategies should consider mitigations that can increase the biomass and evenness of native species and a reduction of grazing pressure to restore the wetland and conserve biodiversity.