Native plant diversity in tropical savannas decreases when exotic pasture grass cover increases

Theinvasionofexoticplantsintosavannaecosystemscandisruptthenaturalpatternofvegetation.Indiancouch BothriochloapertusawasintroducedintoAustraliaasaspeciesforrehabilitationofdegradedgrazingland.Inthisstudythe effect of increasing B. pertusa cover on native plant diversity and possible mechanisms of its spread were examined. Forty sites were sampled in uncleared Queensland rangelands with a range of B. pertusa and Bothriochloa ewartiana (a native species)cover.ThemeannumberofnativespeciesperquadratdeclinedwithincreasingB.pertusacoverbutremainedstable oversites with increasingB.ewartianacover. Mean species richnessaccumulated at a significantly lower rate for sites with B.pertusapresent.Canonicalanalysisofprinciplecoordinatessuggestedthatthreegroupsofsites,rangingfromlowtohigh species richness and cover of native plants, were correlated along gradients of B. pertusa cover, grazing intensity and basal area of dead trees. Generalised linear modelling indicated significant negative relationships between B. pertusa cover and totalgroundcover,forbsandperennialgrassrichnessandcover,andcoverofninenativeperennialtussockgrasses(Aristida leptopoda,B. decipiens, B. ewartiana, Chrysopogon fallax, Dichanthium fecundum, D. sericeum, Heteropogon contortus, H.triticeusandThemedatriandra).ThisstudysuggeststhatanincreaseinB.pertusaisassociatedwithasubstantialchange inthecoverandspeciesrichnessofnativeplantcommunities.Thiswasconsideredtooccurviacompetitiveeffectsaswellasa result of cattle grazing, rainfall deficit and tree death. Regardless of the mechanism, continued spread of exotic pasture species has the potential to cause significant changes to rangeland biodiversity.