Multispecies mutualisms: Trait-mediated and density-mediated indirect effects

Background/Question/Methods .  Just as herbivore feeding can influence plant interactions with other herbivores and mutualists, patterns of host-use by one mutualist may alter subsequent plant-mutualist interactions.  However, surprisingly little is known about the ecological and evolutionary consequences of multispecies mutualisms.  This is astonishing considering that almost 90% of flowering species require animal vectors for pollination, and many of these same plant species also require animal vectors to mediate subsequent seed dispersal.  Patterns of host use by seed dispersers may be influenced by pollination mutualisms through at least two common pathways: density-mediated indirect effects and trait-mediated indirect effects.  I will explore the mechanisms driving these two pathways and the potential ecological and evolutionary outcomes for the host.  I will then use the native spring ephemeral plant Trillium erectum in the USA and its associated pollinators and ant seed dispersers as a case study to investigate how pollination mutualisms at the flowering stage alter the likelihood and intensity of ant-plant seed-dispersal mutualisms and how shifts in these interactions translate into changes in plant demography and altered selective landscapes.  Using field manipulations of pollination interactions, chemical analyses (of the seed elaiosomes), and behavioral assays (of the ants), I will tease apart the relative importance of density-mediated versus trait-mediated effects of pollination on the likelihood of seed dispersal.
Results/Conclusions .  Taken together, consideration of the broader context in which organisms interact and evolve is fundamental to our understanding of how multiple-species interactions shape genetic and biological diversity at ecological and microevolutionary scales.