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Criterion 1 Indicator 8
Rationale
 
Number of Forest-Dependent Species Occupying a Small Portion of Their Former Range
 
 
 
In evaluating the condition of a wildlife or plant species, it is essential to know the natural geographic range of the species, the current range of the species, and how well individuals are distributed across the landscape within that range. The geographic range of a species represents the broadest possible area where a species can exist, and is generally determined by major environmental patterns such as ecoregion, climate, and elevation. When changes occur in the range of a species, they may indicate significant changes in the size of the population or in the availability of habitat. A continuous decline in geographic range may signal that the species is in trouble, losing viability and at greater risk of extinction.
 
Within a species’ geographic range, the number of individuals and their distribution over the landscape affect the likelihood that an individual will find a mate, the intensity of competition between species, and the probability that a species will continue to exist. For most forest-dependent wildlife, the distribution of individuals within the geographic range is dynamic. Population gaps form and disappear as natural disturbances and forest succession shift habitat availability. In assessing the condition of a wildlife species, biologists must be able to determine the difference between these natural population fluctuations and long-term declines.
 
Monitoring the distribution of wildlife species benefits the public in several ways.
  • Many wildlife species provide measurable or potential economic benefits to the state. Monitoring these populations can help ensure the proper management of these natural resources.
  • Population monitoring will aid in preventing future wildlife crises and forestall the economic and social costs of species recovery plans.
  • A change in the distribution of a forest-dependent species may indicate underlying changes in forest structure, composition, or productivity, at a landscape or regional scale.
  • Monitoring changing patterns in species distribution can provide conservation biologists with important clues about the cause of declining population viability.

Can This Indicator Be Quantified
  Based on a review of current and historic range maps, wildlife inventories, and previous assessments, we have found sufficient information to conclude that at least 11 forest-dependent species have been extirpated from a significant portion of their previous range since European-American settlement in Oregon. These species are:
 
  • Northern spotted owl (Strix occidentalis caurina)
  • Great gray owl (Strix nebulosa)
  • Common nighthawk (Chordeiles minor)
  • Yellow-billed cuckoo (Coccyzus americanus)
  • Lewis’ woodpecker (Melanerpes lewis)
  • Mountain quail (Oreortyx pictus)
  • Gray wolf (Canis lupis)
  • Wolverine (Gulo gulo)
  • Fisher (Martes pennanti)
  • American marten (Martes americana)
  • Grizzly bear (Ursus artos)
 
These 11 species represent a subset of a more extensive list of species reported to have declining ranges by the Oregon Department of Fish and Wildlife (Marshall, 1992). This list represents those species with particularly strong evidence for a diminished range or extirpation based on empirical data. The ODFW and other investigators also have inferred range declines based on a decrease of a particular vegetation type (e.g., old-growth forest).

Trends
To date, assessments that have reported declines in species’ geographic ranges have not been based on data derived from randomized or extensive systematic sampling (e.g., ONHP, 1998; Marshall, 1992; Puchy and Marshall, 1993; Blaustein, et. al., 1995; Maj and Garton, 1994). A literature review did not find a single example of a wildlife monitoring study that conformed to assumptions for statistical inference and had a sufficient sample size to detect and measure a change in geographic range. Generally, range declines have been determined based on observations compiled over time from a variety of separate sources. A further discussion of available data and difficulties is presented in the "Data Source and Availability" section.

Data Source and Availability
  For the information sources used in Indicators #8 and #9, a list of attributes has been compiled in Appendix B. Information sources for these two indicators have been combined in this appendix because a single database or research program often provides both distribution and abundance information. There are likely to be hundreds of scientific papers relevant to the assessment of wildlife ranges and abundance in Oregon. Field notes and checklists can provide additional information. Unfortunately, we could not find a compendium of population data that would facilitate a statewide assessment of Indicators #8 and #9.
Species distribution information is most often available at a local scale, including source information for museum specimens, wildlife research records, anecdotal observations, and local systematic inventories. Currently, several monitoring programs are acquiring data that will support analysis of population trends. Most of these programs focus on species with traditional, economic importance (i.e., ODFW annual game and furbearer statistics), or federally listed threatened and endangered species (e.g., Lint, et. al., 1999; Madsen, et. al., 1999).
 
The North American Breeding Bird Survey (BBS), started by the U.S. Geological Survey in the mid-1960s, is an example of a comprehensive monitoring study of birds. There are currently 115 active BBS survey routes well-distributed throughout Oregon. Data collection and analytical methods for this survey have been peer-reviewed (Geissler and Sauer 1990; Link and Sauer 1994; Sauer, et. al., 1994). The current BBS database appears to be useful for an exploratory analysis of geographic range changes during the last 30 years. However, there do not yet appear to be adequate sample sizes for many avian species to support hypothesis testing for population distribution trends. Such tests should become more feasible as more data is acquired.
 
A similar monitoring study for amphibians has recently been started by the U.S. Geological Survey (USGS 1999b). A more extensive list of population distribution information has been compiled in Appendix B.

Reliability of Data
   
We have found very few surveys that have been deliberately conducted to examine patterns of species distribution across a species’ entire geographic range. The BBS and ODFW big-game inventories are two programs for which it may be possible to derive estimates of data accuracy or precision. The geographic ranges of most species have been described using a compilation of information for which statistical methods of inference probably would be inappropriate.
 
Although statistical methods are useful in measuring data accuracy, they are not the only way to establish the reliability of information used in biodiversity or natural resource analyses. The breadth and interdisciplinary nature of bioregional assessments typically cause them to be conducted outside the traditional confines of the scientific method. Swanson and Greene (1999) describe several alternative techniques to assure credible bioregional assessments in the face of uncertain or incomplete data, including risk analysis, population viability analysis (PVA), spatially explicit habitat models, and peer review. Other investigators have explored methods to integrate observational records and checklists from independent sources into an analytical framework (Droege, et. al., 1998; Dunn, et. al., 1996).
 
In this exploratory assessment, a twofold approach was used to determine declines in geographic ranges. First, species distribution maps from a variety of publications were used to delineate the current extent of a species range (e.g., Nussbaum, et. al., 1983; Summers and Miller, 1993; Verts and Carraway, 1998). To describe the historical range for each species, maps (e.g., Bailey, 1936; Maj and Garton, 1994) and less spatially precise information from scientific papers were used (e.g., Rymon, 1969; Olterman, 1972). Anecdotal accounts were also used (e.g., Gabrielson and Jewett, 1940).
 
This information was used to determine if species were present in Oregon during the period 1850 to 1950, to the finest spatial resolution that the data would support. Historic maps and tables were compared to the current geographic range for the species. A species was determined to have a declining range if there was qualitative evidence of a range decline greater than 25 percent between 1850 and 1950.
 
However, for some species, well-distributed observations did not exist for 1850 to 1950. Commonly, these were species that are small, secretive, or not considered to have economic importance. For some species, range declines are reported based on evidence derived from other wildlife population or biodiversity assessments (e.g., Blausteinm, et. al., 1995; Marshall, 1992). These assessments typically have supported a determination of range decline based on evidence that the abundance or distribution of a significant habitat component is diminished (e.g., large diameter snags, old-growth forest).
 

Scale
   
An inventory and monitoring program would be useful in learning more about which wildlife species, if any, are disappearing from parts of their former range. In order to determine over how large an area and for how long such a program should be carried out, it is critical that the program’s goals be clearly defined. Haufler (1999) has noted some of the items that must be considered: the extent of the planning landscape, minimum resolution of data, and duration of the plan. It is also crucial that program staff determine what criteria will trigger a management action or classification status (i.e., "species having a declining range"). For example, if the goal is to detect changes in the population size and geographic range of selected wildlife species, it will be important to define what constitutes the minimum significant change in the parameter (e.g., a 20 percent decline in the species’ range over 25 years). The time and money available are also considerations that affect the intensity and sampling frame of the monitoring program.

Recommended Action for Data Collection
   
Appendix B identifies several programs and sources of wildlife distribution information that may be useful in describing or monitoring animal populations. However, current estimates of geographic ranges usually are not based on extensive surveys. Biologists have developed modeling strategies to describe animal distributions when empirical data is incomplete. Csuti, et. al., (1997) demonstrated an approach in which habitat maps were made from ODFW data; these maps were then used to refine wildlife distribution maps that were based on recorded observations. Scott, et. al., (1999) reviewed several applications of gap analysis used to predict distribution of terrestrial vertebrates at a regional scale.

Definitions
  Biodiversity — The variability among living organisms from all sources, including terrestrial, marine, and other aquatic ecosystems, and the ecological complexes of which they are a part; this includes diversity within species, between species, and of ecosystems (Heywood and Baste, 1995).
 
Biological population — A group of organisms of one species, occupying a defined area and usually isolated to some degree from other similar groups (Lincoln, et. al., 1998).
 
Extirpation — Extermination of the population of a given species from an area (Lincoln, et. al., 1998).
 
Gap analysis — A process in which areas of high biodiversity are identified and mapped using vegetation cover types as candidate areas for reserved status (paraphrased from Short, et. al., 1996).
 
Habitat — The space used by an organism, together with the other organisms with which it coexists, and the landscape and climatic elements that affect it; the place where an animal or plant normally lives and reproduces (United Nations Environment Programme, 1995).
 
Species — A group of individuals that has their major characteristics in common and are potentially interfertile (FEMAT 1993).
 
Statistical inference — The process of predicting or estimating population parameters on the basis of sample data; inductive statistics (Lincoln, et. al., 1998).
 
Sustainability / sustainable use — The use of components of biodiversity in a way and at a rate that does not lead to the long-term decline of biological diversity, thereby maintaining its potential to meet the needs and aspirations of present and future generations (The Convention on Biological Diversity, Nairobi, 1992).
 

Selected References
  See "Selected References" under Indicator #9.