The Macroecology of Global Diversification: Investigating the Spatial Patterns of Macroevolution of Amphibians
Creators: name: Author, Unknown
Publisher: name: University of Lincoln
Year: 2023

description: The diversification of life on Earth has been ongoing for ?4.5 billion years. Throughout this time multicellular life has evolved, forming complex body plans, and from this a vast diversity of vertebrates. The distribution of this diversity is not equal, with biodiverse hotspots and environments depauperate of species. A substantial number of vertebrates are the amphibians, a species-rich group of ?8,000 species. Originating ?330 million years ago they are derived from the earliest land-dwelling tetrapods. However, they are also the most at threat of extinction of any vertebrate class. In this thesis I investigate the diversification dynamics of amphibians from two aspects. Firstly, I investigate the spatial pattern of recent speciation rates across the most expansive phylogenetic tree to date. I test the latitudinal and elevational speciation gradient for over 6,000 species, finding that contrary to the evolutionary hypothesis that the species richness gradients are formed via higher speciation, speciation rate is near constant across space. Environmental variables are then used to test whether these patterns of speciation rate across space can be explained by species ecology. Amphibian diversification is positively affected by UV-B. This is due to UV-B having a positive relationship to recent speciation rate across Anura and Caudata species. Gymnophiona showed a contrasting relationship of speciation and ecology to the other amphibians orders with positive NPP effects on speciation rate. Most environmental factors did not show substantial effects on amphibian speciation rate at broad-scale analyses. When investigating amphibian genera, there were few similar findings between groups, but there was also differences between speciation rate metrics. Thus environmental predictors of speciation rate do not explain variation in the data well and indicate that over macroecological scales species-specific speciation rate is largely independent of temperature, productivity, environmental stability and heterogeneity. Whether the latitudinal diversity gradient was formed from high tropical diversification followed by a slowdown to produce the equal rates across space found in this study is not known. The answer lies in whether tropical hotspots can saturate, at which point amphibian speciation is suppressed. In order to understand whether ecological saturation is a valid hypothesis for amphibian diversification the second part of the thesis tests the hypothesis that diversification is governed by a process of nichefilling. By investigating island amphibians, niche-filling could be detected in spatially-bounded insular groups. Diversification tempo consistently showed a slowdown towards the present in these groups, which could be explained by ecological limits to island diversity. Ecological limits are shown to arise due to competitive interactions between species within a radiation and ecologically limiting factors, which were productivity, island area and precipitation. The theory that dimorphic species diverge across niche space saturating more niches was rejected as sexual size dimorphism had no significant relationship to island diversity. Overall, this thesis demonstrates that macroevolution and ecology are interconnected. However, disentangling the effects at both large and small scales is difficult. Ecological factors can explain spatial heterogeneity and diversification slowdowns in diversity for island radiations, but global macroecological patterns of speciation rate show a more complex dynamic.