Cenozoic topography, monsoons and biodiversity conservation within the Tibetan Region: An evolving story

Abstract

The biodiversity of the Himalaya, Hengduan Mountains and Tibet, here collectively termed the Tibetan Region, is exceptional in a global context. To contextualize and understand the origins of this biotic richness, and its conservation value, we examine recent fossil finds and review progress in understanding the orogeny of the Tibetan Region. We examine the deep-time origins of monsoons affecting Asia, climate variation over different timescales, and the establishment of environmental niche heterogeneity linked to topographic development. The origins of the modern biodiversity were established in the Eocene, concurrent with the formation of pronounced topographic relief across the Tibetan Region. High (>4 km) mountains to the north and south of what is now the Tibetan Plateau bounded a Paleogene central lowland (<2.5 km) hosting moist subtropical vegetation influenced by an intensifying monsoon. In mid Miocene times, before the Himalaya reached their current elevation, sediment infilling and compressional tectonics raised the floor of the central valley to above 3000 m, but central Tibet was still moist enough, and low enough, to host a warm temperate angiosperm-dominated woodland. After 15 Ma, global cooling, the further rise of central Tibet, and the rain shadow cast by the growing Himalaya, progressively led to more open, herb-rich vegetation as the modern high plateau formed with its cool, dry climate. In the moist monsoonal Hengduan Mountains, high and spatially extensive since the Eocene but subsequently deeply dissected by river incision, Neogene cooling depressed the tree line, compressed altitudinal zonation, and created strong environmental heterogeneity. This served as a cradle for the then newly-evolving alpine biota and favoured diversity within more thermophilic vegetation at lower elevations. This diversity has survived through a combination of minimal Quaternary glaciation, and complex relief-related environmental niche heterogeneity. The great antiquity and diversity of the Tibetan Region biota argues for its conservation, and the importance of that biota is demonstrated through our insights into its long temporal gestation provided by fossil archives and information written in surviving genomes. These data sources are worthy of conservation in their own right, but for the living biotic inventory we need to ask what it is we want to conserve. Is it 1) individual taxa for their intrinsic properties, 2) their services in functioning ecosystems, or 3) their capacity to generate future new biodiversity? If 2 or 3 are our goal then landscape conservation at scale is required, and not just seed banks or botanical/zoological gardens.

A Middle Eocene lowland humid subtropical “Shangri-La” ecosystem in central Tibet

“…S16 ), not solely through a process of tectonic “uplift” with Tibet behaving as a coherent entity, but by a process of gradual evolution of compression-driven landscape modification and “bathtub sedimentation” ( 47 ). † These complex landscape changes would have provided a heterogeneous environmental niche matrix within which numerous speciation events would have taken place ( 48 ). In the future, the ancient complexity of Tibetan topography needs to be considered when studying the evolution of the modern plateau and its influence on climate and biodiversity ( 20 ).…”

Insights into the drivers of radiating diversification in biodiversity hotspots usingSaussurea(Asteraceae) as a case

“…(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint this version posted March 16, 2021. ; https://doi.org/10.1101/2021.03.15.435394 doi: bioRxiv preprint Pliocene (Xing & Ree, 2017;Spicer et al, 2020). During the Quaternary glaciation, the Hengduan Mountains with its deep valleys would have provided numerous micro-refugia within the altitudinal and aspect heterogeneity (Sun et al, 2017;Spicer et al, 2021).…”

The topographic evolution of the Tibetan Region as revealed by palaeontology

“…The interactions between topography, climate and biodiversity in the Tibetan region are complex, but a dynamic high-relief orography offers high levels of niche heterogeneity favouring multiple speciation events (e.g. Spicer 2017, Spicer et al 2020aRahbeck et al 2019). Numerous molecular phylogenetic analyses suggest intense speciation in the Miocene and many authors have attempted to make links between this diversification and the perceived 'uplift of the Tibetan Plateau' (Renner 2016;Lu et al 2018).…”