Global macroecology of ant-plant mutualisms: The role of biotic interactions in shaping species diversity and composition patterns
by Yangqing Luo
Date of Examination:2023-09-18
Date of issue:2023-12-11
Advisor:Prof. Dr. Holger Kreft
Referee:Prof. Dr. Teja Tscharntke
Referee:Prof. Dr. Dirk Hölscher
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EnglishUnderstanding the mechanisms underlying patterns of species distribution and diversity is a central goal in ecology and biogeography. Biotic interactions have been acknowledged as important factors affecting species diversity at local scales. However, the importance of these interactions at larger scales and their possible interplay with other biogeographic and environmental factors is not well-understood yet. Ant-plant mutualisms, detectable through plant structures such as domatia, extrafloral nectaries (EFNs), and elaiosomes, are well studied in terms of their systematic distribution and ecological functions, and have a worldwide distribution spanning both mainland and islands. This makes them ideal systems for studying the ecological and biogeographical processes that shape plant diversity gradients at large scales. To explore the influences of biotic and abiotic environmental factors at macroecological scales, I analyzed a comprehensive global dataset containing distributional information on over 19,000 ant-associated flowering plants in Chapter 2. In Chapter 3, which focused on islands specifically, I further analyzed the distribution of ant-protected plants and interacting ants on 230 islands to unravel how biotic drivers and their interplay with other biogeographic factors shape island community composition. In Chapter 4, I focused on the compositional representation of elaiosome-bearing plants on 289 oceanic islands compared to their source pools, and investigated the potential influences of abiotic environmental factors and island isolation. I show that, at the global scale, the diversity of interacting ants is positively correlated with the species richness of ant-associated plants, after controlling for the confounding abiotic effects and phylogenetic dependence among regions. Despite the dominant influence of contemporary climate on ant-associated plant diversity, the significance of biotic interactions remains at large scales. Furthermore, arboreal ants, in combination with island isolation, play a key role in shaping the distribution of domatium-bearing plants on islands, although their presence shows no significant correlation with the distribution of EFN-bearing plants. This difference can be attributed to the different characteristics of the two interactions, with ant-domatium associations being more specified and obligatory, whereas ant-EFN associations are generalized and facultative. Finally, elaiosome-bearing plants, typically dispersed by ants, show proportionally balanced distributions on most oceanic islands compared to their mainland source pools. The proportional representation of elaiosome-bearing plants shows a weak correlation with island isolation, but correlates significantly with climatic differences between islands and their source pools, revealing that drier islands have higher proportions of elaiosome-bearing plants. My thesis thus provides new insights into the influence of biotic and abiotic environments, as well as dispersal, in shaping plant biogeography at large scales. It provides compelling evidence that biotic interactions have a significant influence on the spatial distribution of angiosperms beyond local scales, both on the mainland and on islands. In addition to dispersal, a thorough assessment of the abiotic environment can contribute to a deeper insight into the composition of island flora. A comprehensive understanding of the complex interplay between biotic interactions and other biogeographic factors can improve our understanding of the drivers of large-scale species diversity patterns.
Keywords: domatia; extrafloral nectaries; elaiosomes; global biogeography; island disharmony