Avian assemblages of invasive Australian Acacia thickets in the Western Cape

[摘要] ENGLISH ABSTRACT: Human-modified habitats form increasingly large components of landscapes, threateningbiodiversity and creating challenges for conservation. In some cases altered habitats formentirely novel ecosystems that may support new combinations of species and speciesabundances, and create habitat space in otherwise transformed landscapes. In the WesternCape of South Africa, woody invasive species contribute to landscape-level habitattransformation and form novel ecosystems. Invasive Australian Acacia species are especiallyproblematic in lowland areas where they create dense thickets and substantially transformboth biotic communities and abiotic processes. Despite the prominence of Acacia thicketsacross the Western Cape, their ability to support native fauna is not well understood and theobjective of this study was to assess the significance of Acacia thickets as habitat for theregion's avifauna. Birds were surveyed in Acacia thickets in the south-western Western Capein three seasons to examine species richness, abundance and functional abundance.Furthermore, I examined the extent to which differences in patch-level vegetation structurealter bird communities. Between survey sites and seasons, significant variation was observedin assemblage richness, density, median body size and biomass. Variation in vegetationdensity, stem density, mean vegetation height and total canopy cover best explained variationin bird assemblages. Eighty species were estimated to utilize Acacia thickets and assemblageshad a mean density of 7.78 birds per ha. The most abundant feeding guilds were the mixedfeeders and insectivores. The median body size observed was 15.2 g and the body sizefrequency distribution of all species in Acacia spanned a similar range compared to the bodysize frequency distribution for the species list for the entire Western Cape. The mean biomassof bird communities was 0.224 kg per ha. Using data on bird density and biomass, Acaciathickets across the Fynbos Biome support and estimated average of over 21 million birds witha combined biomass of over 600 thousand kg. This study found that Acacia thickets in theWestern Cape support a subset of the region's birds with the most abundant species beingsmall mixed feeders, which are also frequently urban-adapted. Compared with other habitattypes, Acacia support bird assemblages with moderate species richness and density. Thisstudy shows that Acacia thickets, as a novel habitat, provide a significant amount of habitatspace in a highly transformed landscape and highlights the need for comprehensive evaluation of altered habitats before assumptions are made about their ecological value.

[发布机构] Stellenbosch University