Soil Fungi and the Effects of an Invasive Forb on Grasses: Neighbor Identity Matters

We studied the effects of soil fungi on interactions between Centaurea melitensis, an exotic invasive weed in central California, and two co-occurring grasses, Nassella pulchra and Avena barbata. The fungicide benomyl reduced the abundance of arbuscular mycorrhizal (AM) fungi in plant roots but did not affect non-AM fungi. Centaurea plants grown alone were \u3e50% smaller with the resident microbial community intact than when benomyl was applied. When grown with Nassella, the effect of benomyl was reversed. Centaurea grew almost five times larger with the resident microbial community intact. Fungicide had no effect on the biomass of Centaurea grown with Avena, but biomass of Centaurea was significantly lower when grown with Avena than when grown with Nassella or alone. Photosynthetically fixed carbon may have been transferred from Nassella via soil fungi to Centaurea, constituting a form of soil fungi-mediated parasitism, but such a transfer did not occur from Avena to Centaurea. Second, Nassella may have been more inhibited by soil pathogens in the presence of Centaurea than when alone, and the inhibition of Nassella may have released Centaurea from competition. A third possibility is that Nassella has strong positive effects on the growth of soil fungi, but the positive feedback of beneficial soil fungi to Nassella is less than the positive feedback to Centaurea. Regardless of the mechanism, the difference in soil fungicide treatment effects on competition between Centaurea and Nassella vs. Centaurea and Avena has important implications for the invasion of California grasslands

Centaurea in the Columbia basin ecoregion Disturbance, invasion, and competition

To assess the potential role of biological soil crusts and native perennial grasses in the establishment and resource acquisition of three invasive species of Centaurea in the Columbia Basin Ecoregion, I conducted two different multi-year, multi-site, field-based investigations. In one study, C. maculosa, C. diffusa, and C. solstitialis were experimentally invaded into nine habitat types across three disturbance regimes. In another study, plot-level species removals allowed inference of the competitive effects of native bunchgrasses on Centaurea species and of Centaurea species on the native bunchgrasses. Recruitment of Centaurea species was approximately 1-2% in non-disturbed (control) plots compared to 5-10% in the disturbance treatments. Regionally, within the disturbance treatments, there were no overall differences in establishment between plots where the soil crust was removed and plots where the plants and the soil crusts were removed although species and treatment differences did occur at some sites. Rates of establishment for all three Centaurea species were positively correlated with precipitation across years and sites. Experimental removal of native bunchgrasses did not result in elevated leaf water potential of Centaurea species relative to control plots (grasses and Centaurea individuals present) at any sites for any of the Centaurea species tested. Midday leaf water potentials for Centaurea species decreased from -1.0 MPa in May to -2.5 MPa in July regardless of native bunchgrass presence. Midday leaf water potentials for native bunchgrasses decreased from -1.5 MPa in May to approximately -4.0 MPa in July. Native bunchgrasses maintained greater leaf water potentials in plots where Centaurea individuals had been removed compared to bunchgrasses in control plots early in the season (0.16 MPa mean difference, p = 0.007) and late in the growing season (0.70 MPa mean difference, p < 0.0001). In a greenhouse study, Festuca idahoensis individuals grown with Centaurea maculosa produced 0.54 (+/- 0.1) grams of root mass in the 10-20 cm depth range compared to 1.02 (+/- 0.2) grams of root mass at the same depths, for Festuca idahoensis individuals grown with a conspecific. These data suggest that Centaurea maculosa achieves its competitive effect through negatively affecting the rooting depth of Festuca idahoensis individuals

Cheaters must prosper: reconciling theoretical and empirical perspectives on cheating in mutualism.

Cheating is a focal concept in the study of mutualism, with the majority of researchers considering cheating to be both prevalent and highly damaging. However, current definitions of cheating do not reliably capture the evolutionary threat that has been a central motivation for the study of cheating. We describe the development of the cheating concept and distill a relative-fitness-based definition of cheating that encapsulates the evolutionary threat posed by cheating, i.e. that cheaters will spread and erode the benefits of mutualism. We then describe experiments required to conclude that cheating is occurring and to quantify fitness conflict more generally. Next, we discuss how our definition and methods can generate comparability and integration of theory and experiments, which are currently divided by their respective prioritisations of fitness consequences and traits. To evaluate the current empirical evidence for cheating, we review the literature on several of the best-studied mutualisms. We find that although there are numerous observations of low-quality partners, there is currently very little support from fitness data that any of these meet our criteria to be considered cheaters. Finally, we highlight future directions for research on conflict in mutualisms, including novel research avenues opened by a relative-fitness-based definition of cheating

Cheaters must prosper: reconciling theoretical and empirical perspectives on cheating in mutualism

Cheating is a focal concept in the study of mutualism, with the majority of researchers considering cheating to be both prevalent and highly damaging. However, current definitions of cheating do not reliably capture the evolutionary threat that has been a central motivation for the study of cheating. We describe the development of the cheating concept and distill a relative‐fitness‐based definition of cheating that encapsulates the evolutionary threat posed by cheating, i.e. that cheaters will spread and erode the benefits of mutualism. We then describe experiments required to conclude that cheating is occurring and to quantify fitness conflict more generally. Next, we discuss how our definition and methods can generate comparability and integration of theory and experiments, which are currently divided by their respective prioritisations of fitness consequences and traits. To evaluate the current empirical evidence for cheating, we review the literature on several of the best‐studied mutualisms. We find that although there are numerous observations of low‐quality partners, there is currently very little support from fitness data that any of these meet our criteria to be considered cheaters. Finally, we highlight future directions for research on conflict in mutualisms, including novel research avenues opened by a relative‐fitness‐based definition of cheating