Experimental rewilding enhances grassland functional composition and pollinator habitat use

Abstract

Semi-natural grasslands are rich in biodiversity and thus important habitats for conservation, yet they are experiencing rapid declines due to agricultural intensification and abandonment. Promoting a more diverse mammalian herbivore community, including large and megaherbivores, may result in positive cascade effects for biodiversity and ecosystem functioning. Therefore, reintroducing an ecologically functional substitute of an extinct large herbivore could mitigate current biodiversity declines and foster semi-natural grassland conservation. To test this hypothesis, we set up a 3-year rewilding experiment where 12 feral horses were introduced in three 10-hectare enclosure replicates (four horses per enclosure). We used community-weighted mean plant functional traits to elucidate plant community changes induced by grazing through time. We also investigated the effects of this experimental treatment on insect pollinated plants and on pollinator habitat use. The grassland community exerted a mixed tolerance/avoidance response to grazing. This resulted in plant functional compositional changes which favoured prostrate plant species with higher specific leaf area, characteristic of ruderal communities. Plant species richness was higher in grazed compared to ungrazed areas. Butterfly and bumblebee habitat use, as well as feeding and resting activities were also higher in grazed areas. Moreover, the number of pollinators increased with plant species richness. Synthesis and applications. This study demonstrates that, to enhance the diversity of a given herbivore community with ecological replacements of extinct wild horses can have significant effects on the functional composition of grasslands. It can also mitigate plant species declines, in particular bee-dependent plants, and boost pollinator habitat use. Novel management alternatives are urgently needed to reverse the negative effect of land abandonment in European agricultural landscapes. Thus, rewilding interventions with large mammalian herbivores may offset current biodiversity declines by maintaining important functional links between plants and pollinators in grassland ecosystems.