The ongoing global decline in insect abundance and biomass is now well documented. To what degree this decline in insect abundance and biomass causes losses in biodiversity is still unclear, mainly due to the lack of appropriate quantitative long-term studies that allow for precise estimates of richness over time. In conservation ecology, the species accumulation curve (SAC) is a common method to assess the effectiveness of protected areas and to compare richness estimates at a standardised sample size. However, extrapolation and richness estimation are only possible in cases of asymptotic convergence, which was not achieved in many studies. Here, we construct temporal series of SACs from long-term surveys (1950–2022) of Austrian butterflies and macro-moths to assess temporal trends in species richness and dominance inside and outside protected areas across an altitudinal gradient. Irrespective of altitude, we detected a trend towards increasing rarity and dominance outside PAs, but not towards decreasing total richness. These trends were less visible inside PAs, although in lowland PAs sample-size-normalised richness also decreased over time. Evenness in PAs remained constant over time and was generally higher than outside PAs. Both, protected and unprotected areas suffered from high temporal species turnover. We conclude that the Lepidoptera assemblages of our study region underwent severe and ongoing structural rearrangements with unknown consequences for the sustained occurrence of many now-rare species. Temporal series of spatial species accumulation curves are able to detect such structural changes. The protected areas of our study region succeeded in conserving assemblage structure and compositional variability. Fewer species than outside PAs became rare. However, total richness is still higher outside PAs, and negative population trends were also visible inside PAs.