Bird distribution in riparian vegetation in the extensive natural landscape of Australia’s tropical savanna: a broad‐scale survey and analysis of a distributional data base

Abstract

Aim (a) To characterize the riparian bird assemblage, and its variation, in a large area of northern Australia; (b) to examine the distinctiveness of this assemblage in relation to the broader landscape; (c) to consider the influence of disturbance on this assemblage; (d) to examine temporal variability in the riparian assemblage, and especially evidence for seasonal movements between riparian and non-riparian areas.Location c. 620,000 km 2 of the seasonal tropics of the Northern Territory, Australia.Methods (a) Synchronous sampling of birds in riparian and adjacent non-riparian areas at 100 sites stratified across 13 catchments and an extensive rainfall gradient. (b) Repeat visits to 13 of these sites at contrasting seasons. (c) Analysis of a larger distributional database to assess the relative occurrence of records in riparian areas relative to nonriparian areas.Results Species richness and the total abundance of birds was significantly greater in riparian zones than in matched non-riparian areas, especially where the riparian zones contained extensive cover of rain forest plants and Melaleuca . Similarity in bird species composition between riparian zones and adjacent non-riparian areas was generally low, and this distinction was greatest in lower reaches of the rivers and where the riparian zone contained no eucalypts. Bird species composition varied gradationally from riparian zones in high rainfall areas, through riparian zones in low rainfall areas and non-riparian zones in high rainfall areas, to non-riparian zones in low rainfall areas. Many species occurred widely across the riparian sites sampled. Of ninety-four species recorded from more than five sites, forty-five species were significantly more abundant in riparian zones than in matched non-riparian zones, whereas this pattern was reversed for only twelve species. There was little association between foraging group and preference for riparian zones. Species had highly idiosyncratic distributions across the riparian samples, with the most common trend being an association with mean annual rainfall. Many species were significantly more closely associated with riparian zones in lower rainfall areas than in higher rainfall areas. Indeed, many species typical of higher rainfall areas extended into lower rainfall areas only, or mainly, along riparian strips. There was some temporal fluidity in bird species composition of riparian zones, suggesting seasonal movements between riparian zones and the surrounding landscape. There was little evidence that disturbance was a major factor influencing the distribution of riparian birds, probably because other major geographical and environmental gradients probably dwarfed the influence of the relatively minor variation between samples in disturbance. Main conclusionsThe bird fauna of riparian areas is distinct from that of the surrounding savannas, and especially so in lower rainfall areas. Riparian vegetation allows many species to extend their distributions into lower rainfall areas. The riparian assemblage is l...

The Role of Habitat Heterogeneity in Structuring Mangrove Bird Assemblages

“…The relative abundance of food resources near tidal creeks may be one reason for high bird species richness in the mangroves (Figure 1). An increase in bird species richness in riparian zones compared to nonriparian zones was also observed in Australian savanna, due to the greater plant species richness and structural complexity of riparian zones [30].…”

“…Besides being an important factor in contributing to the increase in species richness and diversity, habitat structure is also an important determinant influencing habitat selection and distribution of species, especially in complex habitats such as tropical forest [29]. For example, in the Northern Territory, Woinarski et al [30] reported that species richness and abundance of birds was significantly greater in forested riparian zones compared to non-riparian habitats, and that species composition also varied within habitats depending on their relative complexity.…”

Riparian Zones Increase Regional Species Richness by Harboring Different, Not More, Species

“…Specifically, riparian habitats are often cited as harboring higher numbers of species than adjacent upland habitats within the same geographic location (Naiman et al 1993, NRC 2002. This pattern has been observed in some systems (McIntyre 2000, Woinarski et al 2000, Catterall et al 2001, Brown and Peet 2003, but not in others (McComb et al 1993, Hughes and Cass 1997, Treonis et al 1999, Catterall et al 2001, Pearson and Manuwal 2001. In some systems, in fact, the opposite gradient (higher upland diversity) has stronger support (McGarigal and McComb 1992, Kirkman et al 1998, Burnham 2002, Schade et al 2003.…”

The impacts of fire on birds in Australia's tropical savannas

“…These highly fragmented environments now harbour much of the relictual biota of northern Australia and support biotic communities very different to the far more pervasive savanna woodlands (Specht 1958;Taylor and Dunlop 1985;Russell-Smith 1991;Russell-Smith et al 1992;Woinarski 1993Woinarski , 2004Woinarski et al 2000a).…”

“…Vegetation types that contrast with the tropical savanna matrix add a further mix of bird species to the regional pool, but because of the small extent and patchiness of these habitats, their bird species richness is generally low. This is particularly so for rainforests and riparian forests, whose bird communities are attenuated from higher to lower rainfall areas of northern Australia, and from east (Cape York Peninsula) to west (Kimberley) (Johnstone and Burbidge 1991;Woinarski 1993;Woinarski et al 2000a). A small number of bird species are closely associated with Melaleuca woodlands and forests (e.g.…”