Long-term (25 yr) measures of weather and populations of the 28 most frequently trapped Namib Desert Tenebrionidae (darkling beetles) revealed a population-sustaining effect of fog for eight species using specialized behavioural mechanisms to collect fog water. During a prolonged (18 yr) low-rainfall interval, populations of these fog-collecting species maintained more of their relative abundance than all but one species that does not collect fog. Fog-collecting adaptations include complete change in diel activity period, sand trench and sand parapet building, and fog basking. All eight fog-collecting species persisted during the dry period, whereas 17 of the 19 species lacking fog-collecting adaptations disappeared or declined to less than 1% of their mean abundance. Populations of two species reported to collect fog facultatively during their usual diel rhythm declined as rapidly as species lacking fog-collecting adaptations. These demographic comparisons of fog-collecting species and species that do not do so depended upon monitoring highly variable population fluctuations under the particular climatic events that occurred during this study. The population consequences of adaptations to collecting fog water could not have been identified by any study lasting less than 17 years.