Most discussion and management of biological diversity occurs at the local population level, which can be defined as that group of organisms of one species that live in a specific area and that tend to interact more frequently with each other than with individuals from other populations. Loss of diversity always occurs first with the extinction of local populations, and, if the process continues long enough, will ultimately lead to extinction at the regional and global scales. Among animals, most changes in the biological diversity of western rangelands have involved local extinctions, and I summarize the general factors that determine whether a population will either persist through time or decline in numbers to local extinction. These include not only events that directly impact local birth and death rates, but also the ability of individual to move between populations. I then discuss the changes that have occurred in the diversity of the bird community in a riparian area along the Lower Truckee River in west-central Nevada that was studied by Ridgeway in 1868 and over a century later by Klebenow and Oakleaf in 1972-76. A comparison of the species that were located in each study indicates that nearly half of the avian diversity that was originally present in this habitat has now disappeared. An analysis of the birds that are now locally extinct suggests four changes in the environment have been important: (1) loss of the total amount of habitat or subhabitats, such as marshy areas near the river; (2) loss of specific resources within the habitat, such as many native fish; (3) changes in the structure of the remaining habitat, such as a loss of ground cover needed by ground-nesting species to protect their young from predators; and (4) loss of connectivity between remaining habitat patches. Restoration of local biological diversity in riparian habitats, for both birds and other animals, will require management actions to address each of these factors.
Stacey, Peter B.
"Diversity of rangeland bird populations,"
Natural Resources and Environmental Issues:
Vol. 4, Article 5.
Available at: http://digitalcommons.usu.edu/nrei/vol4/iss1/5