U.S. Department of Agriculture: Animal and Plant Health Inspection Service


Date of this Version

July 2006


Published in International Biodeterioration & Biodegradation 58 (2006) 2–8.


Population monitoring is a valuable component to managing invasive rodent populations. Indices can be efficient methods for monitoring rodent populations, as more labor-intensive density estimation procedures often are impractical or invalid to apply. Many monitoring objectives can be couched in an indexing framework. Indexing procedures obtain maximal utility if they exhibit key characteristics, including being practical to apply, being sensitive to population changes or differences in the target species, having an inherent variance formula to estimate precision of index values, and relying on as few assumptions as possible. Here, a general indexing paradigm that promotes the characteristics that make indices most useful is specifically applied to rodent monitoring scenarios. Observations are made at stations located throughout the area of interest. Stations can take many forms, depending on the observations, and range from points for visual counts to tracking plots (or tiles), bait blocks, chew cards, camera stations, trap lines and many others. Thus, a wide variety of observation methods for many animal species can fit into this format. Observations are made at each station on multiple occasions for each indexing session. Collection of geographic location data for each station is encouraged. No assumptions of independence are made among stations or observation occasions, and measurements made at each station are required to be continuous or unboundedly discrete. The formula for a general index is presented along with a derived variance formula. Issues relevant to the application of this methodology to rodent populations, and indices in general, are discussed.