Australian Field Ornithology 2018, 35, 119–125 http://dx.doi.org/10.20938/afo35119125

Estimating the population sizes of rainforest of the Tweed Caldera

Elliot C. Leach1*, Chris J. Burwell1, 2, Richard G. Sharpe3, Darryl N. Jones1 and Roger L. Kitching1

1Environmental Futures Research Institute, Griffith University, Nathan QLD 4111, Australia 2Biodiversity Program, Queensland Museum, South Brisbane QLD 4101, Australia 3Australian Rivers Institute, Griffith University, Nathan QLD 4111, Australia *Corresponding author. Email: [email protected]

Abstract. Climate change is likely to negatively impact populations of rainforest birds along Australia’s eastern seaboard. To this end, accurate estimations of species’ current population sizes are necessary for future monitoring and conservation. Here, we used distance sampling protocols to generate abundance and density estimates for 11 species of birds occupying the Gondwanan rainforests of the Tweed Caldera on the Queensland–New South Wales border. Data were collected along two elevational gradients in subtropical rainforest between September 2015 and October 2016. Half-normal functions with cosine adjustments were used to calculate the density of each species per km2. We used a GIS to determine the total area of suitable habitat available in protected areas adjacent to the sampling locations (~425 km2) and then extrapolated the density estimates to generate estimates of population size for each target species in the study area. Estimated population sizes ranged from 473 719 (Yellow-throated Scrubwren Sericornis citreogularis) to 21 841 (White-throated Treecreeper Cormobates leucophaea). The standard error (SE) associated with the population estimate of the White-throated Treecreeper was large, as was the SE associated with the Eastern Yellow Robin Eopsaltria australis (estimated population 235 084), Green Catbird Ailuroedus crassirostris (69 731) and Grey Fantail Rhipidura fuliginosa (124 534). Accurate abundance estimates (SE <20% of total abundance) were obtained for the Brown Gerygone mouki (193 919), Brown Thornbill Acanthiza pusilla (198 801), Golden Whistler Pachycephala pectoralis (72 730), Lewin’s Honeyeater Meliphaga lewinii (190 389), Silvereye Zosterops lateralis (92 268), White-browed Scrubwren Sericornis frontalis (94 426) and Yellow-throated Scrubwren (473 719). Species- specific surveys would provide more accurate results, but given the declines expected for many species in the region, and the current lack of estimates of population sizes in these protected areas, the baseline data presented here are of importance from ecological and conservation perspectives.

Introduction rainforests of the Tweed Caldera region of south-eastern Queensland and north-eastern New South Wales were Accurate estimation of population size is of fundamental recognised by BirdLife Australia as part of the Scenic Rim importance in conservation biology (Newson et al. 2008). Key Biodiversity Area in 2009 (BirdLife International 2017) At a broad scale, the New Atlas of Australian Birds (Barrett and support populations of several rare and threatened et al. 2003) and the 2015 State of Australia’s Birds report species. These habitats, along with other montane (BirdLife Australia 2015) quantify the population sizes rainforests on the eastern coast of Australia, are imperilled of each of Australia’s resident species. Considerable by ongoing climate change (Laurance et al. 2011). effort has been made to calculate the population sizes of In 2015–2016, ECL conducted avian biodiversity Australian birds on smaller scales in a wide range of habitats surveys in the Gondwanan rainforests. Here, we use these (reviewed in Recher 1989). However, comparatively little data, which were collected under a distance sampling work has been conducted in rainforests (but see Pattemore protocol, to estimate the population sizes of rainforest & Kikkawa 1974; Kikkawa 2002; Shoo et al. 2005, 2009). birds occupying the Tweed Caldera. This represents the Bird species restricted to the montane rainforests along first attempt to quantify the population sizes of rainforest Australia’s eastern seaboard are more likely than others to birds in this region using distance sampling and, along with suffer population declines from climate change (Williamset other studies (Leach 2016; Leach et al. 2018a,b), aims to al. 2003; Laurance et al. 2011). An understanding of avian inform future monitoring and conservation projects. population sizes in these habitats (which often fall within the boundaries of protected areas) is therefore necessary for effective conservation. Methods Distance sampling (Buckland et al. 2005; Thomas Study area et al. 2010) is one of the most widely used methods for estimating population size and has been employed on Fieldwork was carried out between September 2015 and a systematic basis in the Wet Tropics World Heritage October 2016 in north-eastern New South Wales in the Area over recent decades (Anderson 2011). However, Border Ranges National Park (28°23′S, 153°3′E) and no such sampling regime exists in the Gondwana Mebbin National Park (28°27′S, 153°10′E) (Figure 1). Both Rainforests of Australia World Heritage Area (hereafter are part of the Gondwanan rainforests and, together, protect Gondwanan rainforests), despite the importance of this 15 940 ha of subtropical rainforest (McDonald 2010). At region to Australian biodiversity (Kitching et al. 2010). The low elevations, the average annual rainfall in the region is 120 Australian Field Ornithology E. C. Leach et al.

in September and October 2016. In total, we obtained 11 monthly datasets suitable for analysis.

Analysis Generally, distance sampling requires a minimum of 60 detections per species for accurate estimation of the detection function (Buckland et al. 2005). However, as few as 35 detections per species have been used by previous researchers working on Australian rainforest birds (Anderson 2011). We chose to use a minimum threshold of 45 detections per species. From the raw data, we removed groups that were comprised of >10 individuals (n = 5) because of the potential inaccuracy of these observations. This resulted in a dataset of 11 species suitable for population estimation. Following the approach of Anderson (2011), we first inspected the distance–frequency histograms for each species. Raw data for each species were then assigned to 10-m distance ‘bins’ to minimise heaping effects. Truncation was applied when necessary (for example, when many detections were assigned to Figure 1. Map of the study area on the Queensland–New the 40–50-m distance bin) in order to avoid problems South Wales border. Survey sites were located in five when fitting the tail of detection functions (Buckland elevational bands (300, 500, 700, 900 and 1100 m asl) along two elevational gradients (‘Sheepstation Creek’ et al. 2005). To achieve consistent convergence, models for and ‘Bar Mountain’; SC and BM, respectively). There were each species were fitted using half-normal functions with two replicate sites (indicated by A and B) located at each cosine adjustments. The goodness of fit of the models was elevation on each gradient. assessed using chi-squared tests. Finally, the reliability of population estimation for each species was assessed through the calculation of standard errors. We sought to extend the area in which we could estimate 1555 mm (McDonald 2010). Based on temperature data population size of the 11 species by including adjacent collected at each site using temperature loggers (DS21 rainforest reserves, primarily Lamington National Park in Thermochron® iButton®, Maxim Integrated, California, Queensland. To this end, an extract from an Australian USA), the average maximum summer temperature during national Digital Elevation Model (DEM) surrounding the the sampling period was 29°C, and the average minimum Lamington, Border Ranges and Mebbin National Parks winter temperature was 6°C. The vegetation of the study was obtained via the Geoscience Australia Elevation area is described in detail by McDonald (2010); in general, Information System web tool (http://www.ga.gov.au/elvis). it is classified as warm subtropical rainforest. The DEM was interrogated in a GIS to produce five height Two elevational gradients (named ‘Sheepstation Creek’ bands, separated by 200 m, from 200 m to 1200 m asl. and ‘Bar Mountain’), encompassing the full elevational This represented the full elevational range of rainforest in distribution of rainforest in the region, were established the region that was adjacent to that sampled. The height before sampling commenced. Each gradient consisted bands were projected to Map Grid of Australia Zone 56 of two replicate sites in each of five elevational bands on the Geocentric Datum of Australia 1994. The total area ~300, 500, 700, 900 and 1100 m above sea-level (asl), in of suitable habitat in the region based on this projection 2 a design further described in Leach (2016); all sites were was computed. This area (~425 km ) was then used in separated by a minimum of 400 m. In total, there were our analysis to expand from a local to a regional scale of 20 sites (Figure 1). population estimation. All analysis was performed using the package Distance Distance sampling protocol (Thomas et al. 2010) in R statistical software version 3.4.2 (R Core Team 2017). ECL made point counts at each site, beginning on 29 September 2015 and continuing on a monthly basis until 7 October 2016. Point counts were of 10 minutes’ duration Results and were conducted within 3 hours of dawn in fine weather. During this time, all birds seen or heard within a radius In total, ECL conducted 220 point counts and recorded of 100 m from the central point were recorded. Where 3116 birds, representing 63 species. Of the 11 species possible, distance to each individual bird was recorded with sufficient numbers of detections for analysis (Table 1), using a laser range finder. If this was not possible, or if an Lewin’s Honeyeater Meliphaga lewinii was most frequently individual bird was heard but not seen, the distance to the detected, being recorded 280 times. Half-normal functions individual was ‘binned’ (rounded) to the nearest 10 m. Birds with cosine adjustments were used to estimate the total in groups were recorded but were treated as individuals abundance and density of the target species (Table 1) in analysis. Inclement weather and road closures within in the study area, which was made up of ~425 km2 of the national parks meant that sampling at the same time suitable habitat. An example of a fitted detection function each month was not possible; as a consequence, fieldtrips is shown in Figure 2 (fitted detection functions for each in March and April 2016 were combined as were those of the 11 species are provided in Appendix 1). All models Estimating population sizes of rainforest birds, Tweed Caldera 121

Table 1: Estimated total abundance (total number of individuals in the population) and density (number of individuals/ km2) of 11 species of rainforest birds occupying the Tweed Caldera on the Queensland–New South Wales border; n = number of individuals detected during fieldwork, p = average probability of detection of the species, SE = standard error, CV = coefficient of variance, and * = reliable abundance estimate (SE <20% of total abundance estimate).

Species n p Abundance Density

Abundance SE CV Density SE CV

Green Catbird Ailuroedus crassirostris 56 0.31 69 731 18 299 0.26 164 43 0.26

White-throated Treecreeper Cormobates leucophaea 48 0.75 21 841 6796 0.31 51 16 0.31

Lewin’s Honeyeater Meliphaga lewinii* 280 0.40 190 389 22 526 0.12 447 53 0.12

Brown Gerygone Gerygone mouki* 262 0.61 193 919 31 408 0.16 456 74 0.16

Yellow-throated Scrubwren Sericornis citreogularis* 178 0.14 473 719 88 947 0.19 1113 209 0.19

White-browed Scrubwren Sericornis frontalis* 79 0.26 94 426 16 426 0.17 222 39 0.17

Brown Thornbill Acanthiza pusilla* 124 0.15 198 801 36 887 0.18 467 87 0.18

Golden Whistler Pachycephala pectoralis* 105 0.40 72 730 10 576 0.14 171 25 0.14

Grey Fantail Rhiphidura fuliginosa 81 0.19 124 534 31 526 0.25 293 74 0.25

Eastern Yellow Robin Eopsaltria australis 76 0.12 235 084 55 725 0.24 552 131 0.24

Silvereye Zosterops lateralis* 135 0.38 92 268 15 451 0.17 217 36 0.17

converged and satisfied the goodness-of-fit assumptions, Discussion but some abundance estimates were more reliable than others. The average detection probability of the 11 species The broad-scale distributions of Australian birds are varied between a high of 75% (White-throated Treecreeper relatively well understood (Barrett et al. 2003), but finer- Cormobates leucophaea) and a low of 12% (Eastern scale patterns, particularly for local population sizes, are Yellow Robin Eopsaltria australis). The species with the less well known. A better understanding of fine-scale highest estimated abundance was the Yellow-throated population sizes and trends is essential for effective Scrubwren Sericornis citreogularis, though the standard conservation (Newson et al. 2008). Using a distance errors associated with this estimate were high; the White- sampling protocol (Buckland et al. 2005), we estimated throated Treecreeper was the least abundant of the the population sizes of 11 bird species in the Gondwanan 11 species. rainforests. Because of logistical constraints, however, we could estimate the abundance of only relatively common species in the study area. Standard errors associated with the abundance estimates varied widely. We considered abundance estimates reliable when the associated standard errors were <20% of total estimated abundance. Under this criterion, we accurately estimated the population sizes of seven of the 11 rainforest species under investigation: Brown Gerygone, Brown Thornbill, Golden Whistler, Lewin’s Honeyeater, Silvereye, White-browed Scrubwren and Yellow-throated Scrubwren (Table 1). These species were relatively easy to detect during fieldwork as they were usually conspicuous while foraging and often vocalised. Some of the remaining species (Eastern Yellow Robin and Grey Fantail) were also easily detected, but others were more difficult to detect if not vocalising (Green Catbird and White-throated Treecreeper). The reliability of abundance estimates for each species could certainly be improved with greater survey effort and replication. However, this paper can be seen as a first step towards improved knowledge of the population sizes of rainforest birds in the northern areas of the Gondwanan rainforests. The species investigated in this paper are common rainforest birds, with relatively extensive distributions, and are insectivorous or omnivorous (see Leach et al. Figure 2. Distance histogram and fitted detection function 2018a). Common species provide important ecosystem for the Green Catbird. Photo: Duade Paton services, and also define the character of ecosystems 122 Australian Field Ornithology E. C. Leach et al.

(Gaston 2010; Inger et al. 2015). Such species are often abundance throughout the seasons (or years), allowing overlooked in favour of those of higher conservation researchers to monitor population trends over longer time priority (Gaston 2008) but, recently, worrying trends in the scales (Leach et al. 2018b). Estimating the abundance of abundance of common species have become apparent rarer species in the region should also become a research (Ford 2011; Inger et al. 2015). Therefore, the baseline priority, and will require specific, targeted surveys and more assessments of population size in the present study intensive sampling effort. Such data are of vital importance provide a reference point for comparisons in future avian if we are to better understand the changes in biodiversity monitoring and conservation projects in the Gondwanan that are occurring as a result of the changing climate. rainforests. Future estimations of population size for the 11 species examined in the present paper could be used Acknowledgements to identify population declines in the study area. The IUCN Red List Criteria, which are used to determine the current All fieldwork was undertaken under Scientific Licence SL101514 issued by the New South Wales National Parks and Wildlife conservation status of a species, could then be applied Service, and staff of the Kyogle National Parks and Wildlife to the population in question. For example, an estimated Office provided ECL with assistance during fieldwork. ECL also decline in population size of >30% would serve to classify a received assistance from numerous volunteers—special thanks species as Vulnerable in the study area (Mace et al. 2008). go to Jess Mackie. 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Appendix 1. Fitted detection function for 11 species of birds in the Tweed Caldera region of south-eastern Queensland and north-eastern New South Wales Estimating population sizes of rainforest birds, Tweed Caldera 125

Appendix 1. continued