Modeling Sighting Heterogeneity and Abundance in Spatially Replicated Multiple-Observer Surveys
RICHARD J. BARKER,1 DAVID M. FORSYTH,2 MATTHEW WOOD,3
11 Department of Mathematics and Statistics , University of Otago, P.O. Box 56, Dunedin , New Zealand
2Department of Sustainability and Environment , Arthur Rylah Institute for Environmental Research , 123 Brown Street , Heidelberg, VIC 3084 , Australia
3Australian Ecological Research Services Ltd., 341 Princes Highway , Portland, VIC 3305, Australia
Failure to account for dependencies among observers in multiple-observer capture-recapture studies will lead to biased inference yet methods that account for dependencies are poorly developed. We combined hierarchical capture-recapture models and finite sampling theory to infer population abundance of koala (Pbascolarctos cinereus) in the 5,812-ha Mount Eccles National Park, southeastern Australia. We used plot replication to build a Bayesian hierarchical model of koala sightings by multiple observers. Given the number of koala present on a plot, we modeled counts of koala by a multinomial log-linear model with interaction terms to account for between-observer dependencies. We modeled total numbers using a Poisson log-linear model with covariate and random effects. We estimated the number of koala as 1.9 koala/ha (95% CI = 1.6, 2.4). We found evidence of between-observer interactions, highlighting the importance of having at least 3 observers in multiple observer studies.