The use of genetic and species diversity as illustrated by the higher plants D 1 D 1.1 Introduction The problem at the very heart of conserving species diversity can be shown by the example ofthe conifers (Pinaceae) and orchids. The Pinaceae family comprises worldwide just some 250 species (WCMC, 1992) that are the dominant form of vegetation on 19 million km 2 ofthe Earth’s surface, for example in the boreal coniferous forests. By contrast, there are 25,000–35,000 species of orchid (WCMC, 1992), but in no part ofthe world is any vegetation determined in its structure or biochemical cycles by orchids. The question therefore arises, does humankind need the 35,000th orchid, and – if there is no direct need – what are the reasons for worldwide endeavours to preserve this species for the future? This chapter gives an overview ofthe Earth’s biological diversity at the level of genetic and species diversity. We first discuss the use of species diversity for the example ofthe higher plants, andthen go on to present selected issues of concern in more detail. D 1.2 The bases of genetic and species diversity andtheir geographic distribution Any differentiation within a species begins with a DNA mutation that only rarely proves to be of direct advantage in evolutionary terms. More frequently this advantage only emerges after a longer period or when environmental conditions change (pre-adaptation). The establishment of barriers to crossbreeding marks the transition from a population into a new and distinct species (Box D 1.2-1). Genetic diversity is almost impossible to measure. That is why various molecular biological indicators are generally used when making statements in this regard (detailed explanations in Bisby, 1995 and Mallet, 1996). The origins of life lie approximately 4 thousand million years in the past. Since that time the number of species has constantly increased, even though there have also been mass extinctions in the course ofthe Earth’s history (Fig. D 1.2-1). But the humaninduced extinction rate we see today is 1,000–10,000 times higher than any natural background rate (Barbault and Sastrapradja, 1995; May and Tregonning, 1998). Worldwide, approximately 1.75 million species have been described (Table 1.2-1). This represents Box D 1.2-1 Mechanisms that lead to species diversity as illustrated by the impact of fire On the territory ofthe Republic of South Africa there are approx 23,500 plant species, of which 80 per cent are endemic. This is particularly true ofthe Cape peninsula, which is famous for being a flora kingdom in its own right, the Capensis. But even there, one cannot find 23,000 species in any one area under investigation (eg 1 hectare).The local diversity (termed α-diversity) is relatively low and constant (5–30 species per m 2 ). But then on each mountain one finds a completely new flora (there is a high β-diversity, the measure of regional diversity). The reason lies in the differentiation ofthe landscape by fire as a natural on-site factor. Fires occur in limited areas, whenever sufficient biomass has accumulated (every 30–40 years). After a fire, it is those species that are best adapted to the fire that germinate. The seedlings have relatively little competition, so any mutation has a good chance of survival. If after several years the plants bloom on this burned area then in each population a limited exchange of genes with neighbouring populations and, thus, the opportunity to stabilize mutations through inbreeding occurs. This leads over longer periods to genetic isolation and speciation in limited populations, ie endemisms (Bond, 1983). Similar mechanisms lead in arid regions to the formation of new species since every time it rains isolated populations emerge andthese remain isolated for a time from neighbouring populations.
32 D The use of genetic and species diversity Number of families 900 600 300 Cambrian phase Palaeozoic phase 1 2 3 4 Mesozoic-Cenozoic phase 5 Modern fauna Figure D 1.2-1 Change in global species diversity illustrated by the marine animal families. Extinctions: 1. end-Ordovician 2. late Devonian 3. end-Permian 4. end-Triassic 5. Cretaceous-Tertiary. Source: Barbault and Sastrapradja, 1995 Palaeozoic fauna Cambrian fauna 0 600 V C O S D C P TR J K T PALAEOZOIC 400 200 Geological time [million years] MESOZOIC CENO- ZOIC 0 probably just 14 per cent ofthe total number of species on Earth, the total being estimated at 13.6 million.The plants and vertebrates are relatively well described. There are particularly large gaps in our knowledge of microorganisms andthe arthropods. It is above all the insects that dominate species diversity on Earth. Among the beetles alone there are twice as many species as there are among plants, and ten times the number of vertebrate species. Table D 1.2-1 Estimates ofthe number of species worldwide. The certainty ofthese estimates has been categorized as follows: good = within a factor of 2; moderate = within a factor of 5; low = within a factor of 10; very low = not in the same order of magnitude. Source: after Heywood, 1997 Division Described Estimated number of species [in 1,000] Proportion Certainty species [% working of assessment [in 1,000] Lower level Working Upper level estimate] estimate Viruses 4 50 400 1,000 2.9 very low Bacteria 4 50 1,000 3,000 7.3 very low Fungi 72 200 1,500 2,000 11.0 moderate Monocellular organisms 40 60 200 200 1.5 very low Algae 40 150 400 1,000 2.9 very low Vascular plants 270 300 320 500 2.3 good Nematodes 25 100 400 1,000 2.9 low Crustacea 40 75 150 200 1.1 moderate Arachnida 75 300 750 1,000 5.5 moderate Insects 950 2,000 8,000 100,000 58.7 moderate Molluscs 70 100 200 200 1.5 moderate Vertebrates 45 50 50 55 0.4 good Others 115 200 250 800 1.8 moderate Total 1,750 3,635 13,620 110,955 100.0 very low