The Australian Empty Quarter: epitome of a nutrient-desert
Everyone knows that Australia is the driest continent. However, how many know that 'desert' in Australia refers less to a lack of water than to a lack of nutrients?
On most maps of land use in Australia, a certain large-scale pattern is obvious. However, it does not seem to have been pointed out as a distinctive geographical feature of Australia.
This is a broad vertical band across the western half of the continent, centred just west of the border between Western Australia (https://en.wikipedia.org/wiki/Western_Australia#/media/File:Western_Australia_in_Australia.svg) and South Australia/Northern Territory.
The climate varies from tropical in the north to temperate in the south.
In this broad longitudinal band there is, to this day, negligible farming or other utilisation of the land (see https://www.agriculture.gov.au/abares/aclump and https://www.veganaustralia.org.au/impact_of_a_vegan_agricultural_system_on_land_use and https://www.researchgate.net/figure/A-2000-01-national-scale-land-use-map-illustrating-the-broad-distribution-of-pastures_fig1_230817486 and https://www.moffittsfarm.com.au/2014/01/04/land-use-australia-2005-2006-source-abares/ and https://ars.els-cdn.com/content/image/1-s2.0-S0959378016300231-gr1.jpg and https://www.researchgate.net/figure/Sample-locations-and-land-use-types-in-Australia-The-agricultural-areas-of-Australia-are_fig1_288059143 and https://www.researchgate.net/figure/a-Carnahan-classification-of-current-vegetation-for-Australia-AUSLIG-1990-and-b_fig1_236770005 and https://www.creativespirits.info/aboriginalculture/land/aboriginal-homelands-outstations).
This feature needs a name, and I suggest we call it the Australian Empty Quarter.
What is most surprising about the Australian Empty Quarter is how densely-vegetated it is with evergreen perennial plants (https://diamantina-tour.rezdy.com/95387/great-victoria-desert-nullarbor-expedition?lang=ja and https://www.worldatlas.com/articles/where-does-the-great-victoria-desert-lie.html and https://www.abc.net.au/science/photos/2012/11/21/3636626.htm?xml=3636626.mediarss.xml#bigpicturepos and https://www.istockphoto.com/photo/marble-gum-gm532574825-55893698 and https://upload.wikimedia.org/wikipedia/commons/8/81/Eucalyptus_gongylocarpa.jpg and https://www.exploroz.com/wildflowers/395+marble-gum and https://www.alamy.com/stock-photo-camel-crossing-the-great-victoria-desert-western-australia-97792062.html).
The driest part of Australia, which truly does has sparse vegetation, occurs far to the east. It constitutes a different geographical feature, viz.
- the low-lying plain around Lake Eyre (http://www.truganinaweather.com/weather-education/australian-climate-maps.htm and https://en.wikipedia.org/wiki/Lake_Eyre_basin), which includes
- the Strzelecki Desert (https://en.wikipedia.org/wiki/Strzelecki_Desert), and
- the Sturt Stony Desert (https://en.wikipedia.org/wiki/Sturt_Stony_Desert).
The Australian Empty Quarter, despite looking far more luxuriant than the conventional image of desert, is poor habitat for the largest-bodied Australian animals.
This is illustrated by the scarcity/absence of
- the emu (Dromaius novaehollandiae, https://upload.wikimedia.org/wikipedia/commons/f/ff/Emu_distribution.png and https://upload.wikimedia.org/wikipedia/commons/8/8b/Dromaius_novaehollandiae_map_distribution_2.svg), and
- kangaroos (mainly Osphranter rufus, http://www.ecoclimax.com/2017/05/kangaroo-distribution-in-australia-vs.html and https://www.echidnawalkabout.com.au/macropods-kangaroos-of-australia/ and https://bio1152.nicerweb.com/Locked/media/ch52/kangaroo.html).
Various other familiar species of animals show a similar gap in their distributions, e.g.
- Gymnorhina tibicen (https://www.researchgate.net/figure/Magpie-plumage-variants-and-their-distribution-across-the-Australian-continent_fig1_232248336 and https://edition.cnn.com/2019/09/15/australia/australia-magpie-death-intl-hnk-scli/index.html), and
- Grallina cyanoleuca (map in https://www.inaturalist.org/taxa/8583-Grallina-cyanoleuca, and scroll to map in https://animalia.bio/magpie-lark).
(It is noteworthy that feral populations of the dromedary, Camelus dromedarius, have fared well in the Australian Empty Quarter, https://www.inaturalist.org/journal/milewski/58214-diet-of-the-feral-dromedary-camelus-dromedarius-in-australia-part-1#.)
The converse pattern applies to certain slow-metabolising forms, e.g.
- Myrmecobius fasciatus (see Figure 4 in https://www.pnas.org/content/112/15/4531/tab-figures-data), and
- Notoryctes spp. (https://upload.wikimedia.org/wikipedia/commons/9/96/Marsupial_mole_distribution_map.svg and https://www.inaturalist.org/journal/milewski/96648-is-the-australian-mole-notoryctes-a-supermole#).
The most significant plants in the Australian Empty Quarter are hummock grasses belonging to the genus Triodia (https://en.wikipedia.org/wiki/Triodia_(plant)). These are adapted so extremely to nutrient-poverty and fire that they have minimal value for herbivores (https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.2007.00017.x). Their leaves are woody, resinous and unpalatable.
The ultimate environmental reason for the Australian Empty Quarter is a combination of extreme flatness and underlying bedrock of nutrient-poor sediments such as sandstone.
This land has been geologically stable since the time of dinosaurs. Thus, the soils - sandy and poor to start with - have become monotonously depleted of phosphorus and zinc in particular.
This poverty limits herbivory so much that vegetation is left to grow until wildfire returns (https://www.agric.wa.gov.au/rangelands/spinifex-rangeland-pastures-and-fire-western-australia#:~:text=Fire%20has%20a%20significant%20influence,that%20are%20less%20fire%2Dadapted.). Combustion has replaced digestion as the most important recycler of nutrients.
The Australian Empty Quarter is best understood as a nutrient-desert rather than a rainfall-desert, because, even if it were well-watered, it would remain largely devoid of large animals.
Although this zone is semi-arid, it is ecologically aligned with the African Empty Quarter (https://www.inaturalist.org/posts/54390-why-was-angola-the-empty-quarter-for-african-large-mammals#), rather than the conventional concept of a desert.
As an example of a nutrient-desert, the Australian Empty Quarter is far more extreme than the Kalahari-derived sand-sheet, covered with miombo woodland (https://en.wikipedia.org/wiki/Miombo), in Angola. In the latter, the incidence of large animals is minimal by African standards, but categorically greater than in any part of Australia.
One of the principles illustrated by these 'Empty Quarters' is as follows:
There are some climates just too dry for the vegetation to maintain cover over the land, and this is the conventional concept of desert. However, no soils on this Earth are so nutrient-poor that woody plants cannot grow on them, albeit slowly.
Thus arises the combination we see in the Australian Empty Quarter: a vegetated plain effectively useless to man and (indigenous) beast.
Comentarios
The following shows the locations of the major 'deserts' in Australia. However, of these only the relatively small Sturt Stony Desert and Strzelecki Desert are arid enough to conform to the concept of desert on other continents. Even the Simpson Desert is no drier than parts of the Kalahari or Sahel in Africa, which are usually regarded as semi-deserts: https://www.backyardchickens.com/hredirect2.php?c=5495231.
The Great Victoria Desert is acknowledged as the most extensive 'desert' in Australia. However, what does not come out in the usual accounts is that it is also by far the most luxuriantly vegetated 'desert' on Earth, having far more vegetation than, say, the semi-arid zone of North Africa called the Sahel: https://en.wikipedia.org/wiki/Great_Victoria_Desert.
The following is misleading unless one understands that, in Australia, 'desert' is a function not of aridity alone but instead a combination of nutrient-poverty and semi-aridity: https://en.wikipedia.org/wiki/Deserts_of_Australia.
The following map of variation in rainfall from year to year (https://www.researchgate.net/figure/Rainfall-variability-map-for-Australia-based-on-data-between-1900-and-1996-Variability_fig2_319404345) shows a significant anomaly, relevant to the topic of this Post. The Australian Empty Quarter, instead of having particularly variable rainfall (presumably equivalent to proneness to drought), has rainfall more reliable than that in areas to the west and to the east.
Agregar un comentario