Background: Demographic models are widely used in conservation and management, and their parameterisation often relies on data collected for other purposes. When underlying data lack clear indications of associated uncertainty, modellers often fail to account for that uncertainty in model outputs, such as estimates of population growth.udMethodology/Principal Findings: We applied a likelihood approach to infer uncertainty retrospectively from pointudestimates of vital rates. Combining this with resampling techniques and projection modelling, we show that confidenceudintervals for population growth estimates are easy to derive. We used similar techniques to examine the effects of sample size on uncertainty. Our approach is illustrated using data on the red fox, Vulpes vulpes, a predator of ecological and cultural importance, and the most widespread extant terrestrial mammal. We show that uncertainty surrounding estimated population growth rates can be high, even for relatively well-studied populations. Halving that uncertainty typically requires a quadrupling of sampling effort.udConclusions/Significance: Our results compel caution when comparing demographic trends between populations withoutudaccounting for uncertainty. Our methods will be widely applicable to demographic studies of many species.
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