Development of insects in newly established grassland over six years: Increased conservation benefits through diverse plant communities

Agri-environmental schemes seek to mitigate the loss of biodiversity by subsidizing the restoration of grassland in Europe. However, little is known about the colonization of newly established grasslands by insects, especially over periods exceeding 1–2 years. Therefore, we investigated the development of species richness, abundance and species assemblages of wild bees, syrphids, butterflies, orthopterans and true bugs in two types of newly established grassland strips in an agricultural landscape in Austria over a six-year time span. We compared currently subsidized plant-species poor grassland strips (SG; Ecological Focus Areas) with newly established grassland strips (NG) that were created with a diverse, regionally adapted seed mixture, to find out whether diverse seed mixtures can ensure lasting improvements in the insect species richness of grassland strips over a 6-year period. We used permanent seminatural grasslands (old grassland OG) as reference habitat. From the first year after establishment, we found no differences in syrphid assemblages between OG and NG. In contrast, true bug assemblages in NG differed significantly from those in OG still five years after establishment. Wild bee, butterfly and orthopteran species composition differences between grassland strips (NG, SG) and permanent grasslands (OG) were decreasing over time. However, we found no evidence of a successional change in insect species assemblages in NG and SG. NG promoted a significantly higher wild bee richness and equivalent syrphid, butterfly, orthoptera and true bug richness compared with OG. Over the study period, NG showed sustained insect diversity, outperforming plant species-poor SG in promoting wild bee, syrphid and true bug richness. Therefore, we suggest that diverse and regional seed mixtures could enhance the effectiveness of grassland restoration for insect conservation and could make re-sowing within the first 6 years after establishment unnecessary.