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Title: Aerial surveys adjusted by ground surveys to estimate area occupied by black-tailed prairie dog colonies

Author
item SIDLE, JOHN - Forest Service (FS)
item Augustine, David
item JOHNSON, DOUG - Us Geological Survey (USGS)
item MILLER, STERLING - National Wildlife Foundation
item CULLY, JACK - Us Geological Survey (USGS)
item READING, RICHARD - Denver Zoological Gardens

Submitted to: Wildlife Society Bulletin
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/12/2012
Publication Date: 5/28/2012
Citation: Sidle, J., Augustine, D.J., Johnson, D.H., Miller, S.D., Cully, J.F., Reading, R.P. 2012. Aerial surveys adjusted by ground surveys to estimate area occupied by black-tailed prairie dog colonies. Wildlife Society Bulletin. 36:248-256.

Interpretive Summary: State and federal agencies attempt to measure the area of black-tailed prairie dog (BTPD) colonies in North America to monitor the status of the species. Aerial surveys are one way to estimate the area of BTPD colonies in a large area such as a state. Although BTPDs build large mounds at their burrow entrances that are visible to observers from aircraft, the aerial observers are unable to determine if the burrows are currently occupied by BTPD. We compared aerial and ground surveys for two National Grasslands in Colorado. We show that depending on how the ground survey data are used, the aerial survey overestimated BTPD colony area by 94% on the Comanche National Grassland and 58% on the Pawnee National Grassland. To address this bias, we present a ground survey technique that involves (1) sending ground observers to some of the sites identified as having burrows during the aerial survey on the ground, and (2) recording the proportion of the aerial intercept that intersects occupied colony and the proportion that intersects an uninhabited colony site. For the improved estimate of BTPD colony area derived from this method, we describe how to calculate the mean and confidence interval for the final estimate.

Technical Abstract: State and federal agencies use estimates of the area of black-tailed prairie dog (BTPD) colonies in North America as a metric to assess the status of the species. Aerial surveys using line-intercept methods are one approach to estimate the extent of BTPD colonies in a large geographic area such as a state. Although BTPDs construct conspicuous mounds at burrow openings, aerial observers have difficulty discriminating between areas with burrows occupied by living BTPDs (colonies) versus areas of uninhabited burrows (uninhabited colony sites). Consequently, aerial line-intercept surveys can potentially overestimate the extent of BTPD colonies unless adjusted by an on-the-ground inspection of a sample of the aerial line-intercepts. However, the way in which ground survey data are collected and applied can strongly influence final estimates. To illustrate this issue, we compared aerial line-intercept surveys conducted over 2 National Grasslands in Colorado with independent ground-mapping of known BTPD colonies. Aerial line-intercepts adjusted by ground surveys using a single activity category adjustment approach, which has been used in previous large-scale surveys, overestimated colonies by at least 94% on the Comanche National Grassland and 58% on the Pawnee National Grassland. To address this bias, we present a ground survey technique that involves (1) visiting a subset of aerial intercepts classified as occupied colonies plus a subset of intercepts classified as uninhabited colony sites on the ground, and (2) based on ground observations, recording the proportion of the aerial intercept that intersects a colony and the proportion that intersects an uninhabited colony site. For the improved estimate of BTPD colony area derived from this method, we describe how to calculate the variance for the final estimate based on combined variances in parameters estimated from both the aerial survey and the ground survey.