CAUSES OF BIODIVERSITY LOSS
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INTRODUCTION
The term ‘Biodiversity’ is a contraction of “biolog ical diversity” and refers to the number, variety and variability of living organisms. In its widest sense, therefore, it is synonymous with “Life on Earth”. It embraces two different concepts: one is a measure of how many different living things there are and the other is the measure of how different they are. Although many definitions of biodiversity exist, the most often cited is provided by the “Convention on Biological Diversity” (UNEP, 1992).
According to the Convention on Biological Diversity, “ biological diversity means the variability among living organisms from all sources including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems” (Johnson, 1992). The term “biodiversity ”, thus, refers to the variety of all life on earth, and explicitly recognizes how the interaction of the different components of ecosystems results in the provision of essential ecosystem services on the one hand and social and recreational opportunities on the other, including being a source of inspiration and cultural identity (Commonwealth of Australia, 1996). This definition places emphasis on variability or heterogeneity, rather than on the objects displaying that variability. It addresses this variability at three hierarchical levels - genes, species and ecosystems. The three levels of biodiversity are 1) diversity between species (species diversity), 2) diversity of genes within a species (genetic diversity), and 3) variability in the habitats occurring within a region (ecosystem diversity) (Chaudhuri and Sarkar, 2002).
Species diversity
The species is the basic unit of classification in biology. Although a species might be defined as a group of similar organisms that interbreed or share a common lineage of descent, there is no universal agreement on how to define a species. Even when the species is the basic unit, it represents only one level of a complex phylogenetic hierarchy: related species are grouped in genera, related genera in families, families in orders, and so on, up to the highest level, the kingdom, of which five are generally recognized at present (animals, plants, fungi, bacteria and protoctists). More schematically, the levels of biodiversity are listed in Table. 1. Species richness measures the number of species within a given area, giving equal weight to each one. This measure can be used at different geographical levels (a given area, a country and, ultimately, the world). It is still the most straightforward and, in many ways, the most useful measure of biodiversity.
The number or richness of species is obviously a most incomplete measure of biodiversity. It is complemented by:
- Species diversity, which measures the species in an area, adjusting for both sampling effects and species abundance.
- Taxic (taxonomic) diversity, which measures the taxonomic dispersion of species, thus emphasizing isolated evolutionary species. The basic idea behind this measure is that biodiversity might be better measured at higher taxonomic levels (e.g. genera or families). The explanation is that an area with, say, ten species in the same genus is less diverse than an area with ten species, each belonging to a different genus.
- Functional diversity, which assesses the richness of functional features and interrelations in an area, identifying food webs along with keystone species and guilds.
However, not only diversity is of importance. Species endemism, that is whether a species is restricted to (“endemic to”) a n area under discussion, is equally vital. For example, islands typically have fewer species than equivalent-sized continental areas. They also usually have a higher percentage of species found nowhere else. In other words, they have lower species richness and higher species endemism.
Genetic diversity
Genetic diversity is the variation of the set of genes carried by different organisms: it occurs on a small scale among organisms of the same species, among closely related species such as those in the same genus, and among more distantly related species, in different families, orders, or kingdoms (Table. 1).
Genetic diversity might be characterised by a range of techniques: by observation of inherited genetic traits, by studying the chromosomes and their species specific karyotype, and by analysing the DNA information using molecular technology.
Global genetic diversity is extremely large. It has been estimated that there are some 109 different genes present in the world’s biota. The number of possible combinations of gene-sequence variants in a population is so great that it cannot even be expressed in a meaningful way.
This amazing variation in the genetic code offers opportunities for evolutionary change, the survival of species, adaptations to a changing environment, and the formation of new species. More recently, biotechnology and crop or breed improvement programmes rely on the identification of genetic material giving rise to desirable traits, and the incorporation of this material in appropriate organisms.
Ecosystem diversity
Species exist in natural settings, within functioning communities and ecosystems, interacting with other species and the abiotic environment. Ecosystems function as entities with system-wide properties. Care about diversity must, therefore, also focus on system-wide aspects, such as dying coral reefs. Different classification systems exist to describe ecosystem diversity. On a world scale, bio-geographic zones, biomes, eco-regions, and oceanic realms are used. On a smaller scale, one deals with landscapes, ecosystems and communities (Table 1). Qualification of ecosystems on a global scale faces problems. A major reason for this is that they do not have a clearly delineated identity. They do not, in general, exist as discrete units, but represent different parts of a highly variable natural continuum. To study ecosystem diversity at different levels, geographic information systems (GIS) are increasingly used, both during assessment and as a basic management tool.
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