Spatial phylogenetic theory and the scaling of diversity in the Earth microbiome
Microbes are everywhere, in the guts and on the skin of every organism, deep in the oceans and high in the atmosphere, in all soils from the Amazon rainforest to the Siberian tundra, and even deep in the Earth's crust. While microbial ecology today advances in great strides, several outstanding problems remain in trying to understand microbial communities and processes that drive their dynamics, structure, and diversity. One such problem lies in our lack of an analytical and predictive theoretical framework that can accommodate for highly heterogeneous, growing, interacting, evolving, and diversifying communities where abundances, relationships, and novelty can change and intermingle over time. This is particularly relevant in the microbial world where mutation and horizontal gene transfer can work as diversifying mechanisms on a similar time scale as other demographic processes. In an era of genomics and metagenomics a second problem presents itself as the sheer size and velocity of contemporary datasets. Efficiently extracting meaningful quantities and low dimensional summaries from these datasets is highly non-trivial but central to our understanding of the link between patterns and processes.
We present planet scale empirical patterns of microbial phylogenetic diversity across tens of thousands of sites and for several different types of biomes obtained from the Earth Microbiome Project dataset and analyze them using insights coming out of recent advances in spatial phylogenetic theory. In particular we show how the scaling behavior of the microbial edge-abundance distribution varies geographically and how it changes following variable coarse-graining in space and evolutionary time. We then map the phylogenetic beta diversity decay with distance given any focal spatial location within and across microbiome types, thus uncovering deep evolutionary patterns of microbial diversification, universality, and candidate processes that gave rise to them. Finally we explore the microbial edge area relationship and its implications for conservation.