Biodiversity and Conservation

Two phrases that have become commonly used by biologists and natural resource managers, and now also by policy makers, the media and the general public, are biological diversity (typically shortened to biodiversity) and biological integrity. These phrases represent concepts that are critical to developing strategies for the preservation of biotic systems, yet they are often used imprecisely, without a true understanding of the underlying concepts. The Office of Technology Assessment, in a report titled "Technologies to Maintain Biological Diversity," defines biological diversity as "the variety and variability among living organisms and the ecological complexes in which they occur." Thus biological diversity, or biodiversity, is really much more than just the species richness (the number of species) or species diversity (a measure of both the number of species and their relative abundances) at a given site. Biological diversity is a more comprehensive concept that considers several types of classification. For example, we can consider not only the number of species at a site, but also the representation of different higher levels of taxonomic classification such as the number of families or orders present. Likewise, we can consider genetic variability within a population or among different populations. Genetic variation among populations is reflected in "subspecies" designations, and these local populations or subspecies are often threatened or endangered. Finally, we can consider ecological diversity at a number of different levels. One example of this is the number of ecosystems represented within a landscape, such as the presence of the habitat mosaic within the Middle Rio Grande Valley. All of these concepts are important when considering biodiversity.

Biological integrity refers to the wholeness of a system, which includes the presence of all of the appropriate components and of all appropriate processes. Thus not only must the correct native species of plants, animals and microorganisms be present, but also the numerous processes, such as nutrient cycling or energy flow, should be occurring at their correct rates. This reflects the capacity of the system to recover after disturbances. The biological integrity of the Rio Grande riparian forest has been greatly compromised since the water in the river has been regulated. Natural processes such as flooding, which affects, for example, rates of decomposition of dead plant material and recruitment of young woody plants, have been altered. Thus the riparian forest now is not self-sustaining. Conservation and restoration efforts focused on promoting the biological integrity of a system are much more effective than those designed simply to save individual species. This is an important concept along the Rio Grande.

Keystone species are important to the maintenance of local biodiversity. A keystone species is a species that has strong regulatory effects on the composition of the community, even through it may be in relatively low abundance compared to other members of the community. The presence of the keystone species in some way determines which other species of organisms can be present. For example, in montane river systems beavers act as keystone species through their engineering feats: the construction of beaver dams and ponds creates habitat for many other aquatic plants and animals. When beavers are removed, the ponds are not maintained and many other species are eventually lost. Other examples are large herbivores such as elephants and rhinos in the African savannas, sea otters in the great kelp forests off the western coast of North America, and starfish in the intertidal zone. In the local bosque, the Rio Grande cottonwood is considered a keystone species because it provides the physical structure for the forest itself. Without cottonwoods, certain species of animals, such as cavity-nesting birds or insects that feed on cottonwoods, will not be supported. The presence of cottonwoods creates the physical conditions that allow other bosque plants to grow. A complete loss of cottonwoods will certainly change the structure and composition of the local riparian forest.