Impacts of the invasive tree Acacia mearnsii on riparian and instream aquatic environments in the Cape Floristic Region, South Africa

Railoun, Moegamad Zaid (2018-03)

Thesis (MScConsEcol)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: In this study, I compared the notoriously invasive wattle species Acacia mearnsii, to two native woody species in terms of patterns leaf litterfall and nutrient resorption in riparian environments, and the decomposition of the leaf litter in aquatic environments and in stream macroinvertebrate communities in mountain streams in the Fynbos biome of the CFR. More explicitly, the study assessed: (1) leaf litter fall between A. mearnsii and co-occuring native species on an monthly basis (2) the nutrient (N and C) concentrations dropped in leaf litter inputs monthly (3) the amount of nutrients (N and P) resorbed between species before senescence (4) the decomposition rates between A. mearnsii and fynbos species in away and home environments to test the Home Field Advantage (HFA) hypothesis and, finally (5) the macroinvertebrate assemblages in different leaf bags in home and away environments to test macroinvertebrate litter affinity effects instream. The results in the study indicate that A. mearnsii had seven to times times higher leaf litterfall rates in the Wit and Du Toit‟s River compared to co-occuring native species in invaded and near pristine riparian zones. Acacia mearnsii had two peaks in litterfall, one at the end of the dry season in mid-autumn, and the other in mid-summer. A. mearnsii also kept a higher foliar N concentration than co-occuring native species, which gives the wattle species a competitive advantage. Native species exhibited low nitrogen concentrations which are reflected annually. In addition, the results indicated that co-occuring natives efficiently recycles nutrients before leaf abscission, for instance through high P resorption efficiencies. Acacia mearnsii was not as efficient in recycling nutrients, most notably N, but was more efficient in recycling P, suggesting it may require more P than can be readily supplied from the soil. The results indicate that the studied species had high resorption parameters (proficiency, A. mearnsii and efficiency in native species), which indicated a P limited landscape. This can be an important reason in the success of Acacia spp. in South African landscapes and particularly in riparian zones. The results also indicated that A. mearnsii and fynbos species differed locally at all sites in instream decomposition rates, with A. mearnsii decaying at a much faster rate. The difference in decay rates was attributed to differences in litter quality characteristics between native and invasive species (N concentration and C:N ratio). The faster decay rates in A. mearnsii due to leaf litter with high N and P can have a detrimental effect on in stream functionality therefore affect the species diversity of aquatic biota. The macroinvertebrate litter affinity effects were tested and showed no preference to home turf litter or introduced littertype regardless of the local environment at each invasion status. Functional feeding groups increased at both Wit River site, as macroinvertebrates were season-dependent on leaf litter and additionally resources A. mearnsii site may hold. Conversely, at the Du Toit‟s River low invertebrate diversity and abundances and was regulated by stream characteristics and site geomorphology at both reaches. Furthermore, seasonal hydrological regime could have accounted for macroinvertebrate species abundance and diversity at each river as there was a selective pressure on communities to utilize resources. The research contributes to a more comprehensive understanding of nutrient cycling, acquisition and conservation strategies of native compared to invasive species in the Fynbos biome in South Africa. Additionally it also gives insight into how invading species could potentially modify aquatic ecosystems and change macroinvertebrate communities in disturbed environments. Invaders can strongly affect multiple services in an ecosystem therefore it is imperative that these mulitiple roles should be asssed and managed as environmental change (i.e, drought) could cause a long lasting effect on ecosystems holistically (riparian areas, in stream biogeochemistry and aquatic assemblages).

AFRIKAANSE OPSOMMING: In hierdie studie het ek die berugte indringende wattle spesie Acacia mearnsii teenoor twee inheemse boom spesies in term van patrone van blare val en voedingstowwe resorpsie in rivieroewers omgewings en ontbinding proses van blare in akwatiese en makro-ongewerweldes gemeenskappe binne bergagtige strome in die Fynbos bioom van die KFO. Meer uitdruklik, die studie beoordeel: (1) blaar val patrone tussen die A. mearnsii en mede-voorkomende inheemse spesies op n maandelikse bases (2) die voedingstowwe (N en C) konsentrasies in blaar val maandeliks (3) die hoeveelheid voedingstowwe (N en P) wat geabosrbeer word tussen blaar spesies voor veroudering (4) en die ontbinding tariewe of proses tussen A. mearnsii en fynbos spesies in naby tuis omgewings en weg van n tuis omgewing om die Home Field Advantage (HFA) hipotese te toets en uiteindelik die makroongewerweldes versameling in verskillende blaar sakkies in tuis en weg van die tuis omgewings om die makro-ongewerweldes blare affiniteit binne stroom te toets. Die resultate in die studie dui aan dat A. mearnsii sewe tot tien keer hoër blaar val hoeveelheid in beide die Wit en Du Toit‟s Rivier in vergelyking met die mede-voorkomende inheemse boom spesies binne indringende en byna ongerepte rivieroewers omgewingssone. Acacia mearnsii het twee pieke in blaar val, waar een voorkom aan die einde van die droe seisoen in middel herfs en die ander een middel somer in Desember. Acacia mearnsii het hou ook n relatiewe hoër blaar N konsentrasie as die mede-voorkomende inheemse spesies wat die wattle spesies n mededingende voordeel gee. Die inheemse spesies stal uit n laer stikstof konsentrasie wat aan gedui word maandeliks. Daarbenewens, die resultate dui aan dat die mede-voorkomende inheemse spesies doeltreffend voedingstowwe herwin voor blaar afsnyding, byvoorbeeld deur hoe P resorpsie doeltreffend te gebruik. A. mearnsii was nie so doeltreffend in die herwinning van voedingstowwe veral N, maar was meer doeltreffend in die herwinning van P wat aandui dat die spesie meer P vereis as wat dit beskikbaar is van die rivieroewers omgewing. Die resultate dui ook aan dat die bestudeerde spesie „n hoër resorpsie grens het veral in vaardigheid in die A. mearnsii and doeltreffendheid in die inheemse boom spesies, wat aandui „n P limitasie in die rivieroewers omgewing. Dit kan uiters die belangrikste rede wees vir die sukses van die Acacia spp. in Suid Afrika rivieroewers omgewings. Die resultate dui ook aan dat A. mearnsii en fynbos species verskillend plaaslik by al die studie plekke in die ontbinding proses binne in die stroom gebiede met die A. mearnsii specie wat die vinnigste ontbind oor tyd. Die verskille in die ontbindings van die blaar spesies was aangedui deur die verskille in blaar kwaliteit tussen die inheemse en die indringende spesies (N konsentrasie en C:N verhoudings). Die vinnige ontbinding proses in die A. mearnsii weens die blaar val wat hoë N en P inhoud besit kan dalk n nadelige impak het op binne stroom funksie en as gevolg van dit mag die spesie se diversiteit en akwatiese biota affekteer of beinvloed. Die makroongewerweldes blare affiniteit was ook getoets en die resultate wys geen voorkeur vir blaar tipe van sy tuis omgewing of van die blaar tiepe wat voorgestel was in die omgewing nie. Die funksionele voedings groepe het vermeerder by beide, Wit Rivier studie plekke omdat die makroongewerweldes was seisoenaal afhanklik van die blaar val asook die hulpbronne wat A. mearnsii indringende plekke hou. By die Du Toit‟s Rivier was lae nommers van ongewerweldes diversiteit en verspreidings gereguleer deur stroom eienskappe en die verskillende plekke se geomorfologie. Die seisoenale hidrologiese patron kan dalk verantwoordelik wees vir die makro-ongewerweldes spesies se verspreiding en diversiteit by beide riviere as gevolg van n selektiewe drukking deur gemeenskappe om hulpbronne te gebruik. Die navorsing dra by tot „n meer omvattende begrip van die voedingstowwe siklus, verkryging en bewaring strategië van die mede-voorkomende inheemse boom spesies in vergelyking teen die indringende wattle spesie A. mearnsii in die Fynbos bioom in Suid -Afrika. Daarbenewens gee dit ook „n insig op hoe indringende spesies die potensiaal het om akwatiese ekosisteme dalk te verander en ook die makro-ongewerweldes gemeenskappe binne stroom. Indringende spesies kan verskeie impakte het binne „n ekosisteem daarom is dit uiters belangrik dat die verskeie impakte moet beoordeel word en ook bestuur word. Veranderinge in omgewegings (bv., droogte) kan dalk „n blywende negatiewe effek het op ekosisteme in „n meer holistiese manier (rivieroewers omgewings, binne stroom biogeochemie en akwatiese versamelings).

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103942
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