Grassland Degradation Changes the Complexity of Ant-Hemipteran-Plant Tritrophic Mutualisms

Ants, plants, and hemipterans in tritrophic mutualisms represent closer approximations to real ecosystems compared to twofold mutualisms, playing a critical role in ecosystem functioning. Although habitat degradation is a useful framework for investigating the stability of mutualisms, few studies have focused on such mutualistic interactions in degraded grassland. In this study, we conducted both a field and a greenhouse experiment to assess the effect of grassland degradation on the organization of ant–plant networks and ant-hemipteran-plant tritrophic interactions in the light and severely degraded grassland of Songnen Plain, China. In general, we found that severe degradation of grassland changed the spatial distribution pattern of ant–plant networks from uniform to aggregation and increased the species diversity within these networks and facilitated the Lasius flavus-aphid/mealybugs-Artemisia scoparia tritrophic mutualisms. L. flavus improves individual plant performance by increasing plant height, reducing soil moisture content, and facilitating seed transportation of A. scoparia. These advantages enhance plant fitness and population spread of A. scoparia, consequently boosting its dominance within degraded grassland habitats. In turn, the well-developed root of A. scoparia attracted more L. flavus and aphid/mealybugs by providing living space and food. Our findings enhance the understanding of tritrophic mutualisms and their mechanisms in the context of grassland degradation, thus providing valuable information for the conservation, management, and restoration of degraded grassland.