Thursday, September 24, 2020

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Home Issue 32 A touch of light – rockpools and desert frogs

A touch of light – rockpools and desert frogs

By MIKE GILLAM

All photos © Mike Gillam

Desert tree frogs were my main reason for the day’s walk. These small brown hylids cram themselves into a narrow crevice, preferably moist, to escape the elements – most other desert frogs burrow.

A frog’s permeable skin allows it to quickly absorb water and oxygen but this attribute also works in reverse, so water loss is a real hazard for amphibians. Frog skin differs greatly from mammals: the stratum corneum is much thinner and this hugely increases their vulnerability to dehydration and environmental pollutants. Frogs also possess glands in the skin which can exude a moisturising mucus. 

Pressed tightly together the desert tree frog group reduces water loss by minimising their total surface area that’s exposed to the air. As frogs on the outside become stressed they push and elbow their way back into the centre of the cluster and this forces others to take a turn on the outer edge.

We walked upslope to study the crevasse from the high southern rim. On the return we’d climb down into the dry drainage channel and investigate the chain of pools. The crevasse extends some 200 metres to the summit of the range, a deceptively short distance for the hours it would take to descend, negotiating frequent boulder hazards, chokes of vines and fallen trees.

The absence of fish and paucity of aquatic vegetation in the higher elevation pools were noteworthy. While hylid frogs also inhabit the lower river channel, in these rock pools both species appeared more visible and abundant. The larger river pools support predatory herons and cormorants, a constant hazard for frogs active in daylight. The absence or presence of fish, voracious predators of tadpoles are perhaps more significant.

At the head of the paved and walled crevasse, the bedrock drainage forked, its two diagonal arms separated by an island of stranded rock. The left arm of the fork is deeper and the right is wider. Both arms Y intercept and harvest sheet flow from large areas of the mountainside, a broad summit and barrel-shaped flanks tilting towards the central crevasse.

Huge boulders have broken away from the fracture zone and these stand in the bedrock, free-standing sculptures in a sunken gallery. Near vertical cliffs are carved and polished where the plunging stream gathers pace in flash floods. Cascades and torrents crash and career around and over immovable midstream boulders that redirect or slow the wildly flowing energy.

Significant drops in elevation occur at six or seven points in the crevasse and here the torrent crashes over ledges and gouges out basins. Places of turbulence, fallen trees and eddies do little to absorb the speed of descent during floods but rock falls along the sides of the crevasse hold linear terraces of soil. These support a stunning array of desert plants in naturally raised garden beds.

While a major flow must gouge out some vegetation, doubtless these micro environments also benefit from minor rainfall events that disturb little, gently top up occasional rock pools and don’t always reach the river valley below. Unfortunately, the botanical richness of this well-watered crevasse also includes some healthy tussocks of buffel grass which we carefully remove and bag.

Walking and climbing while causing some damage to the brilliant array of plants, no matter how slight, came with a strong sense that we were intruders. Certainly the litany of broken branches and bruised lichens would recover in time but when does the impact become unconscionable.

I could well imagine significant damage resulting from a larger group, charged with a spirit of conquest and adventure. Sunscreens and insect repellents released by some sweaty hikers as they treat themselves to a spa bath would wreak havoc in such tiny and fragile environments.

Large sun-drenched riverine lagoons are often complex. The structural diversity and scale of the habitat from seepage inflows, highly oxygenated to stagnant environments, overhanging riparian vegetation, rafts of aquatic plants, myriad vertebrate and invertebrate denizens, can leave the casual observer with ecosystem overload.

In contrast the higher elevation catchment pools, mostly smaller than a plunge pool and devoid of significant vegetation, offer greater intimacy. Most are not particularly species rich and are unlikely to contain life-forms unrepresented elsewhere. In effect these are microcosms that are more readily understood and appreciated by the casual observer.

The aquatic community, the combinations, the conspicuous abundance of some species and the absence of others are part of the wonder I experience every time I find a new pool. (A friend once told me that a new species of freshwater sponge had been found in a rock pool at Watarrka National Park – amazing!)

At a microscopic level there would be much more to hold the interest of a freshwater biologist wielding a dip net but to the naked eye there was plenty to keep me gainfully employed.

While the crevasse rock-pools differed markedly from the adjacent river lagoon, each also showed subtle variations, levels of sunlight and shade, temperature and oxygen, nutrients and minerals. Typically, rock pools are clearer and largely free of the tannins produced by overhanging riparian vegetation such as Eucalypts and Melaleucas etc.

From top to bottom the crevasse supported a chain of six rock pools of varying size and at first glance, different assemblages of dominant species. The highest pool was dry when I visited but there was partial shade and enough residual moisture to support lush purple flowering native ‘mint’, a feature shared with pool number six at the very base of the crevasse.

The next pool (P2) down the order was about the size of a domestic spa bath designed for eight people sitting in a circle. I chose this pool for closer attention. The water was very clear, there was minimal algae and no nearby vegetation.  The scoured basin was 80 cm deep and partially protected by a rocky buttress and overhang. This setting would create a backwash or eddy of relative calm when the torrent was flowing.

I sat for an hour and was impressed by the hylid frogs that appeared not to have a care in the world. Predation by waterbirds and waders was much less of an issue in such a tight environment. After dark the slender pythons would be a serious threat and I wondered if these isolated frog populations had adjusted their behaviour accordingly, with a bias towards diurnal foraging?

There were two species, the spectacular green tree frog with white spots, Litoria gilleni, and the much smaller and brownish in colour desert tree frog, Litoria rubella. Both were basking in the sun to raise their body temperature, waiting in ambush for insects, and occasionally plunging into the frigid pool to rehydrate.

Even the spotted diving beetles, doubtless widespread downstream, were much more conspicuous here, a bit like looking at mosquito larvae in a petri dish.

Pool 3 was about 10 m2 and a large swarm of whirly gig beetles swirled and stirred the surface. The pool was a little more exposed to aerial predation, the walls significantly wider and sunlight levels higher so vegetation had returned and with it a solitary damsel fly. Despite its obvious richness, the situation was not open enough for waterbirds that demand open flyways and a clear view of approaching predators.

Pool 4 was tiny and pool 5 was the largest with immense overhanging fig trees on the western wall; I’d need a week just to scratch the surface. There was more light and more algae and visibility was poorer but backswimmers (see image at bottom) and whirly gigs were very common.

At the base of the crevasse Pool 6 was situated where the minor drainage feature plunged over the final three metre cliff and scoured out a pool within clear sight of the main river channel. A nearby trough provided water for cattle and livestock. Nevertheless, it was also open to livestock, its banks and approaches quite muddy and churned up.

Obvious signs of nutrient enrichment included animal dung and pea green water. A very minor seepage fed into this pool from the much larger pool 5 above and this subtle refreshment probably saved it from utter ruin.

A quick inspection indicated a lack of aquatic life but the excessively fertilised native mint, Stemodia viscosa, was thriving. Given its prevalence I’m quite certain the so called ‘blistered Pyrgomorphs’ already present in small numbers would be abundant up and down the crevasse in the coming spring. This grasshopper’s latin name draws attention to the raised lumps or ‘pustules’ hence Monistria pustulifera! In recognition of its exquisite beauty I’m going to promote the alternative common name, the painted grasshopper (see the image of mating pair second from top).

Returning to upper rockpool 2, we had settled in to watch. The frogs were most active on a smooth damp ledge, a polished forecourt with options to leap in and out of the facing pool or retreat into deep horizontal crevices behind, the squeeze spaces where they huddle. I imagine the crevices would also offer anchorage for the adult frogs during those occasional flash floods that wash their tadpole progeny down the mountainside.

Reflected light levels were very low in this section and I cursed my laziness bringing a light carbon fibre monopod instead of the heaviest field tripod in the world stored in the vehicle. I couldn’t approach without unsettling the frogs and stirring up the crystal clear water so I opted to photograph them from the opposing bank with the intention of cropping tightly.

The need for reasonable depth of field and a steady shutter speed was testing and I applied every trick in the book, weight bearing down on a lowered monopod, bracing body and elbows forming endless triangles, Zen breathing, engaging mirror lock up; all to enable me to lower the shutter speed to ridiculous levels. I consoled myself that the out-of-focus condition is momentary and the pain in my hunched back and neck would not last either.

Reviewing the dozens of below standard images, I was interrupted by the arrival of finches in a nearby tree. Desert etiquette demands that all wildlife especially birds have total right of passage to water and so we murmured our apologies and moved back out of the way. The finches came in two bursts, first eight or nine Nyi Nyi followed shortly after by a pair of painted finches, each descending only briefly to drink, more out of habit than dire necessity.

The frogs were enjoying considerable success catching blowflies and the light continued to fall so I hauled out the flash and a reflector to provide bounce, with a touch of regret. The highly reflective wet highlights of the frogs were going to prove a headache. Frogs are wet and shiny. Reflective highlights are an intrinsic part of the frog’s character but I felt the highlights were distracting and came at the expense of the beautiful white spots on the green tree frog.

The oval shaped and yellow spotted diving beetles were equally fascinating. They were a rarity in such ideal conditions and I was excited to have my first opportunity to photograph them. With smooth dark bodies and yellow cream spots, they were accomplished swimmers underwater although they lacked some grace on the surface, their movements a little jerky, their batteries in need of re-charging. After hours of practice photography, I was rewarded with a moment of serendipity.

Several of the frogs waiting patiently on the ledge had caught blowflies with surprising speed and I turned my attention to a large green frog watching a fly as it wandered closer and closer. The frog was very still and tensed as the fly moved within range. In the blink of an eye it lunged and head butted the fly into the water. I lost focus and was very surprised the frog did not pursue the helpless fly, now vibrating its wings and doing circuits on the surface of the pool.

Come on, I pleaded, seeing a viable image floating past but the frog had assumed statue mode, its front facing eyes and stereo vision fixated on a new fly.

Then a diving beetle approached on the surface and began investigating the stricken fly and my odds improved (see image above left). A second diving beetle passed underneath, its appearance reminding me of a miniature whale shark. Dragonflies were predictably absent in the shady rock pools although a great many wasps visited pool 2, a curious addition explained by a nearby overhang that contained thousands of mud nests in close proximity to a ready supply of damp clay on the water’s edge.

Suffice to say this particular crevasse is situated on a pastoral property not a national park. This fact combined with the ‘inaccessible’ terrain probably gives it a better chance. I don’t wish to infer that such places are mismanaged on national parks because the park system has more than its share of true believers albeit under-resourced. It certainly doesn’t follow that tourism imperatives always prevail over conservation.

Self-respecting park planners work collaboratively with rangers who know their parks intimately. Both have the same priorities and work to deftly subvert the singular economic focus of politicians. Typically, they identify ‘sacrifice zones’ where the impacts of people can be mitigated, usually in large scale scenic landscapes with serious holding capacity.

Serving the misguided paradigm that “parks are for people”, these iconic attractions are usually well known and difficult to hide. Through the careful placement of facilities such as walking tracks, promotions and signage, the park planner skilfully leads people away from the rich and vulnerable ‘in between’ sites.  The fragile crevasse I’m describing is just such a site. Located on a national park it would be a case of ‘move along, nothing to see here!’

Unfortunately, our dedicated land managers are facing defeat because everyone, caring and uncaring alike, is using Google Earth. Special interest groups are demanding more and more access to parks for adventure tourism. Unscrupulous individuals are illegally cutting tracks for mountain biking and expecting Governments, not to prosecute, but to incorporate, repair the damage, accept the drain on their own budgets, legalise and look the other way.

It’s way past time to use Google Earth and drones to map out fragile and vulnerable locations, restrict access and on occasion, declare them off limits.

The crevasse like many others, needs minimal management to avoid gradual ruin. A small herbicide free weeding team, moving with the grace and care of ballerinas, could achieve much in half a day every couple of years. The crevasse could be inexpensively fenced at its mouth to exclude cattle and horses. A tax deductible advantage could be awarded to pastoral properties that act as land managers for such places.

Meanwhile self-motivated people all over the country are doing their best to assist in the conservation of overlooked and miraculous places that seem to fall through the land management cracks of government agencies.

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