Abstract: The National Park of El Kala (PNEK) located in North-eastern of Algeria is one of themost significant reserves of biodiversity in the Mediterranean region. It includes the cork oak, adominant tree species, as well as mixed stands of eucalyptus and acacias planted in 1970.However, during this last decade, it was noted that Acacia decurrens Willd. invaded the park andthreatened the cork oak ecosystem. The principal objective of our research was thus to evaluatethe transformations induced by this acacia on the cork oak symbiotical microflora and on theplant biodiversity. The first results show the negative effect of acacia on cork oak growth, itsmycorrhizal statute as well as on the morphotypic diversity of its ectomycorrhizes. The plant andfungal biodiversity fall significantly in the sites invaded by this acacia species.