Ant-plant Mutualism as a Driver of Caterpillar Community Abundance, Diversity, and Specialization
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Authors
Coronado, Stephanie M.
Issue Date
2024
Type
Dissertation
Language
Keywords
Ant-plant Protection Mutualisms , Community Ecology , Insect-plant Interactions , Mutualism , Specialization , Tropical Diversity
Alternative Title
Abstract
Understanding how species assemble into diverse communities—and why some communities are more diverse than others—is a central goal of ecology. Much of this complexity is likely due to the interplay among species interactions, environmental resources, and community traits at different scales, ranging from individuals to landscapes. Over the past few decades, ecologists have begun to realize the importance of mutualistic interactions as drivers of diversity. This dissertation investigates how mutualism responds to resource availability, and how and when such responses lead to changes in the diversity and composition of the communities in which mutualisms are embedded. Specifically, we examined variation in ant-plant protection mutualism strength across multiple scales and the consequences for the abundance and diversity of ant-plant herbivore communities. Ant-plant protection mutualisms, in which plants host colonies of ants that defend plants from herbivory, are an ideal system in which to examine the effects of mutualism on community assembly. In tropical regions, ant-plants sustain a diverse community of caterpillar herbivores, which allowed us to test how ant defense affects caterpillar community diversity, specialization, and species traits. By experimentally manipulating ant presence on ant-plants, we were able to separately examine the direct effects of resource availability on caterpillar community assembly and the indirect effects of resource availability that operate through ant-plant mutualisms. In Chapter 1, I examine ontogenetic variation in an ant-plant mutualism. Young ant-plants often lack ant colonies, potentially due to constraints imposed by size and corresponding resource pools. We examined whether juvenile ant-plants have alternative defense strategies prior to ant occupation. We surveyed juvenile ant-plants of the genus Cecropia across a variable landscape in northwest Costa Rica, measuring ant colonization rates, defensive leaf traits, spider visitation, and caterpillar herbivory. Our results suggest that smaller, uncolonized juvenile Cecropia are resource-constrained and are generally unable to invest in alternative defenses. Spider abundance was high on ant-uncolonized trees, yet spiders did not reduce herbivory. In contrast, tree size, light, and soil fertility increased leaf defensive phenolic compounds, which were effective at reducing herbivory. This chapter suggests that defense strategies in young Cecropia do not trade off, but instead juvenile trees invest more in all defense strategies and in growth when resources are high. In Chapter 2, I investigate whether ant-plant interactions can promote the diversity and specialization of herbivore communities in a seasonally dry tropical forest in Mexico. We used a factorial, common-garden experiment with the ant-plant Cordia alliodora to test how ant defense and water addition affected the diversity and specialization of caterpillar communities. We found that although ant defense reduced caterpillar abundance and diversity at the scale of individual trees, ant defense increased caterpillar community diversity at the scale of forest plots. The presence of ant defense increased caterpillar community evenness and the abundance of dietary-specialist caterpillar species. Because caterpillar species appeared to have different levels of tolerance to ants, and ant defensive activity varied strongly among the ant-defended trees, our results suggest that variation in ant defense at the scale of individual trees allows for niche partitioning among caterpillar species. Finally, in Chapter 3, I examine whether the effects of ant defense on caterpillar community diversity that we observed in Chapter 2 are altered by the size and component traits of caterpillar species pools. We established two common garden experiments with Cecropia in two distinct forests, a seasonally dry forest and a rainforest in Costa Rica. Experimental trees had ants either excluded or introduced. We found that, similar to our results in Chapter 2, in the seasonally dry forest, ants increased caterpillar community diversity and evenness at the scale of forest plots. However, ants did not affect the diversity of caterpillar communities in the rainforest. Ant defense was equally effective at reducing caterpillar densities in both forest plots, but the caterpillar species pool was larger and more even in the rainforest than in the dry forest. Ant defense primarily increased the evenness of caterpillar communities by reducing the most abundant, gregarious caterpillar species, which represented a smaller proportion of the rainforest species pool. This study suggests that regional-level species pool characteristics can determine the impact of species interactions on community assembly. This dissertation provides evidence that mutualism shapes community assembly. Moreover, these results suggest that mutualism is an essential driver of biodiversity. This dissertation is also a testament to the value of field experiments and the careful consideration of scale in multitrophic interactions.