Nullarbor of Australia is the most remarkable frog-free zone on Earth

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The following map shows the approximate global distribution of frogs:
http://en.wikipedia.org/wiki/Frog#/media/File:Distribution.anura.1.png.

This map provides a useful overview.

However, it is misleading in two ways, one relatively inconsequential, and the other more remarkable.

Examples of minor inaccuracies include

• Western Sahara, where several spp. of anurans are indigenous,
• southern Algeria, where e.g. Bufotes boulengeri (https://www.inaturalist.org/taxa/992054-Bufotes-boulengeri) is indigenous, and
• the northernmost tip of the South Island of New Zealand, where Leiopelma hamiltoni (https://www.inaturalist.org/taxa/23273-Leiopelma-hamiltoni) is indigenous.

The more remarkable inaccuracy is on the Nullarbor Plain of southern Australia, which in fact lacks any frogs over a surprisingly large area (https://museumsvictoria.com.au/media/4263/403-408_mmv74_tyler-and-prideaux_3_web.pdf.).

The Nullarbor Plain is one of the most extensive examples of a karstic plain on Earth (https://austhrutime.com/nullarbor_plain.htm#:~:text=At%20about%20250%2C000%20km2,the%20west%20coast%20across%20it.).

It has an area of 200 thousand square kilometres, about the size of England.

The Nullarbor Plain (https://en.wikipedia.org/wiki/Nullarbor_Plain) is devoid of frogs within a mainland. This distinguishes it from several large, frog-free islands and peninsulas, including

• the island of Newfoundland (https://en.wikipedia.org/wiki/Newfoundland_(island) and (https://www.gov.nl.ca/ffa/wildlife/all-species/amphibians/#:~:text=Four%20species%2C%20including%20American%20Toad,Peeper%20are%20native%20to%20Labrador.)), off easternmost Canada,
• the island of Tierra del Fuego, at the southernmost tip of South America,
• most of the South Island of New Zealand (150 thousand square kilometres), and
• the peninsula of Kamchatka (https://en.wikipedia.org/wiki/Kamchatka_Peninsula), which lacks indigenous frogs, although it has salamanders (https://en.wikipedia.org/wiki/Salamander#/media/File:Cypron-Range_Caudata.svg and https://sg.sgu.ru/en/articles/annonated-list-of-amphibian-and-reptile-species-of-the-far-east-russia).

The following map (http://en.wikipedia.org/wiki/Nullarbor_Plain#/media/File:IBRA_6.1_Nullarbor.png) shows the precise extent of the Nullarbor Plain within Australia. For North American readers, this is about the same as the sizes of each of the following states: Minnesota, Utah, Idaho, Kansas, Nebraska, and South Dakota (https://en.wikipedia.org/wiki/List_of_U.S._states_and_territories_by_area).

Comparing the extents of the various frog-free zones cannot easily be done by just looking at a world map, because of the two-dimensional distortions inevitable in cartography. The relative sizes of areas are subject to a latitudinal effect.

Newfoundland island (http://en.wikipedia.org/wiki/Newfoundland_(island)#/media/File:Newfoundland_map.png.), devoid of indigenous amphibians, has an area of 109 thousand square kilometres. This may look as extensive as the Nullarbor Plain, but in fact is only about the size of Ireland.

The Australian area is particularly extensive, even if one allows for the likelihood that a few species of frogs encroach marginally on the Nullarbor Plain.

The spp. occurring immediately adjacent to the Nullarbor Plain are:

• Neobatrachus sutor/sudellae/aquilonius (https://bie.ala.org.au/species/https://biodiversity.org.au/afd/taxa/953a5af4-2932-4c8b-8f33-850b5f8f3fed and https://en.wikipedia.org/wiki/Shoemaker_frog and https://museum.wa.gov.au/explore/frogwatch/frogs/centralian-trilling-frog and https://www.frogid.net.au/frogs/neobatrachus-aquilonius),
• Notaden nichollsi (https://terrestrialecosystems.com/non-cocooning-desert-frogs/ and https://en.wikipedia.org/wiki/Desert_spadefoot_toad),
• Ranoidea maini (https://en.wikipedia.org/wiki/Main%27s_frog and https://museum.wa.gov.au/explore/frogwatch/sheep-or-mains-frog and https://www.researchgate.net/publication/276352664_Cyclorana_maini_and_Notaden_sp_additions_to_the_frog_fauna_of_South_Australia and https://www.frogid.net.au/frogs/cyclorana-maini), and
• Ranoidea rubella (https://en.wikipedia.org/wiki/Desert_tree_frog).

Apart from the Nullarbor Plain, all the frog-free areas on Earth have relatively simple explanations in terms of climate and geography, as follows:

• aridity in the case of parts of the Sahara, the Arabian deserts, and the Atacama desert,
• frigidity in the case of the Arctic, and
• a combination of marine isolation and a cold climate in the case of Newfoundland, Tierra del Fuego, Kamchatka, and the South Island of New Zealand.

None of the above explanations apply to the Nullarbor Plain, which

• lies at the same latitude as southernmost Brazil, and
• has a mild, semi-arid climate, similar to that of the Little Karoo (https://en.wikipedia.org/wiki/Karoo#Little_Karoo) in South Africa, where various species of frogs occur.

Why, then, is the Nullarbor Plain - on which the most typical plant is the 1 m-high shrub Maireana sedifolia (https://www.inaturalist.org/taxa/338307-Maireana-sedifolia) - devoid of frogs?

Contributing factors include

• the particular proneness of Australia to to prolonged drought, and
• the geological peculiarity, on the Nullarbor Plain, of an extensive substrate so porous that, even after rain, pools tend not to last long (https://www.researchgate.net/publication/246599692_Karst_evolution_of_the_Nullarbor_Plain_Australia and https://link.springer.com/article/10.1007/BF00766986).

However, frogs occur in an even drier area: the Lake Eyre Basin (https://en.wikipedia.org/wiki/Lake_Eyre_basin and https://en.wikipedia.org/wiki/Neobatrachus and https://www.publish.csiro.au/zo/ZO97062 and https://www.inaturalist.org/taxa/23611-Litoria-rubella). This is the driest part of Australia, with mean annual rainfall about 150 mm per year, considerably less than the 250 mm on the Nullarbor Plain (https://enviro-friendly.com/information/average-rainfall/).

Thus, climate alone cannot explain the dearth of frogs on the Nullarbor Plain. We must consider other aspects of the substrates, including their depth, topographic uniformity, and nutritional value.

Frogs on treeless plains under semi-arid climates tend to depend on subterranean dormancy for most of the year (https://www.rswa.org.au/publications/Journal/86(3)/v86(3)Thompsonetal.pdf).

Dormancy during dry times, on semi-arid plains, depends on the adults burrowing to a certain minimum depth. Soils on the Nullarbor Plain are extensively shallow over the limestone bedrock, tending to preclude survival of adults.

Furthermore, the parent rock here, hence the soils, are poor in phosphorus and zinc.

These calcareous, loamy soils on the Nullarbor Plain are not as nutrient-poor as those producing the various forms of sclerophyllous, fire-prone vegetation so widespread in semi-arid Australia (e.g. https://en.wikipedia.org/wiki/Great_Victoria_Desert).

However, at the same time, they are not as nutrient-rich as the loamy soils in adjacent areas with bedrock other than limestone. These tend to support the moderately nutrient-rich shrub, Maireana pyramidata, in loamy bottomlands (https://en.wikipedia.org/wiki/Maireana_pyramidata#/media/File:Maireanapyramidata1.png and https://www.inaturalist.org/taxa/934705-Maireana-pyramidata).

When enough rain falls to produce shallow pools, survival of frogs depends on tadpoles being able to develop to the stage of metamorphosis, before the water drains or evaporates.

The rate of growth of tadpoles tends to depend on the concentration of crucial nutrients in the pool and its detrital layer.

The tadpoles of Neobatrachus take at least four months to develop to metamorphosis (https://www.frogid.net.au/frogs/neobatrachus-sudellae). Those of Ranoidea maini take only about one month, but require water deep enough for gregarious swimming.

Therefore, I hypothesise as follows:

What tends to preclude the life-cycle of even arid-adapted frogs, on the Nullarbor Plain, is a combination of

• moderate nutrient-poverty, limiting the rate of growth of tadpoles,
• short duration of rain-pools, limiting the periods over which tadpoles can grow,
• flatness precluding the formation of drainage lines, hence any pools deeper than a few centimetres, and
• shallowness of soils, limiting the ability of adults to survive in burrows when drought resumes.

Publicado el 14 de agosto de 2022 por milewski