Adventures in the Wild

Convergence - the Albany Pitcher Plant

I've always been fascinated by evolutionary convergence, the phenomenon whereby distantly related species, responding to similar selective pressures, evolve similar or even identical solutions to the same problem.

In swamps around Albany, where the nutrient levels are very low, the Albany pitcher plant Cephalotus follicularis has come up with the ingenious solution to its nutrition problems of developing pitchers in which it traps insects that then supplement the meagre supply it gets from the soil. But, of course, you've heard this story before. In South East Asia, members of the genus Nepenthes have done exactly the same thing, as have Sarracenia the trumpet pitchers in the Americas.

Cephalotus follicularis is the sole species in its family Cephalotaceae confined to the south coast of Western Australia. Its closest relatives are apparently members of the Saxifrage family Saxifragaceae and so it has evolved insect trapping pitchers completely independently of Nepenthes and Sarracenia. Some of the features of such plants are obvious design features, the swollen receptacle for the digestive fluids with the lid to keep out the rain. But other features are not quite so obvious.

The margins of the pitcher in Cephalotus follicularis are strongly ribbed. Presumably this plays some role in for the same design reappears in some members of Nepenthes.

Nepenthes villosa another narrowly restricted species, endemic to the upper slopes of Mount Kinabalu on the island of Borneo. Althogh found in a completely different habitat to the Albany Pitcher Plant, this species has found a similar insect trapping solution, even down to the use of a strongly ribbed margin on its pitchers.

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.

Limestone Forest - another world

Today I decided to go for another walk in the forest that lines the trail between Baie d’Upi and Baie d’Oro. It had rained during the early hours of the morning, and so I woke up and made doubly sure that none of my things were touching the sides of the tent, dreading that I might wake up with everything soaked. When I was in Nouméa, thinking that it was the last time I would use it, I washed my tent in the washing machine. It came out so clean and silky that I wondered if the detergent had perhaps washed off some waterproofing. But in the morning all was still dry, so it would appear that all I had washed away was 14 years of grime.

With the overnight rain, the forest was all steamy in the morning sun, pungent with the smells of earth and decay. The first part of the trail south of Baie d’Oro, where the land is flat, runs through a forest in the process of transformation into a coconut plantation. Conversion here is not a dramatic process, but a gradual favouring of coconuts over the indigenous vegetation, with large trees felled in the traditional way, by ringbarking them and then lighting fires around their base until they fall over where they appear to be left to rot into the earth.

In the interim it’s still an incredibly rich environment, full of the calls of endemic birds, flitting between the coconuts and the pines.

As soon as the trail left the flat however, it entered the forest proper. Yesterday I had run into an Australian woman Rosemary and her teenage son Paul, so I don’t remember seeing very much on the walk through this forest. But it’s a world that repays close attention.

A world of rich, lush green, where miniature green butterflies pollinate miniature green passionflowers;

where tiny snails are lost on enormous green leaves;

where diminutive skinks fight over little flecks of sunlight, and when approached hold their heads still, regarding the intruder, while their tails quiver as if to say “Pick me! Pick me!”

This mantle of green is thrown over the most unlikely substrate, a rocky plateau of limestone rock, where tree roots disappear down into holes in the earth,

and miniature caves provide refuge for enormous crabs, the primary consumers of the leaf litter here.