Response of aquatic macroinvertebrate and aerial odonate assemblages to the removal of invasive alien trees in the Western Cape

[摘要] ENGLISH ABSTRACT:Invasive alien trees, especially Acacia species, are a great threat to biodiversity in South Africanrivers. The national Working for Water Programme is addressing the alien problem by removingalien trees from river margins. The concern is that this may be creating even further disturbanceby affecting water quality and habitats. In particular, how is this affecting rare and endemicspecies? Benthic macro invertebrates and aerial odonates were assessed along five Western Caperivers in alien-invaded, cleared and natural sites between December and March. The aim was toassess the effect of both alien invasion and alien clearing on aquatic invertebrates. Odonata, beingeasily sampled, were also assessed as they are severely affected by the lack of understorey plantsin shaded, alien-invaded sites. Odonata and benthic macroinvertebrates were treated as twoseparate studies and no attempt was made to correlate the results obtained from both studies.SASS5, a qualitative, rapid bioassessment technique, based on the sensitivity of the familiespresent, was used as a measure of river health and, indirectly, of water quality. SASS indicated adecline in water quality conditions after alien clearing, a likely response to the greater insolationas well as erosion of cleared banks, resulting in elevated temperatures and suspended solids andlowered oxygen levels. Community responses to alien disturbance and other environmentalfactors were analysed using PRIMER and CANOCO software. Assemblages of Odonata werefound to provide a rapid, cost-effective means of assessment and monitoring. Potential indicatorand detector species were also identified using the lndicator Value method. Assemblage patternsof aquatic invertebrates, however, were dominated by between-river and seasonal effects.Therefore, while SASS detected overall changes in river health, without identifying the cause,potential aquatic detector taxa were identified that may be useful for linking general disturbanceto alien invasion or clearing. All indicator and detector taxa identified in both studies can be usedfor long-term monitoring and for identifying biodiverse areas for clearing or protection. In bothstudies, sensitive, endemic taxa were lost after clearing, being replaced by more tolerant,widespread taxa. Recovery of the biota appears to follow the recovery succession of thevegetation, with most sensitive or endemic taxa only appearing after the recovery of indigenousplants. A number of recommendations are made regarding the restoration of both water quality(particularly with respect to shade and soil erosion) and biotope availability (particularly in termsof the recultivation of indigenous plants) in order to achieve biodiversity objectives. The role ofcatchment management, prioritisation procedures and long-term monitoring are also discussed.

[发布机构] Stellenbosch University