How convergent are perennial daisies in mediterranean-type climates? part 2

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...continued from https://www.inaturalist.org/journal/milewski/73287-how-convergent-are-perennial-daisies-in-mediterranean-type-climates-part-1#

NUMBERS OF GENERA AND SPECIES:

Under mediterranean-type climates, the floras of perennial daisies show the following decreasing order in richness of spp.: South Africa (more than 600 spp.) > California (about 250 spp.) > Chile > South Australia > Western Australia.

The numbers of genera follow the same order, except in the case of the two richest floras. This is because California has by far the most genera: 71, compared with 54 in South Africa.

The region in South Africa is less extensive than e.g. Sardinia (https://en.wikipedia.org/wiki/Sardinia) or Sicily (https://en.wikipedia.org/wiki/Sicily). Yet, it contains up to 620 spp. of perennial daisies (readers, please correct this by eliminating any annual/biennial spp. listed in part 1).

This forms an extreme contrast with Western Australia. In this region, there are few indigenous genera (14) and spp. (26 at most) of perennial daisies (and even annual spp. are few).

An obvious biogeographical pattern in California is that the flora of daisies is richest at high altitudes in the Sierra Nevada range (https://en.wikipedia.org/wiki/Sierra_Nevada) - the highest peak of which (4421 m. a.s.l.) is also the highest point in the contiguous United States of America. Many of these spp. do not extend down to the mediterranean-type climate (the altitudinal threshold of which I assume to be 2000 m a.s.l.).

GROWTH-FORMS WITHIN THE PERENNIAL CATEGORY:

The daisies in the various regions differ far more than predicted by the principle of evolutionary convergence.

In particular,

  • the floras of daisies, at the generic level, are largely different among the regions,
  • the floristic differences are accompanied by surprising differences in growth-forms and other ecological features, and
  • even where the genera are in common, the spp. tend not to resemble each other in growth-forms.

Shared genera include

A remarkable genus, in the context of phylogenetic sharing among the regions with the mediterranean-type climate, is Senecio (https://en.wikipedia.org/wiki/Senecio).

This genus is indigenous to all these regions on Earth, including the Mediterranean Basin itself. However, Senecio has speciated and radiated ecologically in South Africa - far more than even in California. For example, at least three spp. of Senecio in South Africa have foliar succulence (https://www.inaturalist.org/observations/99438247) under the mediterranean-type climate.

The growth-forms of perennial daisies are largely different in the various regions, as follows.

In Western Australia, the flora of daisies and the range of growth-forms are both limited.

In California, the indigenous daisies tend to resemble the confamilial weeds and ruderal pioneers (https://en.wikipedia.org/wiki/Ruderal_species) in ploughed, mined, and urbanised landscapes worldwide. This includes typical thistles (https://en.wikipedia.org/wiki/Thistle), no genus or species of which is indigenous to the other regions in this study.

In South Africa, a considerable number of spp. (e.g. in genera Euryops, https://www.inaturalist.org/observations/20174572, Metalasia, https://www.inaturalist.org/taxa/589916-Metalasia-densa, and Felicia, https://www.inaturalist.org/taxa/528743-Felicia-filifolia) have an ericoid growth-form (https://en.wikipedia.org/wiki/Ericoid).

These are 'sclerophyllous' low shrubs with small, terete or cylindrically rolled (but not spinescent) leaves, resembling Erica (https://en.wikipedia.org/wiki/Erica_(plant)).

In California, only Ericameria contains ericoid spp. (https://www.inaturalist.org/observations/144172914 and https://www.inaturalist.org/observations/138187830).

In Australia, ericoid daisies are absent. This growth-form is extremely common in the vegetation, but is occupied by families such as Myrtaceae (e.g. https://en.wikipedia.org/wiki/Baeckea), to the exclusion of daisies.

I have found limited information on the spectrum of growth-forms in the case of Chile. However, several spp. of daisies are drought-deciduous (https://en.wikipedia.org/wiki/Drought_deciduous) under the mediterranean-type climate in Chile and California.

Deciduousness (including winter-deciduousness) seems unusual among daisies, and drought-deciduousness - which generally correlates with nutrient-rich soils - is rare in South Africa and absent in Australia, under the mediterranean-type climate.

The genus Chaetanthera (https://www.inaturalist.org/journal/milewski/73287-perennial-daisies-in-mediterranean-type-climates-part-1/edit) is remarkable for

  • its morphological plasticity, and
  • having speciated in the central part of the country.

However, only two perennial spp. of Chaetanthera occur under the mediterranean-type climate in Chile. Thus, Chile lacks the multi-spp. 'evolutionary radiations' so obvious in the lists of genera that I have presented for California and, particularly, South Africa.

POSSIBLE EXPLANATION:

It is clear that, in defiance of theoretical predictions, indigenous daisies differ among the various regions with mediterranean-type climates. Most of the genera occur only in a particular region, and even the shared genera express themselves differently in the various regions.

These differences could be analysed in enough ways to fill a whole book. However, I choose to focus on the extreme richness of perennial daisies in South Africa vs the extreme poverty of perennial daisies in Western Australia.

The floristic poverty of daisies is one of the most remarkable features of Western Australia under the mediterranean-type climate. It seems particularly anomalous because, among all the regions on Earth with mediterranean-type climates, Western Australia and South Africa seem to be most analogous in geographical situation and the widespread incidence of sandy substrates.

This disparity in the floras cannot be explained by geographic isolation.

For example, the remote Galapagos archipelago (https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1903.tb04962.x) contains about as many genera and spp. of perennial daisies as the southwest botanical province (https://en.wikipedia.org/wiki/Botanical_Provinces_of_Western_Australia), which is fully contiguous with the continent of Australia.

However, the disparities may partly be explained by nuances within an ecological syndrome based on flat topography, senile substrates, nutrient-poor soils, and intense wildfires (https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.2007.00017.x).

A clue is that the typically Australasian genus Olearia (https://en.wikipedia.org/wiki/Olearia) contains more spp. in South Australia than in Western Australia, under the mediterranean-type climate. This is associated with the slight renewal of nutrients brought by the topography (Mt Lofty range,https://en.wikipedia.org/wiki/Mount_Lofty_Ranges, and, marginally, Flinders range, https://en.wikipedia.org/wiki/Flinders_Ranges) and Murray River (https://en.wikipedia.org/wiki/Murray_Mouth), in South Australia.

It is hard to generalise about a group as diverse as daisies. However, I offer the following conceptual framework.

Many of the important families under the mediterranean-type climate in southwestern Australia, and to a lesser extent southwestern South Africa, have rhizal adaptations to regimes of poverty of phosphorus, zinc, and other nutrients, and to depletion of organic nitrogen by intense combustion.

For example, Proteaceae (https://en.wikipedia.org/wiki/Proteaceae) and Restionaceae (https://en.wikipedia.org/wiki/Restionaceae) have cluster roots (https://en.wikipedia.org/wiki/Cluster_root#:~:text=Cluster%20roots%2C%20also%20known%20as,just%20beneath%20the%20leaf%20litter.), and Fabaceae have nitrogen-fixing nodules (https://en.wikipedia.org/wiki/Root_nodule).

One of the distinctive aspects of daisies is that they generally lack rhizal specialisations, beyond the arbuscular mycorrhizae (https://en.wikipedia.org/wiki/Arbuscular_mycorrhiza) present in most plants. In this sense, they tend not to be particularly adapted to the nutritional regime epitomised by southwestern Australia, where soils are extremely depleted and wildfires are extremely pervasive.

Daisies instead tend - as a bold generalisation - to be adapted to regimes of physical disturbance of soils, whether by animals or by inanimate means such as wind, water, or gravity. The light, wind-dispersed seeds typical of daisies are suited to prompt arrival wherever patches of disturbed ground arise, ranging in size from molehills through landslides to windblown dune-systems.

One of the puzzling differences between Australia and southern Africa is in the incidence of fossorial and subterranean animals such as mole-rats (https://en.wikipedia.org/wiki/Blesmol).

These animals occur widely in South Africa, even on soils derived from sandstone or quartzite, whereas they are absent or relatively scarce even on deep loams in Australia.

Furthermore, South Africa is unusually rich in other digging mammals such as golden moles (https://en.wikipedia.org/wiki/Golden_mole) and the aardvark (https://en.wikipedia.org/wiki/Aardvark), which lack counterparts even in California and Chile.

All the regions studied here, except for those in Australia, have mole-rats or analogous rodents (https://en.wikipedia.org/wiki/Tuco-tuco and https://en.wikipedia.org/wiki/Botta%27s_pocket_gopher).

Soil-disturbing animals affect the environment not only physically but also nutritionally. This is because turnover of topsoil and litter - like tilling in horticulture/agriculture - tends to promote mineralisation. This favours unspecialised rhizal systems, such as those of most daisies.

According to this framework, there is a nearly categorical difference, environmentally, between South Africa and Western Australia.

The regime in Australia typically minimises the normal rate of recycling of nutrients, punctuated by occasional deposition of ash by wildfires that consume green crowns as well as dead plant-parts. This combustion-based, extremely episodic reycling is wasteful of nitrogen, sulphur, selenium, and other nutrients, volatilised in smoke or subsequently leached from the soil.

This framework may help to explain why nitrogen-fixing plants have a far greater incidence in Australia than in South Africa, even where climates are similar. Just as southwestern Australia is poor in daisies, it is rich in both

  • nitrogen-fixing leguminous and casuarinaceous plants, and
  • ectomycorrhizal plants (https://en.wikipedia.org/wiki/Ectomycorrhiza), in which the rhizal mutualism with powerful fungi retrieves nitrogen from litter - in a way unrecorded in any plant in South Africa under the mediterranean-type climate

A relationship worth testing, in future, is that between daisies and nitrogen-fixing plants in Australia and South Africa. I predict that in kwongan (https://en.wikipedia.org/wiki/Kwongan), fynbos (https://en.wikipedia.org/wiki/Fynbos), and other relevant types of vegetation, there is an inverse correlation between nitrogen-fixing plants (Fabaceae, Casuarinaceae, Zamiaceae, Myricaceae, etc.) and daisies.

I further predict that this inverse correlation applies to the similar climates in Chile and California - where Rhamnaceae (e.g. https://en.wikipedia.org/wiki/Trevoa and https://en.wikipedia.org/wiki/Ceanothus) have partly usurped the nitrogen-fixing role from Fabaceae..

Posted on December 13, 2022 02:17 AM by milewski milewski

Comments

@tonyrebelo

Here is a challenging question for you. Are any spp. of Asteraceae in the southwestern Cape deciduous?

Posted by milewski about 1 year ago

Thinking about burrowing animals in Australia, vs mole-rats. Below is by no means an extensive list, just random examples that popped into my head.
We of course have wombats, a bit bigger I guess 😉
Kangaroos dig the top surface extensively, as do White-winged Choughs & Echina's.
I would guess that the bulk of our larger burrowing animals are our reptiles; skinks, dragons, snakes & goanna's.
In our neck of the woods (Murray Mallee), the region is covered with a limestone shelf, sometimes on the surface, other time burried under 60m of ground. This limestome was created from shells where much of the region was an ocean millenia ago; the limestone streches for thousands and thousands of square kms. The underground limestone leaches up making nearly impenetrable calcrete surface rock. As such, animals like wombats make the best burrows by finding a hole in the calcrete from a mallee tree trunk that has long since rotted away.
So our burrowing animals need to be smaller; ie inverts eg spiders, ants, mole-crickets, beetles, etc. Many times they only have an inch of sandy loam to burrow in.
Everywhere we go here there are thousands of wolf spider holes.
Seed has the potential to fall in these disused burrows & germinate.
We have significant quantities of Daisy-bushes (Olearia);
Olearia muelleri would be our predominant Olearia species, thousands of specimens, www.ellura.info/Shrub.html#Olearia-muelleri
2ndly O. magniflora, then 3rdly O. brachyphylla.
But I wouldn't say the ground is disturbed. It's got an excellent microbiotic crust, which helps retain mosture. We get ~275mm avg rainfall p.a.
Mallee stops Olearia from thriving, so we see Olearia more in lower story senna & saltbush areas.
The other type of landscape we see in the semi-arid areas of South Australia are old sand dunes. Not the Saudi-arabia kind, you actually wouldn't recognise them as such. But each one seems to have it's own biosphere that's separated from the adjacent dunes by the lower levels (often filled with mallee). The species variation between crest of adjacent sand dunes can be quite staggering, compared to places like Ellura which is much flatter.

Posted by ellurasanctuary about 1 year ago

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