The relative importance of species richness and functional diversity for ecosystem functions performed by dung beetles (Coleoptera: Scarabaeidae) in South Africa.

Thesis (PhDConsEcol)--Stellenbosch University, 2020.

ENGLISH ABSTRACT: The ongoing and accelerating loss of biodiversity due to anthropogenic activities has highlighted the effects of declining biodiversity on ecosystem functioning. In rangelands, livestock grazing along with variation in soil texture and climatic conditions can have significant impacts of biodiversity through alteration of habitat structure. In the semi-arid Karoo, South Africa, rangelands are extensive, and much of this ecosystem has been disturbed by livestock. This study aimed to determine how variation in land use (rangeland for livestock or protected area) and rainfall (100 to 520 mm.year-1 in the Nama Karoo and 20 to 290 mm.year-1 in the Succulent Karoo) influence the functionally important dung beetle diversity (i.e. species richness, abundance, functional diversity and body size, and their assemblage composition, and the ecosystem functions they provide in the semi-arid Karoo. Here, I investigate the relative importance of long-term (mean annual precipitation), medium-term (yearly difference from mean) and short-term (recent rains) on dung beetle species richness, abundance and assemblage composition. First, I assessed whether variation in land use, herbivore stocking rates, vegetation cover, soil texture and rainfall affect dung beetle communities and functional diversity. Dung beetle abundance and functional diversity showed contrasting responses between protected areas (PAs) and livestock farms, with PAs having a significantly greater abundance of dung beetles and lower functional diversity. Neither species richness nor body size was influenced by land use, although richness and abundance increased with higher herbivore stocking rates and rainfall. Species richness was best predicted by short-term rainfall and abundance by long-term rainfall. I further assessed whether variation in land use and rainfall affect dung beetle community composition. Dung beetle species composition varied between farms and PAs, and also varied with MAP, stocking rates, vegetation cover and rainfall season. I Identified 15 indicator species that explained this variation in dung beetle species composition which were associated with the different environmental variables, furthermore, four of these species were associated with two environmental variables. The response of dung beetle species to environmental variables was influenced by their functional traits, particularly reproduction capacity and dispersal ability. There was greater reproduction capacity for dung beetles in the PAs than on farms. Greater dispersal ability was also associated with PAs as well as the summer rainfall region and greater vegetation cover. I also assessed beta diversity across the rainfall gradient, and these analyses indicated that beta diversity is driven by assemblage nestedness, with all species at the driest sites being a subset of the assemblage in the wettest sites. I then investigated the role of dung beetle communities (in terms of their abundance, species richness, body size and functional groups (rollers or tunnellers)) and the influence of environmental variables on the ecosystem functions that they provide. The rate at which dung was removed increased with increased species richness in both farms and PAs and increased with an increase in rainfall, with dung removal responding to medium-term rainfall. Different functional groups proved to be more efficient at different functions. Rollers and tunnellers efficiently facilitated the removal of dung and plant productivity, respectively. This demonstrated that the different functional groups made different contributions to different ecosystem functions (i.e. dung removal or nutrient cycling), and that conservation of a functionally complete species assemblages is essential to maintain optimum ecosystem functioning. Both dung beetle communities and their ecosystem functions showed varied responses to rainfall. This suggests that to fully understand dung beetle community structure and their contribution to ecosystem function, ecological studies need to include all species community attributes when assessing the impacts of land use and climate changes. Moreover, to fully grasp their contributions to ecosystem functions, these assessments should not rely on only one of their ecosystem functions.

AFRIKAANSE OPSOMMING: Die deurlopende en versnellende verlies van biodiversiteit as gevolg van menslike aktiwiteite het beklemtoonde effekte op ekosisteemwerking. In weidingslande, kan veewyding tesame met variasie van grondtekstuur en klimaatsomstandighede beduidende impakte hê op biodiversiteit, deur middel van veranderde habitatsstruktuur. Weidingslande neem groot areas in beslag in die halfdroeë Karoo van Suid-Afrika, en ‘n groot deel van hierdie bioom is alreeds versteur deur veeboerdery. Die doel van hierdie studie was om te bepaal hoe verskillende grondgebruike (weidingslande vir vee of beskermde gebiede) en reënval (100 tot 520 mm.jaar-1 in die Nama Karoo en 20 tot 290 mm.jaar-1 in die Vetplant Karoo) die funksioneel-belangrike miskruier diversiteit (d.w.s. spesierykheid, veelheid, funksionele diversiteit en liggaamsgrootte), en hul spesiesamestelling, en die ekosisteemfunksies wat hul uitvoer in the halfdroeë Karoo, beinvloed. Hier ondersoek ek die relatiewe belangrikheid van langtermyn (gemiddelde jaarlikse reënval), mediumtermyn (jaarlikse verskille teenoor die gemiddeld) en korttermyn (onlangse reën) op miskruier spesierykheid, veelheid en spesiesamestelling. Ek het eerstens geassesseer of variasie in grondgebruik, herbivoor aanvullingskoers, plantbedekking, grondtekstuur en reënval miskuier gemeenskappe en funksionele diversiteit affekteer. Miskuier veelheid en funksionele diversiteit het tussen beskermde gebiede (BGe) en veeplase teenstrydige reaksies getoon, waar BGe beduidend hoër miskruier veelheid en laer funksionele diversiteit getoon het. Spesierykheid en liggaamsgrootte was nie deur grondgebruik beïnvloed nie, alhoewel spesierykheid en veelheid toegeneem het tesame met vee-aanvullingskoers en reënval. Beste voorspellings vir spesierykheid was volgens korttermyn reënval, terwyl die beste voorspellings vir veelheid volgens langtermyn reënval was. Ek het verder geassesseer of variasie in grondgebruik en reënval miskruier gemeenskapsamestelling affekteer. Miskruier spesiesamestelling het gewissel tussen plase en BGe, asook MAP, aanvullingskoers, plantbedekking en reënvalseisoen. Ek het 15 aanwyserspesies geïdentifiseer wat hierdie variasie in miskruier spesiesamestelling verduidelik, wat geassosieer was met veskillende omgewingsveranderlikes. Verdermeer was vier van hierdie spesies geassosieerd met twee omgewingsveranderlikes. Die reaksies van miskruier-spesies teenoor omgewingsveranderlikes was beïnvloed deur funksionele karaktereienskappe, spesifiek voorplantingskapasiteit en verspreidingsvermoë. Daar was hoër voorplantingskapasiteit vir miskruiers in BGe in vergelyking met plase. Beter verspreidingsvermoë was ook geassosieer met BGe, asook die somerreënvalgebiede en hoër plantbedekking. Ek het ook beta-diversiteit oor die reënvalsgradiënt geassesseer, en hierdie analises het getoon dat beta-diversiteit aangedryf word deur genestheid van gemeenskappe, waar die gemeenskappe in droeë gebiede ‘n onderafdeling van dié gemeenskappe in natter gebiede is. Hierna het ek die rol van miskruier-gemeenskappe (in terme van spesierykheid, veelheid, liggaamsgrootte en funksionele groepe (rollers of tonnelgrouers)) en die invloed van omgewingsveranderlikes op die ekosisteemfunskies wat hul voorsien, ondersoek. Die koers waarteen mis verwyder was het toegeneem met toenemende spesierykheid vir beide plase en BGe, asook toenemende reënval, waar misverwydering gereageer het op mediumtermyn reënval. Verskillende funksionele groepe was verantwoordelik vir verskillende funksies. Rollers en tonnelgrouers het afsonderlik doeltreffende verwydering van mis en plantprodukte gehandhaaf. Dit dui aan dat verskillende funksionele groepe verskillende bydrae maak tot ekosisteemwerking (d.w.s. misverwydering en voedingstof-sirkulering), en dat die bewaring van ‘n gemeenskap waar alle funksionele groepe ingesluit is van hoë belang is om optimale ekosisteemwerking te handhaaf. Miskruier gemeenskappe en hul funksies in hul ekosisteme het verskeie reaksies getoon teenoor reënval. Dit dui aan dat ekologiese studies alle eienskappe van miskruier-gemeenskap moet insluit wanneer die impakte van grondgebruik en klimaatsverandering ondersoek word, om sodoende goeie verstand van miskruier gemeenskapstruktuur en hul bydrae tot ekosisteemfunksionering te kry. Om hul bydrae ten volle te verstaan, moet hierdie assesserings nie alleenlik op een ekosisteemfunksie staatmaak nie.

Doctoral