Abbreviations LAI = leaf area index; LWR = leaf weight ratio; PAR = photosynthetically active radiation; RWR = root weight ratio; SWR = stem weight ratio Abstract Question: Resistance of the native community has been identified as an important factor limiting invasion success and invader impact. However, to what extent resistance interacts with disturbance to control invasion success remains unclear. We studied the interaction between biotic resistance, fire and smallscale disturbances mimicking those of large mammalian herbivores (hoof action and grazing) on invasion success of the alien shrub Chromolaena odorata.Location: Hluhluwe-iMfolozi Park, South Africa. Methods:We performed a seedling transplant experiment in a savanna grassland. We manipulated the grass layer by clipping (grazing), created small-scale soil disturbances (hoof action) and transplanted seedlings under native tree canopies to assess potential nursing effects. Subsequently, we burned half of our plots. We assessed the effect of fire and small-scale disturbances on seedling growth, survival and biomass allocation.Results: Seedling survival of C. odorata was 26% in undisturbed savanna grassland, reducing to 5% post-fire. Small-scale disturbances increased seedling biomass and survival and modified biomass allocation, whereas fire greatly reduced seedling survival. Root allocation increased in response to grass clipping, while stem allocation decreased. Tree shading increased seedling survival in the absence of fire, but greatly reduced post-fire survival.Conclusions: Grass communities in savannas exert a high level of biotic resistance to C. odorata invasion. However, small-scale disturbances, mimicking those of large mammalian herbivores, can facilitate the invasion of C. odorata in savanna grassland both directly, by creating micro-sites for establishment, and indirectly, by reducing the negative effect of fire. Thus, small-scale disturbances may aid the long-term persistence of woody species invading grass-dominated systems.

Question Biotic resistance is defined as the reduction in invasion success caused by the native community through competition, herbivory and/or pathogens. Biotic resistance has mostly been studied during the initial stages of invasion. However, to what extent biotic resistance hampers survival, or persistence, of invaders in the longer term is often not known. We studied how native grassland communities affected growth, reproduction and survival during the adult life stage of the high‐impact woody invader Chromolaena odorata under different water availability treatments. Location Hluhluwe‐iMfolozi Park, South Africa. Methods We performed a 2‐years full‐factorial field experiment in a savanna grassland where we manipulated water availability and neighbouring vegetation; in one‐third of the plots vegetation was cleared and planted with C. odorata monocultures, in one‐third C. odorata shrubs were planted in grasslands, and one‐third were unplanted grassland controls. Results Growth and reproduction of adult C. odorata were greatly reduced due to competition with native grasses, but not survival of C. odorata. Shrub survival was high and did not differ in plots with and without grass. Water availability did not affect growth, reproduction and competitive ability of C. odorata, but higher water availability did increase the competitive ability of the grasses. Soil moisture levels were lower in grasslands planted with C. odorata compared to unplanted controls, independent of the water treatment, suggesting higher water use of C. odorata compared to native grasses. Conclusions Savanna grasslands have a strong competitive effect on invasion by the exotic woody shrub C. odorata, reducing growth and reproduction, but not survival of the shrub. We found no evidence that biotic resistance was stronger under more unfavourable abiotic conditions, as C. odorata was equally impacted in all water treatments. The high survival rates of C. odorata suggest that competitive interactions are not likely to prevent invader persistence in the landscape. Invader persistence is important in determining longer‐term invasion success as well as invader impact, and the concept of persistence should not be overlooked in studies on invasive species.