The hidden multitrophic diversity of specialized grass-shoot insects – neglected by grassland management

Abstract

Current biodiversity research often focuses on large-scale biodiversity patterns, thereby neglecting specialized endophytic species hidden inside plant tissue, which are negatively affected by traditional grassland management. Here, we analyzed grass shoot-inhabiting herbivore-parasitoid communities of ten perennial and five annual grass species that are abundant in Germany. We dissected grass shoots and reared all insects to characterize these plant-herbivore-parasitoid food webs, which depend on the overwintering of intact grass shoots developing in unmown habitats. Our results support the hypothesis that both mean shoot length of each grass species and the annual-perennial dichotomy explain differences in species richness. Annual grasses were not attacked at all, while the shoot length of perennial grass species showed a positive linear relationship with herbivore species richness, explaining 96 % of the variance. Shoot length was also closely related to the species richness of the total number of herbivore and parasitoid species (R²= 0.93). On average, 11.5 herbivore and 29.9 parasitoid species were found per perennial grass species. 31 % of the insects were monophagous, i.e. restricted to only one host grass species, and 32 % oligophagous, i.e. restricted to grasses (Poaceae). The increase in species richness with grass shoot length is in line with the hypothesis that higher resource heterogeneity, apparency and productivity of the host plant species enhance the diversity of consumers and their associated enemies. In bipartite network analyses, the interestingly high number of herbivore-parasitoid compartments increased with shoot length, likely due to the coevolution of these highly specialized tritrophic communities and adaptive radiation among grass species. The annual-perennial dichotomy was not related to differences in biomass, plant chemistry, architecture or habitat age. In grassland management, the endophagous insect community has been largely ignored, despite the, on average, high richness per grass species (41.4 insect species) with 63 % being grass specialists. The need for unmown long-term refuges, protecting intact grass shoots for persistent insect populations, is often overlooked. Future grassland management should pay more attention to these neglected communities of specialized grass-shoot insects.