In order to monitor species diversity, surveying indicators in habitats has often been recommended as more cost efficient than assessing species directly. In this study data from the Swiss National Forest Inventory (NFI) and the Biodiversity Monitoring Program (BDM) were used to verify the correlation of species density for vascular plants, mosses, and molluscs with 58 variables of forest structure, site conditions and forest management. The analyses show that site factors, in particular the biogeographic regions, the altitude, slope and the soil acidity, explain 18 to 49% of the observed variance in species density, depending on the species group (taxon). Of all the factors influenced by management, only the availability of light (stand density) was found to play an important role primarily on vascular plants. In addition the density of molluscs is positively correlated to shrub cover. However, none of the regression models tested explains more than 54% of the variance of species density. Therefore, the authors conclude that the species richness of the species groups investigated can be assessed reliably only by direct survey. The analyses confirm that certain data assessed in forest inventories is ecologically very important and relevant for environmental policy.