Background/Question/Methods Partners in a mutualism may not benefit each other at all densities. At high densities, mutualists may cost their partners more than benefit them, thus decreasing their population density. In the mutualism between arbuscular mycorrhizal fungi and plants, plants infected with mycorrhizal fungi often have greater fitness than uninfected plants because the fungi provide the plants with nutrients. However, effects of different densities of fungi on plant success are less well known. In a greenhouse experiment, we investigated the effects of different inoculum densities of one species of fungus, Archaeospora trappei, on the survival, growth, and reproductive success of one species of plant, Plantago aristata. We investigated whether plant fitness increased as the density of fungi in the inoculum increased or whether plant fitness was greatest at an intermediate density of fungi. As a part of this study, we investigated whether the density of fungi with which plants were grown affected the germination rate of their seeds.
Results/Conclusions The density of fungi in which parent plants grew affected several components of plant fitness, including the length of time that it took for the plants’ seeds to germinate, but not the seeds’ rate of germination.  In addition, the relationship between number of seeds produced on a typical seed head and the weight of the seed head changed with different fungal densities. The outcome of the interaction between fungi and plants in one generation can be used to predict the long-term population dynamics of the mutualism.