Thomas Encyclopedia of Applied Plant Sciences

1. Auflage 2003
ISBN: 978-0-08-091790-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 2000 Seiten

Autoren/Hrsg.

Thomas, Brian

Leseproben

B
BIODIVERSITY AND CONSERVATION
Contents Plant Diversity, Conservation and Use Germplasm Conservation Recalcitrance Seed Banks Plant Diversity, Conservation and Use
N Maxted and S P Kell, University of Birmingham, Birmingham, UK Copyright 2003, Elsevier Ltd. All Rights Reserved. Plant Biodiversity Introduction One of the critical challenges facing the world today is the conservation of biological diversity, or biodiversity, and the sustainable use of its components for the benefit of humankind. Biodiversity is a finite world resource that has economic and ethical value to humankind, but this resource is being eroded or lost for perpetuity by careless, unsustainable practices. The exponential loss of biodiversity is well documented; what is most worrying is that this process shows no signs of slowing down and threatens to become even more severe in the future. The fundamental importance of these issues was addressed at the United Nations Conference on Environment and Development (UNCED) held in Rio de Janeiro, Brazil in 1992, and has been enshrined in the Convention on Biological Diversity (CBD). Its objectives are: the conservation of biological diversity, the sustainable use of its components and the fair and equitable sharing of the benefits arising out of the utilization of genetic resources, including by appropriate access to genetic resources and by appropriate transfer of relevant technologies, taking into account all rights over those resources and to technologies, and by appropriate funding. The Convention on Biological Diversity is the first comprehensive global treaty for the conservation of biodiversity and sustainable utilization of the earth’s biological resources. However, it goes beyond these goals, to encompass the issues of access to genetic resources and the sharing of benefits arising from the exploitation of those resources, as well as recognizing the need to provide the biologically rich South with the support it needs to implement its provisions. The Convention represents a milestone in biodiversity conservation thinking, reflecting international acknowledgement of the loss of our biological resources and the urgent need for action. It is now recognized as the primary guiding framework for biodiversity conservation and sustainable use. What Is Plant Biodiversity? The widespread use of the term biodiversity began in the 1980s and was launched into popular use by Edward Wilson, the American conservationist, who defined it as: The variety of organisms considered at all levels, from genetic variants belonging to the same species through arrays of species to arrays of genera, families, and still higher taxonomic levels; including the variety of ecosystems, which comprise both communities of organisms within particular habitats and the physical conditions under which they live. Plant biodiversity is that component of biodiversity that is included within the plant kingdom. There are many definitions of biodiversity, but the definition provided by the Convention on Biological Diversity can be adapted to refer to plant biodiversity alone: Plant biodiversity means the variability among living plants from all ecosystems (terrestrial, marine and other aquatic) and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems. Plant diversity exists at all levels of biological organization, as illustrated in Figure 1. Therefore, plant biodiversity encompasses diversity at all levels of biological organization: communities, species, individuals, and genes. However, the most commonly used estimate of biodiversity is in terms of the numbers of species (see Table 1 Since many areas of high plant diversity remain unexplored, or unrecorded, estimates of the total number of plant species are inevitably inaccurate. However, based on the number of currently described species (i.e., those known to science), there is estimated to be roughly a total of 320 000. Table 1 shows the approximate or known numbers of described species in each plant group, and an indication of how well each group is known. Quantifying Plant Biodiversity In order to conserve and use plant biodiversity we must first measure it, and this only becomes possible when quantitative values can be ascribed and compared; therefore, to disentangle the huge array of variation that biodiversity encompasses, we need to distinguish the key elements. It is customary to classify biodiversity into three main groups: genetic diversity, taxonomic (or organism) diversity, and ecological diversity. Within these groups are a number of elements or levels of biodiversity (Table 2), although these elements should not be thought of as being distinct from one another, but as being inextricably linked. Genetic diversity is the heritable variation that is observed within and between populations of organisms. The basic genetic component is the gene. Genes are found in the nuclei of all cells of all organisms. Genes are made up of DNA, and are situated along chromosomes. Ultimately, it is the variation in the sequence of four nucleotide base pairs, which, as components of nucleic acid, constitute the genetic code, which is passed from generation to generation. New genetic variation arises in individuals by gene and chromosome mutations, and in organisms that reproduce sexually, by recombination. There can be various distinct forms of the same gene, referred to as alleles. Figure 1 Diversity from gene to community. Reproduced with permission from Frankel OH, Brown AHD, and Burdon JJ (1995) The Conservation of Plant Biodiversity. Cambridge: Cambridge University Press. Table 1 Species diversity for major plant groups Table 2 The composition and levels of biodiversity Individuals in a population or a species vary genetically for a range of characteristics or traits. Such genetically significant traits might include: height, fecundity, disease resistance, or tolerance to extreme environmental conditions such as drought. This variation, which may for instance be expressed morphologically, behaviorally, or physiologically, is referred to as the phenotype. The phenotype results from a combination of the individual’s genotype (its genetic composition), interacting with the environment in which it is found. The pool of genetic variation present within an interbreeding population is acted upon by selection. It is the genetic variation within and between individuals and populations of the same species, that ensures the species as a whole can adapt and change in response to natural (e.g., changing environment) and artificial (e.g., breeder’s selection criteria) selection pressures. Therefore, genetic diversity enables evolution and adaptation of species within a changing environment, and is essential for the long-term survival of a species. The diversity of organisms is classified using a taxonomic hierarchy, which varies depending upon the particular classification used. However, the taxonomic levels shown in Table 2 are universally accepted, with interim levels occurring, making up a complex classification, e.g., below “family” level there may also be “subfamilies.” Central to the concept of taxonomic diversity is the species, and for practical purposes, species are the most appropriate targets for biodiversity conservation and use. While genes provide the blueprint for the construction of organisms, they are only expressed through the form and function of species. Similarly, ecosystems are essentially manifestations of the interactions between organisms. It follows that neither genes nor ecosystems can be manipulated or managed without attention being given to the requirements of species; they are the entities in nature that adapt and evolve, occupy ecological space, and become extinct. Ecological diversity describes biodiversity from population level and upwards through niches, habitats, ecosystems, landscapes and bioregions, to biomes (Table 2). At the largest scale, a “biome” describes any of a group of major regional terrestrial communities with its own type of climate, vegetation, and animal life. They are not sharply separated but merge gradually into one another. Examples include tundra, temperate deciduous forest, and desert. At the smallest scale, ecological diversity can describe “populations,” which are local communities of potentially interbreeding organisms. In asexual organisms, the term usually refers to a local community of physiologically or morphologically similar individuals of the same species. Conservationists usually refer to habitat, ecosystem, or landscape conservation, but ecosystems have become the accepted unit of conservation within this group. An ecosystem is defined by the CBD as: “a dynamic complex of plant, animal and microorganism communities and their non-living environment interacting as a functional unit.” While it is convenient to think of biodiversity in terms of genetic, species, or ecosystem diversity, it is also important to note the significance of populations. Populations are local communities of potentially breeding organisms. It is within populations of species that we find the genetic diversity that is significant in terms of adaptation and survival. Therefore, while measurements of presence and absence of species may be...