When can we trust population trends? A method for quantifying the effects of sampling interval and duration

Species’ population trends are fundamental to conservation. They are used to determine the state of nature, and to prioritize species for conservation action, for example through the IUCN red list. It is crucial to be able to quantify the degree to which population trend data can be trusted, yet there is not currently a straightforward way to do so. We present a method that compares trends derived from various samples of ‘complete’ population time series, to see how often these samples correctly estimate the sign (i.e. direction) and magnitude of the complete trend. We apply our method to a dataset of 29,226 waterbird population time series from across North America. Our analysis shows that, for waterbirds, if a statistically significant (p