The Geography Of Marginal Plant Diversity

In terrestrial plant communities species richness in terms of alpha diversity generally decreases from the Equator to the poles. It is therefore understandable that biodiversity studies are frequently concerned with species assemblages at low latitudes. Conditions that limit plant distribution and create marginal areas can also be expected to influence biodiversity. Globally, tropical rainforests are commonly considered to be the

Fig. 2.12 Heather burning on a Scottish moor. Burning mature heather (Calluna vulgaris) in early April on the Hill of Alyth, Angus, Scotland.

outstanding regions for plant biodiversity and as such currently command much attention. Understandably, if there is to be a political will to preserve rainforests there is a need to increase public awareness of the species richness of these tropical plant communities. However, anxiety about the fate of the tropical rainforests tends to reduce awareness that there are many other regions where biodiversity is also likely to be at risk, not just from human activities, which are at least partially capable of being controlled, but principally from global climate change which is probably beyond control as long as human populations continue to grow.

There are several notable areas outwith the tropics that are marginal in terms of resources for plant growth and yet are remarkable for their diversity of plant life. Such areas are commonly referred to as biodiversity hotspots (Myers, 2003). They may not all be as rich in species as the tropical regions, but relative to their own area they stand out, either in terms of species numbers or for richness in subspecific variation. Notable

Fig. 2.13 The Lowther Hills, Southern Uplands, Scotland, showing an example of muirburn as practised when the object is to create a moor suitable for game-bird shooting. The burning is carried out frequently (every 8-20 years) and in small patches. This creates the maximum ofyoung nutritious heather which is ideal feeding for grouse. The frequency and intensity ofburning depends on the object in view as any burning regime can have positive or negative effects on different aspects of the moorland ecosystem (see text).

Fig. 2.13 The Lowther Hills, Southern Uplands, Scotland, showing an example of muirburn as practised when the object is to create a moor suitable for game-bird shooting. The burning is carried out frequently (every 8-20 years) and in small patches. This creates the maximum ofyoung nutritious heather which is ideal feeding for grouse. The frequency and intensity ofburning depends on the object in view as any burning regime can have positive or negative effects on different aspects of the moorland ecosystem (see text).

examples of such biodiversity hotspots include the Cape Peninsula of South Africa (and other regions with a similar long-term history of Mediterranean-type climatic stability), and the central Brazilian Cerrado. In addition there are numerous climatically favoured thermal oases in both hot and cold deserts. Even in the High Arctic there are localized floristic hotspots which in comparison with surrounding areas have a high level of species diversity.

2.5.1 The South African Cape flora

The Cape Peninsula with its outstanding flora of over 8500 species of flowering plants has a greater diversity of plant species at a regional level than the richest tropical rainforest (Cowling et al., 1992; Linder & Hardy, 2004). Particularly famous for species richness within the Cape region is the unique Fynbos. The term Fynbos, derived from the Dutch Fijnbosch, is used to describe woody vegetation with thin or fine branches which is a characteristic feature of the varied collection of heath and scrub communities that survive on ancient, nutrient-deficient, acid soils (see Figs. 2.14-2.17). The Cape region as a whole has a rugged landscape with sharply differentiated habitats in close proximity to one another coupled with variable and complex precipitation patterns (Goldblatt & Manning, 2000). In the Fynbos in particular, the soils are particularly nutrient deficient, being developed mainly on nutrient poor quartzite sand. The Fynbos alone has a flora numbering 2285 species and infraspecific taxa, including many endemic species in an area of 471 km2. The flora is also remarkable for the large number of species that are found in just a few genera, with the genus Erica alone having 650 species (Goldblatt & Manning, 2000). The Fynbos can be grouped under five broad types depending on their dominant species (Table 2.2).

Considering the Cape flora as a whole, the long Pliocene-Pleistocene history of climatic stability, like that of southern Europe, North America or southern South America, has probably been a major factor in contributing to the unique species richness of this region (Goldblatt & Manning, 2000). The extraordinary species richness of the Fynbos is due in part to topographic and geological diversity and this has contributed to both beta and gamma diversity. Using the

Fig. 2.14 Location of the Fynbos and Succulent Karoo within the Cape Floral Kingdom. (Reproduced with permission from Cowling & Richardson, 1995.)

definitions given in Table 2.1 the Fynbos can also be claimed to be richer in species per unit area (greater alpha diversity) than most tropical rainforests. In the case of the Cape Fynbos vegetation, species richness is attributed largely to a high degree of endemism, which appears to be aided by development of specific ericoid mycorrhizal-mediated nutrient uptake, which explains edaphic specialization and speciation particularly in the Ericaceae and Fabaceae (Cowling et al., 1992).

The Cape region is also a very ancient landscape that has maintained global migration routes along the African mountain chains. Consequently, long-term species migration has resulted in the acquisition of groups with geographical affinities using both the Gondwana track (Antarctic) which includes all southern hemisphere connections as well as African and boreal tracks. The latter includes taxa such as Ranunculus, Anemone, Galium, Dipsaceae, etc., which have had access to South Africa along the East African Highlands (Linder et al., 1992). Comparison of the nature of endemism in Mediterranean heathlands with the South African Fynbos and a similar community in Australia, the Kwongan, shows that in both continents more than 90% of the endemic species are edaphic

Fig. 2.15 Restiod Fynbos vegetation near the South African Cape. The dominant white flowered species is Cape snow (Syncarpha vestita, Asteraceae) growing among clumps of Restionaceae. (Photo R. J. Mitchell.)

specialists. There are nevertheless notable differences between the two regions in the biological profiles of the endemics. South African endemic species are more likely to be low shrubs with soil-stored and ant-dispersed seed while Australian endemics are concentrated among low to medium-height shrubs with either canopy-stored or soil-stored seed (Cowling et al., 1994).

The Succulent Karoo communities on the west coast of southern Africa (see Fig. 2.17) are similar to the Fynbos, in that they are floristically part of the Greater Cape flora and considered to be one of the Earth's 25 biodiversity hot spots (Myers, 2003). Like the Fynbos the species-rich vegetation of the Succulent Karoo (about 5000 species) contains more than 40% endemic species. The rainfall is usually less than 250 mm and falls predominantly in winter. In contrast to the Fynbos the soil is nutrient rich and the succulent vegetation is not fire prone. The Aizoaceae (ice plants) which dominate the succulent vegetation of the Karoo are represented in this region by 1750 species in 127 genera. A study of the possible causes of this extraordinary diversity (Klak et al., 2004) has suggested that this group has diversified both recently and rapidly, with the estimated age for this radiation lying between 3.8 and 8.7 million years ago, yielding a per-lineage

Table 2.2. Summary of principal types of Fynbos vegetation occurring in South Africa

Proteoid Fynbos consists of bushy vegetation usually taller than 1.5 m and exhibits much colour in winter due to the large number of Protea species in flower. The principal locations are at altitudes lower than 1000 m and are most common at the base of mountains in areas with deep colluvial soils.

Ericaceous Fynbos, with a large number of species from the Ericaceae and resembling the temperate heathlands of Europe; this type occurs in permanently moist and cool environments on seaward-facing slopes and the upper regions of coastal mountains.

Dry Fynbos, found where the soil conditions are too dry to sustain shallow-rooted vegetation and consequently is dominated by small ericaceous shrubs able to extract moisture from lower regions of the soil with deep roots. This vegetation type is common on inland mountains and also on coastal sand dunes and the borders of the Succulent Karoo (see below).

Restiod Fynbos describes communities where the precipitation is not adequate for woody plants such as proteoids and ericoids. The Restionaceae, a southern hemisphere monocot family consisting of rhizomatous xeromorphic herbs with photosynthetic stems, is typical of nutrient-poor soils. There are over 180 endemic Restionaceae species in the South African Cape region where they characterize the Fynbos in areas where rainfall is low but predictable and where shallow-rooted shrubs absorb most of the moisture leaving little for deeper rooted plants. Alternatively, restiods also flourish where drainage is blocked by a water-impermeable layer (Fig. 2.17).

Graminoid Fynbos, typical of the eastern Fynbos where rain falls mostly in summer and conditions are particularly suitable for tropical grasses.

Source: Adapted from Cowling & Richardson (1995).

Fig. 2.16 Botanists examining South African Cape heathland. The dominant purple flowering species in the fore- and middle-ground is Erica melanthera. (Photo R. J. Mitchell.)

diversification rate of 0.77-1.75 per million years. Diversification of the group appears to be associated with the origin of several morphological characteristics and one anatomical feature, namely the development of wide-band tracheids in the Ruschiodeae clade which are absent in all sister taxa. These specialized cells are considered as preventing tracheid collapse and therefore provide an adaptation for resisting water stress. The lack of this feature in species-poor clades of the Aizoaceae has been taken to suggest that this characteristic is a key innovation that may have facilitated this rapid radiative evolution (Klak et al., 2004).

2.5.2 Mediterranean heathlands

Mediterranean heathlands, like the South African Fynbos, are also noted for their species richness and high levels of endemism, but differ in having lower numbers of species per genus (Ojeda et al., 1995). As with the Fynbos the biodiversity of these sites is due in part to their antiquity. The European Mediterranean region, owing to its climatic stability, has been a refu-gium for many species throughout the Pleistocene and still retains a high number of endemic species. Again, like the South African Fynbos, the ancestors of the species and populations found in this region today have been present ever since the pre-Pleistocene climatic cooling period created the summer-dry Mediterranean

type climate some 2-5 million years ago and made fire a driving force in the evolution of this distinctive flora.

In the European Mediterranean heathlands a wide variety of imprecise terms are used to describe the fire-prone, dwarf-shrub zone. These include garrigue, and maquis. Both are spread over vast areas and are derived from the burning and grazing of former forests. The taller form of scrub with species of broom (Cistus spp.), tree heather (Erica arborea), rosemary (Rosmarinus officinalis), and myrtle (Myrtus communis) is usually termed maquis while the term garrigue denotes a low-growing vegetation often containing many prickly dwarf shrubs with drought-resistant foliage but with a species composition that varies depending on local conditions. In Spain the tomillares is a form of garrigue with several species of thyme, sage and lavender, while in Italy gorse (Ulex europeaus) provides a link with the heaths of western Europe and Great Britain.

The more recent history of the Mediterranean garrigue and maquis can be traced from the earliest human occupation of the region. Archaeological botanical studies looking at plant remains in charcoal deposits can detect vegetation changes beginning in the middle Neolithic, when deciduous oaks declined and were replaced by evergreen oaks. The Chalcolithic (Greek khalkos + lithos, copper + stone) period is a phase in the development of human culture in which the use of early metal tools appeared alongside the use of stone tools (c. 6500-5500 BP). This period of human cultural development also witnessed the arrival and spread of meso-Mediterranean and thermo-Mediterranean plant formations, creating first the maquis, which then led to the development of garrigue vegetation during the Bronze Age (Heinz et al., 2004). The degradation stages of this vegetation follow the sequence deciduous forest ! evergreen forest ! maquis

! garrigue ! steppe.

The fire-prone nature of the vegetation stems from a combination of a climate with hot dry summers and the evolution of the vegetation to adapt to the dry season with partially desiccated, sclerophyllous vegetation. Many Mediterranean plants are also characterized by being active producers of volatile organic carbons. These monoterpenes and sesquiterpenes (isoprenes; Fig. 2.18), commonly referred to as essential oils, lend a characteristic odour to the foliage. A long-recognized property of these compounds is to act as grazing deterrents. Some act as direct deterrents. Camphor has long been known to discourage egg laying by moths and the volatile oils in peppermint are claimed to discourage geese from grazing this plant. In other cases the mode of action is more subtle. In wild thyme (Thymus vulgaris) the terpene composition varies from plant to plant. Slugs appear to distinguish between individual plants and their terpenes and their preferences change with time. This polymorphism diversifies the attacks of the

Open chain

Monocyclic

Bicyclic a Pinene Camphor

Fig. 2.18 Volatile oils commonly found in plants.

predators and apparently increases the survival chances of certain individuals (Gouyon et al., 1983). A similar situation is found in thyme-feeding aphids (Linhart et al., 2005). It has also been shown that volatile oils reduce heat injury to the electron transfer system and that isoprene production in certain heat-resistant species increases under conditions of high light and high temperatures (Haraguchi et al., 1997). It is not entirely clear how this protection is achieved. One possibility is that membrane stability may be enhanced by these essential oils altering the liability for phase change at high temperatures. Another possibility is that the excretion of the volatile carbon compounds enables the electron transfer system to function without becoming inhibited by excessive carbon accumulation, an explanation that is analogous to the supposed benefits of light respiration.

A downside of this adaptation appears to be the increased sensitivity that volatile carbon producing plants show to ozone pollution. Such a sensitivity to oxidative damage also indicates that the vegetation is prone to fire damage. In some cases the volatile organic compounds may increase the readiness of the foliage to ignite, hence the rapid burning that then ensues and which can pass through the vegetation with great speed. The swiftness with which the fire passes through the vegetation is, however, a phenomenon which aids survival as the brevity of the fire does not incinerate all the wood. The extreme case is the biblical 'burning bush' (probably Dictamnus albus although plants in the genera Bassia (Chenopodiaceae) and Combretum (Combretaceae) are also sometimes known as burning bush). Benzene has been confirmed among the various volatile compounds produced by Dictamnus albus and easily decomposes into chavicol and the very flammable hydrocarbon 2-methyl-1,3-butadiene (isoprene b.p. 340C). The secretion of isoprene, which can be especially intense on hot windless days, leads to formation of the isoprene cloud that may inflame the bush without causing too much harm to the source plant (Fleisher & Fleisher, 2004).

2.5.3 Mediterranean-type vegetation worldwide

A worldwide examination of regions with Mediterranean-type climates, and this includes the South African Fynbos, and the Australian Kwongan, suggests that in

Geraniol Citral

Geraniol Citral

a Pinene Camphor

Table 2.3. Factors which can lead to rapid speciation

(1) Poor soils favour shrubs that are killed by fire and re-establish from seed. The frequent fires that occur in most Mediterranean heathlands also encourage shrubs that can resprout.

(2) Nutrient-poor soils can favour species that rely on seed production and seed conservation in buried seed banks for reproduction. These so-called seeders adopt a strategy that avoids the extra investment in underground organs that takes place in long-lived plants.

(3) The numerical dominance of seeders on poor soil lowers their extinction rates.

(4) Seeders have relatively short generation times, which leads to increased speciation rate.

addition to having soils with low soil pH and low nutrient levels, there are a number of common demographic features that may account for their floristic diversity. In particular, elevated speciation rates coupled with depressed rates of extinction could lead to enhanced species richness with fire as the predominant promoter of diversity.

Various aspects of this situation have been suggested as responsible for the rapid speciation that has taken place in these regions (Wisheu et al., 2000; Table 2.3).

2.5.4 The Brazilian Cerrado

The Brazilian savanna biome usually referred to as the Campo Cerrado (Portuguese Campo, field; cerrado, closed, cf. campo limpo, clean field - grassland) or just Cerrado, is a vast area of dense scrub and woodlands extending over 2 million km2 (Figs. 2.19-2.20) and is frequently so dense that vehicle access is impossible. The Cerrado is another example of a very ancient landscape and extends from the southern margins of the Amazonian forest to outlying areas in the southern states of Sao Paulo and Parana. The antiquity of the Cerrado even suggests that it existed in prototypic form in the Cretaceous before the separation of the South African and South American continents. During the Pleistocene the Cerrado would have expanded as the extent of the Amazonian forest shrank, and then subsequently contracted as the rainforest spread during the Holocene (Prance & Lovejoy, 1985).

The ancient soils on which the commonly twisted and fire-resistant trees of the Cerrado grow are highly acidic and rich in soluble aluminum (Hueck, 1966). Highly toxic levels of aluminium up to 1000 mg kg-1 d. wt have been recorded in the foliage of some species (Geoghegan & Sprent, 1996), which necessitates a high degree of adaptation and probably contributes to the high level of endemism that occurs here at a species level. Considering the hostile nature of the soils the Cerrado has an extraordinarily rich flora. The vegetation comprises about 800 species of trees and large shrubs and several times that number of ground species (herbs and subshrubs).

An important aspect of the Cerrado is that unlike the African savannas it has lost the fauna of large herbivores that would have existed at the time of its creation. The herbivores that remain comprise anteat-ers, armadillos, opossums, some monkeys and various rodents such as agoutis, picas and capybaras (Ratter et al., 1997). The introduction of cattle and horses is likely to have restored the vegetation to the structure it might have had in its earlier history.

When the flora of gallery forests, mesophytic forests and other habitats occurring in the biome are included, the total number of vascular plant species is estimated to reach about 10 000. In common with other savannas the cerrado flora is resistant to fire and shows all the usual morphological features associated with frequent fires: corky bark, xylopodia (lignotubers) and in grasses tunicate (having a dry paperlike covering) leaf bases (Ratter et al., 1997). The flora is dominated by a few endemic genera with hundreds of species. In the genus Mimosa alone there are 189 species, 74% of which are endemic (Simon & Proenca, 2000). The Cerrado also shows high levels of beta and gamma diversity with many subdivisions of recognizable forest and scrub communities (Eiten, 1972).

Fig. 2.19 Open cerrado — a species-rich vegetation maintained by frequent fires. The relative dominance of species is also influenced in certain areas by frost. The dominant trees in the middle and far distance (Vochysia tucanorum) suffer significant damage to stems and leaves and are slow to recover when exposed to frost. Brando and Durrigan (2004) consider that the frequency and intensity of frosts can maintain more open forms of cerrado vegetation even in sites where both water and nutrient availability could support denser vegetation. (Photo Dr P. E. Gibbs.)

Fig. 2.19 Open cerrado — a species-rich vegetation maintained by frequent fires. The relative dominance of species is also influenced in certain areas by frost. The dominant trees in the middle and far distance (Vochysia tucanorum) suffer significant damage to stems and leaves and are slow to recover when exposed to frost. Brando and Durrigan (2004) consider that the frequency and intensity of frosts can maintain more open forms of cerrado vegetation even in sites where both water and nutrient availability could support denser vegetation. (Photo Dr P. E. Gibbs.)

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