Comparison of camera and road survey estimates for white-tailed deer
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Wildlife managers require reliable, cost-effective, and accurate methods for conducting population surveys in making wildlife management decisions. Traditional methods such as spotlight counts, drive counts, strip counts (aerial, thermal, infrared) and mark-recapture techniques can be expensive, labor-intensive, or limited to habitats with high visibility. Convenience sampling designs are often used to circumvent these problems, creating the potential for unknown bias in survey results. Infrared-triggered cameras (ITCs) are a rapidly developing technology that may provide a viable alternative to wildlife managers because they can be economically used with alternative sampling designs. We evaluated population-density estimates from unbaited ITCs and road surveys for the endangered Florida Key deer (Odocoileus virginianus clavium) on No Name Key, Florida, USA (461-ha island). Road surveys (n=253,) were conducted along a standardized 4-km route each week at sunrise (n = 90), sunset (n=93), and nighttime (n=70) between January 1998 and December 2000 (total deer observed=4,078). During this same period, 11 ITC stations (1 camera/42 ha) collected 8,625 exposures, of which 5,511 registered deer (64% of photographs). Study results found a difference (P < 0.001) between methods with road-survey population estimates lower (76 deer) than ITC estimates (166 dee/}. In comparing the proportion of marked deer between the 2 methods, we observed a higher (P < 0.001) proportion from road surveys (0.266) than from ITC estimates (0.146). Spatial analysis of deer observations also revealed the sample area coverage to be incongruent between the 2 methods; approximately 79% of all deer observations were on urban roads comprising 63% of the survey route. Lower road-survey estimates are attributed to 1) urban deer behavior resulting in a high proportion of marked deer observations, and 2) inadequate sample area coverage. We suggest that ITC estimates may provide an alternative to road surveys for estimating white-tailed deer densities, and may alleviate sample bias generated by convenience sampling, particularly on small, outer islands where habitat and/or lack of infrastructure (i.e., roads) precludes the use of other methods.
