The purpose of this chapter is to provide an overview of the changing status of the world's biological diversity. The first sections below provide general information on the nature and scope of biological diversity, and broad trends at genetic and species level. Subsequent sections outline the status of the general ecosystem types that are the subject of thematic programmes established by the Convention, and brief remarks are given on other important habitat types.
The dimensions of biodiversity
This term is used by the Convention to refer to all aspects of variability evident within the living world, including diversity within and between individuals, populations, species, communities, and ecosystems. Differences in pest resistance among rice varieties, the range of habitats within a forest ecosystem, or the global extinction of species of lake fish, all illustrate different aspects of biological diversity. The term is commonly used loosely to refer to all species and habitats in some given area, or even on the Earth overall.
The Convention's approach to biodiversity
The key objectives of the Convention, as set out in Article 1, and outlined in chapter 2 below, are simple in summary but in practice all-encompassing: the conservation of biological diversity, the sustainable use of its components, and the fair and equitable sharing of benefits arising from use of genetic resources. Article 2 of the Convention provides a short description of the term `biodiversity', which refers essentially to the diversity of living organisms, the genes they contain, and the communities to which they contribute.
In addressing the boundless complexity of biological diversity, it has become conventional to think in hierarchical terms, from the genetic material within individual cells, building up through individual organisms, populations, species and communities of species, to the biosphere overall.
Genes provide the blueprint for the construction and functioning of organisms, and their diversity is thus clearly fundamental. The Convention puts due emphasis on genetic resources, i.e. the genetic diversity responsible for key properties of organisms used by humans, for food, medicine or other purposes, and which provides the potential for future modifications to these organisms. However, genes in nature are expressed only through the form and differential survival of organisms, and if attempts are made to manipulate genes, for example in bioengineering, it is important to focus on the requirements of whole organisms if this is to be undertaken successfully. Accordingly, the diversity of organisms tends to be central to biodiversity studies, and species diversity is a generally useful and practicable measure of biodiversity.
At the same time, in seeking to make management intervention as efficient as possible, it is essential to take an holistic view of biodiversity and address the interactions that species have with each other and their non-living environment, i.e. to work from an ecological perspective. By several of its decisions,1
the Conference of the Parties has explicitly recognized the need for this approach. In particular, decision V/6 and its annex provide a description and discussion of the ecosystem approach
, which in effect becomes the paradigm within which the Convention's activities are undertaken.2
It is often useful to address biodiversity issues in sectoral or other non-hierarchical terms. Agricultural biodiversity, for example, comprises those elements at all levels of the biological hierarchy, from genes to ecosystems, involved in agriculture and food production. The Convention has established a work programme on agricultural biodiversity, in recognition of the pivotal role this sector has in the complex area where biodiversity conservation and sustainable development intersect.
Attention may be focused on the biodiversity of a particular class of habitats, such as freshwaters, marine waters, mountains, soil or caves. The Convention has taken this approach in developing programmes of work on the biodiversity of inland waters, marine and coastal waters, forests, and dry and sub-humid lands.
The extent and occurrence of global biodiversity
The defining feature of the planet Earth is that it supports living organisms, and the entire space occupied by such organisms is termed the biosphere.
The part of the planet occupied by living organisms can be pictured as a thin and irregular envelope around the Earth's surface, at most just a few kilometres deep on the globe's radius of more than 6000 km. Because most organisms depend directly or indirectly on sunlight, the regions reached by sunlight form the core of the biosphere: i.e. the land surface, the top few millimetres of the soil, and the upper waters of lakes and the ocean. Bacteria occur almost everywhere, even kilometres deep within the Earth's rocky crust. Active living organisms are usually absent where liquid water is absent, but the dormant spores of bacteria and fungi are ubiquitous, from polar icecaps to many kilometres above the surface of the Earth.
The living organisms in the biosphere are organized in discrete groups. Those that reproduce sexually typically exist as species, i.e. distinctive groups of similar populations that are isolated reproductively from other such groups. Bacteria and many plants spread and reproduce vegetatively, i.e. without sexual reproduction, and the classic species concept is difficult to apply in such cases. The diversity of species, broadly defined, is nevertheless a useful general measure of the biodiversity of an area, country or the world. Globally, around 1.75 million species have been described and formally named to date, and there are good grounds for believing that several million more species exist but remain undiscovered and undescribed (Table 1.1).3
Table 1.1 Estimated numbers of described species, and possible global total
||Estimated total species
||1 000 000
|Protoctists (algae, protozoa, etc)
||1 320 000
||10 600 000
||1 500 000
||1 744 000
||ca.14 000 000
Notes: The `described species' column refers to species named by taxonomists. These estimates are inevitably incomplete, because new species will have been described since publication of any checklist and more are continually being described; most groups of organisms lack a list of species and numbers are even more approximate. Most animal species, including around 8 million of the more than 10 million animal species estimated to exist, are insects. Almost 10,000 bird species and 4,640 mammals are recognized, and probably very few of either group remain to be discovered. The `estimated total' column includes provisional working estimates of described species plus the number of unknown and undescribed species; the overall estimated total figure may be highly inaccurate. Source: UNEP-WCMC, adapted from tables 3.1-1 and 3.1-2 of the Global Biodiversity Assessment
Nearly three-quarters (71%) of the Earth's surface is covered by marine waters. These have an average depth of 3.8 km, and the whole of this region, comprising virtually all the water on the planet, is theoretically capable of supporting active life. Oceans and seas thus make up the vast majority of the volume of the biosphere and by far the most extensive, if most poorly known, main ecosystem type. However, the amount of living material in most of the sea, i.e. that part of the open ocean below the upper hundred or so metres, is low compared with many terrestrial habitats.
Only two to three percent of the total world water volume is non-saline: around two-thirds of this is locked away as ice and around one-third is groundwater in the upper layers of the Earth's crust. Surface freshwaters, i.e. the world's lakes, rivers and wetlands, hold the vanishingly small volume of water remaining, but this supports an important sector of global biodiversity. For example, about 40% of the more than 25,000 fish species known in the world occur in freshwaters, and many isolated water systems, large old lakes in particular, have a large number of species found nowhere else.
Land, bearing the wide diversity of terrestrial ecosystems that humans are most familiar with, as well as surface freshwaters, covers less than one-third (29%) of the Earth's surface. About half of this is below 500 m elevation and the global average elevation is only about 800 m. Most of the world land surface is situated in the northern hemisphere, and the amount north of the Tropic of Cancer slightly exceeds that in the rest of the world combined.
A familiar but important feature of biodiversity is that species are not evenly distributed over the planet. Although the information available on the distribution of the world's species is uneven and incomplete, the single most obvious pattern in global biodiversity is that overall species richness tends to increase toward the equator. At its simplest, this means that there are more species in total and per unit area in the tropics than in temperate regions, and more here than in polar regions. This variation in species number is strongly correlated with global variation in incident energy and water availability. These factors potentially lead to increased net primary production (NPP) by photosynthetic organisms, and a possible explanation for variation in species number is that this broader resource base may allow more species to coexist.
Moist forests in the tropics are in general the most species-rich environments on Earth. If recent estimates of the number of as yet unknown species, mainly insects, in tropical forests are accepted, these regions, which extend over perhaps 7% of the world's surface, may hold up to 90% of the world's species. If tropical forest small insects are discounted, then coral reefs and, especially for flowering plants, areas of Mediterranean climate in South Africa and southwest Australia, may be similarly rich in species.
Map 1 represents an index of diversity based on richness and endemism in the four terrestrial vertebrate classes and vascular plants in most countries of the world. Further details of this National Biodiversity Index
are provided in Annex 1. Map 2 shows selected regions of importance for both birds and plants.
|Map 1. Biodiversity at country level
|Map 2. Selected regions of high biodiversity value
1 Full information on the decisions adopted by the Conference of the Parties can be found in the `Handbook of the Convention on Biological Diversity' published simultaneously with the `Global Biodiversity Outlook'.
2 See Table 3.4 in Chapter 3
3 See UNEP (1995) Global Biodiversity Assessment
(henceforth ` Global Biodiversity Assessment
'), chapter 3