17.17: Species Diversity and Biodiversity

Essential to Know

• Species diversity is a well-defined scientific term that expresses a combination of species richness (number of species) and evenness (degree to which the community has balanced populations with no dominant species).
• High species diversity is generally equated with healthy ecosystems, but there are exceptions.
• Biodiversity is a poorly defined term that refers to a combination of genetic diversity (genetic variation within populations of a species), species diversity (species richness and evenness within a community), ecosystem diversity (variations in communities of species within an ecosystem), and physiological diversity (variations in feeding, reproduction, and predator avoidance strategies within a community or ecosystem).

Understanding the Concept

The term biodiversity entered common usage when the global community of nations recognized the need to preserve species from extinction and unique habitats or ecosystems from destruction. The term has no precise definition but is based on the much more precisely defined technical term species diversity, which has been in use for many years.

Species diversity is a measure of species richness and evenness. Richness describes the number of individual species, whereas evenness expresses the degree to which a community has balanced populations in which there is not a small number of numerically dominant species. Several statistical indices have been developed to express both species richness and evenness as a single species diversity number.

Species richness is important because the larger the number of species present in a community, the more robust the community is considered to be. If there are many species, a community disturbance (such as a change in temperature or salinity) may lead to the exclusion or even extinction of one or a few species, but most species are likely to survive. Evenness is important because if the community is dominated by only a few species and these species are excluded or rendered extinct by a disturbance, competition between the remaining species to become the new dominant species may cause severe ecosystem instability. Alternatively, an uneven community may reflect effects of a disturbance, such as pollution, that favors dominance by tolerant species.

Although communities are generally thought to be desirable and stable if they contain many species and have strong evenness, those characteristics may not always be ideal. For example, coral reefs that are undisturbed by storm wave damage for many decades become dominated by just a few species of hard corals, and most other hard coral species are excluded by competition with these dominant species. Thus, the beautiful, untouched hard coral communities present in a few sheltered locations have low species richness, and species are unevenly distributed. These reefs have low species diversity (at least of corals).

In contrast, a coral reef that was damaged several years earlier by a hurricane has more species of corals (and possibly other species) and greater evenness, or higher species diversity. Few of us would consider this damaged environment an ideal situation, but the periodic disturbance may be necessary to restore and preserve the diversity of coral reefs since high species diversity is generally considered a positive attribute of any ecosystem.

During the hundreds of millions of years that life has existed on the Earth, virtually all the species that have lived have become extinct and been replaced by others. Because this natural process still continues today, none of the species now on the Earth are likely to be living a few million years in the future. Human disturbances, including pollution, have caused many species to become endangered or extinct. The rate of species extinction due to human disturbances is estimated to be many times faster than natural extinction, and possibly is much faster than the rate at which new species can evolve to replace those that are lost.

Only recently have scientists begun to realize that probably the best way to control the extinction rates of species is to control losses of their habitat and disturbances of their community structure. In response to the need to consider habitat and community structure in protecting individual species, a new concept was developed called “biodiversity.” Biodiversity is still often interpreted as species richness, but it should include many attributes of natural ecosystems. For example, biodiversity can be separated into four components:

• Genetic diversity, or variation in the genes within a species or population. High genetic diversity is thought to maximize the potential for new species development. It is also thought to maximize the potential for species survival when the species is subjected to an environmental disturbance because some members of the species may be more resistant.
• Species diversity, or species richness and evenness. High species diversity is thought to maximize the stability of the ecosystem and its resistance to environmental disturbances.
• Ecosystem diversity, or variation in the communities of species within an ecosystem. High ecosystem diversity is thought to reflect the availability of a wide variety of ecological niches. If the range of niches is large, environmental disturbance is unlikely to alter the ecosystem in such a way that more than a few species lack a niche within which to survive.
• Physiological diversity, or variation in physiological adaptations to feeding, reproduction, and predator avoidance within a community or ecosystem. High physiological diversity is thought to reflect a greater ability of the community within an ecosystem to adjust to environmental disturbances and maintain its stability.

A global agreement was established in 1993 to preserve biodiversity. Because biodiversity is a poorly defined term that encompasses many different characteristics of organisms in their living environment, this agreement will continue to be very difficult to translate into management actions. Protection of biodiversity may mean very different things to different nations or individuals.