Eric Duell, from the Kansas Biological Survey & Center for Ecological Research, discusses his article: ‘Mycorrhizal-herbivore interactions and the competitive release of subdominant tallgrass prairie species‘
Tallgrass prairies of central and eastern North America are characterized by diverse plant communities consisting of grasses and forbs (often called wildflowers or broadleaves) which possess a variety of life history traits. Of course, these plants do not operate in a vacuum, and are constantly interacting with other organisms, both harmful and helpful. In our work, we explored the intricate interactions between plants, soil microbes, and herbivores in tallgrass prairie ecosystems. These relationships, which play a crucial role in maintaining plant communities and biodiversity, are complex and often difficult to decipher. One particularly important group of microbes are known as arbuscular mycorrhizal (AM) fungi (Figure 1). In short, AM fungi provide plants with difficult-to-access nutrients, such as phosphorus. In return, the plant provides the AM fungi with carbon in the form of sugars produced during photosynthesis. Our study aimed to shed light on the underexplored interactions between above- and belowground herbivores and AM fungi, which are vital for plant health and ecosystem stability.
Figure 1. Spores (reproductive structures) of AM fungi. Photo by Liz Koziol.
Experimental Setup
In our experiment, we grew experimental tallgrass prairie plant communities in large containers (~50 kg) filled with sterile prairie soil. These contained an assemblage of eight tallgrass prairie grass and forb species. The setup was designed to test various combinations: half of the containers included AM fungi, half did not; half had native root-feeding soil nematodes, half did not; and half experienced grasshopper herbivory, while half were free from aboveground herbivory.
We carefully prepared the soil, inoculating it with AM fungal spores and nematode communities to recreate natural prairie conditions. We ensured that the microbial communities were as close to field conditions as possible. Plants were then introduced and allowed to establish before being subjected to herbivory treatments.
Grasshoppers, specifically the two-striped grasshopper (Melanoplus bivittatus); Figure 2), a common invertebrate herbivore in tallgrass prairies, were used to simulate aboveground herbivory. The effects of this herbivory were closely monitored, with the team measuring leaf damage and overall plant health.
Key Objectives
Our study investigated:
 1) The independent and interacting effects of above- and belowground herbivores on AM symbiosis.
2) The effects of these herbivores and mycorrhizal fungi on plant community structure.
3) The influence of mycorrhizal responsiveness on plant tolerance to herbivory and the resultant shifts in plant community composition.
Findings and Implications
AM Fungi as a Central Player: The results underscored the pivotal role of AM fungi in tallgrass prairies. AM symbiosis significantly influenced the dominance of C4 grasses, such as big bluestem (Andropogon gerardii) and Indiangrass (Sorghastrum nutans), with these grasses thriving in the presence of AM fungi and grasshopper herbivory but not when nematodes were present. This suggests that AM fungi help C4 grasses withstand aboveground herbivory better than belowground attacks.
Figure 2. Two-striped grasshopper (Melanoplus bivittatus). Photo by Eric B. Duell.
Herbivore Effects: The study revealed that herbivore effects were additive and could alter plant community dynamics. For example, C3 grasses showed competitive release in the absence of AM fungi, a trend reversed when grasshoppers were introduced. Forbs, or broad-leaved herbaceous plants, displayed species-specific responses. For instance, false boneset (Brickellia eupatorioides), which does not have a strong positive response to AM fungi, increased its biomass in the absence of AM fungi and with grasshopper herbivory, whereas AM fungi boosted the biomass of other forbs like black-eyed susan (Rudbeckia hirta) and azure blue sage (Salvia azurea) unless grasshoppers were present.
Interaction Dynamics: The interactions between AM fungi and herbivores were complex. Grasshopper herbivory generally promoted the dominance of AM-dependent plants, while nematode herbivory reduced it. This suggests that aboveground and belowground herbivores affect plant-microbe interactions differently, with potentially significant implications for ecosystem management and restoration efforts.
Significance of the Study
This research is crucial for understanding how different types of herbivores and symbiotic fungi interact to shape plant communities. Such knowledge is vital for conserving and restoring tallgrass prairies, which are among the most endangered ecosystems in the world. By elucidating the roles of these interactions, the study provides valuable insights into the mechanisms maintaining plant diversity and ecosystem function.
The findings highlight the importance of considering both above- and belowground interactions in ecological research and management. For instance, strategies to enhance plant resilience in prairies might involve fostering beneficial AM fungal relationships and managing herbivore populations.
Future Directions
The study opens several avenues for further research. Investigating the long-term effects of these interactions and their responses to environmental changes, such as climate change or invasive species, could provide deeper insights into ecosystem dynamics. Additionally, exploring similar interactions in other ecosystems can help generalize the findings and apply them in different conservation contexts.
In conclusion, this study underscores the complexity and significance of plant-microbe-herbivore interactions in tallgrass prairies. It highlights the need for integrated management approaches that consider these multifaceted relationships to maintain and restore healthy, resilient ecosystems.