Plant invasion and fragmentation indirectly and contrastingly affect native plants and grassland arthropods

Plant invasion and habitat fragmentation have a detrimental effect on biodiversity in nearly all types of ecosystems. We compared the direct and indirect effects of the invasion of the common milkweed (Asclepias syriaca) on biodiversity patterns in different-sized Hungarian forest-steppe fragments. We assessed vegetation structure, measured temperature and soil moisture, and studied organisms with different ecological roles in invaded and non-invaded sites of fragments: plants, bees, butterflies, flower-visiting wasps, flies, true bugs, and spiders. Temperature and soil moisture were lower in invaded than in non-invaded area. Milkweed had a positive effect on plant species richness and flower abundance. In contrast, we mainly found indirect effects of invasion on arthropods through alteration of physical habitat characteristics and food resources. Pollinators were positively affected by native flowers, thus, milkweed indirectly supported pollinators. Similarly, we found higher species richness of herbivores in invaded sites than control sites, as species richness of true bugs also increased with increasing plant species richness. Predators were positively affected by complex vegetation structure, higher soil moisture and lower temperature. Furthermore, increasing fragment size had a strong negative effect on spider species richness of non-invaded sites, but no effect in invaded sites. Especially, grassland specialist spiders were more sensitive to fragment size than generalists, whereas generalist spider species rather profited from invasion. Although milkweed invades natural areas, we did not identify strong negative effects of its presence on the diversity of the grassland biota. However, the supportive effect of milkweed on a few generalist species homogenises the communities. The rate of invasion might increase with increasing fragmentation, therefore we recommend eliminating invasive plants from small habitat fragments to preserve the native biota. Focusing also on generalist species and revealing the indirect effects of invasions are essential for understanding the invasion mechanisms and would support restoration efforts.

Post-restoration grassland management overrides the effects of restoration methods in propagule-rich landscapes

Grassland restoration is gaining momentum worldwide to tackle the loss of biodiversity and associated ecosystem services. Restoration methods and their effects on ecological community reassembly have been extensively studied across various grassland types, while the importance of post-restoration management has so far received less attention. Grassland management is an important surrogate for natural disturbances, with which most ancient grasslands have coevolved. Thus, without the reintroduction of management-related disturbance, restoration targets are unlikely to be achieved in restored grasslands. In this study, we aimed to explore how 20 yr of management by mowing once a year or light cattle grazing affects restoration success in Palearctic meadow-steppe grasslands restored by either sowing native grasses (sown sites), applying Medicago sativa as a nurse plant (Medicago sites), or allowing spontaneous succession (spontaneous sites). We found that, following mowing, sown sites maintained long-lasting establishment limitation, while Medicago sites experienced a delay in succession. These limitations resulted in low total and target species richness, low functional redundancy, and distinct species and functional composition compared to reference data from ancient grasslands. Spontaneous sites that were mowed reached a more advanced successional stage, although they did not reach reference levels regarding most vegetation descriptors. Sown and Medicago sites that were grazed had higher total and target species richness than those that were mowed, and showed restoration success similar to that of spontaneous sites, on which grazing had only moderate further positive effects. Grazed sites, irrespective of the restoration method, were uniformly species rich, functionally diverse, and functionally redundant, and thus became important biodiverse habitats with considerable resilience. We conclude that an optimally chosen post-restoration management may have an impact on long-term community reassembly comparable to the choice of restoration method. Restoration planners may, therefore, need to put more emphasis on future management than on the initial restoration method. However, our findings also imply that if local constraints, such as potentially high invasive propagule pressure, necessitate the application of restoration methods that could also hinder the establishment of target species, the long-term recovery of the grassland can still be ensured by wisely chosen post-restoration management.