Conservation and Sustainable Use of the Biosphere - WBGU

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Conservation and Sustainable Use of the Biosphere - WBGU

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 of the 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 of the 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 of the 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 of the Earth’s biological

diversity at the level of genetic and species

diversity. We first discuss the use of species diversity

for the example of the higher plants, and then go on

to present selected issues of concern in more detail.

D 1.2

The bases of genetic and species diversity and

their 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 of

the 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 of the Republic of South Africa there are

approx 23,500 plant species, of which 80 per cent are

endemic. This is particularly true of the 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

of the 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 and these remain isolated

for a time from neighbouring populations.

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