Alien species, such as Tithonia diversifolia, if introduced to areas outside of their natural distribution range, subsequently become invasive, threatening biodiversity and agricultural productivity. A field survey to determine the potential allelopathic effects of T. diversfiolia on species composition was conducted in Goromonzi, Zimbabwe using two different sizes quadrats. The quadrats were randomly placed at different locations as follows: under the canopy of T. diversifolia, 10 meters away, 20 meters away and 30 meters away from T. diversifolia infested area. The quadrats were randomly thrown four times to represent four replications. Four different weed species (Cynodon dactylon, Hyparrhenia spp, Melinis repens and Sporobolus spp.) were identified using species index catalogue. Results postulate that there was a significant difference (P >0.01) between species found under the canopy and those found away from T. diversfiolia plants. T. diversifolia affects species composition, as distance from infested area increases species richness and evenness. Therefore, T. diversfiolia affects species diversity, and it is possible that this could be due to allelochemicals produced by the invasive plant. Future studies can quantify allelopathic compounds in the invasive plant and test allelopathic effects of the compounds on the four weed species.

Keywords: Alien species, Allelochemicals, Invasion, Species evenness, Species richness