<p>Population size of an endangered species and its trend are two key parameters that define its conservation status, and eventually determine the need for and design of future conservation actions. The assessment of its conservation status can be misled by various sources of uncertainty, if these not properly considered in monitoring programmes. We analysed the potential consequences of variations and uncertainty in estimates of the population size of the narrow endemic species for assessing their conservation status, using <i>Petrocoptis grandiflora</i> Rothm. as a model species. After accounting for semantic uncertainty, we found that the published estimates of <i>P. grandiflora</i> population size in different dates, based on density and area, are highly variable. Differences in the estimated population size by site were up to five-fold, and all the estimates differed from a complete census across its distribution area carried out in this study. The trend in population size based on two previously published estimates (2009 and 2017) was highly negative, and led to unfavourable-inadequate assessment of the conservation status. However, the trend in population size based on two partial counts (2007 and 2023) was stable, and would have led to a favourable assessment. Furthermore, after eliminating spatial uncertainty, we found that Natura 2000 coverage for the species is less than 50%. Given the profound consequences that different uncertainty sources could have for conservation management if unaccounted for, we conclude that the population size of endangered chasmophytes should be determined by tailored methods, optimised for the particular objectives of each monitoring programme.</p>