Quantitative Assessment of Grassland Quality: Acoustic Determination of Population Sizes of Orthopteran Indicator Species

The population sizes of 10 sympatric grasshopper species in an example grassland biotope were quantitatively determined using their species-specific song patterns. The numbers of singing males, the diurnal song activity, the song quality, the audible distance for the different species, and their sex proportion provided the basis for the calculation of grasshopper numbers. By this, densities and species compositions could be estimated simultaneously. The calculated numbers were strongly supported by the results of direct catches in small areas. For assessment of the quality of biotopes and their sub-biotopes, the numerical proportion between euryoecious and xerophilic grasshoppers (independent of the species) was calculated and proved to be an excellent parameter. This provided a rapid, well-reproducible, and detailed method to measure the eutrophication of grassland habitats. With this method, we could also quantitatively assess the effects of protective measures.