The nature of biodiversity

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Biodiversity can be defined as the variety and abundance of species, their genetic composition, and the communities, ecosystems and landscapes in which they occur (Maclaren, 1996). The term is sometimes used very narrowly to refer to species-richness, but should be employed to describe the full complexity of life on Earth. Such descriptions still have far to go; as Box 7.1 points out, some 200 000 fungi have so far been described but these represent only a fraction of those believed to exist. Localized species-richness at a particular place is called alpha diversity, but the biotic community often changes in a traverse of the landscape as soil, slope or disturbance such as fire changes, frequently creating locally different habitats within a forest: beta diversity measures the extent of such change along a gradient and can be thought of as the diversity within a landscape. Gamma diversity is similar to alpha diversity but is a measure of species richness across a range of habitats within a larger geographical area and is often used to show regional diversity, which may include forests and other types of vegetation. Temporal diversity refers to the change of species over time. In practice alpha diversity is often calculated as a diversity index (such as the Simpson Index or the Shannon-Weiner Index), incorporating the number of species within an area and the evenness of spread of individuals across those species. Whittaker et al. (2001) gives a good review of the subject. Kimmins (1992) found that many temperate forests have low alpha but high beta diversity, while the reverse is true for many tropical rain forests, although seasonal rain forests can have both high alpha and beta diversity (Zanne and Chapman, 2005). Thus even within forests biodiversity varies greatly in different parts of the world.

Biodiversity within particular populations and habitats may be very important for their continued success and survival in the face of both competition and disease. A species or population showing a considerable range of genetic makeup may well have at least some strains able to resist incoming disease, while others may respond better to particular competitive or habitat changes. Such genetic variation within a species and the continuing evolution within particular populations can eventually culminate in forms so different that they are recognized as new and different species. The value of genetic variation is discussed by Booth and Grime (2003) in the introduction to a paper on the effects of genetic impoverishment on plant community diversity. They point particularly to comments by Harper (1977, p. 707) that 'Diversity of a plant community is inadequately described by the number and abundance of the species within it' and that 'a major part of the community diversity exists at the intraspecific level' (i.e. within a species). Their own experiments were concerned with 11 long-lived herbaceous species growing on an area of ancient limestone pasture in North Derbyshire. These were used to create a number of model communities identical in species composition but widely contrasted in genetic diversity. These communities were allowed to develop in microcosms containing natural rendzina soil (see Section 2.2.1) and exposed to a standardized regime of simulated grazing and trampling. Though a gradual loss of species diversity occurred in all three treatments employed it was, at the end of the 5-year experiment, highest in the most genetically diverse communities, a result of significance to all ecosystems including woodlands. Evidence of the type shown here and in Section 6.1 demonstrates that while we should be concerned over the loss ofspecies, we should also be highly concerned over the more insidious and often largely unnoticed loss of genetic diversity.

6.3.2 Geographical patterns of old and young species in African forests

Given the high biodiversity of the world's forests (an estimated 90% of all terrestrial species) and the fact that over 8000 tree species are thought to be threatened with extinction (see Section 1.1.1), plus the role forests play in human welfare (Section 1.2), forests are obviously an important priority for conservation. However, forests are more than mere repositories of species; they can also be an important source of new species, vital to the evolutionary development of forests in the future. In their consideration of this problem

Figure 6.2 Map of central Africa showing geographical variation in the ratio between numbers of species in young and older groups of bird species. Slanting lines interrupting the signature indicate that the number of species is too low for a reliable calculation. (From Fjeldsa and Lovett, 1997. Biodiversity and Conservation 6, Fig. 3. With kind permission of Springer Science and Business Media.)

Figure 6.2 Map of central Africa showing geographical variation in the ratio between numbers of species in young and older groups of bird species. Slanting lines interrupting the signature indicate that the number of species is too low for a reliable calculation. (From Fjeldsa and Lovett, 1997. Biodiversity and Conservation 6, Fig. 3. With kind permission of Springer Science and Business Media.)

Fjeldsa and Lovett (1997) concentrated on the evolution of both birds and plants, which has been studied by many workers over a wide area. DNA evidence has been particularly valuable in determining when different species first evolved. There was, for example, a speciation burst in the bushshrikes (Malaconotinae) in the Miocene (5-24 Ma), and an explosive radiation of starlings (Sturnidae) in the Plio-Pleistocene (1-4 Ma). Figure 6.2 illustrates avian results in part of the area covered. Close examination of montane forest habitats in East Africa shows that they have high concentrations of species with restricted distributions. Peak concentrations of neoendemics, recently evolved species of local origin, occur in the same mountains as clusters of distinctive old species with relictual distributions.

Africa, in common with the rest of the world, has experienced many changes in its climate over the years, and an early theory was that speciation occurred as a result of temporary fragmentation of the main rain-forest blocks in Pleistocene forest refuges. The combined results of many workers in diverse fields have led to a re-interpretation of the spatio-evolutionary pattern of forest-adapted species in tropical Africa. The current view is that there are a number of montane areas of particular importance as evolutionary centres (or 'evolutionary fronts') from which large numbers of young species have migrated (Fig. 6.3). Vertical shading on the main diagram indicates 'museum areas' of lowland forest where species of potentially diverse origins accumulated

Importance Forest With Diagram

• o The principal and secondary centres for evolution h HiIi|i|i| Long-term accumulation of species of new species of forest birds and plants

Interchange during very humid climatic periods * Several biogeographically relict species

Figure 6.3 A re-interpretation of the spatio-evolutionary pattern of forest-adapted species in tropical Africa, with a smaller diagram illustrating the traditional interpretation of speciation on the left. (From Fjeldsaa and Lovett, 1997. Biodiversity and Conservation 6, Fig. 6. With kind permission of Springer Science and Business Media.)

• o The principal and secondary centres for evolution h HiIi|i|i| Long-term accumulation of species of new species of forest birds and plants

Interchange during very humid climatic periods * Several biogeographically relict species

Figure 6.3 A re-interpretation of the spatio-evolutionary pattern of forest-adapted species in tropical Africa, with a smaller diagram illustrating the traditional interpretation of speciation on the left. (From Fjeldsaa and Lovett, 1997. Biodiversity and Conservation 6, Fig. 6. With kind permission of Springer Science and Business Media.)

during the upper Tertiary (1.8-23 Ma), with little speciation in the Pleistocene (the last 1.8 million years), and with species-richness patterns that may reflect rainfall and habitat heterogeneity. The asterisks show places where the existence of several relict species indicates long-term diversity. In contrast, black areas are centres of endemic species representing strong radiations in the Plio-Pleistocene; open areas represent less important centres. This illustrates the vital importance of these lowland African forests for the conservation of biodiversity, above and beyond just the number of species they contain.

6.3.3 Rediscovery of lost species: The Wollemipine and the Gondwana bequest

The excitement generated by the discovery of a modern representative of a group long thought to be extinct - a 'living fossil' as Charles Darwin put it - is always considerable. Perhaps the most striking in the twentieth century was that in 1938 of the coelocanth fish Latimeria chalumnae which had four limblike fins on its underside and most striking resemblances to fossil forms from Devonian rocks at least 380 million years old. To botanists the discovery in

1945 of living trees of the deciduous dawn redwood Metasequoia glyptostro-boides in the Hupeh and Szechuan provinces of China was just as important. Metasequoia, of which 10 fossil species are now known, had been described from fossil material in 1941 and was widespread in the northern hemisphere in Cretaceous times more than 65 million years ago. The insights which the living dawn redwoods gave into the evolutionary development of the evergreen giant sequoia Sequoiadendron giganteum and the coastal redwood Sequoia sempervirens were particularly important.

In 1994 came the discovery by David Noble of what is now called the Wollemi pine Wollemia nobilis, in the desolate canyons of Wollemi National Park in the Blue Mountains about 100 km north-west of Sydney, Australia (Woodford, 2002). The 'canyon band' is some 6 km wide and stretches for nearly 200 km; some of its estimated 500 canyons have never been visited by non-indigenous people and the danger of being caught in rising waters caused by torrential rains is such that the name wollemi is believed to be derived from the Darkinjung word wollumnii - aborigine for 'watch your step'. Desolate though these deep water-carved sandstone gorges are, it is almost certainly the exceptionally moist nature of their microclimate and isolation which have led to the survival of these unusual trees.

Noble was leading a group through an area new to him when he suddenly came upon some very tall strange trees growing in a slightly more open area. Their apices were well above the rest of the warm temperate rain forest dominated by coachwood Ceratopetalum apetalum and sassafras Doryophora sassafras. The stiff yellow-green adult leaves were unusually arranged in four rows along the tops of the branches. In contrast, the soft, green juvenile leaves found on the topmost branches were arranged in two rows along the branches and were dark green on the upper surfaces but had white waxy undersurfaces. The mature leaf outlines had a distinct resemblance to those of Agathis jurassica which had flourished over 150 million years before. Even the bark was unusual in having a bubbly surface which, with its subtle gradation of colour, has been described as resembling a settled swarm of bees, quite unlike any existing member of the Araucariaceae, in which family it belonged as subsequent collection of reproductive material made clear. Both male and female cones are borne high up at the ends of very long thin lateral branches of the same tree. Seed cones were first collected by a forest ranger suspended at the end of a cable beneath a helicopter. They are now caught in fine mesh nets below the canopy and many plants have been raised from seed (Fig. 6.4) while others have been grown from cuttings. In its natural environment it appears that dense shade and competition with other plants make development of a seedling into a tree most unlikely, though seedlings grow well and rapidly in cultivation.

Gondwana Biodiversity Development

Figure 6.4 Seedling of Wollemi pine Wollemia nobilis growing on the forest floor amidst leaf debris of coachwood and sassafras in Australia. Note the two cotyledons and double row of juvenile leaves. Very few seedlings survive the low light levels and generally unfavourable conditions on the floors of their native canyons. (Drawn by John R. Packham.)

Figure 6.4 Seedling of Wollemi pine Wollemia nobilis growing on the forest floor amidst leaf debris of coachwood and sassafras in Australia. Note the two cotyledons and double row of juvenile leaves. Very few seedlings survive the low light levels and generally unfavourable conditions on the floors of their native canyons. (Drawn by John R. Packham.)

If this new species is in the Araucariaceae why does it deserve to be put into a new genus? The answer to this becomes apparent when the spiny female cones and their cone scales are examined. In Agathis, the exterior of whose female cone is smooth, the ovule rests on top of the scale and when the mature seeds fall they spin like rotors, because each of them has a wing on one side. In contrast the ovule ofAraucaria is embedded in the scale and the seed is not winged, while the cone scales are sharply pointed. The Wollemi pine differed from both in that each seed was winged all round and rested on a pointed cone scale.

DNA studies by Hill showed that in the long-distant past Araucaria and Agathis diverged from a common ancestor, and that Wollemia branched off the Agathis line later still. Moreover, genetic constitutions of the wild Wollemi pines turned out to be remarkably uniform. When Peakall (1998) plotted the DNA peaks forming a considerable proportion of the complete genome - the total set of the genetic information within an organism - he was amazed to find that the DNA peaks from different trees exactly overlapped each other. This was true even if one of the two trees came from stand one and the other from stand two which, although they are only 3 km apart, are effectively completely isolated genetically. DNA peaks of the seedlings were also similar. This is a very unusual situation, especially as low genetic diversity normally implies that populations are especially vulnerable to infection. In contrast, plots of DNA peaks in various species of both Araucaria and Agathis showed that different trees of the same species had marked differences. This is not surprising: for example trees of the Moreton Bay or Hoop pine Araucaria cunninghamii, which with the Monkey Puzzle is an important source of timber, have continued to evolve against different environmental backgrounds in Queensland and New South Wales.

It turned out that the distinctive form of the Wollemi pine pollen had been seen many times before in geological borings. The presence of these tiny fossil pollen grains, which had received the name Dilwynites, showed that Wollemia and/or its close relatives had existed in the Cretaceous at least 91 million years ago and that its distribution had at one time extended to both the central Australian Desert and Antarctica. There are, moreover, two types of this fossil pollen grain, D. granulatus (whose pollen is almost identical to Wollemia) and D. tuberculatus; proof that the Wollemi pine was once part of a bigger and more diverse group of trees. It was also much more widely distributed within Australia; in 1986 cores from rocks 50 million years old in the central Australian desert yielded fossil pollen of both Dilwynites and of Fischerpollis halensis, a possible species of the Venus flytrap, the modern version of which (Dionaea muscipula) is now restricted to south-east USA. At that time annual rainfall in the area may have been as high as 1800 mm, easily enough for the Wollemi pine. Subsequently Fischerpollis pollen has also been found in the states of Victoria and south Australia.

The Araucariaceae first appear in the fossil record around 245 million years ago, growing in the northern hemisphere in the Triassic period. By the beginning of the Jurassic (205 Ma) its representatives were present in Australia as well as the northern hemisphere. The cosmic catastrophe which probably wiped out the dinosaurs at the end of the Cretaceous, 65 million years ago, seems also to have eliminated the Araucariacean forests of the northern hemisphere apart from a few minor populations in south-east Asia, leaving those of the southern hemisphere as a Gondwanaland survival.

Jones et al. (1995) played a major role in investigating and elucidating the nature of these strange ancient Wollemi pines, some of which live to an age of at least 400 years. The slender trees of today, the tallest of which currently reaches 38 m (125 ft), are in fact difficult to age as they throw up numerous basal shoots so producing a natural 'coppice' of shoots of varying age, a most unusual feature in a conifer. One fallen trunk has been dated at 350 years old, but as older trunks are gradually replaced with younger ones, the individual rootstock from which it came may have been much older. The Royal Botanic Gardens, Sydney, has subsequently assisted in the protection and conservation of the only three groups of Wollemi pines - with 23, 17 and 3 trees respectively -to be discovered. This species is of major value to conifer collectors and a major commercial launch was made in 2005/6 when many thousands ofyoung trees were made available for sale. It has so far proved tolerant of temperatures down to —5 °C.

As mentioned above, the Araucariaceae as a whole form an important part of the flora contributed by Gondwanaland (see Section 2.5.2); Araucaria and Agathis are both of major value in forestry and as specimen trees. Some of the 18 species of Araucaria, whose native trees are now confined to the southern hemisphere and found in South America, Australasia and the islands of the South Pacific, have been very widely planted. In this genus male and female cones are usually on separate trees but are sometimes found on separate branches of the same tree. The rather bizarre monkey puzzle A. araucana with its sharply pointed leaves is found in many public parks the world over (Hora, 1981).

The Bunya pine Araucaria bildwilli has its present centre of population in the Bunya Mountains of south-east Queensland, but was far more widespread in the Jurassic (135-205 Ma) when it extended into the northern hemisphere. These trees were at one time widely planted in Australian parks but their seed-cones, which can reach 40 cm in diameter and weigh over 11 kg, proved to be a major health hazard. They fall suddenly and with an impact sufficient to have killed a dinosaur in former times! The seeds are an important article of diet for the aborigines. Tall and imposing Norfolk Island Pines A. heterophylla are commonly planted in parks and along seafronts. They are also very attractive when small and are often grown as pot plants.

Agathis, the most tropical of all the conifer genera, has 15-20 species that are found in the wettest tropical rain forests of the Malay archipelago, Sumatra, the Philippines and Fiji, with outliers in the subtropical forests of Queensland and northernmost New Zealand. Mature kauris are often very large - in New Zealand A. australis reaches a height of over 30m with a trunk diameter greater than 3 m (Salmon, 1991) - and their timbers are amongst the most valuable softwoods in the world. That of most species is strong and remarkably free of knots because the trunk sheds its lower branches as it grows upwards. Fast-growing plantation forests of A. dammara in Java yield much Kauri timber. Though care is now taken to conserve them, the A. australis Kauri stands in North Island, New Zealand, were initially over-exploited both for their timber and the resin (kauri gum) which the trees of this and other species yield spontaneously and around sites of injuries. Fossil gum in peat bogs where the tree no longer grows is particularly prized; both forms are used as a basis for varnishes, linoleum and paints. The so-called Kauri of the East Indies A. brownii is one of many introduced trees growing in Madeira.

The Wollemi pine and its relatives are a mere fraction of the Gondwanaland bequest, indeed the long period that Australia - the driest of the continents -has been isolated has led to a most distinctive flora and fauna. Its climate varies immensely, from arid desert to rain forest and swamp, from baking heat to cold mountains. Of its flowering plant flora of some 20 000 species, 85% are endemic (found only in Australia), as are 500 (roughly a third) of the genera. Daytime temperatures can fluctuate by as much as 40 °C, and high evaporation rates have led to the development of scrub, mallee (small multi-stemmed trees) and open sclerophyll (leathery-leaved) woodlands often dominated by gum trees Eucalyptus spp. (Bopp, 2005). Cronin (1987, 1989) provides good illustrated accounts of some typical wild flowers, and of the palms, ferns and allied groups. Some species of the tree fern genera Dicksonia and Cyathea reach considerable heights in the understorey of eucalypt forests. Australian palms vary widely in their size, properties and appearance. Some have a crownshaft embracing the leaf bases at the top of the tall stem, others do not, while the leaves themselves may be pinnate or fan shaped. The slender stems of climbing palms, such as 'vicious hairy Mary' Calamus radicalis, are equipped with hooked prickles that enable them to climb over surrounding trees. The northern kentia palm Gronophyllum ramsayi of the open forests and adjacent islands in the far north of Australia has a trunk up to 35 m tall and 30 cm in diameter. Very many Australian plant communities are adapted to fire, and the grass tree Xanthorrhoea australis which commonly occurs in eucalypt forests, is often known as blackboy because of the blackened appearance of its trunk, from the top of which fresh leaves develop when the rains come. It is not a true tree since its 'stems' consist not of wood but of tightly packed leaf bases.

6.3.4 The Brazil nut: edible product of a complex ecosystem

In many ecosystems certain species can be lost without great impact on the way the ecosystem works. This is because several species may fulfil the same functional role (such as pollinators or food for carnivores) so if one is lost this functional redundancy allows the other species to fill the gap (see also Section 9.5). On the other side of the coin are keystone species which play a key role in an ecosystem, the loss of which affects many other species and can lead to severe loss or change in diversity. Often, however, the inter-relationships between species can be much more complex, forming a web where each relies on the other so that all are essential and no one species is the sole keystone. This is beautifully illustrated in forests by the Brazil nut tree Bertholletia excelsa. One of the tallest and most magnificent trees in the Amazonian rain forest, it produces its capsules - each of which is slightly larger than a cricket ball and contains 10-25 seeds (the nuts) - at the top of the canopy (Fig. 6.5a). These capsules are often 50 m above the ground so in January and February the nut gatherers are at some risk from the falling fruits as they compete with

Figure 6.5 (a) Fruit of Brazil nut tree Bertholletia excelsa sawn through to reveal the seeds within. Besides being common in the upland non-flooded regions of the Amazonian forests of Brazil, Peru and Bolivia, these trees are also found in Colombia and Venezuela. (b) An eighteenth century engraving of an agouti Dasyprocta aguti, a rodent which feeds on the nuts and whose neglected caches give rise to new seedlings. (Both these illustrations are from Trees, the journal of the International Tree Foundation - formerly known as 'Men of the Trees' - which does such a good job in planting trees in many places in desperate need of them, such as the Sahel south of the Sahara Desert.) (From Prance, 2003. Trees 63.)

Figure 6.5 (a) Fruit of Brazil nut tree Bertholletia excelsa sawn through to reveal the seeds within. Besides being common in the upland non-flooded regions of the Amazonian forests of Brazil, Peru and Bolivia, these trees are also found in Colombia and Venezuela. (b) An eighteenth century engraving of an agouti Dasyprocta aguti, a rodent which feeds on the nuts and whose neglected caches give rise to new seedlings. (Both these illustrations are from Trees, the journal of the International Tree Foundation - formerly known as 'Men of the Trees' - which does such a good job in planting trees in many places in desperate need of them, such as the Sahel south of the Sahara Desert.) (From Prance, 2003. Trees 63.)

the agoutis Dasyprocta aguti that both feed on and disperse the seeds (Fig. 6.5b). The agoutis are large rodents similar to guinea pigs that consume seeds and flowers on the forest floor. The fruit are technically known as pyxidia, which are capsules that normally open by a lid coming away from the top; in this case, however, the fruit do not open and seed release is dependent upon the very sharp teeth of the agouti that enable them to penetrate the wall of the capsule. The agouti then caches the seeds in the forest floor to eat later, just like the many corvid birds around the world do with pine seeds and acorns (Box 4.1). Being rich in vitamins, selenium, the amino-acid leucine, oil and protein, the nuts are very valuable to the agoutis and to humans. Attempts made to produce nuts from plantation trees have not been very successful because of the natural complexity of their ecology, as outlined by Prance (2003).

The trees flower from October to December and it takes 14 months for the fruits to mature after pollination. The large and complex flowers possess a hood that prevents most animals from reaching the floral organs; indeed only large bees are capable of lifting it and gaining access to the nectar on the inside of the hood. The commonest of the pollinating bees are the euglossine or orchid bees, whose males gather scent from the orchid flowers, pack it into pouches on their hind legs and use it to attract the female bees during mating. The success of Brazil nut production is thus linked with the epiphytic orchids that grow well on forest trees, but have not usually been present in plantations where the lack of suitable pollinators has been critical. The pollinating bees in the forest canopy, the presence of epiphytic orchids on neighbouring trees, and the agoutis scavenging on the forest floor, all play a vital role in the production of this valuable sustainably produced non-timber forest product.

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