UC Irvine

Mutualisms – interactions between two species that benefit them both – play a central role in structuring ecological communities and ecosystems. However, from an ecological perspective, mutualism strength (weakly to strongly beneficial) is highly variable across biotic and abiotic contexts. Global changes including climate change and biodiversity loss may be important drivers of variation in mutualisms. However, compared to other species interactions, we know little about the effects of global change on mutualisms or the cascading ecological consequences. To fill this gap, my dissertation assesses (1) the effects of climate change and biodiversity loss on ant-aphid mutualisms, (2) the mechanisms underlying such effects, and (3) the broader population-level consequences of variation in mutualisms for the species involved in the interaction. Ant-aphid mutualisms are experimentally tractable systems in which ants protect aphids against predators and parasitoids, and in return, aphids excrete a sugar-rich reward that ants consume. By experimentally manipulating aphid multi-trophic interactions along replicated climatic gradients, I found that aphids are more abundant at lower elevations where it is more arid (Chapter 1), and this is best explained by changes in interactions with natural enemies and mutualist ants (Chapters 1 and 2). Although the negative effects of natural enemies increase at lower elevations, the positive effects of mutualist ants increase even more strongly, thus increasing aphid colony growth rates and abundance (Chapters 1 and 2). These results illustrate that multi-trophic interactions, and mutualisms in particular, are important drivers of species responses to changes in climate. In addition, with observational demographic studies, I investigated the effects of mutualist ant identity and diversity on aphid population dynamics. I found that aphids frequently associate with multiple ant species, both simultaneously and sequentially across their lifetimes. Aphids do not benefit from simultaneous mutualist ant diversity, likely because competing ants are ineffective mutualists. However, because ants differ in availability, aphids benefit from more consistent mutualist services with sequential ant diversity. In summary, by documenting the population-level effects of mutualisms and how these effects are mediated by climate change and biodiversity loss, my dissertation provides a window into the multi-trophic mechanisms by which global change structures ecological communities.