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Taxonomy and Ecology of Pseudophryne australis


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Posted by Richard Wells on March 17, 2002 at 20:46:25:

Hello all,
For those interested in the Family Myobatrachidae I offer you my considerations regarding the genus Pseudophryne in Australia - specifically on Pseudophryne australis - which were recently published in the AUSTRALIAN BIODIVERSITY RECORD, 2002 (No 4): 1-28 (March, 2002)
A copy of this article is attached below, but owing to its size, I shall send it in parts:


Kind Regards from

Richard Wells


AUSTRALIAN BIODIVERSITY RECORD
______________________________________________________
2002 (No 4) ISSN 1325-2992 March, 2002
________________________________________________

Notes on the Red-crowned Toadlet Pseudophryne (Pseudophryne) australis (Gray, 1835) - an endangered frog from the Sydney Basin of New South Wales.

by

Richard W. Wells
“Shiralee”, Major West Road, Cowra, New South Wales, Australia

Introduction
Recently, at the very time I was discussing the survival status of the Red-crowned Toadlet, I heard a newsflash on the radio that reported that an Asteroid over one kilometre wide had just missed hitting the Earth ! It made me recall with disturbing unease another intergalactic visitor only a few years ago - and I’m sure some of you will also remember the event. The world’s astronomical fraternity issued a brief press release that reported another near-miss from an even bigger Asteroid that could have likely killed the entire planet - it had just missed hitting us at over a hundred thousand kilometres an hour. The really worrying thing about this space stuff is that the most-experienced of the most-vigilant - the world’s leading astronomers - actually didn’t know it was heading our way until they saw it pass Planet Dumbo!

This situation though is really typical of extinction at any level. At one point in time a species can be quite abundant, and then all of a sudden its gone - a bit like the old analogy of a candle glowing its brightest just before it goes out. The Red-crowned Toadlet and numerous other species now appear to be on the slide towards extinction and I am wondering how we didn’t see it coming sooner.

Up until a few years ago I, like a number of other herpetologists, was unconvinced that the species was in any significant danger. Vast areas of its habitat had been protected in National Parks so why worry? However, increasingly vocal concerns about this species’ conservation by some amateur herpetologists did make me get out of the Past and have another look at the ridges as they are Today and guess what I found? I discovered that there probably is a case for concern about this species’ survival status, even though it may be considered a locally common species over some parts of its range. Hence, I have prepared an introductory overview of the Red-crowned Toadlet derived from both my own experiences and from the available data in the limited literature on the species. Readers are referred to the References section for sources of the information used in this article, as well as for further reading material on this species.

And in the Beginning...
there were Frogs, Frogs and More Frogs!
As most of my earliest field work in herpetology was either among the woodland of the Cumberland Plain or the granite areas of the Central Tablelands and Southern Highlands of NSW, it was comparatively late in my herpetological experience that I managed to uncover the Red-crowned Toadlet, Pseudophryne australis.

It was during 1965 on a field trip to Waterfall on the southern outskirts of Sydney that I first came upon the species. I was collecting along a ridge near the falls at the end of the track to the east of the railway line (the oldies will know where I mean). These little frogs were under almost every rock I turned on the ridges that day and I had no doubt that they were very common - although at the time I didn’t know what they were called. I was quite surprised at this as I had collected in what appeared to be similar-looking sandstone ridges looking for snakes and had never seen this frog before. What I now realise after tramping over many ridges is simply the fact that not all ridges are equal as far as the Red-crowned Toadlet is concerned.

Then, as now, my main interest was snakes, and most frogs in those days were regarded more for their potential as snake food than for their good looks. And of course, as was the custom of the day, some were promptly collected as potential food items. However, I soon considered that this particular little frog was a fairly toxic sort of creature as a snake snack. Soon after my juvenile Red-bellied Black Snake (Pseudechis porphyriacus) devoured one, the snake promptly died! I didn’t know for sure whether the frog was toxic to the snake, but sure as hell I never let it happen again.

Over the subsequent decade I spent a lot of my life on sandstone ridges and in the process, I managed to observe thousands of Red-crowned Toadlets. They were just so common that I thought that everything about their life history would have been long ago recorded and usually I just about ignored them - but not quite. As well as being very common, they were exceptionally pretty.

Mostly single individuals or pairs were found under a rock, but sometimes small aggregations of 4-8 were found - and sometimes over 10 shared a single slab of rock. Often large aggregations of around a hundred Red-crowned Toadlets could be found along a 20 or 30 metre stretch of a small waterbody breeding after the summer and autumn rains.

As I said above, most discoveries of this frog were usually incidental events to my searches for snakes. I noted their presence usually only because I soon learnt that they lived where the Broad-headed Snake lived. After a while you could tell which rocks were likely to have a Red-crowned Toadlet under them and you wouldn’t bother lifting them, as frog rocks and snake rocks are usually mutually exclusive sorts of real estate.

Over the years I collected only a handful of those thousands of Red-crowned Toadlets that I saw and those only because when I realised that the herpetology of the Sydney Basin was so poorly known I hoped that others might be able to increase our knowledge of the species.

Even by the mid-1970s they were still a very common frog wherever they occurred, but by the early 1980s I noticed that they appeared to be either no longer present at some sites or least now very rare. I thought at the time it was the big drought and possibly it was, but just so much else was happening to the sandstone habitats around Sydney it was hard to identify the precise reasons for the apparent decline.

Nowadays, things appear even more serious and the little frog I found once so commonly 35 years ago is now thought by some to be at risk of extinction. If someone had told me that this could happen to that species back in 1965 I would have laughed in their face.

The following notes on the Red-crowned Toadlet are intended to encourage a bit more interest in this little frog before it is too late, so read on....

Taxonomic Considerations
The Red-crowned Toadlet is a very small, but solidly-built frog of the Family Myobatrachidae. There appears to be some morphological variation over this species’ range that suggests that there are a number of isolates that warrant closer scrutiny.

Some populations appear to have slightly different calls, different dorsal and ventral colour patterns and at least one population appears to reach a larger maximum size.

Consequently, I believe that Pseudophryne australis is at the very least a polytypic species (i.e. one species different subspecies) - but it may also possibly be a composite of at least two separate species.

Regardless of the actual situation, some clearer view of the taxonomy of Pseudophryne australis may be forthcoming soon. Due to recent concerns regarding the survival status of various populations of this species the NSW National Parks and Wildlife Service has supported a detailed genetic analysis of some of the isolates by a scientist attached to the Australian Museum. When their study is completed this should allow a better understanding of the species’ variation.

Despite the above, the following article treats the Red-crowned Toadlet as a single species in keeping with the approach of all other authors to date.

General Morphology
The snout is short, and the canthal region is rounded and there are no vomerine and maxillary teeth. Unlike many other frogs, there is no exposed tympanic membrane and the ear structure is vestigial.

Metacarpal tubercles (2) are present and conspicuous on the hands, but less distinct on the feet. The inner metatarsal tubercle is round, low, and not shovel-shaped.

The skin is mostly smooth to slightly warty along the back, and smooth on the underbody - although the posterior part of the abdomen and the undersurface of the thighs may be slightly granular in texture.

The fingers and toes are free of any webbing, and the limbs are short in keeping with its mode of progression. Red-crowned toadlets tend to crawl or walk slowly when moving about, or at best feebly hop, rather than employ the typical well-developed hopping or leaping locomotion of most other frogs.

Mature specimens are usually only around 20-25 mm in length, with the largest specimens barely reaching 30 mm, making it actually one of Australia’s smallest frogs. Mature females tend to be slightly larger than males.

Colouration of Mature Specimens
As the common name suggests, this species’ most distinguishing feature is its distinctive head colour-pattern. A bright reddish-orange (or rarely pale orange) ‘T’-shaped pattern is always present on the top of its head, and this pattern is the result of a short (and usually thinner) reddish median stripe that runs from the snout to about the centre of the head where it contacts a broader stripe that runs across the head between the eyes (known as an interocular stripe). Occasionally this ‘T’-shaped pattern may be incomplete or less clearly differentiated. In some specimens, the median stripe may be fragmented or restricted to a small patch on the snout, while in others virtually the entire top of the head may be bright red, with the median stripe being very broad and more-or-less merging with the interocular stripe and so forming a triangular pattern of sorts. The dorsal body colour may vary across several shades of brown, but is usually more of a dark brown - and there is often a scattering of small spots or blotches over the back and the sides. These reddish body markings may be very prominent or reduced to being almost absent, but there is always a short reddish stripe or at least a spot along the lower back above the hindlimbs (also known as a urostylar or coccygeal stripe).

The sides of the head and body tend to be somewhat paler than the upper body, being a paler brown than that of the dorsum, with scattered darker brown flecking or blotching. Other individuals may be stunningly patterned with very pale bluish-grey along the sides and on the limbs with a fine peppering of white flecks, and in some specimens, the limbs may be delicately flecked with reddish as well.

Additionally, there are prominent white areas on the thighs and on the upper part of the forelimbs, and the undersurface of the hands and feet are dark brown with the tubercles whitish.

The colouration of the undersurface is similarly spectacular, with large blotches of brilliant white and black forming a variegated or mottled pattern.

Colouration of Larval Specimens
Generally advanced tadpoles are very dark almost black dorsally with a continuous covering of melanophores with numerous small golden chromatophores. The ventral area is also heavily pigmented, with the exception of the median part of the abdomen being clear or transparent. The tail fins are barely pigmented, but the myotome area of the tail is very dark. The distinctive red head colour appears close to the time of metamorphosis. Red-crowned Toadlet tadpoles when fully grown prior to stage 38 may be various shades of grey though, with lighter specimens possessing noticeable darker spotting. These spots however are also present in darker coloured individuals, but less obvious. There is usually a faint or thin reddish mid-dorsal stripe most distinctive about the posterior of the dorsum, and from here it extends along the anterior dorsum of the tail. The venter is transparent with darker edging of melanophores surrounding scattered groups of very fine gold flecks. From about stage 38 a faint or pale greyish-brown triangular crown becomes evident and this gradually changes to its distinctive reddish colouration by stage 42, along with the intensifying of the reddish mid-dorsal stripe on the body.

Although the tadpoles of this species are quite distinctive, they are more often the only species present where they occur so identification shouldn’t pose too much of a problem.

However, on occasions, another tiny species of frog (the Common Froglet, Ranidella signifera) may be sympatric with Red-crowned Toadlets in its breeding sites. Ranidella tadpoles may share a water body with this species and their similar size may cause some initial confusion with identification.

Most notably, the Ranidella tadpole tends to be lighter coloured on the dorsal body, and has distinct darker mottling over both the dorsal part of the body as well as the tail. In general appearance, the tadpole of the Red-crowned Toadlet has a uniformly dark body and an almost transparent tail. The larvae also differ morphologically from Ranidella in possessing five rows of similar-sized teeth (2 upper, plus 3 lower rpws) and the oral papillae are confined to the edges of the mouth.

Distribution
The Red-crowned Toadlet has a very restricted distribution, being only known from a relatively small area of mid-eastern New South Wales.

The idea that the Red-crowned Toadlet has a restricted distribution can be found in a literature extending back over 100 years, (see bibliography, but in particular Tyler, M.J. (1994), Thumm (1997), Thumm and Mahony (1997, 1999) and Cogger (2000) and the references contained therein for more recent mentions).

It is known only from a number of isolated populations that are all contained within the Sydney Geological Basin. Its range is circumscribed by the distributional extremities that range from Pokolbin State Forest in the north, to the Nowra district in the South, and Mt Victoria in the west.

The northern and southern populations appear to be geographically isolated and show some readily apparent morphological variation.

Historical records from beyond the Sydney Basin such as those from the New England region, are now thought to be in error. Old records from an area east of Tamworth have also never been confirmed, but the occurrence in this area of a number of other species that are usually sympatric with the Red-crowned Toadlet may indicate that a northern isolate of P. australis may possibly exist.

Even within its known range however, the species could potentially occur over a wider area than current observation records suggest, as its habitat extends considerably beyond the current limits of specimen records in places. Further survey of potential habitat may identify additional populations and range extensions and would be very worthwhile.

Habitat Preferences
All available evidence indicates that the Red-crowned Toadlet is restricted to the Triassic Hawkesbury and Narrabeen Sandstones of the Sydney Geological Basin. The overall geological characteristics of this region may be found in Branagan, Herbert and Langford-Smith (1976).

The species has been recorded from near sea-level to about 1000 metres in elevation, but most sites are on fairly low coastal ranges under 200 m in elevation.

The habitat may be found in steep escarpment areas and plateaus, as well as low undulating ranges and outcroppings. Ironstone capping is a common feature of many sites where this species has been observed.

Favoured microhabitats for shelter sites are under flat sandstone rocks (‘bush-rock’) either resting on bare rock or damp loamy soils. They have also been found under logs on soil, beneath thick ground litter and in horizontal rock crevices near the ground.

Within these geological formations, this species mainly occupies the upper parts of ridges, usually being restricted to within about 100 metres of the ridgetop. Although they also occur on plateaus or more level rock platforms along the ridgetop this area is usually less preferred than the first tallus slope areas below the upper escarpment or just below benched rock platforms.

Red-crowned Toadlets do not usually live along permanent flowing water courses such as occur in gullies, instead preferring permanently moist soaks or areas of dense ground vegetation or litter along or near head-water stream beds. These are the non-perennial first or second order drainage systems that are at the very upper parts of ridges. They are ephemeral drainages that are commonly called ‘feeder-creeks’ that channel water from the upper most parts of the ridge, benches, cliffs and tallus slopes to the perennial streams in the gullies below. Such watercourses are dry or reduced to scattered shallow pools or ponds for much of the year, and have sustained flow for only a few weeks following the onset of late summer thunderstorms, then ceasing again until another short period of flow that is initiated by the follow-up rains of Autumn. The water quality in these upper feeder-creeks and soaks is very high and is almost totally lacking in nutrients.

The principal vegetation community present in this species’ habitat is Sydney Sandstone Ridgetop Woodland (mainly dominated by Corymbia gummifera and Eucalyptus haemastoma, although a number of different associations within this community are utilised depending upon the area). Other vegetation communities have also been recorded as representing this species’ habitat: viz Sydney Sandstone Gully Forest (dominated by Eucalyptus piperata, Eucalyptus pilularis and Angophora costata, but utilised mainly at the ecotone between this community and the former, rather than in the gullies proper), Coastal Sandstone Heath community (dominated by Banksia spp., Hakea teretifolia, and Baekea spp.), and the Blue Mountains Sandstone Plateau Forest community (dominated by Eucalyptus sieberi and Eucalyptus piperata). Tree cover when present is usually open and low (10-20m), and the understorey is dominated by a complex range of xeromorphic shrubs. Definitions of these vegetation communities and associations may be found in Benson and Fallding (1981), Benson (1986, 1992), Keith and Benson (1988), Benson and Keith (1990), Benson, Thomas and Burkitt (1990), Benson and Howell (1994), Fisher, Ryan and Lembit (1995), and Ryan, Fisher and Schaeper (1996).

The climate of its habitat is extreme with parts of its distribution experiencing highly variable temperature and rainfall patterns. Taken over the last 130 years the annual rainfall of Sydney has varied from around 500 mm. to around 2200 mm., with most years falling within a range of 900mm to 1600mm. The rainfall pattern across this species’ habitat precludes regular seasonal flooding events and this is believed to explain the unusual opportunistic breeding biology of the species.

There is currently no area of Critical Habitat of the Red-crowned Toadlet declared under the NSW Threatened Species Conservation Act.

Reproduction
In keeping with the low rainfall and temporary availability of flowing water, the specialised terrestrial reproductive strategy adopted by the Red-crowned Toadlet is ideal.

Only small nests of low numbers of large eggs are produced, the larvae develop rapidly and metamorphosis takes place quickly. Despite this, recruitment rates are low, as the hostile conditions take a heavy toll on eggs and larvae with many clutches probably being lost to desication with the evaporation of the shallow larval pools. To offset this loss to the population, females can lay multiple clutches during the year, and so take advantage of the sporadic availability of suitable breeding weather.

Most breeding sites appear to be right at the head-waters of first or second order streams, which are for most of the year either totally dry or little more than chains of small pools and dense piles of flood-accumulated leaf-litter. Such microhabitat traps moisture throughout the year, and so forms an essential refuge for the species during periods of low rainfall as well.

Hanging swamps are not used for breeding by this species, but they do breed and refuge in the drainage lines associated with such habitats.

Additionally, on occasions this species can be found breeding along the eroded gutters or verges of unsealed fire trails and the like, where dense accumulations of leaf-litter may form along the edges of the road - much as it does along non-perennial stream beds.

Although mating has only been positively recorded in Spring, mating likely occurs throughout the year depending upon weather conditions.

Breeding is definitely not restricted to one short time of the year, instead occurring opportunistically, mainly from early Spring through Summer and into Autumn. However, the extent or timing of breeding activity may differ from year to year.

This irregular and extended breeding cycle of the Red-crowned Toadlet is probably a consequence of the highly variable and unpredictable rainfall patterns occurring within its habitat.

Males have been recorded calling in every month of the year, but most calling takes place between September and February. Calling occurs either during or following periods of prolonged rainfall, and less often in drier weather and occur both during the day and night.

Most calling by this species occurs during the late afternoon and early evening, either just before, during, or more often just after periods of heavy rainfall. The most intense calling activity occurs following periods of severe thunderstorm activity in Summer. Ambient temperatures recorded during calling range from 5°C to 30°C, but 17°C to 25°C are more usual conditions.

The call of the male of this species has been variously described as a nasal ‘ank-ank’ or singular ‘ank’ repeated slowly, or occasionally as a short metallic ‘erk’ sound, and this is fairly accurate. Others have likened the call to a harsh grating ‘cr-ee-k’ repeated two or three times, or as an ‘ark’ or ‘squelch’ sound, or something like ‘eeeek eek’ repeated several times at variable intervals.

However, it should be appreciated that the call may vary from one part of its range to another. Further, the calls may differ in their intensity depending upon the particular conditions at the time. For instance, those calling from beneath the cover of rocks, crevices or leaf-litter will tend to sound slightly different to those calling from burrows due to amplification etc, and repetition-rates may vary with the prevailing weather conditions as well.

Frogs have also been recorded calling in winter and eggs have been found every month of the year except June. Winter breeding however, would be an unusual occurrence, and probably indicative of the milder weather conditions prevailing in the coastal part of the species’ range during some years. Winter breeding in the plateau population in the Blue Mountains has never been recorded and would be most unlikely due to the very low temperatures experienced there.

Usually small colonies of individuals share a suitable site, so there can be a number of individuals calling within close proximity of one another on occasions. During periods of heightened breeding activity, as in summer thunderstorm periods, numerous specimens may converge on a particular site such as damp creek bed or soak and the calling may be quite intense and prolonged.

Gravid females have been found in every month of the year, and egg-laying occurs in all but the coldest months. Most egg-laying however appears to take place between September and February, and a lesser amount from March to May.

The researches of Karen Thumm of Sydney have significantly advanced our understanding of this species’ breeding biology (see references). Females have been known to lay around 15-20 eggs in a clutch, although as few as 10 or even as many as 66 have been recorded from a single laying. Additionally a single female can produce more than one clutch in a year if weather conditions permit. The period between each clutch varies considerably, but under captive conditions a single female produced 17 clutches of eggs over a 2.5 year period, and the interval between clutches ranged from about 2 weeks to 6 months.

At the time of laying, each egg is around 2.5 mm in diameter with a jelly envelope about 6.5 mm in diameter, and so is quite large relative to the size of the frog.

Many breeding sites have been found to be associated with slow water seepage from ridges upslope and have a soil pH ranging from 5.5 to 6.5. Under these conditions the water is of very high purity. Red-crowned Toadlets have not been recorded breeding in permanently flowing streams or waters that are even mildly polluted.

Eggs are laid in moist situations, often in a shallow soil or humus cavity that is covered with either decaying vegetation or ground plants such as moss. Nests are usually in close proximity to a depression of sorts either already possessing a pool of water, or one that will soon form following rainfall.

A ‘nest’ cavity can be little more than a surface irregularity and may be quite small - around 40 mm in diameter and about 12 mm deep and can be difficult to find amongst damp, decaying leaf-litter. Although often quite damp, nests are not waterlogged while the eggs are developing.

Many nests have been found in very sheltered positions such as beneath thick accumulations of leaf-litter that partly form a dam to small pools of water in the beds of non-perennial streams. Nests have also been discovered on slightly sloping, moist ground just above small temporary rock pools or adjacent to flowing seepages from the base of clifflines.

On occasions, individuals have been discovered nesting deep inside water-soaked sandstone crevices at the base of rock ledges and amongst the boulders surrounding small waterfalls. Nests have also been discovered in depressions under sandstone slabs (or ‘bush-rock’) on rock platforms. In these situations the eggs have been found resting on accumulated damp soil and litter under the slab, and were exposed to water seepage from the ridge upslope.

When breeding in disturbed sites, such as along the verges of fire-trails, nests have been found in similar conditions - i.e. under litter or low vegetation such as grasses, on slightly sloping damp or wet soil upslope from small erosion pools on loamy-clay soil.

As to whether there is any parental care of the eggs or the nest, more data are needed. Mating and egg-laying has been observed to occur at the male calling site, in effect implying that males choose the nesting site then call from this position to attract a female. Some observers maintain that the male may even construct nesting cavities in the soil of suitable nesting areas also. Nests have been discovered with only the eggs present, as well as on occasions with a female present. That males call from the nesting position does not mean that they also guard the eggs, although this has been implied by some observers. Males are seldom found in the nest once the eggs are laid and this probably indicates that they may have no role in guarding the nest. However, as some observers have reported finding males with eggs in nests, it leaves open the question of male protection for the time being. It has also been suggested that females may actively assist the hatching process by moving the eggs closer to water when the time is right.

The actual duration from egg-laying to hatching, and then as larvae to metamorphosis varies somewhat over the range of the species, and in particular depends on weather conditions. Eggs laid in the warmer conditions of summer generally develop much faster than those laid during the cooler Autumn conditions. The usual developmental period from egg to metamorphosis is thought to range from about 1-2 months, with the embryo reaching about 17 mm in length by the time of hatching. The tadpole attains a maximum length of about 25 mm by the time of metamorphosis, and the juvenile stage is usually attained by around the age of 2 months, and at this stage juveniles are about 8 mm in length. However, development of the embryos may take from about 30 days to as much as 180 days. If the weather has been too cold, embryos may over-winter and hatch out in Spring. A period of rainfall triggers hatching, and the larvae are then washed into a nearby pool or more or less remain in the area of the nest as it is progressively inundated. Follow-up rains are needed in some areas as summer larval pools can rapidly evaporate in the hot weather, and in so doing kill many Red-crowned Toadlets tadpoles before they are able to metamorphose.

In more protected situations where areas under large sandstone slabs may be utilised as nesting sites, the rocks may also cover depressions in the underlying rock which can also act as reservoirs for run-off water and so form small pools where the larvae may complete their transformation even during extremely dry conditions.

Such under-rock pools of water may persist during even severe drought conditions and act as very important refuges for many species of ridge top fauna - but in particular, the Red-crowned Toadlet.

Most pools utilised by this species’ larvae are quite small in area (often less than a metre across) and very shallow (less than 30 cm deep, and are often much shallower at only around 5-10 cm deep). Sometimes the tadpoles may utilise little more than tiny, water-filled depressions that have been created by a small rock barrier to flow or the accumulations leaf-litter that drifts into small piles to form temporary dams on the bed of a creek.

As more rainfall increases the flow, larvae may be washed into more substantial pools that can see them through to metamorphosis, but should the rainfall be inadequate, many larvae perish as the smallest of the pools evaporate.

Common characteristics of larval pools and their immediate surrounds, are rocky substrate conditions with associated dense piles of decaying leaf-litter (flood debris), and an almost total lack of vegetation.

Known prey for post-metamorphic Red-crowned Toadlets are termites, mites, pseudoscorpions, collembolans, ants and small cockroaches, although they would likely eat most small invertebrates that would be encountered.

Larvae however, are thought to be mainly detritus-feeders due to the nature of the larval pools. Although typical stream-side vegetation of ferns and sedges is always near larval pools, the actual environment of the pools where the larvae develop is biologically very poor habitat. It is possible that the larvae don’t actually eat at all prior to metamorphosis, subsisting entirely on stored yolk from the embryonic phase in their life cycle.

The Red-crowned Toadlet has adapted well to the harsh environmental conditions of Sydney’s sandstone ridge country. It is a long-lived species able to withstand prolonged periods of drought through its nocturnal and largely fossorial lifestyle and its use of a range of moisture-trapping microhabitats such as deep rock crevices as refugia. It is the only species of frog in the Sydney Basin that has adapted specifically to the sandstone ridgetop environment with its highly unpredictable rainfall patterns.


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