The ICESCAPE Software Users Guide (Appendix 1) describes a parametric bootstrap (Davison and Hinkley 1997) model for implementing the general abundance estimator proposed by Southwell (2004) for Antarctic land-breeding predators. The software, which was developed as a suite of routines in the R language for statistical computing (R Development Core Team 2008), aims to adjust raw count data for availability and perception bias, and account for sampling fractions less than unity, in order to standardise estimates of breeding populations derived from count data collected under variable conditions. In Antarctica, adjustment for availability is particularly important, because difficulties in accessing breeding sites often lead to counts of land-breeding predators being taken at suboptimal times in a breeding cycle when the availability fraction is substantially different from unity. Adjustment for availability standardises counts to a common reference point of breeding chronology, and is achieved by applying an adjustment factor based on time series of availability throughout a breeding season. Such time series are typically collected at only a limited number of sites, so a search algorithm is used to determine surrogate availability information for a site when none exists. Importantly, standardisation in this way allows site-specific estimates to be aggregated to achieve region-scale population estimates. ICESCAPE uses a bootstrap resampling framework to propagate uncertainties associated with adjustments for availability bias, perception bias and sampling fraction through to final standardised population estimates.
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