The Bird Community of an Acacia-Dominated Secondary Rainforest: a Brief Case Study
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Australian Field Ornithology 2015, 32, 59–68 The bird community of an Acacia-dominated secondary rainforest: A brief case study Amanda N.D. Freeman The School for Field Studies, Centre for Rainforest Studies, P.O. Box 141, Yungaburra QLD 4884, Australia Email: [email protected] Summary. Secondary forests can assist in restoring ecosystems affected by habitat loss and fragmentation and may provide a cost-effective alternative to large-scale planting. In the Wet Tropics bioregion, abandoned farmland frequently develops successional forest dominated by Hickory Wattle Acacia celsa but there has been little research on its value to wildlife. We examined the bird communities of younger (17–25 years) and older (50+ years) Acacia-regrowth forests and compared them with those in contiguous cleared land and never-cleared rainforest over an 18–day period in November 2003 in one location on the Atherton Tablelands, Queensland. Cleared sites had the lowest bird species richness and contained a mix of grassland, eucalypt, mixed forest and rainforest-dependent species. Bird species richness was highest in the never-cleared forest. Cleared sites had significantly fewer mixed forest and rainforest-dependent bird species than never-cleared forest. In both regrowth age classes and in never-cleared forest, there were few grassland and eucalypt forest bird species and similar proportions of mixed forest and rainforest-dependent bird species. Cleared sites and younger regrowth had significantly fewer endemic bird species than never-cleared forest, and six rainforest-dependent species were not recorded in regrowth. Although regrowth as young as 17–25 years provides habitat for a range of rainforest species, even at 50+ years it may not support some rainforest species, including some regional endemics of conservation concern. Introduction The potential of secondary rainforest to facilitate the recovery of faunal communities in fragmented agricultural landscapes and to provide relatively low-cost landscape restoration has not been widely discussed in the ecological literature (Bowen et al. 2007; Chazdon et al. 2009). In Australia’s tropical regions, most regrowth has occurred where agricultural production has been reduced or abandoned, usually on land of very low agricultural value. In the Wet Tropics region, secondary forest canopies are often dominated by wattles, such as Hickory Wattle Acacia celsa. Hickory Wattle is an early-to-intermediate successional species that is rare in mature rainforest. Structural features of these secondary forests include a dense, closed, very smooth canopy; a sparse seedling and sapling understorey, with abundant leaf-litter; and rare or absent strangling figs Ficus spp., walking stick palms, tree-ferns, vines, lianas, epiphytes, and rattan palms Calamus spp. (Graham et al. 2001). Allowing natural vegetation succession to occur on formerly cleared land can be viewed as a method of passive ecosystem restoration (Shono et al. 2007), and regrowth forest may provide habitat for native biota. The outcomes for Australian 60 Australian Field Ornithology A.N.D. Freeman rainforest fauna in regrowth forest are, however, largely unknown, and the bird communities of regrowth forests in the Wet Tropics have received limited attention (e.g. Crome 1990; Laurance et al. 1996). This study examines the bird communities of an Acacia-dominated secondary (regrowth) rainforest at mid elevation [700–800 m above sea-level (asl)] in the Wet Tropics bioregion of Queensland. Global climate change poses a serious risk for some Wet Tropics endemic bird species and other taxa that are restricted to such high elevations (Williams et al. 2003; Shoo et al. 2005). The occurrence of rainforest birds, and particularly endemic species, in this regrowth rainforest is thus examined, as such forests may provide refugial habitat and increase functional connectivity in landscapes, hence assisting dispersal of climate-sensitive species between areas of higher elevation. This study provides an example of a secondary forest where the prospects for conservation of primary forest species are maximised as the study site is contiguous with an extensive block of primary rainforest and some regrowth vegetation is >50 years old—factors that are thought to positively influence the accumulation of primary forest species in regrowth habitat (Chazdon et al. 2009). Methods Study area This study was conducted at The School for Field Studies, Centre for Rainforest Studies, property (17°12′S, 145°41′E) on the Atherton Tablelands in the Wet Tropics region of north- eastern Queensland. The property is on mainly granitic soil, 700–800 m asl and is 62 ha in area. Vegetation consists of cleared areas and various ages of unmanaged, secondary regrowth forests dominated by Hickory Wattle and never-cleared simple notophyll vine forest (Type 8 of Tracey 1982) contiguous with a large tract of continuous primary rainforest (Wooroonooran National Park, 76 000 ha). The property’s land-use history is complex. Aerial photographs show that much of the property was cleared, some repeatedly, before the current owner purchased it in 1988. Since then, no further clearing has occurred, though small areas have been kept clear by slashing and/or mowing. Rainforest on the property has been selectively logged, probably in the 1960s–1970s when adjacent forest was selectively logged (Laurance et al. 1996). Mapping of vegetation Ages of regrowth vegetation were ascertained from aerial photographs of the property taken in 1942, 1952, 1965, 1978, 1986, 1992, and 1997. The photographs were scanned and rectified using the Image Analysis extension in ArcView 3.2. Bird survey sites were then selected in vegetation classified as: (1) Cleared. Open grassland with a few trees, introduced shrub Lantana Lantana camara and tall grasses prevalent. (2) Young regrowth (17–25 years). Acacia-dominated canopy, Lantana common, frequent gaps, open understorey, heavy leaf-litter. (3) Older regrowth (50+ years). Canopy comprising a mixture of late-successional rainforest plant species with Acacia emergents, rainforest understorey, rattan palms and vines prevalent. Birds in Acacia-dominated secondary rainforest 61 Atherton Tablelands Wooroonooran National Park SFS property boundary 0.5 km Survey sites Vegetation Cleared Cleared Young regrowth Primary forest Older regrowth Secondary forest Never cleared Figure 1. The School for Field Studies (SFS) property, Atherton Tablelands, Queensland, showing vegetation cover and the location of survey sites. (4) Never-cleared rainforest. Canopy comprising a mixture of late-successional rainforest plant species with no Acacia, dense rainforest understorey, rattan palms and vines abundant. Within each vegetation class, bird survey sites (n = 3–11; see Figure 1, Appendix 1) with a radius of 25 m were selected; these sites contained only one vegetation age category and were far removed from boundaries between age categories. Where a large number of sites in an age category met the selection criteria, the sites surveyed were chosen randomly. Though other ages of regrowth occur on the property, these each cover <2 ha and could not meet our site-selection criteria. Bird surveys One 20-minute area survey of all birds utilising the habitat was conducted at each survey site. The same three to four observers, always including ANDF (an experienced observer who verified identification of bird species), moved around the survey sites recording all individual birds heard and seen. Surveys took place between 0600 h and 1200 h or between 1600 h and 1730 h, on 10–15 November and 26–28 November 2003. Different vegetation categories were visited on each day and at different times to limit the effects of variations in time and weather. Although the 20-minute area-search method was used at all sites, the number of sites surveyed varied between vegetation categories and may have affected the detection of individual bird species. Taxonomy follows Christidis & Boles (2008), and scientific names are given in Appendix 1. 62 Australian Field Ornithology A.N.D. Freeman Functional classification of species Bird species were classified into a priori functional ‘habitat’ groups following the methods of Catterall et al. (2012), based primarily on published descriptions of their use of uncleared vegetation types within the study region: (1) Rainforest-dependent (RF) species are largely confined to, or dependent on, rainforest. (2) Rainforest Wet Tropics (RWT) species are a subset of RF species, comprising endemic species whose distribution is completely within the Wet Tropics. (3) Mixed Forest (MF) species occur mainly in a range of forested habitats from rainforest to more-open-canopied eucalypt communities. (4) Eucalypt Forest (EF) species are typically found in eucalypt communities, and occur only occasionally in denser forest or less-wooded habitats. (5) Grassland/Wetland (GW) species occur mainly in grassland, wetland or water. Results A total of 58 bird species was recorded in this study (Appendix 1). Of the 46 species recorded in never-cleared forest, ten (22%) were also recorded at cleared sites, 31 (67%) in younger regrowth and 27 (59%) in older regrowth. Twenty-four of the species recorded in never-cleared forest were categorised as RF species (including RWT). Of these 24, three (13%) were also recorded at cleared sites, 17 (71%) in young regrowth and 15 (63%) in older regrowth. Bird species richness was greatest in the never-cleared forest (46 species) and six rainforest-dependent species (Double-eyed