Kwongan - a coral reef on land

A coral reef on land. Kwongan heath at Ledge Point, Albany with species of Dampiera, Gastrolobium and Melaleuca growing in close proximity. 

I’m currently in Western Australia exploring the wonders of the Southwest Botanical Province. The primary driver for my visit was to attend the conference of the Taxonomic Database Working Group, an international conference devoted to the development of standards and technologies for the free exchange of biodiversity data. I had attended the conference the last time it was held in Australia, in Sydney in 2001, and after bouncing around the globe the conference was to be held in Australia again, this time in Fremantle. However, as keen as I was to attend the conference, I was also excited to visit Perth again, for the first time in over 9 years. When discussing the trip with others beforehand many people volunteered opinions on Perth, both positive and negative. I quite like Perth, but I realised in having these discussions that for me, talking about Perth is a proxy for the whole southwest part of Western Australia. In the same way that a large part of the appeal of living in Sydney is the encircling belt of sandstone wilderness within the Greater Blue Mountains World Heritage area, the major appeal of a visit to Perth is the chance to visit the southwest botanical province, the richest floral area in the country.

Western Australia is known as the wildflower state, but many may not realise how apt that description is, for in the southwest of the state, stretching in an arc from Kalbarri in the north to Esperance in the east, the winter rainfall zone with its mediterranean climate has a flora that puts on a wildflower spectacle that has to be seen to be believed. Within an area of roughly 350,000 km2 are found some 5,571 species of vascular plants making it the most speciose part of Australia, dwarfing the famed rainforest flora of the Wet Tropics of Queensland with 3,181 species. Moreover the heaths, or kwongan to use their Nyoongar name, of the richest parts of the southwest such as Mt Lesueur, the Stirling Ranges or the Fitzgerald River NP have so many species found in small plots that they compete with the species rich rainforests of Borneo and the Amazon. This diversity is all the more surprising, for the southwest of Western Australia is one of the flattest parts of what is already a very flat continent, so that this speciation has occurred in an environment with little obvious environmental variation.

It has been known for a long time that species diversity is highest in the tropics, and many theories have been devised for why this might be so, focussing on the high productivity of the tropics where the climate is favourable for growth year round. However, the Cape region of South Africa with some 9,000 species and the southwest of Western Australia with 5,571 species, both areas far from the tropics and with strongly seasonal Mediterranean climates have a plant diversity that matches that of most tropical forests. Moreover, species diversity in both places is actually highest in the heaths, vegetation forms of low shrubs on infertile soils with resultant low primary productivity. Some authors such as Tim Flannery have even suggested that it is this low fertility that fosters the high diversity, by preventing the domination of the ecosystem by one species. But these theories too are incomplete, for just as there are high diversity areas outside the tropics, there are also high diversity forests on fertile substrates, and likewise low diversity ecosystems on infertile ones.

I always imagined that the patterns of diversity in the world might be too complex to explain and so elude any simple explanation. But then I read the paper of Dynesius & Jansson. In their 2000 paper, Evolutionary consequences of changes in species' geographical distributions driven by Milankovitch climate oscillations, they made the observation that we live in an ice age world where, at least for the last 2.5 million years, the earth’s climate has repeatedly swung between glacials and interglacials. They made the further observation that areas of the earth differ dramatically in their response to such glacial cycles. In the most extreme case, the northerly parts of Eurasia and North America were covered with enormous glaciers extinguishing all terrestrial life which had to migrate south or perish. However, in the tropics, the change in temperatures was much less so that in some parts of the world, species might be able to persist with little or no need to migrate. The beauty of their hypothesis is that, when the earth’s climate during the height of the last glacial cycle is modelled, it turns out that the Cape region of South Africa as well as southwestern Western Australia also showed very little change in temperature from the modern day. As a result species in these areas were able to persist and over successive glacial interglacial cycles accumulate species through a long process of more and more refined adaptation to the local environment. Europe, by contrast, is populated by a rather limited range of highly adaptable, highly mobile species, which manage to survive only through massive shifts in range each time the climate shifts from warm to cold or back again.

It is worth noting that Dynesius and Johansson's hypothesis most strongly applies to the Quaternary, during which the glacial-interglacial cycle has been the dominant feature of the earth’s climate. As a result, it can only be invoked as an explanation for increased rates of speciation during that period. Indeed, the diversity of the flora of the southwest of Western Australia at the family level is relatively low, with many of the species being found in a few dominant families. In contrast to the Wet Tropics of Queensland with its many families of primitive flowering plants, most of the radiations in southwest WA are relatively modern. While the number of families and genera is low, within many of these genera speciation has been incredible with staggering numbers of species in dominant genera such as  Eucalyptus, Acacia and Banksia.

Banksia coccinea

Banksia formosa

Banksia grandis

Banksia praemorsa

Banksia gardneri var. gardneri Not only is the southwest of Western Australia incredibly rich in Banksia species, but the species here seem to be the strangest of the genus, with peculiarities such as Banksia gardneri, a prostrate species of Banksia whose stems creep along the ground so that its flowers emerge directly from the sand. Kevin Thiele, of the Western Australian Herbarium calls this the wacky factor. As a broad generalisation, if there is any genus of plants shared between southwestern and eastern Australia, the species in the east will be dull, while the species in the southwest will be wacky, their bizarre beauty developed to baroque extremes.

Comments

[Jarrett]

Only your reliable, calm scientific voice can be trusted to deploy terms such as "wacky factor." Though I'm inclined to post a few B ericifolia flowers to illustrate, for your readers, how "dull" the eastern Banksias are. :)

Wonderful photos and narrative.

[Jarrett]

And I love the caution with which you seem to approach the B gardneri, as though it might run away.

[Teresa Gilman]

".....for the past 2.5 bajillion years earth's climate has swung between glacials and interglacials...."

Now, is this a trick concept or what? I looked up interglacial and it said "formed or occurring between two glacial epochs"

OK.

So, what are they saying then in their mobiusstriptalk?

FA

[Philip Gleeson]

Ok, Jarrett, so eastern Australian Banksias are still rather weird, but nonetheless there isn't anything quite like Banksia coccinea and certainly nothing quite so odd as the several species of prostrate Banksia in the southwest. The point could be much more strongly illustrated with Xanthosia or Darwinia or even Eucalyptus.

Teresa, the important point to note is that during the Quaternary, due to the influence of Milankovitch cycles on the earth's climate, the earth has swung in and out of ice ages roughly every 100,000 years. Given that this period of time is relatively short compared to the lifetime of most species, which is on the order of 1 to 3 million years, then this climatic cycle is an important determinant of the earth's current biodiversity. Put another way, every species you see in existince today has to have had the ability to survive through ice ages. If they live in a part of the earth where the influence of ice ages is extreme, such as Europe or North America, this means that they have to have the ability to migrate over large distances every time the cycle changes from one to the other. However, in some climatically blessed parts of the globe, the tropics in general, but also a few odd places such as South Africa and southwest Western Australia where the surrounding oceans moderate the climate, the species don't have to move nearly so far. As a result, these parts of the earth have been able to accumulate species to become far richer in biodiversity than other parts of the globe.