Agricultural intensification, afforestation and land abandonment are major drivers of biodiversity loss in semi-natural grasslands across Europe. Reversing these losses requires the reinstatement of plant-animal interactions such as pollination. Here we assessed the differences in species composition and patterns of plant-pollinator interactions in ancient and restored grasslands and how these patterns are influenced by landscape connectivity, across three European regions (Belgium, Germany and Sweden). We evaluated the differences in pollinator community assemblage, abundance, and interaction network structure between 24 ancient and restored grasslands. We then assessed the effect of surrounding landscape functional connectivity (i.e. green infrastructure, GI) on these variables and tested possible consequences on the reproduction of two model plants, Lotus corniculatus and Salvia pratensis. Neither pollinator richness nor species composition differed between ancient and restored grasslands. A high turnover of interactions across grasslands was detected but was mainly due to replacement of pollinator and plant species. The impact of grassland restoration was consistent across various pollinator functional groups, whereas the surrounding GI had differential effects. Notably, bees, butterflies, beetles, and dipterans (excluding hoverflies) exhibited the most significant responses to GI variations. Interestingly, networks in restored grasslands were more specialised (i.e. less functionally redundant) than in ancient ones and also showed a higher number of insect visits to habitat-generalist plant species. Landscape connectivity had a similar effect, with habitat-specialist plant species receiving fewer visits at higher GI values. Fruit set in S. pratensis and L. corniculatus was unaffected by grassland type or GI. However, the fruit set in the specialist S. pratensis increased with the number of pollinator visits, indicating a positive correlation between pollinator activity and reproductive success in this particular species. Synthesis and applications. Our findings provide evidence of the necessity to enhance ecosystem functions while avoiding biotic homogenization. Restoration programs should aim at increasing landscape connectivity which influences plant communities, pollinator assemblages, and their interaction patterns. To avoid generalist species taking over from specialists in restored grasslands, we suggest reinforcing the presence of specialist species in the latter, for instance by means of introductions, as well as increasing the connectivity to source populations.

[Methods] We sampled plant-pollinator interactions five times at each of the 24 focal grasslands throughout the main flowering periods of 2018. We shifted geographically with the flowering period (May 3rd-June 6th-in Germany, June 14th-July 8th in Belgium and July 10th-August 9th in Sweden), from 9 a.m. to 6 p.m. on sunny days with low wind, and above 15ºC. We gathered flower visitation data along three parallel linear transects (80m long and 3m wide) over 45 min (15 min per transect). We recorded the identity and number of insect contacts to flowers, considering only those (i.e. potential pollination events) when an insect clearly touched the flower reproductive organs.

Swedish Environmental Protection Agency: 2016–01948, Belgian Federal Science Policy Office: BELPSPO/BR/ 175/A1/FUNgreen, Federal Ministry of Education and Research: FKZ: 01LC1619A, Ministerio de Ciencia e Innovación: PCIN- 2016-077.

Peer reviewed