Master Thesis Defense by Riz Fernando Noronha

Title: Self-Organized Soil: Spatial Ecology and Birth-Death Processes 

Abstract:

Soil is a living, dynamic, ecosystem. It provides a habitat for an immense amount of organisms from all across the tree of life: 59% of living species on Earth are estimated to live in soil, making it the most biodiverse singular habitat on the planet. A single gram of soil can host 6000 different bacterial genomes, several meters of fungal hyphae, and several other biological entities like protists, nematodes and mites. Physical properties of soil, such as permeability and particle sizes, have been studied for over a century. Soil appears to follow some fractal statistics, and for several years, fractal methods have been applied to soil science in order to explain them. However, viewing soil as purely inorganic matter does not provide a comprehensive understanding of the biomaterial, as soil structure is found to be self-organizing, and not static. While recent efforts are proving exceedingly valuable in quantifying the effects a microbial ecosystem has on soil, there remains a paucity of studies exploring a two-way interaction. 

In this work, we develop a simple cellular-automata model to model dynamics between worms and soil. The model is self-organizing, and successfully replicates the spatio-temporal heterogeneity seen in real soil ecosystems. Our model emphasizes the importance of spatial structure on biota by observing survival in a parameter region where the worms die out in the mean-field. Furthermore, our model allows for a simple and elegant solution of the parasite problem, where spatial structure allows faster-replicating parasites to coexist with their hosts. At certain parameters, the soil particle size distribution in our model follows a power law, consistent with empirical evidence. The model that we create emphasizes the mutual feedback between spatial structure and species coexistence: the worms organize the soil, and in turn, the soil helps the worms survive. A further investigation of the power law led us to a simple model which can be reduced to the directed percolation universality class, where we appear to observe two new critical points in the system.