What Is Biodiversity?
In essence, biological diversity or biodiversity is the sum of all life on Earth. It includes the vast array of life forms, their individual genetic makeup, their life processes, and their interrelationships in communities and ecosystems. Peter H. Rogan, Director of the Botanical Garden offers this more eloquent definition:
“At the simplest level, biodiversity is the sum total of all the plants, animals, fungi and microorganisms in the world, or in a particular area; all of their individual variation; and all of the interactions between them. It is the set of living organisms that make up the fabric of the planet Earth and allow it to function as it does, by capturing energy from the sun and using it to drive all of life’s processes; by forming communities of organisms that have, through several billion years on Earth, altered the nature of the atmosphere, the soil and the water of our planet; and by making possible the sustainability of our planet through their life activities now.”
According to Edward O. Wilson, a world-renowned scientist and researcher, biodiversity emerged as a scientific discipline during the last 25 years in response to two important events: the realization that human activity threatens the extinction of many plant and animal species either directly or indirectly, by habitat destruction, and the recognition that we have the ability to end that process at minimal cost to human welfare. Biodiversity studies are a hybrid discipline, drawing on both evolutionary biology and biotechnology in “the systematic examination of the full array of organisms and the origin of this diversity, together with the technology by which diversity can be maintained and utilized for the benefit of humanity.”
Levels Of Biodiversity
Biodiversity is commonly studied at three levels: genetic, species, and ecosystem diversity.
Genetic diversity refers to the different genetic makeup up of individual plants, animals, fungi, and microorganisms. It includes both genetic variation within a single species and between different species.
Species diversity is, simply, the variety of species. It includes all the differences within and between species populations, as well as among different species. Species diversity is often used to mean “species richness,” or the number of different species present in a specific habitat. However, it can also be measured in terms of either “species abundance” – the relative population sizes among various species – or “taxonomic diversity” — the genetic linkages between different groups of species. (To learn more about how organisms are classified, follow links from our Biodiversity Reference Material Page to the Sidwell School’s Classification Lab website)
Most discussions on biodiversity focus on species diversity. Evolution occurs at the species level, and the origination and extinction of species are the primary factors impacting biological diversity.
Ecosystem diversity is more difficult to define than the other two levels of biodiversity. It comprises the many differences among ecosystem types, including diversity of habitats and ecological processes. In practice, ecosystem diversity can only be evaluated on a local or regional basis, rather than on a global scale; however, even this assessment is difficult due to the ever-changing boundaries of ecological communities and ecosystems.
Number of Species
Approximately 1.7 million species have so far been identified and scientifically described, but this represents only a fraction of life on Earth. The United Nations’ 1995 Global Biodiversity Assessment estimated the total number of species at 14 million, although other estimates range from 5 million to more than 100 million!
Microorganisms and insects are thought to comprise the largest numbers of species. Insects alone are estimated to account for between 2 and 100 million species, with a working estimate within the scientific community of 8 million. Of the 100 species of animals described to date, approximately 750,000 are insects.
Scientists are working to classify and describe additional species, but it is a slow and painstaking process. For example, as many as half the plant species of Colombia’s Chocó region, including trees and shrubs, do not have a scientific name. On occasion, even new species of mammals are discovered, such as the Chinese muntjac, a kind of deer, and the sun-tailed guenon, a monkey found in Gabon. Most recently, in April 2000, the Amazon National Research institute announced the discovery of two new monkey species in northwestern Brazil.
The history of life on Earth spans approximately 3.75 billion years, the last 600 million years of which has seen a tremendous growth in biodiversity. At that time, the Earth underwent a major ecological transformation. The concentration of oxygen in the atmosphere increased dramatically, and a shield of ozone was created in the stratosphere, providing protection from harmful ultraviolet radiation. These changes permitted the emergence of larger animals in the Earth’s oceans and proliferation of plants, and later, animals on land surfaces. With the exception of a plateau reached during the Mesozoic Era, this diversity of life forms continued to grow, and Earth’s biodiversity is now at or near its peak.
Every life form plays a role in ecosystem processes, although both the nature and scope of its functions will vary. The contribution of a single species to biodiversity is frequently measured in two ways. The first is its taxonomic significance; that is to say that the more different a species is – or the more isolated in the taxonomic hierarchy used to classify species – the greater its contribution to the overall measure of global biodiversity. The second measure focuses on the impact of a particular species on an ecological community. So-called “keystone species” are those whose role within the ecosystem is so great that their removal from the local or regional ecosystem will have a disproportionate effect.
Recent experimental studies have shown that overall ecosystem productively declines as the number of species in the community decreases. This loss of productivity is most dramatic in managed ecosystems – such as crop growing lands and timber plantations – where the variety of species is particularly small. The findings of the one available long-term field study on plant species richness also indicate that reductions in plant species diversity lowered the ecosystem’s resistance to drought.
Researchers have identified two possible explanations for this relationship between plant diversity and productivity. The first, called the “sampling effect,” relies on basic probability, i.e., greater the number of species present, the more likely some will be species with higher levels of contributions to ecosystem functioning. The second proposed explanation is the “complementarity effect,” where increased diversity leads to an increase in the number of species which are complementary rather than competitive in their use of available resources. This creates a more efficient and productive ecological community.
Ecosystem processes are driven by the combined activities of all the species which comprise that community, and reductions in species diversity can severely diminish its productivity. However, it is as yet impossible to predict how the loss of any single species will impact the overall ecological balance – or how it may affect the welfare of man.