The Role of Biome Shifts in Lineage Diversification

Abstract

This thesis examines the role of biomes in lineage diversification. It explores whether biome conservatism, the tendency to remain in ancestral biomes, constrains diversification, and tests whether biome shifts are linked to characteristics of particular biomes, clades or traits. This work focuses on a series of radiations in Australia and New Zealand. Using the hyper-diverse genus Acacia in Australia, Species Distribution Models (SDM) were used to predict distributions and niche traits of 481 species in 19 clades across two biome typologies. Diversification was not constrained to any biomes, with most species (94%) occupying multiple biomes, but diversification was greatest in those biomes currently occupying larger areas. New Zealand groups (Poaceae, Melicytus, Myrsine and Pseudopanax) with small scale radiations (< 25 species) were then investigated in relation to occupancy of the three main biomes (Forest, Open and Alpine). A temporal sequence of biome availability in New Zealand allowed an examination of diversification in the context of the directional transition from forest to more open biomes. A combination of methods including SDM, biogeographical models, and trait measurements of plants grown in a common garden were utilised to explore the importance of biome shifts during diversification, the relationship between trait shifts and biome shifts, and ask if biome conservatism was prevalent in the different clades. Biome conservatism did not constrain diversification in New Zealand lineages. Biome shifts were generally frequent and more closely related to extrinsic biome factors like biome age, biome availability and relative environmental similarity between biomes, rather than to intrinsic features of lineages, such as clade size, diversification rate or age. Traits of species differed predictably by biomes occupied, and biome shifts between highly contrasting biomes (Forest and Alpine) were accompanied by biologically important trait changes. An intermediate number of biome shifts promoted maximum diversification, while low and very high biome shift frequencies dampened diversification. Diversification in New Zealand lineages typically started in Forest, before shifting into different biomes. Multiple biome occupancy and cross-biome diversification were common features of these lineages. Biome conservatism, based on the proportion of taxa occupying the ancestral biome, was evident in many lineages, despite more recent shifts into different biomes. Current methods for assessing biome conservatism are methodologically problematic in lineages that contain species that occupy multiple biomes. A new biome conservatism index is proposed, based on calculating the proportion of taxa that occupy the ancestral biome. This work demonstrates that biomes are a useful habitat scale for examining eco-evolutionary processes shaping the diversification of lineages.