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.