Wednesday, 26 July 2017

Improving inference for aerial surveys of bears: The importance of assumptions and the cost of unnecessary complexity.

 2017 May 25;7(13):4812-4821. doi: 10.1002/ece3.2912. eCollection 2017 Jul.

Author information

Central Alaska NetworkU.S. National Park ServiceFairbanksAKUSA.
Southwest Alaska NetworkU.S. National Park ServiceAnchorageAKUSA.
Department of Natural Resource ManagementSouth Dakota State UniversityBrookingsSDUSA.
U.S. Fish and Wildlife ServiceHadleyMAUSA.
Present address: U.S. National Park ServiceKingstonRIUSA.
Western Alaska Landscape Conservation CooperativeAnchorageAKUSA.
Present address: U.S. National Park ServiceAnchorageAKUSA.


Obtaining useful estimates of wildlife abundance or density requires thoughtful attention to potential sources of bias and precision, and it is widely understood that addressing incomplete detection is critical to appropriate inference. When the underlying assumptions of sampling approaches are violated, both increased bias and reduced precision of the population estimator may result. Bear (Ursus spp.) populations can be difficult to sample and are often monitored using mark-recapture distance sampling (MRDS) methods, although obtaining adequate sample sizes can be cost prohibitive. With the goal of improving inference, we examined the underlying methodological assumptions and estimator efficiency of three datasets collected under an MRDS protocol designed specifically for bears. We analyzed these data using MRDS, conventional distance sampling (CDS), and open-distance sampling approaches to evaluate the apparent bias-precision tradeoff relative to the assumptions inherent under each approach. We also evaluated the incorporation of informative priors on detection parameters within a Bayesian context. We found that the CDS estimator had low apparent bias and was more efficient than the more complex MRDS estimator. When combined with informative priors on the detection process, precision was increased by >50% compared to the MRDS approach with little apparent bias. In addition, open-distance sampling models revealed a serious violation of the assumption that all bears were available to be sampled. Inference is directly related to the underlying assumptions of the survey design and the analytical tools employed. We show that for aerial surveys of bears, avoidance of unnecessary model complexity, use of prior information, and the application of open population models can be used to greatly improve estimator performance and simplify field protocols. Although we focused on distance sampling-based aerial surveys for bears, the general concepts we addressed apply to a variety of wildlife survey contexts.


Ursus arctos; apparent bias; availability bias; brown bear; detection probability; distance sampling; informative prior; mark–recapture distance sampling; open N‐mixture model; perception bias; precision