Multiple mutualist interactions: Effects on plant fitness and arthropod communities
There is a wealth of studies exploring the benefits that mutualists convey to their hosts and the conditions in which these benefits are most important to fitness. Yet, in natural systems many species are actually associating with multiple mutualist partners simultaneously, although studies more typically focus on a single partner at a time. Only including a single mutualism could lead to a biased perspective to how a mutualist influences a host’s fitness response to abiotic and biotic pressures. In addition to its mutualism with rhizobia, the legume Chamaecrista fasciculata forms a mutualistic interaction with ants, providing ants with nectar in exchange for defense against herbivores. Although they provide the plant with very different benefits, these two mutualisms may be connected, with one influencing the effects of the other and even the mutualist abundances. In a full factorial field experiment, I have explored how rhizobia and ants independently and interactively influence the growth and fitness of C. fasciculata as well as how these mutualists may affect the co-occurring arthropod community.
In this study, C. fasciculata receive a very large fitness benefits from associating with rhizobia, but a cost of associating with ants that is amplified in the presence of rhizobia. Due to decreased nodulation in the presence of ants but a greater relative fitness benefit from rhizobia, trade-offs in allocation between rhizobia and extrafloral nectar are likely driving this pattern. However, there is a greater rate of rhizobia contamination in the presence of ants, indicating that ants facilitate successful rhizobia inoculation on a plant by transporting rhizobia through the soil. The ant community is also benefited by the presence of rhizobia with ants preferentially tending plants with rhizobia, but these ants reduce allocation to rhizobia. Additionally, this study demonstrated that rhizobia and ants interact to influence the abundance and diversity of herbivores and arthropods found on the plants. Interactions commonly considered to be mutualistic symbiotic relationships can actually range from mutualism to parasitism, with the same particular species interaction representing both ends of that continuum depending on the context. Therefore, when assessing the effects of mutualists on population and community patterns, it is important to consider a wide range of species interactions.