Vertically-transmitted symbionts and the dynamics of host populations and communities
How does a vertically-transmitted symbiont affect the population dynamics of its host?
The costs and benefits of symbiotic interactions may vary with host and symbiont ontogeny. Effects of symbionts at different stages of host development or on different host demographic rates do not contribute equally to fitness. Although rarely applied, a population dynamics approach that integrates over the host life cycle is therefore necessary for capturing the net costs or benefits and, thus, the mutualistic or parasitic nature of symbioses. Using the native, disturbance-spe
cialist grass Agrostis hyemalis, we asked how a symbiotic endophyte affected the population dynamics of its host and how imperfect vertical transmission influenced symbiont frequency in a late successional environment. A size-structured integral projection model (IPM) parameterized with experimental field data showed that greater rates of individual growth and reproduction for endophyte-symbiotic (E+) hosts outweighed their lower rates of survival, leading to a net positive effect of symbiosis on equilibrium plant population growth (slower rate of extinction). Given that populations undergoing successional transitions are unlikely to be at an equilibrium size structure, we also conducted transient analysis that showed an initial short-term cost to endophyte symbiosis. We used a megamatrix approach to link E- and E+ IPMs via imperfect vertical transmission and found that this parameter strongly influenced the frequency of symbiosis via complex interactions with host demographic rates. Overall, our population dynamics approach improves the ability to characterize the outcome of symbiotic interactions, and results suggest that particular attention should be paid to interactions between the rate of vertical transmission and host demography.
How do a microbial symbiont and abiotic context interact to affect a tri-trophic community?
Plant–microbe protection symbioses occur when a symbiont defends its host against enemies (e.g., insect herbivores); these interactions can have important influences on arthropod abundance and composition. Understanding factors that generate context-dependency in protection symbioses will improve prediction
s on when and where symbionts are most likely to affect the ecology and evolution of host species and their associated communities. Of particular relevance are changes in abiotic contexts that are projected to accompany global warming. For example, increased drought stress can enhance the benefits of fungal symbiosis in plants, which may have multi-trophic consequences for plant-associated arthropods. Here, we tracked colonization of fungal endophyte-symbiotic and aposymbiotic Poa autumnalis (autumn bluegrass) by Rhopalosiphum padi (bird-cherry-oat aphids) and their parasitoids (Aphelinus sp.) following manipulations of soil water levels. Endophyte symbiosis significantly reduced plant colonization by aphids. Under low water, symbiotic plants also supported a significantly higher proportion of aphids that were parasitized by Aphelinus and had higher above-ground biomass than aposymbiotic plants, but these endophyte-mediated effects disappeared under high water. Thus, the multi-trophic consequences of plant-endophyte symbiosis were contingent on the abiotic context, suggesting the potential for complex responses in the arthropod community under future climate shifts.
I completed this work at Rice University under the guidance of Dr. Jenn Rudgers. The results of this work are published in Arthropod-Plant Interactions.