Parthenium hysterophorus is an alien annual herb that aggressively threatens biodiversity of Queen Elizabeth National Park (QENP) in Uganda. Occurrence, abundance and impact of P. hysterophorus on plant species diversity were examined. An observational inspection survey assessed the occurrence of P. hysterophorus while Quadrats sampled vegetation in P. hysterophorus invaded and uninvaded sites of Mweya Peninsula and along Kazinga Channel Track. Plant species richness, dominance, evenness and diversity of invaded and uninvaded sites were statistically different at P < 0.05. Species richness (R) and dominance (D) were higher in invaded sites (R = 58, D = 0.62) than uninvaded sites (P = 0.043; R = 39, P = 0.04; D = 0.46). Consequently, species diversity of other plants became less (1-D = 0.38) in invaded than uninvaded (P = 0.039;1-D = 0.55). Also, P. hysterophorus significantly starts to reduce species diversity and richness at very low levels of abundances, as low as (4.6%) and (7.7%), respectively, and dominates at a relatively higher level (40.2%). It was concluded that P. hysterophorus in QENP, negatively affects the plant species diversity at low levels of abundances through dominance.
Key words: Parthenium hysterophorus, environmental impact, species abundance, species richness, weed spread.