What drives study-dependent differences in distance-decay relationships of microbial communities?

Aim: Ecological communities that exist closer together in space are
generally more compositionally similar than those far apart, as defined by
the distance-decay of similarity relationship. However, recent research
has revealed substantial variability in the distance-decay relationships
of microbial communities between studies of different taxonomic groups,
ecosystems, spatial scales, as well as between those using different
molecular methodologies (e.g. high-throughput sequencing versus molecular
fingerprinting). Here, we test how these factors influence the strength of
microbial distance-decay relationships, to draw generalisations about how
microbial β-diversity scales with space. Location: Global. Time period:
Studies published between 2005-2019 (inclusive). Major taxa studied:
Bacteria, Archaea, and microbial Eukarya. Methods: We conducted a
meta-analysis of microbial distance-decay relationships, using the Mantel
correlation coefficient as a measure of the strength of distance-decay
relationships. Our final dataset consisted of 452 data points, varying in
environmental/ecological context or methodological approaches, and used
linear models to test the effects of each variable. Results: Both
ecological and methodological factors had significant impacts on the
strength of microbial distance-decay relationships. Specifically, the
strength of these relationships varied between environments and habitats,
with soils showing significantly weaker distance-decay relationships than
other habitats, whilst increasing spatial extents had no effect.
Methodological factors such as sequencing depth were positively related to
the strength of distance-decay relationships, and choice of dissimilarity
metric was also important, with phylogenetic metrics generally giving
weaker distance-decay relationships than binary or abundance-based
indices. Main conclusions: We conclude that widely studied microbial
biogeographic patterns, such as the distance-decay relationship, vary by
ecological context but are primarily distorted by methodological choices.
Consequently, we suggest that by linking methodological approaches
appropriately to the ecological context of a study, we can progress
towards generalisable biogeographic relationships in microbial ecology.