The Distribution of the Green Python (Morelia Viridis) in Australia
The Distribution of the Green Python (Morelia viridis) in Australia
A thesis submitted for the degree of Master of Philosophy
By Daniel Natusch School of Biological, Earth and Environmental Science University of New South Wales
November 2010 Distribution of the green python in Australia 2
ORIGINALITY STATEMENT
I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.
Daniel James Deans Natusch
November 2010
Distribution of the green python in Australia 3
ABSTRACT
The green python (Morelia viridis) is an iconic snake species that is highly sought after in the captive pet trade and therefore the target of illegal collection. Despite their popularity and an increase in wildlife conservation in recent years, some important ecological attributes of green pythons remain unknown. This makes their effective conservation management difficult. The aim of this research was to determine the detailed distribution, relative abundance and demographic status of the green python in Cape York Peninsula, Australia. Presence/absence surveys were conducted throughout the potential range of the green python and intensive mark-recapture surveys in the areas where there have been previous records. Green pythons were located in the Iron, McIlwraith and Kawadji-Ngaachi ranges of Cape York, distributed over an estimated area of 2289 km2 where they frequented rainforest habitats and adjacent vine thickets. However, despite anecdotal records, they were not found in the Lockerbie Scrub or Jardine River Catchment. Green pythons were encountered most frequently in the Iron Range area where their density was estimated to be 540 km-2 compared with an estimated density of 200 km-2 in the McIlwraith Range. Mature individuals comprised 83% of snakes caught in the McIlwraith Range, whereas only 56% of those captured in the Iron Range were mature. This study has shown that green pythons are common in suitable local habitat. The differences between their abundance and population structures in the Iron and McIlwraith ranges may be due to differences in prey abundance and the possible impact of poaching. The current distribution of green pythons in Australia is consistent with a rainforest connection with New Guinea during the Pleistocene and it is postulated that the two populations have been separated for at least the last 250000 years. The detailed distribution of green python populations in Cape York, combined with baseline data on their demographic variability, now provides a foundation for the informed conservation management of this species.
Distribution of the green python in Australia 4
PREFACE
“Maxwell sold the business in 1997 to devote more time to God, family and Green Tree Pythons”.
This quote was taken from the blurb of renowned green python keeper Greg Maxwell’s book – “The Complete Chondro”. A snake, comparable to a deity and one’s family? While this quote is taken out of context, it highlights the passion that Maxwell and many others share for this species.
All over the world, people hold green pythons in awe as the crème de la crème of reptile-keeping, and with good reason. An animal born either red or yellow before changing to green is not something you see every day. This is what makes this species so incredible. Needless to say, many dream of seeing a green python in its natural habitat.
I was 13 years old when I encountered my first green python in the rainforests of Cape York Peninsula. There, coiled distinctively over a low hanging vine, was a bright green snake. I had encountered many snakes before, but this one with its striking white vertebral stripe and almost fluorescent green skin was mesmerising.
Since this first meeting I have spent much of my time walking through the wilds of Cape York Peninsula. Green pythons have featured highly in my forays, but it was not until a chance meeting with David Wilson in the Iron Range National Park that I started to think of green pythons as more than just “that green snake”. David enlightened me about some of the finer aspects of green python ecology, describing the colour change and explaining that green pythons were subject to poaching because of their popularity in the captive pet trade. Finally, David explained that, until his studies, next to nothing was known about green pythons in the wild.
Distribution of the green python in Australia 5
I left that meeting wondering how a snake that is so popular in captivity has received so little attention in the wild. In Australia, we did not even know the limits of its distribution.
I began my current research program by scanning the internet and reading literature, finding out all I could about green pythons while continuing to explore ever more remote areas of Cape York, searching for these elusive snakes in areas where no one had sought them before.
It was not until the second year of my undergraduate degree that I began to formally investigate the distribution of green pythons in Australia. When describing distribution I do not merely refer to ‘geographic range’, but also ‘how’ green pythons are distributed in particular habitats (i.e. how abundant they are in certain areas and how their population is structured). To this end, I found a supervisor whose passion for the Australian landscape and thirst for knowledge inspired me to begin my research – Mike Archer at UNSW.
Having walked for 1000’s of kilometres, endured far too many encounters with ‘Wait- a-whiles’ (viciously hooked Lawyer vines, Calamus australis), Stinging Nettle trees (Gympie-Gympie, Dendrochnide moroides) and of course green pythons, I am beginning to understand a lot more about these magnificent animals and their habitat. This thesis is a synthesis of much of what I have discovered through the course of this research.
Distribution of the green python in Australia 6
ACKNOWLEDGEMENTS
Conducting research in Cape York Peninsula is not possible without the help and assistance of many people. With the support of the following individuals, this work was completed safely and successfully and, through their endeavours, made more enjoyable than otherwise possible.
Mike Archer, my supervisor, provided me with the opportunity to conduct this project as part of a formal research program at UNSW. Together with advice from my co- supervisor, Sue Hand, their continued interest, patience and help were significant reasons why this work was able to be completed.
I am indebted to all the people who assisted during fieldwork. Mandy Deans, Bill Koutsamanis, Frank Maltzahn, Matthew McCloskey, Rowan McIntyre and David Natusch gave up there time to spend hours walking through the forest with me. Special mention must go to Jess Lyons, Lachlan McIntyre and Sara Spence who spent entire wet seasons looking for snakes and without whom this project could not have been completed.
Thank you to the plethora of people who helped me with generous logistical support while working in Cape York. Kieran Aland, Brian Benham, Wayne Butcher, Patrick Couper, Don DeBusch, Naomi Hobson, Tim Jaffer, Rod Mueller, Scott Templeton, James Walker and Terry and Davida Webb all helped to make this project possible. My very special thanks go to both John and Jackie Mulholland, who provided me with a home while in Cape York and without whose generous and ongoing support I would still likely be sitting in the forest somewhere!
Special thanks to the numerous Traditional Owners with whom I worked. Their dedication to the conservation of their country, combined with their knowledge, inspired me to make this project the best it could be. Thank you all for the support, guidance and laughs!
Thank you to all the people who provided support and advice for this project. Thank you to Matt Hunt in the School of Biological, Earth and Environmental Science, for
Distribution of the green python in Australia 7
always being a tome of upbeat and forever helpful information. Thank you to Jonothan Webb for discussions and help with population analysis and Robert Heinsohn and David Wilson for support and advice.
Thank you to the Queensland Parks and Wildlife Service for collaboration in this study. This work was conducted under the Department of Environment and Resource Management Scientific Permit numbers WISP04847807 and WITK04847707 and completed in accordance with the University of New South Wales Animal Ethics Committee procedures – permit number 07/60A. This project was funded with the help of the University of New South Wales, Australian Geographic Society, the Linnean Society of New South Wales and SeaSwift Far North Queensland.
I dedicate this work to my father, David Natusch, without whose unconditional support, guidance and humour this study could not have been envisioned – let alone successfully completed.
“... I see that the floodplain has decided to become a floodplain” - David Natusch
Distribution of the green python in Australia 8
CONTENTS
ORIGINALITY STATEMENT ...... 2 ABSTRACT ...... 3 PREFACE ...... 4 ACKNOWLEDGEMENTS ...... 6 CONTENTS ...... 8 FIGURES ...... 10 TABLES ...... 11 APPENDICES ...... 11 1.0 INTRODUCTION ...... 12 1.1 The green python ...... 12 1.2 Green pythons in Cape York Peninsula ...... 15 1.3 Previous estimates of distribution, abundance and habitat use ...... 17 1.4 Aims ...... 18 1.4.1 Specific aims ...... 18 2.0 METHODS ...... 19 2.1 Study Sites ...... 19 2.1.1 Lockerbie Scrub ...... 19 2.1.2 Jardine River Catchment ...... 21 2.1.3 Iron Range ...... 21 2.1.4 Kawadji-Ngaachi Ranges ...... 21 2.1.5 McIlwraith Range ...... 22 2.2 Survey methods ...... 23 2.2.1 Presence/absence surveys ...... 23 2.2.2 Intensive surveys ...... 23 2.3 Analysis ...... 24 3.0 RESULTS ...... 26 3.1 Distribution ...... 27 3.2 Density, abundance and habitat preference ...... 30 3.3 Population demographics ...... 33
Distribution of the green python in Australia 9
4.0 DISCUSSION ...... 35 4.1 Distribution ...... 35 4.2 Population characteristics and habitat use ...... 40 5.0 CONCLUSION ...... 43 5.1 Extrapolation of population size ...... 44 5.2 Threats ...... 46 5.2.1 Feral animals ...... 46 5.2.2 Road kill ...... 47 5.2.3 Fire ...... 47 5.2.4 Poaching ...... 48 5.3 Conservation implications ...... 48 REFERENCES ...... 50 APPENDICES ...... 58
Distribution of the green python in Australia 10
FIGURES
Figure 1. The three colour phases of the green python. Green pythons are born either red or yellow and change to green...... 14 Figure 2. Green pythons are collected in New Guinea by poachers and shipped overseas in large numbers...... 15 Figure 3. The present day land boundaries (dark grey) of Australia and New Guinea and the extent of land (pale grey) following a 50 m drop in sea level (Voris, 2000)...... 16 Figure 4. The distribution of closed forests and the five sites surveyed for green pythons on Cape York Peninsula...... 20 Figure 5. The distribution of habitats suitable to green pythons (light grey) and survey sites where green pythons were found to be present (●) or absent (噘)...... 26 Figure 6. The distribution of suitable habitats in the Iron-McIlwraith Range region (light grey). Presence (●) and absence (噘) sites are mapped without rainforest creek lines (regional ecosystem 3.3.1) (a) and presence sites are mapped showing connecting creek corridors (dark grey) (b)...... 27 Figure 7. Two examples of suitable habitats in which green pythons were found. Evergreen to semi-deciduous notophyll vine forest (left) and deciduous vine thicket (right) – both found in the McIlwraith Range...... 29 Figure 8. The age class distributions of female (black columns), male (white columns) and juvenile (grey column) green pythons captured in the McIlwraith (a) and Iron ranges (b)...... 34 Figure 9. Two examples of rainforest vegetation on the McIlwraith and Kawadji-Ngaachi ranges, showing unsuitable grass and woodland habitats which limit the dispersal of green pythons between suitable habitats in Cape York...... 36 Figure 10. Densities of green pythons are not actually this high. These two are obviously onto something good. Seemingly oblivious to one another, they have set up ambush less than 50 cm apart...... 41
Distribution of the green python in Australia 11
TABLES
Table 1. Transect summary and survey effort at the three sites intensively surveyed for green pythons in Cape York ...... 25 Table 2. Area of each habitat type in which green pythons were found on Cape York and their extent in protected areas...... 28 Table 3. The relative abundance and density of green pythons at the three sites that were intensively surveyed in Cape York...... 32 Table 4. Extrapolation of density estimates in the Iron and McIlwraith Range National Parks ...... 45
APPENDICES
Appendix I. Regional ecosystems surveyed in Cape York in which green pythons were
not found (Sattler and Williams 1999)...... 58
Distribution of the green python in Australia 12
1.0 INTRODUCTION
Understanding species distribution and abundance patterns is a fundamental goal of ecology (Krebs 1972; Ricklefs and Miller 2000). However, detailed understanding of the distribution and demographic status of even the most vulnerable species in Australia has not been established (Cogger et al. 1993). This is particularly true for non-mammalian and non-avian species such as reptiles, fishes and invertebrates, which are often perceived as somehow less important, and thus receive relatively little attention (Bonnet et al. 2002a; Pawar 2003). This knowledge gap presents a variety of hurdles for the management of such species, seriously limiting the effectiveness of strategies intended to assist in their conservation (Rodrigues et al. 2006).
Nevertheless, recognition of a current global biodiversity crisis has lead to an urgency in assessing the conservation status of many species. As such, it is the aim of this thesis to provide detailed distribution information for the green python (Morelia viridis) in Australia. This information will hopefully improve understanding about the current status of this species which should in turn improve the effectiveness of programs designed to secure this iconic Australian reptile into the future.
1.1 The green python
Schlegel first described the green python from the Aru Islands, West Papua, Indonesia in 1872. It was not until 1929, however, that the presence of green pythons in Australia was made known to westerners when it was discovered by the naturalist Donald Thompson (Thompson 1935). Today, green pythons are known to be distributed in Cape York Peninsula, Australia, and throughout New Guinea, including many of its offshore islands (O'Shea 1996; Wilson and Swan 2008). Known as Mo-Idji to the Aboriginal people of Cape York, Arerurang to the Papuan people of New Guinea and affectionately as the Chondro to those who keep them in captivity, this species requires little introduction to traditional peoples or herpetologists.
Distribution of the green python in Australia 13
Green pythons are a relatively small (<2m) rainforest species, and are Australia’s most arboreal python (Greer 1997). In New Guinea they are found in lowland and montane rainforests up to 2000 m above sea level, as well as in secondary regrowth areas (O'Shea 1996). In Australia, green pythons are most frequently recorded in evergreen to semi-deciduous notophyll vine forest and are thought to have strong associations with bamboo thickets (Thompson 1935; Covacevich et al. 1982). The sexes are broadly similar in body size. However, dissection of a small number of museum specimens indicated the minimum length at sexual maturity as 86 cm snout to vent length (hereinafter SVL) in males and 99 cm SVL in females (Shine and Slip 1990). Based on growth rate data, it is predicted that males mature at roughly 2.4 years and females at 3.6 years (Wilson et al. 2006a). Captive specimens are known to live in excess of 20 years (Maxwell 2005). Field studies in Australia by Wilson et al. (2006b) have shown that average movements are roughly equal between the sexes. However, females have a defined home range of 6.21 ± 1.85 ha while males appear not to have a definable home range. Little is known of the reproductive biology of green pythons in the wild. However, extrapolation from the time when small individuals are found suggests hatching occurs in late November (Wilson et al. 2006a).
Perhaps the most extraordinary aspect of green python biology is their fascinating ontogenetic colour change. Juveniles are born either bright ‘banana’ yellow or a mauve ‘brick’ red and in Australia have been shown to change to brilliant green (Fig. 1) at around 60 cm in length (Wilson et al. 2006a). To date, only the yellow juvenile phase has been recorded in Australia, with red juveniles being restricted to northern New Guinea (unpub. data).
Distribution of the green python in Australia 14
General green python, Where they are found, colour change
Figure 1. The three colour phases of the green python. Green pythons are born
either red or yellow and change to green.
In Australia, it has been suggested that these colours have allowed adult and juvenile snakes to utilise distinct microhabitats and therefore a different suite of prey (Wilson et al. 2007). Wilson et al. (2007) found that juveniles hunt during the day for diurnal lizards in rainforest edges and canopy gaps. It has also been suggested that yellow provides relative camouflage from predators in this sun- dappled environment. Conversely, adult green pythons ambush terrestrial rodents in closed canopy rainforest at night, spending the day resting in the canopy where green provides greatest camouflage (Wilson et al. 2007).
It is these outstanding colours and the bizarre colour change that have captured people’s imaginations and have resulted in the high demand for this species in captive collections (Kivit and Wiseman 2005; Maxwell 2005). Due to this demand, green pythons are also taken illegally from the wild in large numbers (Auliya 2003; Maxwell 2005; TRAFFIC 2009; Fig. 2). In Australia, poaching from the wild for the captive pet trade is a major enforcement issue (Rawlings and Donnellan 2003). Although it is unknown how many green pythons have been taken from the wild, their high market value and potential for over-exploitation has prompted the need for accurate ecological knowledge of this species.
Distribution of the green python in Australia 15
Figure 2. Green pythons are collected in New Guinea by poachers and shipped overseas in large numbers.
1.2 Green pythons in Cape York Peninsula
Like their rainforest habitat, green pythons are widespread in New Guinea (McDowell 1975; O'Shea 1996). However, in Australia they are restricted to small patches of rainforest in Cape York Peninsula (Wilson and Swan 2008). The current distribution of rainforest on Cape York is a function of a combination of historical geographic and climatic processes. During the glacio-eustatic cycles of the Pleistocene, a warmer, wet climate was accompanied by high sea-levels, whereas periods of relative aridity saw decreased sea levels (Nix and Kalma 1972). During periods of low sea level, the Sahul Shelf (a submerged section of the continental Sahul Shelf between Australia and New Guinea) was exposed, connecting northern Australia to southern New Guinea (Fig. 3). Indeed, a lowered sea-level of only 10 m, was sufficient to expose the 150 km wide Torres Strait and create a ‘bridge’ connecting New Guinea to Cape York Peninsula. This bridge was present for 91% of the last 250,000 years (Chappell and Shackleton 1986; Voris 2000)
Distribution of the green python in Australia 16
Figure 3. The present day land boundaries (dark grey) of Australia and New Guinea and the extent of land (pale grey) following a 50 m drop in sea level (Voris, 2000).
At specific periods during this time rainfall was sufficient to support a belt of rainforest that linked the rainforests of Cape York and New Guinea allowing biotic interchanges between Queensland and New Guinea (Nix and Kalma 1972; Kikkawa et al. 1981; Flannery 1988). Rainforests currently cover only 5.6% of Cape York’s area (Neldner and Clarkson 1995) and do not occur as a contiguous block, each being divided by areas of open habitat (Fig. 9). Of the five main rainforest areas in Cape York (Fig. 4), green pythons have been reliably recorded from only the Iron and McIlwraith ranges. There are also anecdotal reports of this species from the rainforests of the Lockerbie Scrub (Waldren 1996; Kend 1997; S. Templeton pers. comm. 2009) and Jardine River Catchment (Anonymous pers. comm. 2007), however, these are yet to be substantiated.
The majority of green pythons are recorded from the rainforests of the Iron Range. This may be because the Iron Range offers the most suitable habitat and therefore harbours the greatest abundance of green pythons or, it may be a reflection of the ease of access. The Iron Range area can be accessed relatively easily and is well known for
Distribution of the green python in Australia 17
its diversity of animal species, being visited by many amateur naturalists and tourists each year. Because of this, the Iron Range National Park has been the site of numerous scientific studies (Heinsohn and Cermak 2008) and its flora and fauna have been extensively surveyed (Leung et al. 1994; Abrahams et al. 1995). Conversely, access to other areas of rainforest habitat in Cape York, such as the McIlwraith Range and the Jardine River Catchment, is relatively limited and this may be why there is a lack of green python records from these locales.
1.3 Previous estimates of distribution, abundance and habitat use
Wilson and Heinsohn (2007) predicted the distribution of green pythons in Cape York by combining a small number of records with climatic modelling and vegetation mapping. Records were obtained from the Lockerbie Scrub, Iron Range and McIlwraith Range. Climatic modelling estimated green pythons to have a range of 300 km2 centred on the Iron Range area, whereas vegetation mapping increased the predicted area of occupancy to 3127 km2, the majority of which was in the Iron- McIlwraith Range region.
In addition, surveys in the Iron Range National Park by Wilson and Heinsohn (2007) provided the first estimates of the abundance of green pythons in Australia. The majority of snakes recorded in their study were found in evergreen to semi-deciduous notophyll vine forest (regional ecosystem 3.12.3), where green pythons were estimated to have a density of 400 - 500 km-2. However, green pythons were also found to be present in rainforest regrowth, and beach vine thicket.
Additional occurrence records obtained by Wilson and Heinsohn (2007) were overlain onto regional ecosystem maps for Queensland and yielded another six habitats that were potentially suitable for green pythons. Importantly, the majority of these habitats were woodlands, suggesting, for the first time, the use of marginal, non-rainforest vegetation by this species.
Distribution of the green python in Australia 18
1.4 Aims
Although we now have the predicted distribution range of green pythons in Cape York, it is not known if, and how, green pythons are distributed within their estimated area of occupancy. Similarly, green pythons may be at their highest densities in the Iron Range region, having very small or no populations in other areas, or, they may be uniformly distributed throughout suitable habitat in Cape York. This knowledge gap makes the long term conservation management of green pythons difficult because effective protection strategies cannot be adequately inferred from insufficient data (Grumbine 1990; Rodrigues et al. 2006). Because it is unknown to what extent green pythons are distributed, both geographically and spatially, in terms of habitat use, relative abundance and population demography, I aimed to complement the work of Wilson and Heinsohn (2007) and provide this information for the whole of the Australian range of the species.
1.4.1 Specific aims
There are four specific aims that this thesis addresses:
1) to test the distribution predictions of Wilson and Heinsohn (2007) and provide more accurate estimates of geographic range based on extensive surveys;
2) to ascertain by field work the different habitats used by green python and understand which habitats appear to be the most suitable for this species;
3) to understand the relative abundance and population demography of green pythons in the Lockerbie Scrub, Iron Range and McIlwraith Range; and
4) to use these data to provide recommendations about how to optimise and implement the most effective conservation programs.
Distribution of the green python in Australia 19
2.0 METHODS
2.1 Study Sites
The study area extended from the tip of Cape York Peninsula to the southern extremity of the McIlwraith Range (Fig. 4). Temperatures are high year round and most rain falls between December and April. For most of the rest of the year it remains dry (Frith and Frith 1995). Most of the peninsula is flat and the dominant vegetation is tropical woodland. On the east coast, however, the northern outlier of the Great Dividing Range adds topographic, climatic and biological diversity to the region, which receives enough year-round rainfall to sustain tracts of complex vine forest (Luly et al. 2006)
The vegetation of Cape York has been classified by Neldner and Clarkson (1995) and correlated with a distinct combination of geology and landforms to form regional ecosystems (Sattler and Williams 1999). Because green pythons are most frequently recorded from rainforest, topographic and regional ecosystem maps for Cape York were used to identify habitats potentially suitable for green pythons. Suitable habitat considered large enough to sustain a green python population was identified in the following five locations (Fig. 4).
2.1.1 Lockerbie Scrub
This is Australia’s most northerly rainforest. The surrounding area is dominated by tropical woodlands with approximately 130 km2 of semi-deciduous notophyll vine forest on the Carnegie Range (Neldner and Clarkson 1995). Rainforest grades into deciduous vine thickets and open forests surrounding perched dune lakes near Newcastle Bay and discrete areas of beach rainforest occur on sheltered dunes on the periphery of the Carnegie Range. The Lockerbie rainforests receive a mean annual rainfall of 1744 mm (Bureau of Meteorology 1887-1955).
Distribution of the green python in Australia 20
Figure 4. The distribution of closed forests and the five sites surveyed for green pythons on Cape York Peninsula.
Distribution of the green python in Australia 21
2.1.2 Jardine River Catchment
The Jardine River rainforests are distributed in patches between 11˚ 40’ 50”S and 11˚ 00’ 00”S and receive a mean annual rainfall of 1682 mm (Bureau of Meteorology 1887-1929). The most highly developed vine forest, simple notophyll/microphyll vine forest, is located on the upper slopes and broad ridge tops in the east of the study area grading into sclerophyll scrubs and heaths on shallower or low fertility sands (Lavarack and Stanton 1977). These forests contain numerous palms that, in parts, form extensive areas of mesophyll palm forest dominated by the Fan Palm Ptychosperma macarthurii (pers. obs.).
2.1.3 Iron Range
The Iron Range rainforests are intermingled with various woodland and heath communities and evergreen to semi-deciduous notophyll vine forest is the predominant vine forest type. Higher elevations, and the granite slopes of the Tozer and Dorriwill ranges, support a simple evergreen notophyll vine forest, while beach vine thicket grows in coastal areas on the periphery of complex vine forest (Stanton and Fell 2005). The mean annual rainfall at this site is 2112 mm (Bureau of Meteorology 1956-2010).
2.1.4 Kawadji-Ngaachi Ranges
This series of coastal ranges includes the Hemming, High, Chester, Meston, Adam, Howard and Macrossan ranges. The Kawadji-Ngaachi Ranges run parallel to the Iron and McIlwraith ranges from the mouth of the Nesbit River in the south (13˚ 30 00) to the mouth of the Lockhart River in the north (13˚ 00 00). They are separated from the Iron and McIlwraith ranges by the lowlands of the Lockhart River Valley and are also separate from one another. The steep slopes of each range are covered with a mix of tropical woodland and low, windswept, evergreen to semi-deciduous notophyll vine
Distribution of the green python in Australia 22
forest, becoming exclusively vine forest on the higher ridges. There are no rainfall data for this area.
2.1.5 McIlwraith Range
The McIlwraith Range is a large granite plateau rising to 824 m. Evergreen to semi- deciduous notophyll vine forest is the dominant vegetation type on the plateau, with mesophyll vine forest predominating in the higher rainfall areas (Webb and Tracy 1981). The vine forests on the higher rainfall east coast extend to the foot of the range whereas those on the west taper to varying degrees of deciduousness (Lavarack et al. 1990). The mean annual rainfall for the McIlwraith Range is 1684 mm (this record covers only four years and was taken from a site on the dryer western slope of the range so is not likely to be representative of the entire study area (Lavarack et al. 1990)).
Distribution of the green python in Australia 23
2.2 Survey methods
2.2.1 Presence/absence surveys
Between November 2007 and March 2010, surveys were conducted for green pythons in all areas of potentially suitable habitat in Cape York Peninsula (Fig. 4). Each site was surveyed over two nights and if green pythons were not located they were deemed to be absent. The distances walked during surveys at each separate location depended on the availability of suitable habitat. Surveys were stratified to concentrate on areas of closed forest and their immediate surroundings and were conducted during the wet season (December to April) when green pythons are most easily located. All surveys were made on foot, beginning after 2000 hrs and snakes were located with the aid of a hand-held spotlight. Most green pythons were found near the ground while hunting; however a few were also located resting or hunting in trees up to 4 m above the ground. The position of each snake found was recorded using a Global Positioning System.
2.2.2 Intensive surveys
Intensive surveys were carried out in areas where green pythons had previously been recorded (Wilson and Heinsohn 2007). These included the rainforests of the Lockerbie Scrub, Iron Range and McIlwraith Range. Each site was surveyed during the wet season between December and April and transects were set up in the most common and accessible habitats at each site. Emphasis was placed on selection of vine forest habitats having associated open forest areas to establish habitat preference.
Transect numbers and lengths differed between each habitat type due to logistic constraints (Table 1). To enable calculation of the survey area, the width of each transect was determined by averaging the distance at which a green python could reliably be sighted from the centreline at 200 randomly distributed points within each habitat type (Table 1).
Distribution of the green python in Australia 24
Each transect was surveyed at one week intervals and all transects were surveyed within a one week period, or cycle. The times taken to traverse each transect were recorded to determine encounter rates and the distance of each snake from the transect centreline measured. All green pythons were captured by hand and marked using a combination of scale clipping (Brown and Parker 1976) and insertion of a Passive Integrated Transponder tag (Gibbons and Andrews 2004) so that individuals could be identified if relocated. Individual green pythons were measured to the nearest 0.1 cm using a steel ruler and the sex was established by cloacal probing. Juvenile green pythons were not sexed to avoid potential injury. Following their observation and measurement, all snakes were released immediately at the point of capture.
2.3 Analysis
The open-population Jolly-Seber method (Pollock et al. 1990) was used to analyse the recapture histories of each green python population surveyed. This method was chosen because it provided population estimates at the beginning of the study and allowed comparison with the estimates of Wilson & Heinsohn (2007) at their Iron Range study site. It was assumed that both marked and unmarked snakes had the same probability of capture and that probability of survival was the same for all animals. Following the classification of Wilson et al. (2006a), green pythons were divided into three groups based on colour and sexual maturity. Yellow individuals were classed as juveniles; green individuals smaller than the size at sexual maturity (males 86 cm SVL, females 99 cm SVL, Shine and Slip 1990) were classed as immature and green individuals larger than the minimum size at sexual maturity were classed as adults.
Distribution of the green python in Australia 25
Table 1. Transect summary and survey effort at the three sites intensively surveyed for green pythons in Cape York
Number Total Transect Survey Time Date Regional Study area Vegetation description of transect width area searched Surveyed Ecosystem transects length (km) (m) (km2) (hrs) Semi-deciduous notophyll vine forest on lateritic Carnegie 3.5.3 8 21.4 20.4 0.44 89.4 Tableland Lockerbie Evergreen notophyll vine forest on coastal dunes and beach 2007/08 3.2.1 1 1 15.1 0.02 8 Scrub ridges Corymbia novoguinensis or C.nesophila ± C.tessellaris 3.5.5 3 2.1 30 0.06 13.7 woodland on sandplains Evergreen to semi-deciduous notophyll vine forest of the slopes 3.12.3 10 9.1 13.8 0.13 171.6 of the McIlwraith Uplands Deciduous vine thicket dominated by Cochlospermum gillivraei, McIlwraith 2008/09 3.12.21 Canarium australianum and Acacia aulacocarpa on granite 1 2.1 30 0.06 14 Range slopes Eucalyptus cullenii and Corymbia clarksoniana woodland on 3.12.10 2 0.6 30 0.02 8.1 acid volcanic ranges Evergreen to semi-deciduous notophyll vine forest of the slopes 3.12.3 7 12.8 15.1 0.19 166.6 of the McIlwraith Uplands Low microphyll vine forest dominated by Acacia crassicarpa 2009/10 Iron Range 3.2.11 1 1.6 14 0.02 23.3 and Syzygium banksii on coastal dunes Eucalyptus tetrodonta ± Corymbia clarksoniana woodland on 3.3.31 1 1 30 0.03 11.6 coastal plains
Distribution of the green python in Australia 26
3.0 RESULTS
A total of 301 green pythons where located in three broad areas of Cape York Peninsula. Most were captured during intensive wet season surveys. However, 25% were found either opportunistically or during presence/absence surveys. Two of the records were obtained from the Queensland Museum and one from an anonymous naturalist.
Figure 5. The distribution of habitats suitable to green pythons (light grey) and survey sites where green pythons were found to be present (●) or
absent (噘). Distribution of the green python in Australia 27
3.1 Distribution
Green pythons were found in suitable habitat from the Pascoe River in the north to the southern extremity of the McIlwraith Range (Fig. 5). They were not found in either the Lockerbie Scrub or the Jardine River Catchment. Elevations where green pythons were found varied from 1 m above sea-level (a.s.l.) in the Iron Range to 681 m a.s.l. in the McIlwraith Range. They inhabit 14% of the regional ecosystems surveyed in Cape York (Table 2) and 50% of those in the Iron-McIlwraith Ranges region and every closed forest ecosystem (Fig. 6), but were never located in woodlands, swamp, heath or grasslands (Appendix A).
Figure 6. The distribution of suitable habitats in the Iron-McIlwraith Range region (light grey).
Presence (●) and absence (噘) sites are mapped without rainforest creek lines (regional ecosystem 3.3.1) (a) and presence sites are mapped showing connecting creek corridors (dark grey) (b).
Distribution of the green python in Australia 28
Table 2. Area of each habitat type in which green pythons were found on Cape York and their extent in protected areas. (IR = Iron Range; MR =McIlwraith Range; KN = Kawadji-Ngaachi Ranges).
AExtent AExtent within AExtent within Regional Habitat Sites within the the Iron- Description ecosystem type present Iron Range McIlwraith McIlwraith NP (*) Range NP (*) Range region Simple evergreen notophyll vine forest with Acacia spp. or 3.11.3 Rainforest Eucalyptus spp. emergents on exposed metamorphic and granitic IR, MR 48(14) 142(41) 347 slopes Deciduous vine thicket dominated by Cochlospermum gillivraei, 3.12.21 Vine thicket IR, MR 20(11) 109(57) 190 Canarium australianum and Acacia aulacocarpa on granite slopes Evergreen to semi-deciduous notophyll vine forest of the slopes of 3.12.3 Rainforest All 314(24) 633(49) 1304 the McIlwraith Uplands Low microphyll vine forest dominated by Acacia crassicarpa and 3.2.11 Vine thicket IR, KN 18(20) 23(25) 92 Syzygium banksii on coastal dunes Semi-deciduous mesophyll vine forest on alluvia, and metamorphic 3.3.1 Rainforest All 28(8) 164(46) 356 and granitic foothills and lower hillslopes Total of all vegetation types 428(19) 1071(47) 2289
AAll values in km2; taken from Neldner and Clarkson (1995). *Percentage of the total extent of each vegetation type found in the respective protected area
Distribution of the green python in Australia 29
Figure 7. Two examples of suitable habitats in which green pythons were found. Evergreen to semi-deciduous notophyll vine forest (left) and deciduous vine thicket (right) – both found in the McIlwraith Range.
Distribution of the green python in Australia 30
Assuming their occurrence in all suitable habitats within the Iron-McIlwraith Range region, green pythons have a potential distribution area of 2289 km2. The evergreen to semi-deciduous notophyll vine forest (regional ecosystem 3.12.3) appears to be the preferred habitat for this species. In surveys of this habitat, green pythons were found every time but once. The Iron and McIlwraith ranges’ National Parks include 73% of this preferred habitat and 66% of all the regional ecosystems where green pythons were located are protected within these two Parks (Table 2).
3.2 Density, abundance and habitat preference
There was a marked difference between the relative abundance of green pythons between each of the sites intensively surveyed (Table 3). In total, 111 person-hours were spent searching in three habitat types in the Lockerbie Scrub but no green pythons were found. The survey method was changed after eight cycles and the rainforest was searched randomly for a further 78 person-hours but despite this extra effort, no snakes were discovered.
The time spent searching in the Lockerbie Scrub was very much longer than it took to locate one green python in the areas where they did occur. Assuming that the densities of green pythons at Lockerbie Scrub may be similar to those in the McIlwraith Range, there is less than 1% chance, based on encounter rates, that green pythons were overlooked in the Lockerbie Scrub (χ2 = 164, df = 1, P = <0.001).
The average distances that green pythons were observed from the transect in the Iron and McIlwraith ranges were similar (3.5 m, range 0-14.7 m; 3.2 m, range 0-14.1 m, respectively; χ2 = 0.01, df = 1, P = 0.90) and even after accounting for the difference in survey area (Table 1), green pythons were encountered more frequently in the Iron Range than in the McIlwraith Range (χ2 = 32.1, df = 1, P = <0.001, Table 3). In the McIlwraith Range, the density of green pythons was determined for evergreen to semi- deciduous notophyll vine forest only, as captures in deciduous vine thicket were too few (Table 3). The density of green pythons in the Iron Range was higher in rainforest
Distribution of the green python in Australia 31
than in beach vine thicket and both habitats had higher densities than rainforest in the McIlwraith Range (Table 3). By comparison, green python encounter rates in habitats within the Iron and McIlwraith ranges were approximately equal, however between sites, snakes were encountered more frequently in the Iron Range (Table 3). Despite the significant time spent on intensive surveys of woodlands in both the McIlwraith and Iron ranges, no green pythons were located in this habitat type (Table 3).
Of the snakes encountered in the Iron Range during this study, 11 had been previously tagged during the study of Wilson et al. (2006a). Each of these snakes was larger than 100 cm SVL and comprised 9 females (mean SVL = 127 cm; range = 101-137 cm) and 2 males (mean SVL = 113 cm; range 110.5-115.5 cm).
Distribution of the green python in Australia 32
Table 3. The relative abundance and density of green pythons at the three sites that were intensively surveyed in Cape York.
Population Study Regional Snakes Snakes Vegetation description estimate Snakes/hr Snakes/km2 area Ecosystem captured recaptured (S.E) Semi-deciduous notophyll vine forest on lateritic 3.5.3 0 0 - 0 - Carnegie Tableland Lockerbie Evergreen notophyll vine forest on coastal dunes and 3.2.1 0 0 - 0 - Scrub beach ridges Corymbia novoguinensis or C.nesophila ± C.tessellaris 3.5.5 0 0 - 0 - woodland on sandplains Evergreen to semi-deciduous notophyll vine forest of 3.12.3 47 8 25(±23) 0.32 200 the slopes of the McIlwraith Uplands Deciduous vine thicket dominated by Cochlospermum McIlwraith 3.12.21 gillivraei, Canarium australianum and Acacia 3 1 - 0.29 - Range aulacocarpa on granite slopes Eucalyptus cullenii and Corymbia clarksoniana 3.12.17 0 0 - 0 - woodland on acid volcanic ranges Evergreen to semi-deciduous notophyll vine forest of 3.12.3 131 35 104(±90) 1.00 540 the slopes of the McIlwraith Uplands Low microphyll vine forest dominated by Acacia Iron Range 3.2.11 12 6 7(±7) 0.77 320 crassicarpa and Syzygium banksii on coastal dunes Eucalyptus tetrodonta ± Corymbia clarksoniana 3.3.31 0 0 - 0 - woodland on coastal plains
Distribution of the green python in Australia 33
3.3 Population demographics
The age profile of green pythons differed between study areas (Fig. 8). In the Iron Range, 56% of the snakes captured were adults, similar to the number of immature individuals captured, regardless of sex (χ2 = 2.47, df = 1, P = 0.116). In the McIlwraith Range, however, 83% of snakes were adults, outnumbering immature individuals of both sexes (χ2 = 16.9, df = 1, P = < 0.001). Furthermore, the proportion of snakes captured over 120 cm SVL was different between sites (Iron Range = 15%; McIlwraith Range = 32%; χ2 = 6.1, df = 1, P = 0.01). In the Iron Range, juveniles were captured less often than either immature or adult individuals (χ2 = 33.5, df = 2, P = 0.001), however, in the McIlwraith Range they were captured less often than adults only (χ2 = 28.7, df = 2, P = 0.001; Fig. 8).
a)
Distribution of the green python in Australia 34
b)
Figure 8. The age class distributions of female (black columns), male (white columns) and juvenile (grey column) green pythons captured in the McIlwraith (a) and Iron ranges (b).
Distribution of the green python in Australia 35
4.0 DISCUSSION
4.1 Distribution
The surveys completed in the current study have shown that the green python, a species long thought to be rare, is relatively common and widespread within suitable habitat in Cape York Peninsula. Green pythons are distributed in closed forest habitats in the Iron-McIlwraith Ranges region and have been recorded for the first time in the coastal Kawadji-Ngaachi Ranges. Within the Iron-McIlwraith Ranges region there are 2289 km2 of suitable habitat (Table 2). This is a significantly larger area than the bioclimatic prediction of Wilson and Heinsohn (2007), but is less than their predicted distribution based on regional ecosystems. The difference in the area of regional ecosystems used between the two studies arises due to Wilson and Heinsohn (2007) overlaying green python records onto four woodland regional ecosystems. In the present study green python were never recorded from woodlands (Appendix A).
Green pythons appear to have a strong association with evergreen to semi-deciduous notophyll vine forest in the Iron-McIlwraith Ranges region. During presence/absence surveys, there was only one occasion when a green python was not located in this habitat, although the site was connected by suitable habitat to nearby areas where snakes were located. By contrast, green pythons were not found in surveys of 35 regional ecosystems at more than 100 locations, including woodland, swamp, grassland and heath (Appendix A).
The rainforests of the Iron and McIlwraith ranges are functionally separated by woodlands on the Ngalapichi plateau (pers. obs.). This is in contrast to the presumption of rainforest connectivity between the two sites (Legge et al. 2004). Similarly, the rainforested Kawadji-Ngaachi Ranges are separated from one another and from the larger Iron and McIlwraith ranges by the grasslands and woodlands of the Lockhart and Nesbit River valleys (pers. obs.).
Distribution of the green python in Australia 36
Figure 9. Two examples of rainforest vegetation on the McIlwraith and Kawadji-Ngaachi ranges, showing unsuitable grass and woodland habitats which limit the dispersal of green pythons between suitable habitats in Cape York.
Distribution of the green python in Australia 37
It is possible that green pythons use riparian creek corridors to move between rainforest blocks (Fig. 6), as is the case with other rainforest dependant fauna (Jansen 2005; pers. obs.). However, while green pythons were observed in creek edge habitat in this study, they were never found at a greater distance than 500 m downstream from substantial rainforests. Furthermore, no green pythons were found during presence/absence surveys in closed forest habitat on the Lockhart River, Nesbit River and Falloch Creek tributaries, which potentially link rainforest sites, suggesting that dispersal between these three rainforest areas is low.
Green pythons were found on the northern bank of the Pascoe River. However, they were not found in any habitats further north (Fig. 5). Small pockets of rainforest occur around Temple and Shelburne bays, however, fauna surveys have not found green pythons (Leung 1993; Winter and Lethbridge 1995). There is also a small area of suitable rainforest habitat in the vicinity of Carron and Kennedy hills directly north of the Iron Range (Fig. 6). Surveys conducted as part of the present study did not find green pythons in this area, although their occurrence cannot be ruled out given that they were found on the northern bank of the Pascoe River. This would, however, indicate only a very minor range extension.
Despite anecdotal records of green pythons in the rainforests of the Jardine River Catchment and the Lockerbie Scrub, surveys at these sites have never found this species (Grant and Leung 1993; Cohen 1994; Grant and Leung 1994). Further, given the level of search effort undertaken at the Lockerbie Scrub during this study, there is less than 1% chance that green pythons were overlooked. Wilson and Heinsohn (2007) postulated that the Lockerbie rainforests may be too small to support a viable, long- term green python population. However, the Lockerbie rainforests are significantly larger (134 km2) than any of the isolated areas within the Kawadji-Ngaachi Ranges where green pythons are present and many of the small islands of New Guinea (unpub. data), so the absence of this species from the Jardine and Lockerbie areas appears not to be a function of present day forest area.
Distribution of the green python in Australia 38
This begs the question; why are green pythons absent from these sites? Each contains seemingly suitable habitat and both are situated between New Guinea and the Iron- McIlwraith Ranges region where green pythons are present. The answer can be inferred from the historical biogeography of the Australo-Papuan region. A land connection between Australia and New Guinea was in place as recently as the late Pleistocene (Voris 2000). Although these areas shared a land connection, the expansion of rainforest vegetation beyond its current extent has occurred only briefly during periods of climatic suitability (Nix and Kalma 1972; Nix and Kershaw 1988; Kershaw et al. 2003). The difference between rainforest flora (Webb and Tracy 1981; Crisp et al. 2001) and fauna (Kikkawa et al. 1981) in the Wet Tropics and Cape York Peninsula suggest at least two recent rainforest connections across what is now Torres Strait. The first enabled the dispersal of species from New Guinea to the Wet Tropics (and vice versa) while the second, more recent connection evidently enabled an interchange of species only as far as the southern McIlwraith Range.
Various authors have postulated that these rainforest connections persisted relatively recently (within the last 10000 years; Nix and Kalma 1972; Winter 1997). Palynological evidence from the Wet Tropics and Gulf of Carpentaria suggested that within the last 250000 years there have been only two periods when rainforests in northern Australia may have developed beyond their current extent (Kershaw 1978, 1983, 1994; Hopkins et al. 1996; Chivas et al. 2001). The first occurred between 120000 and 130000 ybp (Kershaw 1978; Kershaw et al. 2007a) and the second between 5900 and 6500 ybp (Kershaw 1994; Hopkins et al. 1996). However, it is improbable that green pythons, and other rainforest species, were permitted to migrate between Australia and New Guinea at these times because both were accompanied by the submergence of the Torres Strait land-bridge during a period of elevated sea-level (Geyh et al. 1979; Chappell and Shackleton 1986; Collins et al. 2006; Woodroffe 2009; Yu and Zhao 2010). In addition, recent palynological studies have found that, during the Holocene at least, rainforests in Cape York and Torres Strait would have been distributed as intermittent patches only, and were unlikely to have been more extensive than present day (Luly et al. 2006; Rowe 2007a; Rowe 2007b).
Distribution of the green python in Australia 39
Consequently it appears that rainforest species residing in Australia and New Guinea have been separated for considerably longer than previously thought (McGuigan et al. 2000; Westerman et al. 2001; Hocknull et al. 2007; Norman et al. 2007; Macqueen et al. 2010). Specifically, genetic examination has revealed that green pythons from southern New Guinea share a more recent connection with con-specifics from the Aru Islands than from Australia (Rawlings and Donnellan 2003; T. Morris pers. comm. 2010). These results appear unusual, as a 10 m drop in sea-level is sufficient to connect Australia to New Guinea, whereas the sea-level must fall by more than 40 m to connect the Aru Islands to the New Guinea mainland (Fairbanks 1989; Voris 2000). This result is, however, consistent with the need of green pythons for rainforest habitat. During periods of low sea-level, rainforests were likely to have been more extensive on an Aru connection than on a Torresion connection as rainforest in New Guinea became less fragmented during glacial periods than in Australia (Hope and Tulip 1994; van der Kaars et al. 2000; Hope et al. 2004; Kershaw et al. 2007b).
Thus, if green pythons have been present in Australia since 250000 ybp then they must have been extirpated from the Lockerbie and Jardine River areas, surviving only in the Iron-McIlwraith Ranges region. Based on this information it can be concluded that the absence of green pythons in the rainforests of the Lockerbie Scrub and Jardine River is because these rainforests disappeared completely during the glacial periods since 250000 ybp. This fits with the findings of other studies which have shown that Pleistocene rainforest contraction into moist refugia has resulted in the vicariance and extinction of other rainforest taxa in north Queensland (Dodson 1989; Schneider and Moritz 1999; Hocknull et al. 2007). Unlike the Iron-McIlwraith Range region, the Lockerbie Scrub and Jardine River Catchment areas have relatively low topography and probably did not act as moist refugia during times of rainforest contraction (Webb and Tracy 1981). The absence of green pythons in the Lockerbie Scrub and Jardine River Catchment is consistent with that of other rainforest-dependant fauna which do not occur at these sites (Legge et al. 2004). It appears that if green pythons did disperse into Australia prior to 250000 ybp then the rainforests of the Iron-McIlwraith Range region must have offered sufficiently large refugia to withstand the glacial rainforest contractions of the Pleistocene.
Distribution of the green python in Australia 40
4.2 Population characteristics and habitat use
Green pythons appear to have stable populations in Cape York occurring at relatively high densities. The population estimates, encounter rates and total number of snakes caught show that green pythons are more abundant in the Iron Range than in the McIlwraith Range. This phenomenon has been shown in other snake species over similarly small areas (Parker and Plummer 1987). The density estimates in notophyll vine forest given in this study for the Iron Range area are very close to the estimates of Wilson and Heinsohn (2007) for the same region. The low recapture rate is probably due to green pythons not spending large amounts of time on the ground (Wilson et al. 2006b). This may explain the estimates given by the Jolly-Seber method, which have large associated standard errors because this model is highly sensitive to low recapture frequency (Mazerolle et al. 2007).
The disparity between the abundance of green pythons in notophyll vine forest in the Iron and McIlwraith ranges is noteworthy. The only obvious difference between the two sites is altitude. The altitudes of the Iron Range study site ranged from 0 to 50 m while the McIlwraith site ranged from 350 to 681 m and it may be that elevation has a role in limiting green python abundance. In agreement with this idea, Henderson and Henderson (1995) found that encounter rates of the tropical and arboreal Corallus hortulanus were significantly lower above than below 400 m. By contrast, green pythons in this study were found to be equally abundant between 350 m and 681 m in the McIlwraith Range and are known to occur up to 2000 m elevation in New Guinea (McDowell 1975; O'Shea 1996). It seems improbable, therefore, that an increased elevation of only 300 m would be the cause of this discrepancy.
The relative abundance of snakes in an area is often correlated with the abundance of suitable prey species (Arnold 1972); this has been shown to be true in Australian ecosystems (Bonnet et al. 2002b; Madsen et al. 2006). It may be that the Iron Range has higher densities of prey species than the McIlwraith Range. Although there are abundance estimates for three known prey species in the Iron Range (Antechinus leo, Melomys capensis and Rattus leucopus; Leung 1999a,b,c), we do not have estimates of prey species for the McIlwraith and, therefore, cannot test this hypothesis.
Distribution of the green python in Australia 41
The recapture of 11 of the 207 snakes marked from the study of Wilson et al. (2006a) appears low given that green pythons are relatively long-lived (Maxwell 2005; Wilson et al. 2006a) and because females occupy a home range (Wilson et al. 2006b). Alternatively, it may be that inter-annual dispersal of green pythons is greater than seasonal movements, or, that mortality is relatively high, or both. The disparity in captures of males and females seems to corroborate the findings of Wilson et al. (2006b) who showed that females occupied home ranges while males do not, increasing the likelihood of recaptures in the same areas over long periods of time. In contrast however, one of these females was located more than 10 km south of where Wilson et al. (2006a) conducted their study, and would therefore be a significant distance outside her calculated home range (Wilson et al. 2006b).
Figure 10. Densities of green pythons are not actually this high. These two are obviously onto something good. Seemingly oblivious to one another, they have set up ambush less than 50 cm apart.
Distribution of the green python in Australia 42
Consistent with the findings of Wilson and Heinsohn (2007) in the Iron Range National Park, this study found that green pythons occurred at their highest densities within notophyll vine forest. Green pythons were, however, also present in vine thickets adjacent to rainforest. Although abundance between habitats reflects search effort rather than preference, in the McIlwraith Range green pythons were found only on sections of vine thicket transects that were in close proximity to rainforest. Green pythons were never found at a distance of more than 1 km from this habitat type. The vine thickets in Cape York are relatively open and species-poor compared to nearby rainforests (Stanton and Fell 2005) and it may be that green python ambush sites and prey species are proportionately scarce. By contrast, the 12 snakes found in vine thicket in the Iron Range were all of varying sizes, including juveniles, suggesting a permanent breeding population and not merely emigrants from nearby areas.
In both the McIlwraith and Iron ranges, females outnumbered males where sex could be determined; however this difference in numbers is not considered to be significant (Fig. 8). These findings do not appear similar to those of Wilson et al. (2006a) who reported significantly more immature females than males (63 females; 4 males). There is no known difference in habits or life history of the sexes of immature green pythons that would explain this difference (Wilson et al. 2006a; Wilson et al. 2006b). While the sex divergence in green pythons found in this study reflects the sex difference in body size at sexual maturation, it does not explain the difference found by Wilson et al. (2006a) for immature snakes which, as suggested in their study, may have been due to incorrect sex determination.
Juvenile green pythons were captured relatively infrequently on surveys (Fig. 8). This is probably a reflection of the short life cycle duration spent within this size class and may also be due to differentiation in habitat use by yellow and green snakes (Wilson et al. 2007).
The difference between age profiles in the McIlwraith and Iron Range samples is noteworthy. The results from the Iron Range fit with those of Wilson and Heinsohn (2007), with only ~50% of the sample population being sexually mature. In the McIlwraith , however, the higher proportion of adult snakes is significant and the number of snakes over 120 cm SVL in the sample indicates a greater number of large individuals at this site. Because the Iron Range National Park is the only easily
Distribution of the green python in Australia 43
accessible part of the green python’s range, it may be that collection for the pet trade in this specific area has resulted in the population demographic being skewed towards immature snakes. The number of green pythons taken from the Iron Range is unknown; however, a previously high market value, and resulting collection, may have caused a localised depletion in the adult population around the Gordon’s Creek area in Iron Range National Park. Given a probable non-annual reproduction and potentially slow emigration rate (Wilson et al. 2006a,b), it may be that we are only now observing the recovery of this population as witnessed by the high proportion of immature snakes. Skewed population structures are in accord with observations for other reptile populations recovering from human predation (Reina et al. 2002) and specifically snakes (Webb et al. 2002; Sasaki et al. 2008). Long term population data from both the Iron and McIlwraith ranges would test which explanation was more likely to be correct. Given the currently low market value of green pythons and logistic difficulties involved in finding them in the wild it appears that future take of this species is likely to be low.
5.0 CONCLUSION
Documentation of changes in the distribution and abundance of species is considered a fundamental requirement for understanding the impacts of past actions and in formulating future management plans (Skelly et al. 2003). This study has provided conservation managers with important baseline data from which to compare future studies and conservation strategies for Australian green pythons.
In conclusion, this work has provided the following important data;
1) Green pythons are found to be present in the Iron and McIlwraith ranges and have been recorded for the first time from the coastal Kawadji-Ngaachi Ranges. Despite anecdotal records, green pythons are absent from the Lockerbie Scrub and Jardine River Catchment.
Distribution of the green python in Australia 44
2) Green pythons favour rainforest habitats and adjacent vine thickets. They were never found in grassland, heath, swamp or woodlands. Based on extensive surveys and the area of each habitat in which they were found, green pythons have a potential distribution range of 2289 km2.
3) Green pythons are more abundant in the Iron Range than in the McIlwraith Range. Estimated density at these sites was 540 per km-2 vs 320 per km-2, respectively.
4) The age structure of green pythons in the Iron Range is skewed towards immature individuals, whereas large, mature snakes comprise the majority of the McIlwraith Range sample.
5) It is postulated that the rainforest contractions of the Pleistocene resulted in the extirpation of green pythons from the Lockerbie and Jardine River Catchment and that green pythons in Cape York and New Guinea have been separated for at least 250000 years.
5.1 Extrapolation of population size
As a result of this study and those that have preceded this one, it is possible to extrapolate the total number of individual green pythons in Australia. This population estimate can be made using the Jolly-Seber method as employed in the current study and that of Wilson and Heinsohn (2007). The Jolly-Seber method attempts to determine the residential population in a given area and is highly sensitive to low numbers of recaptures. As such, population estimates have high standard errors, making extrapolation for conservation purposes somewhat tenuous. Thus, I provide each population extrapolation as a range of values.
Distribution of the green python in Australia 45
Table 4. Extrapolation of density estimates in the Iron and McIlwraith Range National Parks
Protected area SDNVF/km2 Density range/km-2 Extrapolation Source
McIlwraith 633 15 - 369 9500 - 234000 This study Range NP
74 - 1021 23000 - 320000 This study Iron Range NP 314 Wilson and 284 - 624 89000 - 196000 Heinsohn (2007)
Freeman and Borsboom (2006) re-classified the conservation status of Australian green pythons from rare to near threatened based on IUCN criteria. This re-classification was based on the postulation of there being fewer than 3000 mature individuals in the wild. The results of the present study suggest that there are considerably more than 3000 mature green pythons in Australia.
Total population size is extrapolated for the single regional ecosystem for which we have green python density estimates (regional ecosystem 3.12.3; 1304 km2). Given that green pythons may occur at their lowest estimated density of 15 individuals per km-2, estimated population is;
20,000 individuals
When using these figures to determine conservation status, one should take note of the following four caveats.
1) These estimates are from a single regional ecosystem only. There are another 985 km2 of habitat in Cape York where green pythons have been located and this study has shown that within these areas green pythons have been found to be relatively common.
2) In the Iron Range study area of 0.19 km2, 166 green pythons were encountered during the present study and Wilson et al. (2006) encountered 207 green pythons. These figures suggest a density of considerably more than the estimate of 100 per km-2 for the Iron Range population.
Distribution of the green python in Australia 46
3) Because of the relatively sedentary nature and short distances travelled by this species (Wilson et al. 2006), it is unlikely that the high number of snakes caught at each study site were due to individuals emigrating from the broader vicinity.
4) On the 16th and final survey cycle in the Iron Range, after having already marked 122 individual snakes in the 0.19 km2 study area, a further 14 green pythons were encountered only 5 of which were recaptures.
5.2 Threats
Australian green pythons inhabit one of the most remote and inaccessible areas in Australia. Nevertheless, they are subject to various threatening processes. These threats, while not being quantifiable, should be considered when assessing the conservation status of green pythons. They include the following.
5.2.1 Feral animals
Green pythons are sympatric with a variety of feral animal species such as pigs, cattle, horses, dogs and cats. Cattle, horses and pigs damage the soils and vegetation within the rainforest. Pigs in particular root up large areas of soil and create situations that predispose these areas following rain to erosion, becoming bogs and to overall degradation. It is unknown to what extent this has an effect on green pythons although it is likely that it will reduce areas suitable for the ambush hunting strategies of these snakes. Pigs, dogs and cats are the three feral species that may have a direct impact on green pythons. I have observed pigs consuming snakes while in Cape York. Although green pythons have not been one of these species, an omnivorous and opportunistic pig would potentially consume a green python perched low to the ground if found. The same applies to a feral dog or cat. Although direct predation by these feral predators on green pythons may be relatively low, their indirect impact through competition for prey items such as rodents, ground birds, lizards and small marsupials may be considerable.
Distribution of the green python in Australia 47
5.2.2 Road kill
The Iron Range rainforests are in the vicinity of the Lockhart River and Portland Roads communities. As such, an approximately 7 km stretch of suitable habitat is bisected by the Portland Roads Road through Iron Range National Park. Studies have shown that vehicles are a significant cause of mortality and therefore threat to snakes (Bonnet et al. 1999; Row et al. 2007). The green python is a slow moving animal and was often observed crossing this road during the current study. Road-killed green pythons, both adult and juvenile, were observed on six occasions. Due to the low traffic volume on the Portland Roads Road, it is likely that only a small number of green pythons are killed by traffic. Further, Rangers in Iron Range National Park have seen few road- killed green pythons (P. Schroor pers comm. 2010). It should be noted, however, that this road has been recently upgraded. Consequently, increased traffic volume and vehicular speed may increase the mortality of green pythons and other species.
5.2.3 Fire
Fires maintain the boundary between rainforests and open vegetation and are a natural component of the landscape of Cape York Peninsula. As juvenile green pythons are thought to use ecotonal and/or edge habitats (Wilson et al. 2007), fires in nearby sclerophyll and grassland communities’ may result in the mortality of juvenile snakes. This may be especially true for late dry season fires which coincide with the hatching period of green pythons (Wilson et al. 2006a). At this time juveniles are small and may not be able to climb larger and higher branches to escape oncoming fire. Indeed, I have observed a juvenile green python that was killed by fire. It appeared that the snake had made no attempt to escape and was found blackened and burnt, still coiled around its original perch. In addition to direct mortality, green pythons may experience increased levels of predation because fires may alter habitat in a way that facilitates foraging by predators (Catling et al. 2001; Webb and Shine 2007). Webb and Shine (2007) found that a population of small-eyed snakes (Cryptophis nigrescens) from south-eastern
Distribution of the green python in Australia 48
Australia decreased by 48% following wildfire, which the authors attributed to increased predation resulting from changes in habitat. Thus, if juvenile green pythons are restricted to ecotonal and/or edge habitats (Wilson et al. 2007) this, combined with their seasonal reproduction may render this species highly susceptible to fires burnt at the wrong time.
5.2.4 Poaching
Poaching is possibly the greatest threat to green pythons, both in Australia and in New Guinea. Although there has not been a single prosecution for taking wild green pythons in Australia (M. Devery pers. comm. 2010), it is known that Australian green pythons are available in the domestic and international pet trade. To quantify the extent of poaching is virtually impossible (Webb et al. 2002) and as such we do not know how many green pythons have been, or are being, taken from the wild in Australia. Ideally, wildlife authorities should propagate this species in captivity and distribute them amongst domestic keepers, thereby reducing the demand for, and financial incentive to seek, wild-caught animals. However, at the time of writing, it is assumed that poaching of Australian green pythons is minimal. Given the recent reduction in market price and financial and logistical difficulties associated with finding this species in the wild, there is little or no gain to be made by collecting wild specimens.
5.3 Conservation implications
The Australian green python population can effectively be treated as a single conservation unit. Although green pythons occur in three areas which are isolated from one another, each area is characterised by the same core habitat and all three are in close proximity. Furthermore, the majority of habitat suitable to green pythons falls within only three tenures, two of which are National Parks.
Green pythons appear to be resilient to stochastic events and contractions in their area of occupancy. The climate fluctuations and eustatic cycles of the Pleistocene which resulted in significant contraction of suitable rainforest habitat has nevertheless not led
Distribution of the green python in Australia 49
to extinction of this species which seems to be particularly good at surviving environmental threats by persisting in small refuges.
Green pythons satisfy none of the IUCN criteria that would qualify them to be regarded as threatened in Australia (IUCN 2001). Ideally, further surveys in the Iron Range area would shed more valuable light on the dynamics of green python populations, clarifying how the demographic composition of these snakes fluctuates in time. In addition, long term monitoring would provide a more reliable foundation for determining what, if any, changes in policy may be required for effective management of green pythons in the face of future threats such as anthropogenic climate change. Given that this may occur at a rate far faster than natural cycles of climate change, green pythons and other rainforest species may be less able to adjust to the consequent habitat changes than they have been able to do in the past.
Distribution of the green python in Australia 50
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APPENDICES
Appendix A. Regional ecosystems surveyed in Cape York in which green pythons were not found (Sattler and Williams 1999).
Regional Habitat Description ecosystem type Eucalyptus tetrodonta, Corymbia stockeri ± C. nesophila woodland on 3.10.10 Woodland plateaus Corymbia stockeri ± Eucalyptus tetrodonta woodland on hills and erosional 3.11.11 Woodland surfaces Eucalyptus platyphylla and E. leptophleba open forest to woodland on hill 3.11.6 Woodland slopes Eucalyptus cullenii ± Corymbia clarksoniana woodland. On acid volcanic 3.12.10 Woodland ranges Corymbia hylandii subsp. peninsularis ± Welchiodendron longivalve 3.12.11 Woodland woodland on Torres Strait Islands Eucalyptus tetrodonta ± Corymbia nesophila woodland on low hills on 3.12.15 Woodland granites Eucalyptus leptophleba ± Corymbia papuana open woodland on igneous 3.12.17 Woodland hills and ranges Eucalyptus leptophleba, Corymbia clarksoniana woodland to open 3.12.18 Woodland woodland on coastal hills Melaleuca viridiflora ± Neofabricia myrtifolia low woodland on granitic 3.12.26 Woodland ranges Themeda triandra tussock grassland on headlands and islands on acid 3.12.31 Grassland volcanic rocks Trasistional Corymbia clarksoniana ± C. tessellaris open forest on coastal ranges and 3.12.8 forest lowlands
3.2.1 Rainforest Evergreen notophyll vine forest on coastal dunes and beach ridges
3.2.15 Woodland Melaleuca viridiflora and Neofabricia myrtifolia woodland on beach ridges Gahnia sieberiana open to closed heath. Drainage swamps in east coast 3.2.33 Swamp dunefields Melaleuca leucadendra ± M. dealbata open forest. In dune swales, and 3.2.4 Swamp swampy areas Riparian Melaleuca argentea and/or M. fluviatilis ± M. leucadendra open forest. 3.3.10 forest Fringes streams and creeks 3.3.42 Woodland Melaleuca viridiflora low woodland in drainage areas
Riparian 3.3.5 Evergreen notophyll vine forest on alluvia on major watercourses forest Imperata cylindrica ± Mnesithea rottboellioides closed tussock grassland 3.3.57 Grassland on coastal plains Corymbia tessellaris and C. clarksoniana open forest on coastal alluvial 3.3.8 Woodland plains
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Eucalyptus tetrodonta and Corymbia nesophila woodland on sandy gently 3.5.10 Woodland undulating rises and low hills Eucalyptus tetrodonta and Corymbia nesophila woodland on lower slopes 3.5.11 Woodland of plains and rises Melaleuca viridiflora and Asteromyrtus brassii woodland on flat sand 3.5.13 Woodland plains Melaleuca viridiflora ± Acacia spp. ± Asteromyrtus symphyocarpa low 3.5.14 Woodland woodland on scattered coastal sand plains 3.5.17 Woodland Melaleuca stenostachya ± M. viridiflora low open woodland on flat plains
Melaleuca viridiflora and M. stenostachya low open woodland on flat 3.5.18 Woodland plains Asteromyrtus lysicephala and Choriceras tricorne open heath on sand 3.5.19 Heathland sheets Eucalyptus tetrodonta, Corymbia nesophila tall woodland on deeply 3.5.2 Woodland weathered plateaus and remnants Trasistional Simple evergreen notophyll vine forest with Eucalyptus pellita on 3.5.21 forest sandstone plateaus Semi-deciduous notophyll vine forest. Restricted to lateritic Carnegie 3.5.3 Rainforest Tableland Themeda arguens and Dichanthium sericeum closed tussock grassland on 3.5.30 Grassland low undulating rises Corymbia novoguinensis ± C. tessellaris woodland on northern Cape York 3.5.5 Woodland Peninsula Eucalyptus tetrodonta ± Corymbia clarksoniana woodland. Mainly occurs 3.5.7 Woodland on sand plains Eucalyptus tetrodonta and Corymbia hylandii subsp. peninsularis 3.5.8 Woodland woodland on rises and erosional plains Semi-deciduous notophyll/microphyll vine thicket on isolated lateritic 3.7.1 Rainforest hillslopes