Species-coexistence in mallee-heath in Western Australia: up to 150 species of plants growing together in one type of shrubland

A familiar concept in ecology is the species-richness of any given community - technically called 'alpha-diversity' (https://en.wikipedia.org/wiki/Alpha_diversity).

Some communities of organisms contain just a few species, while others contain many species which have found a way to coexist in the same habitat. And this variation can be surprisingly great.

Eucalypts (Eucalyptus, https://en.wikipedia.org/wiki/Eucalyptus) growing naturally in the form of shrubs are the characteristic plants over large areas of southwestern Australia, under a temperate climate with rainfall mainly in winter. The climate is similar to those of southwestermost South Africa, southern California, and coastal parts of the Mediterranean Basin. However, the vegetation can be extreme in its species-richness.

This is exemplified by Fitzgerald River National Park (https://en.wikipedia.org/wiki/Fitzgerald_River_National_Park) and its environs, on the southern coast of Western Australia (see https://researchrepository.murdoch.edu.au/view/author/Newbey,_Kenneth.html).

On well-drained lithified dunes just inland from the littoral zone, the mallee vegetation consists of Eucalyptus angulosa (https://www.inaturalist.org/taxa/208143-Eucalyptus-angulosa), Eucalyptus falcata, Melaleuca pentagona (https://www.inaturalist.org/taxa/1255442-Melaleuca-pentagona) and only a few other species. The soil is calcareous sand over limestone, and the number of plant species coexisting in this community is as small as 12.

By contrast:

On the plain a few kilometers inland, a duplex soil (siliceous sand over pale clay) has developed over a siliceous sedimentary rock called spongolite (https://en.wikipedia.org/wiki/Spongolite and https://www.researchgate.net/publication/236842784_Eocene_spiculites_and_spongolites_in_southwestern_Australia_Not_deep_not_polar_but_shallow_and_warm).

Here, mallee shrubs of Eucalyptus pleurocarpa (https://en.wikipedia.org/wiki/Eucalyptus_pleurocarpa and https://apps.lucidcentral.org/euclid/text/entities/eucalyptus_pleurocarpa.htm) form an upper stratum over a knee-high heathy stratum. Here the number of plant species coexisting intimately within a single community can reach up to 150.

The typical soil-profile where species-richness is greatest is 10-15 centimeters of loamy sand (in places containing black, shiny gravel) over a monotone/slightly mottled clay between the depth of 15 centimeters and the depth of 70 centimeters, over weathering spongolite (70-80 centimeters), over spongolite rock.

Another way of describing this soil-profile is that it:

In this exceptionally species-rich vegetation, up to approximately 100 species of small-leafed, evergreen, flammable shrubs (mainly Proteaceae, Myrtaceae and Fabaceae but also Casuarinaceae, Ericaceae, Dilleniaceae, Rutaceae, etc.) share the same stand.

The shrubs in question are typically knee-high, and all have rhizal adaptations - ranging from cluster roots (https://en.wikipedia.org/wiki/Cluster_root) to ectotrophic mycorrhizae (https://mycorrhizas.info/info.html), and from rhizobial to actinorhizal (https://en.wikipedia.org/wiki/Frankia) nodules for fixation of atmospheric nitrogen.

In this exceptionally species-rich heathy stratum, it seems that every new individual plant examined adds another species to the tally.

The following show the general appearance of the community in question: https://www.inaturalist.org/observations/92880446 and https://www.inaturalist.org/observations/88921818 and https://www.inaturalist.org/observations/88491761 and https://www.inaturalist.org/observations/88400105.

The species-poverty of the first community described above, on calcareous dunes, may come as no surprise to naturalists in South Africa, California and the Mediterranean Basin, where introduced eucalypts are notorious for their suppression of other plants.

However, this connotation makes it all the more remarkable that, in the second community described above, Eucalyptus coexists with a shrubby component so species-rich as to exceed all other heathlands (in the loose sense) on Earth - including fynbos (https://www.jstor.org/stable/3235578) in South Africa with its particular reputation for beta-diversity (https://en.wikipedia.org/wiki/Fynbos and https://en.wikipedia.org/wiki/Beta_diversity and https://link.springer.com/chapter/10.1007/978-94-009-9200-9_5 and https://link.springer.com/chapter/10.1007/978-3-642-68935-2_19 and https://link.springer.com/article/10.1007/BF00121419).

Posted on Τρίτη 01 Μάρτιος 2022 23:55:38 UTC by milewski milewski


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A species-area curve for Elim Flats dwarf fynbos on laterite near what is now Agulhas National Park(data collected by Sue Milton) shows a steep rise levelling off by 40 square meters at about 80 plant species, but finally tallying 90 species. These belonged to 66 genera in 30 families. I can compare the flora in this South African sample-plot with that in a remarkably comparable sample-plot in vegetation of similar (minimal) height in kwongan just east of Fitzgerald River National Park (near Hopetoun airstrip), which I studied myself. The special interest of this comparison is how closely the climates and soils are matched. The tally of species in the Australian case was 76 plant species (considerably less than in the South African case), and these belonged to 55 genera in 20 families. No species was shared intercontinentally between these sample-plots, but the number of genera shared was one and the number of families shared was ten.

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Kwongan on skeletal sandy soil quartzite on the Barrens in Fitzgerald River National Park is fairly rich in plant species, but does not rival mallee-heath on duplex soil over spongolite. The vegetation poorest in plant species in or near Fitzgerald River National Park is on calcareous sand over limestone, where there are only 10-12 species in some sample-plots. These are well-drained lithified dunes. Species are e.g. Eucalyptus angulosa, Melaleuca pentagona, and Eucalyptus 'falcata'.

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