Plant-Animal Interactions in a Changing World

Abstract

Plant-animal interactions drive ecological functioning in tropical forests by maintaining biodiversity, influencing plant population dynamics, and providing key ecosystem services. However, the biological mechanisms underpinning these interactions across multiple levels of organization remain poorly understood, in large part due to sampling biases. Moreover, understanding how anthropogenic activities affect plant-animal interactions, and consequently ecological functioning, is urgent in a world undergoing rapid environmental change. Addressing these ecological challenges is critical, particularly in tropical forests with high levels of biodiversity and pressing conservation threats such as Madagascar and Gabon. In Madagascar, I worked with a diverse team to investigate the relationships between lemurs and trees across primary and secondary forests. We combined germination experiments, biodiversity surveys, ethnobiological data, and ecological network analysis. In Chapter 2, germination experiments with eight captive and two wild lemur species revealed that lemur gut-passage primarily enhanced germination success and reduced time-to-germination via seed priming. Disperser identity and functional traits influenced these outcomes; for example, seed size was associated with germination probability, and seeds passed by male lemurs had a 40\% higher germination probability than those passed by females. Plant phylogenetic relatedness further affected germination outcomes. In Chapter 3, network analyses across primary and secondary forests highlighted the pivotal role of small nocturnal lemurs in ecological networks representing lemur seed dispersal, seed predation, and herbivory. Tree centrality—a measure of a tree's importance within a network— varied by land use, interaction type, and ecological traits. In secondary forests, central trees tended to have smaller diameters and lower wood density. In primary forests, non-endemic trees with lower wood density tended to be more central. Multilayer network analysis demonstrates that trees connecting primary and secondary forests were often non-endemic with low wood density. Further, human use of trees was associated with network structure in secondary forests, with central trees more likely to be used by people, particularly in herbivory networks. In Chapter 4, simulated species extirpation demonstrated that primary forest networks were more stable than secondary forests or multilayer networks, even accounting for lemur dietary flexibility. Despite seven lemur species linking forest types, multilayer networks were consistently less stable, emphasizing the vulnerability of human-modified landscapes to species loss.Through complementary research in Gabon, we examined the influence of frugivore-plant interactions on tree diversity and carbon storage. We investigated frugivory interactions across Gabon, combining botanical survey data across 252 1 ha plots with imputed frugivory data. In Chapter 5, results highlighted that frugivore trees contributed disproportionately to tree taxonomic and phylogenetic $\beta$-diversity. Notably, anthropogenic effects were associated with interaction diversity; plots near villages had higher interaction richness and $\alpha$-diversity but contributed less to $\beta$-diversity, suggesting that human activity may drive biotic homogenization across large spatial scales. In Chapter 6, we used simulations to assess how faunal degradation of frugivores affects aboveground biomass. Overall, faunal degradation of endangered species significantly reduced aboveground biomass by limiting the recruitment of large trees. The results varied by taxa; for example, the loss of carnivores and birds increased the aboveground biomass, and the effects of ungulates were mixed. Vulnerability to aboveground biomass loss was higher in low-altitude, low-precipitation areas, primary forests, and plots farther from villages. Accounting for unobserved interactions was critical for robustly assessing the ecological consequences of frugivore declines.Together, these five studies emphasize the multifaceted role of frugivore-plant interactions in tropical forest ecological functioning. Integrating trait-based, phylogenetic, and network approaches advances our mechanistic understanding of plant-animal interactions and their sensitivity to environmental change.