Replacing wild grazers with domestic livestock: effect on dung beetle diversity, biomass, size and ecosystem function in a semi-arid Namibian savannah

Nependa, Uemuvanga Jatandavara Heather (2020-03)

Thesis (MSc)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Anthropogenic activities have parcelled most native mammals into protected areas while cattle and other livestock replace them as the dominant grazers in the surrounding farmlands. This has severe consequences for biodiversity reliant on the resources provided and functions performed by native mammals. These effects can precipitate through multiple trophic levels and ultimately lead to changes in ecosystem processes. A taxon frequently used to measure these changes are dung beetles who are increasingly used as a study taxon—both as a model system for exploring ecosystem functioning and as bioindicators of environmental change. The advantages of using dung beetles are many: they are speciose, abundant in a wide range of terrestrial ecosystems, they respond to environmental gradients and sampling methods are straightforward. Despite their frequent use in land-use and habitat change studies, there is little research on the relationship between dung beetles and the mammals they rely on for dung. Few studies have evaluated both the response of this keystone taxa to land transformation associated with livestock farming and its consequences for ecosystem functions and services. Furthermore, no studies to this effect have been undertaken in the semi-arid Namibian savannahs despite being particularly rich in mammal species and experiencing ever-increasing anthropogenic pressures. Here, I aim to elucidate how land-use change (natural ecosystems into rangeland farming) and the accompanying loss of native vertebrates affects the diversity, development and ecosystem functions provided by dung beetles (Scarabaeidae: Scarabaeinae and Aphodinae). This was accomplished by comparing dung beetle diversity (richness, abundance and biomass), developmental attributes (individual size) and their main ecological functions (secondary seed dispersal and dung removal) between farmlands and protected areas in an area previously neglected in terms of dung beetle diversity studies: African savannahs. The region was very diverse with over a hundred dung beetle species and over 76 000 individuals collected in a relatively small area and during a few months of sampling. Amongst the species sampled, some were collected for the first time in decades and others well beyond their known geographic ranges. At a larger scale, I demonstrated that protected areas harbour far richer vertebrate communities compared to farmlands which had severe effects on dung beetle richness, abundance, biomass and community assemblage composition. Dung beetle species richness was positively correlated with vertebrate richness but not with stocking density, indicating that a rich native vertebrate assemblage is paramount in conserving natural dung beetle diversity. Numerous species were found on farmlands in high abundance, indicating an ability to adapt to these altered conditions, however, the individual development of many of these species was negatively affected due to diminished availability of native dung on farmlands. The native dung was therefore assumed to be of higher quality due to the lack of veterinary pharmaceutical use. Decreases in dung beetle diversity on farmlands lead to decreased ecosystem functioning particularly at the expense of the loss of larger, specialist species. This work, in keeping with the broader literature, shows that rich native vertebrate diversity is not only important to support and promote diverse dung beetle communities but also for maintaining the ecosystem functioning of these threatened savannah landscapes.

AFRIKAANSE OPSOMMING: Antropogeniese aktiwiteite het gelei daartoe dat inheemse soogdiere in beskermde gebiede verpak word, terwyl beeste en ander vee hulle as die dominante beweiders in die omliggende matriks vervang. Dit kan ernstige gevolge hê vir biodiversiteit wat afhanklik is van hulpbronne wat deur inheemse soogdiere voorsien word. Hierdie effekte kan deur verskeie trofiese vlakke presipiteer en uiteindelik tot veranderinge in ekosisteemprosesse lei. Slegs 'n paar studies het beide die reaksie van kern taxa op grondtransformasie beoordeel, asook die gevolge hiervan vir ekosisteemfunksies en -dienste. Geen studies hiroor is onderneem in die semi-droë Namibiese savanne nie, hoewel hierdie areas besondrs ryk aan soogdiere is, maar wat ook toenemende antropogeniese druk ondervind. Hierdie studie het ten doel gehad om te verstaan hoe verandering in grondgebruik, van natuurlike ekosisteme na landelike boerderye, en die gepaardgaande verlies van inheemse gewerwelde diere, 'n invloed het op diversiteit, ontwikkeling, ekosisteemfunksie en die dienste gelewer deur 'n kern takson, miskruiers (Scarabaeidae: Scarabaeinae en Aphodinae). Dit is bewerkstellig deur die veranderinge in die kenmerke van die miskruier-diversiteit (rykheid, getalle en biomassa), ontwikkelingskenmerke (individuele grootte) en hul belangrikste ekologiese funksies (sekondêre verspreiding van saad en misverwydering) tussen landbougrond en beskermde gebiede in die Namibiese savanne te bepaal. Ek het gewys dat die Namibiese savanne baie divers is in miskruiers aangesien meer as 100 spesies en meer as 76 000 individue binne 'n paar maande in 'n betreklik klein gebied versamel is. Talle spesies wat voorheen baie swak bekend was, of spesies wat nie voorheen van Namibië bekend was nie, is versamel. Hierdie studie dra dus aansienlik by tot die bewaring van oënskynlike skaars miskruierspesies. Op ‘n groter skaal demonstreer ek dat beskermde gebiede baie ryker gewerwelde gemeenskappe het in vergelyking met landbougrond, en dit het 'n negatiewe uitwerking op die spesierykheid van miskruiers, hul getalle, biomassa en hul gemeenskapsamestelling. Spesiesrykheid was negatief gekorreleer met die werweldierrykheid, maar nie met veedigtheid nie, wat 'n aanduiding is dat 'n ryk inheemse werweldiergemeenskap belangrik is om natuurlike miskewerdiversiteit te bewaar. Talle spesies is in redelike hoë getalle op landbougrond versamel, wat 'n aanduiding is dat hulle by hierdie veranderde toestande kan aanpas. Ten spyte hiervan is die individuele ontwikkeling van baie van hierdie spesies negatief beïnvloed waarskynlik as gevolg van die lae beskikbaarheid van inheemse hoë gehalte mis op landbougrond. Veranderings in diversiteit lei ook tot afname in die funksionering en ekosisteem dienste (mis verwyder en sekondêre verspreiding van saad) wat deur miskruiers op plaasgrond voorsien word. Groter spesies wat oënskynlik gespesialiseerd was in mis van inheemse gewerwelde diere, was die meeste geraak, wat tot 'n aansienlike verlies aan hul funksies gelei het. Ek toon aan dat ‘n ryk inheemse werweldierdiversiteit nie net belangrik is om die ongeskonde bioverskeidenheid van miskewers te handhaaf nie, maar ook vir die normale funksionering van hierdie bedreigde savanne-landskappe.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108402
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