Lack of taxonomic and distributional information on the majority of taxonomic groups (i.e., invertebrates) in many cases leads to the greatest part of biodiversity being overlooked, and it may vitiate conservation efforts. We stress the need for, and feasibility of, overcoming this drawback in a case study analysis of total local diversity of Coleoptera in a Spanish relict forest (Natural Park of Fragas del Eume). We assessed the reliability of estimates derived from a generalist sampling inventory of the actual scores of richness and abundance of the various beetle families. We found that ≈1,000 species are estimated to live in such a small area (≈90 km²). Because efficiency of the generalist sampling protocol of beetle families was uneven, their inventory completeness varies with population size and heterogeneity of niche occupied. Comparison of the performance of several sampling-effort surrogates indicates that individuals seem to underestimate richness, compared with database records or sampling days, probably because conspecific individuals are spatially aggregated and thus the collection of several specimens does not imply a greater sampling effort than collecting only one specimen. The relationship between richness and abundance is dissimilar among families. Cerambycidae, found to have smaller populations than other families, but representing a large portion of beetle richness, should be considered a conservation priority in the prevention of biodiversity loss, because local extinction of small populations is more probable. Regional or broader scale patterns of beetle biodiversity cannot be addressed due to the lack of comparable inventories. Thus, fieldwork and taxonomic effort on hyperdiverse invertebrate groups should be encouraged and extensively funded to obtain reliable biodiversity information to enhance the effectiveness of conservation policies.