Biodiversity series, Paper no. 1
Department of the Environment, Sport and Territories, 1993
ISBN 0 642 19904 3
Biological diversity or biodiversity refers to the variety of life forms: the different plants, animals and microorganisms, the genes they contain, and the ecosystems they form. This living wealth is the product of hundreds of millions of years of evolutionary history. In places as ancient as Australia, this history can still be seen today in 'living fossils' whose origins date back hundreds of millions of years. Living structures called stromatolites which can be seen in Shark Bay, Western Australia, represent one of the longest continual biological lineages known, some 1900 million years.1 The process of evolution means that the pool of living diversity is dynamic: it increases when new genetic variation is produced, a new species is created or a novel ecosystem formed; it decreases when the genetic variation within a species decreases, a species becomes extinct or an ecosystem complex is lost. The concept emphasises the interrelated nature of the living world and its processes.
Biological diversity is usually considered at three different levels: genetic diversity, species diversity and ecosystem diversity.
- Genetic diversity refers to the variety of genetic information contained in all of the individual plants, animals and microorganisms. Genetic diversity occurs within and between populations of species as well as between species.
- Species diversity refers to the variety of living species.
- Ecosystem diversity relates to the variety of habitats, biotic communities, and ecological processes, as well as the tremendous diversity present within ecosystems in terms of habitat differences and the variety of ecological processes.
Figure 1: Genetic variation within the White-cheeked Rosella. Genetic variation within a species can express itself in many ways. The White-cheeked Rosella, for example, is made up of four varieties, each with its own distinct colour combination and markings. The diagram shows where these varieties are found
Genetic diversity refers to the variation of genes within species. This covers genetic variation between distinct populations of the same species, such as the four varieties of white-cheeked rosella, Platycercus eximius. It also covers genetic variation within a population, which tends to be relatively high in widespread eucalypts such as Eucalyptus cloeziana, E. delegatensis, and E. saligna.2 Genetic diversity can be measured using a variety of DNA-based and other techniques.3
New genetic variation is produced in populations of organisms that can reproduce sexually by recombination and in individuals by gene and chromosome mutations. The pool of genetic variation present in an interbreeding population is shaped by selection. Selection leads to certain genetic attributes being preferred and results in changes to the frequency of genes within this pool.
The large differences in the amount and distribution of genetic variation can be attributed in part to the enormous variety and complexity of habitats, and the different ways organisms obtain their living.
One estimate is that there are 10,000,000,000 different genes distributed across the world's biota, though they do not all make an identical contribution to overall genetic diversity.4 In particular, those genes which control fundamental biochemical processes are strongly conserved across different species groups (or taxa) and generally show little variation. Other more specialised genes display a greater degree of variation.
Species diversity refers to the variety of species. Aspects of species diversity can be measured in a number of ways. Most of these ways can be classified into three groups of measurement:species richness, species abundance and taxonomic or phylogenetic diversity.5
Measures of species richness count the number of species in a defined area. Measures of species abundance sample the relative numbers among species. A typical sample may contain several very common species, a few less common species and numerous rare species. Measures of species diversity that simplify information on species richness and relative abundance into a single index are in extensive use. 5 6. Another approach is to measure taxonomic or phylogenetic diversity, which considers the genetic relationships between different groups of species. These measures are based on analysis which results in a hierarchical classification usually represented by a 'tree' that depicts the branching pattern which is thought to best represent the phylogenetic evolution of the taxa concerned.
Different measures of taxonomic diversity emphasise various taxic characteristics and relationships.7 8 The species level is generally regarded to be the most appropriate to consider the diversity between organisms. This is because species are the primary focus of evolutionary mechanisms and therefore are relatively well defined. At the global level, an estimated 1.7 million species have been described to date; current estimates for the total number of species in existence vary from five million to nearly 100 million.9 In Australia, with an estimated total number of native species (excluding bacteria and viruses) of 475 000, about half are known, but only a quarter formally described.10 Estimations of the number of species can be expected to improve with study into a number of poorly assessed groups: namely microorganisms, fungi, nematodes, mites and insects.
On a broad scale species diversity is not evenly distributed across the globe. The single most obvious pattern in the global distribution of species is that overall species richness is concentrated in equatorial regions and tends to decrease as one moves from equatorial to polar regions. In general, there are more species per unit area in the tropics than in temperate regions and far more species in temperate regions than there are in polar regions. In addition, diversity in land ecosystems generally decreases with increasing altitude. Other factors which are generally believed to influence diversity on land are rainfall patterns and nutrient levels. In marine ecosystems, species richness tends to be concentrated on continental shelves, though deep sea communities are also significant.
Ecosystem diversity encompasses the broad differences between ecosystem types, and the diversity of habitats and ecological processes occurring within each ecosystem type. It is harder to define ecosystem diversity than species or genetic diversity because the 'boundaries' of communities (associations of species) and ecosystems are more fluid. Since the ecosystem concept is dynamic and thus variable, it can be applied at different scales, though for management purposes it is generally used to group broadly similar assemblages of communities, such as temperate rainforests or coral reefs. A key element in the consideration of ecosystems is that in the natural state, ecological processes such as energy flows and water cycles are conserved.
The classification of the Earth's immense variety of ecosystems into a manageable system is a major scientific challenge, and is important for management and conservation of the biosphere. At the global level, most classification systems have attempted to steer a middle course between the complexities of community ecology and the oversimplified terms of a general habitat classification.
Generally these systems use a combination of a habitat type definition with a climatic descriptor; for example, tropical moist forest, or temperate grassland. Some systems also incorporate global biogeography to account for differences in biota between regions of the world which may have very similar climate and physical characteristics.
Australia and its territories encompass an enormous range of terrestrial and aquatic environments, from polar ice-caps to arid grasslands and tropical rainforests, from coral reefs to the deep sea. Each of these comprises a great variety of habitats and interactions between and within biotic and abiotic
components. For example, the spinifex grasslands of the arid zone encompass both treed and treeless communities. Within each spinifex tussock itself there are a range of microhabitats. Different species involved in a range of ecological processes such as seed dispersal (for example, by ant species) and nutrient recycling occur within each microhabitat.
The measurement of ecosystem diversity is still in its infancy. Nevertheless, ecosystem diversity is an essential element of total biodiversity and accordingly should be reflected in any biodiversity assessment.