Herbivory mediates the response of below-ground food webs to invasive grasses

Below-ground food webs in grasslands are affected by both above-ground herbivory and invasive plant species. However, the combined effects of these factors on soil organisms and their interactions with plant communities remain poorly understood. 

We investigated how the invasive African lovegrass (ALG) influenced below-ground food webs in south-eastern Australian grasslands under different herbivory regimes. Using experimental exclosures, we established four treatments varying in herbivore presence (all animals present, non-native mammals excluded, all mammals excluded, all above-ground dwelling animals excluded) across sites dominated either by native kangaroo grass (KG; native sites) or co-dominated by KG and ALG (invaded sites). 

After 4 years, our results revealed that invasive grasses significantly altered the abundance and structure of soil bacteria, fungi, nematodes, arthropods and earthworms. These effects intensified along the gradient of herbivore exclusion. In parallel with herbivore exclusion, accumulated plant litter shifted decomposition from bacterial to fungal-dominated, driven by changes in soil temperature, moisture and substrate, ultimately reshaping the soil biota assemblages. 

Herbivory, particularly by both native and non-native mammals, mitigated many of the adverse impacts of grass species invasion, with native and non-native mammals acting additively. These findings underscore the intricate interplay between invasive grasses and herbivory, emphasising the importance of integrated management strategies to maintain the ecological balance of grassland ecosystems.