Mutualist-Mutualist Communities

The AMF-plant mutualism has been credited with helping plants colonize land, but we still do not understand the mechanisms that maintain this interaction. My research aims to understand the mechanisms of maintenance, and identify the threats to the persistence of the AM fungal mutualism. Research on the maintenance of the AM fungal-plant mutualism is important to all plant terrestrial communities, because AM fungi have been credited with promoting plant productivity, altering competitive outcomes among plant species, and maintaining diversity.

Evolution of AMF-plant Networks
In collaboration with Stuart Borrett (UNC-Wilmington) we are exploring the evolution of network models (Figure 1) to test a series of hypotheses about AM fungal-plant evolution (Bennett & Borrett, submitted to Ecology Letters). Traditional models of the AM fungal-plant interaction propose that plants or fungi can choose the “best” partner, and sanction poorer partners. Under this scenario we should expect to observe a correlation between how partners respond to the association, and how often or how strongly they are associated. However, my work has demonstrated that AM fungi that are strong competitors tend to be poor growth promoters (ie. poor mutualists) demonstrating a lack of correlation between the plant response to the association (growth) and the frequency of association (root colonization) (Bennett & Bever, 2009). My research analyzes the interaction between plants and AMF as a network. Network analysis predicts that in well connected networks containing multiple species of plants and AMF, neither plant nor AMF species should evolve in response to a change by a single partner. Instead of evolution in response to a single partner, mutualists (plant or AMF) should respond to the collection of partners.

Invasive species threaten the persistence of AMF-plant mutualism
A network of interactions among AM fungi and plants predict stability over time, however there are several current threats to this network structure. A novel plant species entering into this network will encounter new AM fungi, and this may create selective pressure for reduced response AMF. To test this idea I grew a suite of invasive and native grasses in two soil communities, and demonstrated that native species have variable responses to soil communities and the AMF within them while introduced species do not (Bennett & Strauss, manuscript complete). In order for introduced species to evolve in response to their mutualistic partners, there must be genetic variation for association with AM fungi, which I have demonstrated in two systems (Bennett & Bever, 2009; Garrido, Bennett et al., 2009). This reduction in response to AM fungal and soil communities may allow invasive species to outcompete native neighbors. My future research will address how introduced species threaten the persistence of the AM fungal-plant mutualism by comparing the response of introduced species in their native and new ranges.

Invasive plants that create monospecific stands produce a system in which one dominant plant partner associates with multiple AM fungi. I hypothesize that this shift in the network structure will allow AM fungi to evolve in response to the single plant partner, however the invasive plant, still associating with multiple fungi, should not evolve in response to its symbionts. In a preliminary test of this question I grew a native and invasive species in soil collected below a native and invaded community, and showed that AM fungi from the invaded site performed best in the root system of the invasive species suggesting evolution to the single host at that site . Given that introduced species threaten the AM fungal-plant mutualism, and therefore every plant community, my current and future research addresses introduced species alteration of soil communities, and AM fungal evolution in response to invasive hosts in multiple systems.