Land-use intensity in semi-natural grasslands indirectly affects arthropod diversity through changes in resource diversity or abundance
Land-use intensification is one of the major drivers for biodiversity loss in many ecosystems. In semi-natural grasslands, land-use components such as cutting, grazing and fertilization have been shown to affect the diversity of plants and arthropods, but the interactions between these drivers and the chain of effects are little known. In this study we tested whether increased land-use intensity affects the diversity of higher trophic levels through changes in resource diversity or biomass. Two models were set up and compared. A) The ‘Resource Heterogeneity Hypothesis’ (RHH) predicts that more diverse resources provide more niches for specialized species at higher trophic levels, i.e. herbivore species richness is increased by increasing plant species richness and predator species richness positively responds to increases in herbivore species richness. B) The ‘More Individuals Hypothesis’ (MIH) predicts that the species richness of consumers increases when resource levels are higher, i.e. plant biomass for herbivores and herbivore biomass for predators, which allows for a higher number of species to persist. The two hypotheses were tested using structural equation modeling based on plant and insect communities from 148 meadows, pastures and mown pastures within three regions in Germany and in two consecutive years.
We found positive effects of land-use intensification on plant biomass but negative effects on the diversity of plants and arthropod herbivores as well as predators. The models revealed different pathways for the effects of land-use intensity on arthropods. For herbivorous insects, the results were consistent with the RHH as we found significant effects on herbivore diversity through changes in plant diversity and no evidence for effects via plant biomass. This result was consistent when herbivore biomass was included in the model. Predators were more strongly affected by changes in herbivore biomass than by herbivore diversity. Hence, effects of intensive land-use on predacious arthropods are more likely following the MIH. These results were consistent in all three studied regions and the two years, despite variation in the strengths of land-use effects on plants and arthropods. We conclude that higher trophic levels in semi-natural grasslands are generally indirectly affected by land-use intensity but that the effects are mediated either by resource diversity or resource biomass depending on the trophic guild.