| Summary: | Thesis (MSc)--Stellenbosch University, 2012. === ENGLISH ABSTRACT: Human-modified habitats form increasingly large components of landscapes, threatening
biodiversity and creating challenges for conservation. In some cases altered habitats form
entirely novel ecosystems that may support new combinations of species and species
abundances, and create habitat space in otherwise transformed landscapes. In the Western
Cape of South Africa, woody invasive species contribute to landscape-level habitat
transformation and form novel ecosystems. Invasive Australian Acacia species are especially
problematic in lowland areas where they create dense thickets and substantially transform
both biotic communities and abiotic processes. Despite the prominence of Acacia thickets
across the Western Cape, their ability to support native fauna is not well understood and the
objective of this study was to assess the significance of Acacia thickets as habitat for the
region’s avifauna. Birds were surveyed in Acacia thickets in the south-western Western Cape
in three seasons to examine species richness, abundance and functional abundance.
Furthermore, I examined the extent to which differences in patch-level vegetation structure
alter bird communities. Between survey sites and seasons, significant variation was observed
in assemblage richness, density, median body size and biomass. Variation in vegetation
density, stem density, mean vegetation height and total canopy cover best explained variation
in bird assemblages. Eighty species were estimated to utilize Acacia thickets and assemblages
had a mean density of 7.78 birds per ha. The most abundant feeding guilds were the mixed
feeders and insectivores. The median body size observed was 15.2 g and the body size
frequency distribution of all species in Acacia spanned a similar range compared to the body
size frequency distribution for the species list for the entire Western Cape. The mean biomass
of bird communities was 0.224 kg per ha. Using data on bird density and biomass, Acacia
thickets across the Fynbos Biome support and estimated average of over 21 million birds with
a combined biomass of over 600 thousand kg. This study found that Acacia thickets in the
Western Cape support a subset of the region’s birds with the most abundant species being
small mixed feeders, which are also frequently urban-adapted. Compared with other habitat
types, Acacia support bird assemblages with moderate species richness and density. This
study shows that Acacia thickets, as a novel habitat, provide a significant amount of habitat
space in a highly transformed landscape and highlights the need for comprehensive evaluation of altered habitats before assumptions are made about their ecological value. === AFRIKAANSE OPSOMMING: Getransformeerde habitatte maak vermeerderend groot deel uit van die omgewing, dit bedreig
biodiversiteit en skep groter uitdagings vir bewaring. In sommige gevalle vorm hierdie
getransformeerde habitatte geheel nuwe ekosisteme wat moontlik nuwe kombinasies van
spesies en spesie volopheid kan onderhou. Verder skep nuwe ekosisteme habitat spasie in
anders veranderde landskappe. In die Wes-Kaap van Suid-Afrika dra die Australiese Acacia
indringer spesies is veral problematies in laagliggende areas, aangesien dit digte ruigtes vorm,
asook beide die biotiese gemeenskappe en die abiotiese prosesse aansienlik transformeer. Ten
spyte daarvan dat daar volop Acacia ruigtes in die WesKaap is, word min verstaan van hul
vermoë om inheemse fauna te onderhou. Die hoofdoel van hierdie studie was om die belang
van Acacia ruigtes as habitat vir die area se voëllewe te bepaal. Voël-opnames in die suidwestelike
dele van die Wes-Kaap is gedoen in Acacia ruigtes oor drie seisoene, om
spesierykheid, volopheid en funksionele volopheid te ondersoek. Verder is die mate waartoe
verskille in die plotte van die plantegroei struktuur, die voëlgemeenskappe verander,
geondersoek. Daar was aansienlike variasie waargeneem in die spesiesamestelling rykheid,
voorkoms digtheid, mediaan liggaamsgrootte en biomassa van die voëls tussen die onderskeie
voëlopnaam plotte en die seisoene. Die variasie in plantegroei digtheid, stam digtheid,
mediaan plantegroeihoogte en totale kroonbedekking verduidelik hierdie variasie in
spesiesamestelling die beste. Tagtig voëlspesies Acacia ruigtes benut en die
populasiesamestelling het ‘n gemiddelde digtheid van 7,78 voëls per ha. Die mees algemene
voel-voeding-guldes was die gemengde-voedsel-vreters en insekvreters. Die median
liggaamsgrootte waargeneem was 15,2 g en die liggaamsgrootte frekwensieverspreiding van
alle spesies in Acacia ruigtes is ooreenkomstig met die liggaamsgrootte
frekwensieverspreiding vir die spesielys vir die hele Wes-Kaap. Die gemiddelde biomassa
van voel gemeenskappe was 0.224 kg per ha. Acacia ruigtes oor die fynbosbioom wat ‘n
geskatte gemiddelde van meer as 21 miljoen voels ondersteun, met ‘n gesamentlike biomassa
van meer as 600 duisend kg. Hierdie studie het bevind dat Acacia ruigtes in die Wes-Kaap ‘n
onderafdeling van die streek se voels ondersteun, met die mees algemene spesies as die klein
gemengde-voedsel-vreters, wat ook dikwels stedelik aangepas is. In vergelyking met ander
habitattipes ondersteun Acacia ruigtes voel samestellings met matige spesierykheid en
digtheid. Hierdie studie toon dat die Acacia ruigtes, as ‘n nuwe habitat, ‘n beduidende
hoeveelheid habitat ruimte in ‘n hoogs getransformeerde omgewing skep en beklemtoon die
behoefte aan ‘n omvattende evaluering van veranderde habitatte, voor aannames gemaak
word oor hul ekologiese waarde.
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