Beyond biogeographic patterns: processes shaping the microbial landscape

Abstract

Recently, microbiologists have established the existence of biogeographic patterns among a wide range of microorganisms. The focus of the field is now shifting to identifying the mechanisms that shape these patterns. Here, we propose that four processes - selection, drift, dispersal and mutation - create and maintain microbial biogeographic patterns on inseparable ecological and evolutionary scales. We consider how the interplay of these processes affects one biogeographic pattern, the distance-decay relationship, and review evidence from the published literature for the processes driving this pattern in microorganisms. Given the limitations of inferring processes from biogeographic patterns, we suggest that studies should focus on directly testing the underlying processes.

Effects of dispersal and selection on stochastic assembly in microbial communities

“…At low dispersal, within-group distance slowly and consistently increased over time (Supplementary Figure S3). This trend is characteristic of stochasticity introduced by drift that can increase in importance under low dispersal rates (Vellend, 2010;Hanson et al, 2012), although we cannot rule out that priority effects also contributed to this pattern. Between-group distance also increased slightly at low dispersal rates, relative to that of communities randomly assembled at initialization, suggesting that some selection occurred.…”

“…In contrast, dispersal can be deterministic-when certain species are better dispersers than others-or stochastic, occurring through passive processes like wind (Nemergut et al, 2013;Lowe and McPeek, 2014). Low or limited dispersal can also introduce stochasticity in microbial communities (Martiny et al, 2006;Bell, 2010;Lindström and Östman, 2011), potentially through increased drift (Hanson et al, 2012;Stegen et al, 2013), whereas high rates of dispersal can induce mass effects. Through mass effects, high dispersal rates can swamp selection, making microbial communities more similar to a regional species pool than expected by chance and less predictable from environmental variables (Leibold et al, 2004).…”

“…We first hypothesize that low dispersal rates cause increases in within-group distance as a result of demographic drift (Hanson et al, 2012), whereas higher dispersal rates result in decreases in within-group distance (Figure 1, compare panels a and b). High dispersal can also 'swamp' the variation caused by local environmental factors through mass effects, but this may be minimal in microbial communities because of fast turnover and selection (Lindström and Östman, 2011).…”

Turnover of soil bacterial diversity driven by wide-scale environmental heterogeneity

“…S ince the first statement of microbial biogeography developed by Baas Becking 1 in 1934 ''Everything is everywhere, but, the environment selects'', it is now well established that soil microbial communities exhibit biogeographical patterns, that is, variations in their distribution across space and time 2 . Although the increasing number of studies of spatial microbial ecology from local to wide scale 3 is helping to identify ecological processes (selection, dispersal, ecological and evolutionary drifts, speciation) impacting microbial diversity, the hierarchy of these processes remains unclear in comparison with that of macroorganisms.…”

“…where z is the turnover rate as in equation (1), w d and w D are the operational taxonomic unit (OTU bin ) similarities between sites located d meters and D meters apart from each other (d 2 and D 2 would correspond to the area of the sampled locations), respectively. Nowadays, TAR is commonly used in this form in most microbial biogeographical studies 3,9,[14][15][16] to assess microbial diversification and its potential relative dependency on ''dispersal'' and ''habitat heterogeneity'' (including habitat diversity and landscape configuration) 4 .…”