Riparian and Upslope Influences on the Regional Avifauna of the Semi-Arid Mulga Lands of South West Queensland
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Riparian areas have been widely recognised by wildlife biologists as a critically important and functionally dominant component of terrestrial landscapes. This viewpoint has its genesis in high concentrations of species and individuals across a wide range of environments and strong interactions between riparian areas and the surrounding landscape. Despite major concerns regarding conservation management in the Australian arid zone, few studies have specifically examined the importance of riparian areas to the terrestrial bird fauna of arid and semi-arid Australia. This research aimed to examine the role of riparian areas in sustaining regional assemblages of terrestrial birds within the Australian arid zone. More specifically I asked: 1. How do riparian and upslope arid zone bird assemblages differ and to what extent are they interrelated? 2. Do these inter-relationships vary temporally with season, rainfall and year? 3. To what extent does surface water influence riparian and upslope bird assemblages? This study was carried out in semi-arid Mulga Lands bioregion of south west Queensland (c. 181000 km2) where bird densities, species richness and composition were compared among 124 sites which were distributed throughout the bioregion and surveyed over two seasons (summer, winter) and two years (1997, 1998). El NiÃ±o-related drought conditions prevailed over both seasons during 1997 but not 1998. Monthly rainfall was not dependent on either season or year. The extent to which the availability of surface water in these areas also influences terrestrial birds was investigated by comparing bird abundance, diversity and species composition at riparian and upslope sites, with and without permanent water. Upslope sites with permanent water were modelled using artesian bore drains. Overall bird densities were twice as high in riparian areas as upslope habitats but about 20% more species were found in upslope habitats. The estimation of species richness in circumstances where there are major differences in abundance emerged as an important issue for riparian-upslope comparisons. Riparian areas were also characterised by higher levels of species dominance and similarity in species composition than upslope areas. Riparian-preferring species accounted for 68% of total bird abundance and many were common in the surrounding landscape. Similarly, many upslope-preferring species were common in riparian areas. The number of species shared between riparian and upslope areas was maximised at riparian sites with permanent water, implying that these areas were of near-universal advantage. These results suggested that riparian habitats of the Mulga Lands exert a fundamental influence on the entire terrestrial avifauna and are therefore important centres of avian biodiversity. Despite high levels of climatic variation but only slight seasonal differences in mean rainfall and plant growth response, I observed a strong summer increase in species richness (overall and among many functional groups) but not in overall abundance. Fewer individuals and species were observed during the drought conditions of 1997. About half of the species (21 of 41) that could be individually categorised showed seasonal or inter-annual differences in occurrence, suggesting extensive inter-bioregional movements. Riparian usage was generally higher during periods of low monthly rainfall, but it was complicated by riparian interactions. Insectivores that forage mainly in the upper stratum, and seed-eaters such as pigeons, parrots and cockatoos, made greater use of riparian areas as rainfall declined, whereas the number of low-feeding insectivore species increased in riparian areas with increasing rainfall. Overall riparian usage was also higher during drought, but not necessarily summer. Species composition was strongly influenced by season, year and rainfall, and there were strong species composition linkages between riparian and upslope bird communities. These results support the proposition that riparian areas have an important if not crucial role in sustaining bird populations, not only during prolonged drought as refuge habitat, but also over much smaller time scales. Birds also responded strongly to the presence of surface water. The relative strength of the effects of riparian status and water availability were similar for most species and functional groups, although where differences were detected all favoured the effect of riparian status. Most species and functional groups showed specialised preferences for specific combinations of riparian status and water availability rather than generalised responses to either or both. Most displayed a dominant preference for riparian or upslope habitats and preferentially sought to meet their need for water within these areas. Because of the specialised responses, the presence of permanent water could only partially explain differences in bird assemblages between riparian and upslope sites. A significant role for higher productivity and/or structural complexity in riparian areas was suggested by strong associations between riparian status and vegetation structure that were only weakly related to the presence of surface water. Small insectivorous passerines, many of which are already uncommon or declining in other bioregions, appear most vulnerable to the planned closure of bore drains. This study suggests that, as far as the Mulga Land birds are concerned, the bird communities of riparian and upslope components of the landscape are functionally interrelated. This is despite strong structural and floristic differences in habitat, and the fact that many bird species show distinct preferences for one habitat or the other. Almost all terrestrial species were found in both the riparian and upslope habitats, although their use appeared to be strongly related to spatial and temporal variations in resource availability. As most birds are capable fliers, and changes in relative abundance were rapid, these patterns are likely to more strongly reflect movement between habitats (and in some cases, bioregions), than differences in recruitment and mortality. In fluctuating and unpredictable environments the ability to move between habitats may be an important adaptive strategy to dampen spatial and temporal variations in resources and facilitate species persistence. The overall picture is one of a shared and responsive avifauna. As many of the specific responses observed in this study appeared to be a predictable outcome of spatial variations in productive potential and temporal variations in resource availability, a conceptual model was proposed to explain spatio-temporal variations in terrestrial bird community organization in the Australian arid zone. The model establishes a graphical response domain, defined by a spatial axis that represents long-term cumulative outcomes of prevailing spatial and temporal productive processes (e.g. spatial variation in nutrient status, soil moisture and vegetation biomass) and a temporal axis that represents short-term availability of productive resources (e.g. rainfall). Within this domain, individual response surfaces were proposed to predict relative site-based differences in overall bird abundance, dominance, species richness, and inter-habitat movement. In addition to responses at average levels of resource availability, the response domain was also used to consider how birds might vary their use of the landscape under two extremes of environmental variability, drought and production pulses after extensive rainfall. The model may also predict assemblage differences in other biomes.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Australian Environmental Studies
Item Access Status
Australian arid zone