Concept · keystone idea · Ch 8
The gradient (reading ecology as a climb)
Climb the short distance from Gulf St Vincent to the Mount Lofty summit and three things change together — rainfall rises, temperature falls, soil turns from alkaline to acid — and every living community on the way is simply the local answer to that one changing ration of salt, water and lime.
First, meet: The Mediterranean bargain (two seasons, not four)
Stand on the lookout at the summit of Mount Lofty, a little over seven hundred metres up and some fifteen kilometres east of the city, and you are not admiring a view so much as reading a diagram. The land falls away below you in one continuous slope, from the forest at your feet down through suburb and plain to the shine of Gulf St Vincent, and everything alive on that slope has arranged itself according to the same short list of rules. That is the gradient, and once you can see it, you can read the whole region from it.
Three things change as you climb, and they change together. Rain increases: the plains scrape by on a few hundred millimetres a year while the high ranges, wringing the moisture out of the same westerly winds, gather something close to twice that. Temperature falls, so the summit forest is a cooler, damper world than the baking flat below it. And soil follows both, more slowly: down on the dry plains, rain has never been enough to rinse salt and lime out of the ground, so the earth stays alkaline and calcareous; up in the wetter hills, the extra water finally rinses it away, leaving soils that are leached and acid by comparison. None of this is a coincidence of three separate stories — it is one story, because in a Mediterranean climate rainfall is the variable that sets almost everything else, including how a soil turns out (Cowling et al. 1996; Hall, Maschmedt & Billing 2009).
Walk that arch and you walk through the book: seagrass and samphire at the gulf, the ploughed remnant of grassy plain, stringybark forest on the range front, fern gully in the wettest folds, and — because the ranges steal rain from the country behind them, leaving a rain shadow on their inland side — mallee scrub over the crest, dry again in a different key. Each community is the local answer to its ration of salt, water and lime, which is why a good field ecologist can look at a single plant and tell you, roughly, where on the slope it belongs. Mediterranean climates like this one are rare on Earth and disproportionately rich in species found nowhere else (Myers et al. 2000), so this modest, driveable slope is Adelaide’s organising idea, and a working model of why these five thin bands of climate matter globally. Learn to read the gradient here and you have a key that opens far more than one gulf and one range.
Go deeper — the mechanism & the evidence
The climb is short in kilometres and steep in consequence because South Australia is a Mediterranean-climate country, and a Mediterranean climate is what does the sorting (Cowling et al. 1996). Moist westerlies coming off Gulf St Vincent are forced up the range front, cooling as they rise and dropping their water — so the Adelaide plains average only a few hundred millimetres of rain a year while the high Mount Lofty summits catch roughly twice that, and temperature falls as the rain rises. Soil is the gradient's third, slower variable, and it is driven by the same rainfall: on the dry plains, low rainfall never flushes carbonate and salt out of the profile, leaving calcareous, alkaline soils (Calcarosols and Chromosols); in the wetter hills, the extra water finally leaches those salts away, leaving acid, leached Kurosols behind (Hall, Maschmedt & Billing 2009) — the same climate-leaching logic J.A. Prescott formalised at the Waite as a rainfall-to-evaporation ratio (Prescott 1950). One further twist: the ranges do not just catch rain, they steal it, so the driest ground of all sits not at the gulf but on the ranges' inland side, in their rain shadow — the arch is lopsided, not a simple ramp.
Sources & further reading
Every claim here is traceable. Each citation links to its entry in the bibliography.
- Cowling, R.M., Rundel, P.W., Lamont, B.B., Arroyo, M.K. & Arianoutsou, M. (1996). Plant diversity in mediterranean-climate regions. Trends in Ecology & Evolution 11(9):362-366. https://doi.org/10.1016/0169-5347(96)10044-6 — Mediterranean summer-drought as the master control sorting plant communities along an environmental gradient.
- Hall, J.A.S., Maschmedt, D.J. & Billing, N.B. (2009). The Soils of Southern South Australia. The South Australian Land and Soil Book Series, Volume 1; Geological Survey of South Australia, Bulletin 56, Volume 1. Department of Water, Land and Biodiversity Conservation, Government of South Australia. https://data.environment.sa.gov.au/Content/Publications/The%20Soils%20of%20Southern%20South%20Australia%20-%20Summary%20of%20Major%20Soil%20Groups.pdf — The plains-to-ranges soil sequence — calcareous, alkaline plains soils giving way to leached, acid soils in the wetter hills.
- Myers, N., Mittermeier, R.A., Mittermeier, C.G., da Fonseca, G.A.B. & Kent, J. (2000). Biodiversity hotspots for conservation priorities. Nature 403: 853–858. https://www.nature.com/articles/35002501 — Why a gradient through a Mediterranean climate zone like this one carries global biodiversity-hotspot stakes.
- Rundel, P.W., Arroyo, M.T.K., Cowling, R.M., Keeley, J.E., Lamont, B.B. & Vargas, P. (2016). Mediterranean Biomes: Evolution of Their Vegetation, Floras, and Climate. Annual Review of Ecology, Evolution, and Systematics 47:383-407. https://doi.org/10.1146/annurev-ecolsys-121415-032330 — How Mediterranean-climate regions worldwide sort their floras along the same rainfall-and-soil axis.
See it in the country
Adelaide rosellaMount Lofty summitRiver red gumShort-beaked echidnaThe gulf-to-range transectWestern grey kangaroo
Grounded in A Coast Running Out of Water. Every figure is stated once in the book's canonical facts register and cited to a published source.