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Indicator 1.08: Population levels of selected representative forest-associated species to describe genetic diversity

An indicator for Criterion 1: Conservation of Biological Diversity

What is this indicator and why is it important?

This indicator uses population trends of selected bird and tree species as a surrogate measure of genetic diversity. Population decreases, especially associated with small populations, can lead to decreases in genetic diversity, and contribute to increased risk of extinction. Many forest-associated species rely on some particular forest structure, vegetation associations, or ecological processes. Monitoring population levels of such representative species will indicate the status of the associations of species associated with specialized conditions. Management use of this indicator will ensure forest health conditions are being monitored and may help avoid species extinction.

What does the indicator show?

Between 1966 and 2006, about 27 percent of forest-associated bird species increased and 25 percent decreased; for nearly one-half the species no strong evidence existed for an increasing or decreasing trend. Most of 38 tree species or species groups analyzed showed increases in number of stems of greater than 50 percent for moderate to large diameter classes (greater than 12 inches in diameter) between 1970 and 2007 (fig. 8-1). State wildlife agency data indicate that populations of many big game species increased in the past 25 years, but forest-associated small game species showed mixed trends.

Figure 8-1: Chart of number of tree species or groups of species by percent change in stem numbers by FIA diameter class midpoints (1970-2006)Figure 8-2: Map of difference between number of bird species with significantly increasing & decreasing population trends (1966-2006)

Are there important regional differences?

The South has the greatest proportion of physiographic regions with higher numbers of bird species with significantly decreasing trends compared to bird species with significantly increasing trends (fig. 8-2). For tree species, the Pacific Coast Region has a greater number of tree species or species groups showing declines in large diameter classes compared to other regions (fig. 8-3).

What has changed since 2003?

Most forest-associated bird species with significantly decreasing population trends between 1966 and 2003 also had decreasing trends between 1966 and 2006. Bird species associated with early successional and wetland habitats are among those with declining population trends; populations of some generalist bird species and some favored by burning have increased (fig. 8-4a). Most tree species showed relatively small changes in stem numbers since 2002, although a few species such as black walnut had increases greater than 15 percent and other species such as jack pine decreased by greater than 25 percent (fig. 8-4b).

Figure 8-3: Charts of number of tree species or groups of species by percent change in stem numbers by FIA diameter class midpoints by region (a) Pacific Coast, (b) Rocky Mountain, (c) North & (d) South (1970-2006)

Why can’t the entire indicator be reported at this time?

Population data are lacking for taxa other than trees, birds, and a small subset of hunted species. We need systematic strategies for monitoring population levels of other taxa and an objective approach for selecting a minimum subset of species that will adequately represent the status of genetic diversity across the full biota.

Figure 8-4: Charts of (a) bird species by population trend (1966-2006) & (b) frequency of tree species or groups of species by relative change in total stem numbers (2002-2007)
Criterion 1 Indicators