The Macroecology of Microorganisms: From Pattern to Process

Microorganisms are the most ubiquitous, diverse, and functionally important organisms on Earth, yet their ecological patterns, and the underlying causative processes that determine their distributions over large spatial scales, remain poorly understood. Therefore, I test for macroecological patterns and processes within microbial communities. I use a combination of data generating approaches including meta-analysis, published sequence datasets and databases, and high-throughput sequencing, coupled with modern statistical methods. Firstly, I show that metagenomic sequencing, is superior to amplicon sequencing as a method of surveying microbial biodiversity, as it detected more diversity at all taxonomic levels. However, cost analysis shows that metagenomics is prohibitively expensive for macroecological studies, where many samples are required. I find that classic macroecological patterns, such as the distance-decay of similarity, are context-dependent and vary according to ecological context, and methodological differences. I therefore make recommendations for future analyses of spatial analyses of microbial communities. Furthermore, I show that whilst microbial communities may exhibit distance-decay relationships, they do not necessarily form biogeographic regions, highlighting a difference in the macroecology of micro- and macroorganisms. I build on this finding by showing that different regional microbial communities can show considerably different responses to the same environmental gradient, hinting that regional communities play an important role in determining microbial community composition at local scales. Finally, I investigate whether regional-scale climatic variables determine the distributions of microorganisms. I show that the climatic drivers and influence of these drivers varies strongly between microbial taxa, suggesting that different microbial taxa have very different macroecologies. I conclude that macroecological patterns in microorganisms may not be as general as in “macroorganisms”, and can be highly context-dependent, varying with taxon, regional metacommunity dynamics, or methodological choices. Careful consideration of these factors is therefore required when attempting to understand how microbial communities will respond to future environmental changes.