Empirical comparison of density estimators for large carnivores

1. Population density is a critical ecological parameter informing effective wildlife management and conservation decisions. Density is often estimated by dividing capture-recapture (C-R) estimates of abundance ( [graphic removed] ) by size of the study area, but this relies on the assumption of geo...

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Published in:	The Journal of applied ecology 2010-02, Vol.47 (1), p.76-84
Main Authors:	Obbard, Martyn E., Howe, Eric J., Kyle, Christopher J.
Format:	Article
Language:	eng
Subjects:	American black bear Animal traps Animal, plant and microbial ecology Animals Applied ecology Bears Biological and medical sciences Black bears boreal forests Boundaries capture of animals carnivore Carnivores Comparative analysis Conservation Density estimation edge effect environmental models Estimating techniques estimation Estimators Fundamental and applied biological sciences. Psychology General aspects geographic closure Mammalia Modelling and Management Population density spatial data spatially explicit capture–recapture Ursus americanus Vertebrates: general zoology, morphology, phylogeny, systematics, cytogenetics, geographical distribution wildlife damage management Wildlife ecology Wildlife management
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Summary:	1. Population density is a critical ecological parameter informing effective wildlife management and conservation decisions. Density is often estimated by dividing capture-recapture (C-R) estimates of abundance ( [graphic removed] ) by size of the study area, but this relies on the assumption of geographic closure - a situation rarely achieved in studies of large carnivores. For geographically open populations [graphic removed] is overestimated relative to the size of the study area because animals with only part of their home range on the study area are available for capture. This bias ('edge effect') is more severe when animals such as large carnivores range widely. To compensate for edge effect, a boundary strip around the trap array is commonly included when estimating the effective trap area ( [graphic removed] ). Various methods for estimating the width of the boundary strip are proposed, but [graphic removed] / [graphic removed] estimates of large carnivore density are generally mistrusted unless concurrent telemetry data are available to define [graphic removed] . Remote sampling by cameras or hair snags may reduce study costs and duration, yet without telemetry data inflated density estimates remain problematic. 2. We evaluated recently developed spatially explicit capture-recapture (SECR) models using data from a common large carnivore, the American black bear Ursus americanus, obtained by remote sampling of 11 geographically open populations. These models permit direct estimation of population density from C-R data without assuming geographic closure. We compared estimates derived using this approach to those derived using conventional approaches that estimate density as [graphic removed] / [graphic removed] . 3. Spatially explicit C-R estimates were 20-200% lower than densities estimated as [graphic removed] / [graphic removed] . AICc supported individual heterogeneity in capture probabilities and home range sizes. Variable home range size could not be accounted for when estimating density as [graphic removed] / [graphic removed] . 4.Synthesis and applications. We conclude that the higher densities estimated as [graphic removed] / [graphic removed] compared to estimates from SECR models are consistent with positive bias due to edge effects in the former. Inflated density estimates could lead to management decisions placing threatened or endangered large carnivores at greater risk. Such decisions could be avoided by estimating density by SECR wh
ISSN:	0021-8901 1365-2664