Insect herbivores reduce plant biomass loss and enhance plant recovery in response to extreme drought

Plant species are responding to anthropogenic climate change by expanding their distributions to higher latitudes and altitudes. This is generating novel communities consisting of a mixture of range-expanding and native plant species. These communities are increasingly subject to extreme droughts and are shaped by herbivory. How drought and herbivores interact to impact biomass production and plant recovery from drought is, however, poorly understood. Here, we conducted a mesocosm experiment to study how grazing by grasshoppers affected biomass production and plant survival of mixed plant communities, that is, consisting of native and range-expanding plant species, under summer drought. Before the summer drought, we exposed mixed plant communities to a 2-week period of feeding by a local meadow grasshopper and had a control without grasshoppers. Communities were grown in soils from the new (Northern) and original range (Southern) of the range-expanding plants and were pre-conditioned by either native or range-expanding plant communities. We show that grasshopper feeding reduced plant biomass loss to drought and enhanced the capacity of plants to recover from drought, both in terms of biomass production and survival. Grasshopper feeding increased the biomass of range-expanders relative to natives, independent of summer drought. In general, soil origin and previous soil conditioning by range-expanding versus native plants did not modify the effects of grasshoppers. We conclude that aboveground herbivory by grasshoppers affects responses of mixed plant communities to drought, increasing both plant community resistance and resilience to drought.