Definition of Biodiversity
4.1 Explain the term biodiversity. Include gene, species and ecosystem levels
The term BIODIVERSITY was first coined by the entomologist E.O. Wilson in 1986. A neologism from biology and diversity, it refers to the variety of life on the planet. There is no single standard definition for biodiversity. 1
1. Biodiversity may be defined as the totality of different organisms, the genes they contain, and the ecosystems they form. 1, 2
2. The Convention on Biological Diversity defines biodiversity as the variability among living organisms from all sources including, among other things, terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are a part; this includes diversity within species, between species and of ecosystems. 3
Biodiversity may be considered at three levels: genetic diversity, species diversity, and ecosystem diversity (see figure 1 below).
Figure 1: Illustrations of various levels of biodiversity 4
- refers to the variety of living species within a geographic area.
(A species may be defined as a group of organisms which are able to interbreed freely under natural conditions to produce viable offspring).3
Species diversity may be measured using the following characteristics:
I. Species richness - the number of species within a particular sample area.
(in combination with)
Species evenness - this refers to the evenness in number of individuals of each species in the area
E.g. for two sample areas X and Y, there are two species, a, and b. In X, there are 92 individuals of species a, and only 8 of species b, while in Y, there are 50 individuals of each species.
If species richness only was used to account for species diversity in X, the diversity might seem lower than Y (although both have the same number of species), because almost all the individuals encountered would be from only one species. Species evenness in conjunction with species richness is thus a more useful indicator of species diversity, because it takes into account rarer species.
II. Relative abundance of species of various categories - (the categories might include size classes, tropic levels, taxonomic groups, or morphological types).
For example, an area with a greater number of closely related species is not as diverse as the same area with the same number of species which are not closely related. An illustration of this point would be an island with two species of birds and one species of lizard – this island would be more diverse than an area with three species of birds and no lizards.3
Therefore, species diversity can be assessed in terms of the number of species or the range of different types of species an area contains.5
Current estimates for the total number of species in existence vary from 5 million to nearly 100 million. About 1.7 million of these species have been identified to date. A complete and current inventory of species is impossible because many invertebrates, microorganisms and lower plants (the three most diverse life-forms) have not yet been identified and named. 2
There has been a definite bias towards describing large organisms, those that are considered attractive or appealing (such as flowering plants and butterflies), those most closely resembling humans (vertebrates, especially mammals), and those that have a direct impact on human activities (such as pests). Organisms that can be studied without complex procedures or expensive equipment have also taken precedence, as have those which are relatively easy to locate. This, however, underestimates the importance of microorganisms including algae, bacteria, fungi, protozoa and viruses, which are vital to life on Earth. So far, less than 3-5 percent of microorganisms have been described. 5
The species level is generally regarded as the most appropriate for considering the diversity between organisms. 2
This is not because species diversity is more important than the other two types, but because:
I. Species diversity is easier to work with.
Species are relatively easy to identify by eye in the field, whereas genetic diversity (see below) requires laboratories, time and resources to identify, and ecosystem diversity (see below) needs many complex measurements to be taken over a long period of time.
II. Species are also easier to conceptualize and have been the basis of much of the evolutionary and ecological research that biodiversity draws on.
Species are well known and are distinct units of diversity. Each species can be considered to have a particular "role" in the ecosystem, so the addition or loss of single species may have consequences for the system as a whole. Conservation efforts often begin with the recognition that a species is endangered in some way, and a change in the number of species in an ecosystem is a readily obtainable and easily understandable measure of how healthy the ecosystem is. 6
- refers to the differences in genetic make-up between distinct species, as well as the genetic variations within a single species. This is the least visible and, arguably, least studied level of biological diversity. 5
Genetic diversity is the variety present at the level of genes. Genes, made of DNA, are the building blocks that determine how an organism will develop and what its traits and abilities will be. This level of diversity can differ by alleles (different variants of the same gene, such as blue or brown eyes), by entire genes (which determine traits, such as the ability to metabolize a particular substance), or by units larger than genes such as chromosomal structure. 6
The amount of diversity at the genetic level is important because it represents the raw material for evolution and adaptation. More genetic diversity in a species or population means a greater ability for some of the individuals in it to adapt to changes in the environment. Less diversity leads to uniformity, which is a problem in the long term, as it is unlikely that any individual in the population would be able to adapt to changing conditions. As an example, modern agricultural practices use monocultures, which are large cultures of genetically identical plants. This is an advantage when is comes to growing and harvesting crops (for example all the plants can be harvested at once), but can be a problem when a disease or parasite attacks the field, as every plant in the field will be susceptible. Monocultures are also unable to deal well with changing conditions. 6
Within species, genetic diversity often increases with environmental variability, which can be expected. If the environment often changes, different genes will have an advantage at different times or places. In this situation genetic diversity remains high because many genes are in the population at any given time. If the environment didn't change, then the small number of genes that had an advantage in that unchanging environment would spread at the cost of the others, causing a drop in genetic diversity. 6
Since the gene is the fundamental unit of natural selection, and thus of evolution, some scientists argue that the real unit of biodiversity is genetic diversity. However, species diversity is the easiest one to study. 1
- encompasses the broad differences between ecosystem types, and the diversity of habitats and ecosystem processes within each ecosystem type. 2
Ecosystem diversity deals with species distributions and community patterns, the role and function of key species, and combines species functions and interactions. The term "ecosystem" here represents all levels greater than species: associations, communities, ecosystems, and the like.
This is the least-understood level of the three described here due to the complexity of the interactions. Trying to understand all the species in an ecosystem and how they affect each other and their surroundings while at the same time being affected themselves, is extremely complex. 6
Difficulties in Examining Ecosystem Diversity
I. The enormous range of terrestrial and aquatic environments on earth has been classified into a number of ecosystems. Major habitat types include tropical rain forests, grasslands, wetlands, coral reefs and mangroves. Measuring changes in the extent of ecosystems is difficult, because there is no globally agreed classification of ecosystems. Thus, ecosystems can be considered on different scales.5
II. Transitions between them are usually not very sharp. A lake may have a very sharp boundary between it and the deciduous forest it is in, but the deciduous forest will shift much more gradually to grasslands or to a coniferous forest. This lack of sharp boundaries is known as "open communities" (as opposed to "closed communities," which would have sudden transitions) and makes studying ecosystems difficult, since even defining and demarcating them can be problematic. 6
III. Species contained within a given ecosystem vary over time.
IV. The classification of the earth’s immense variety of ecosystems into a manageable system is a major scientific challenge. 2
Studies of ecosystem diversity are carried out on different scales: from one ecosystem to an entire region containing many different ecosystems. Regions containing a great variety of ecosystems are rich in biodiversity, but individual ecosystems containing endemic species also make a significant contribution to global biodiversity.
Some of the world's richest habitats are tropical moist forests. Although they cover only 7 percent of the world's surface, these areas contain at least 50 percent, and possibly up to 90 percent of all plant and animal species. Isolated islands such as Jamaica are often rich in endemic species. 5
Another aspect to this level of diversity is
biogeographic diversity which refers to the distribution
of species within habitats or ecosystems.
References
1. Free Archive http://www.freearchive.info/bi/biodiversity.html
2. New South Wales Environmental Protection Agency Biodiversity
http://www.epa.nsw.gov.au/envirom/biodiversity.htm
3. Glowka, L. et al., 1994. A Guide to the Convention on Biological Diversity
Environmental Policy and Law Paper No. 30 IUCN Gland and Cambridge. Xii + 161 pp.
4. Krogh, D., 2000. Biology: a guide to the natural world Prentice Hall, Inc.
5. Pamayanan: News and Views from Pinoy Environmentalists and Advocates Genetic,
Species and Ecosystems Diversity http://www.bwf.org/bk/pamayanan/r-genetic.html
6. The Canadian Biodiversity Website Biodiversity Theory
http://www.canadianbiodiversity.mcgill.ca/english/theory/threelevels.htm#species