Historical Contingency in Community Assembly: Integrating Niches, Species Pools, and Priority Effects

Abstract

The order and timing of species immigration during community assembly can affect species abundances at multiple spatial scales. Known as priority effects, these effects cause historical contingency in the structure and function of communities, resulting in alternative stable states, alternative transient states, or compositional cycles. The mechanisms of priority effects fall into two categories, niche preemption and niche modification, and the conditions for historical contingency by priority effects can be organized into two groups, those regarding regional species pool properties and those regarding local population dynamics. Specifically, two requirements must be satisfied for historical contingency to occur: The regional pool contains species that can together cause priority effects, and local dynamics are rapid enough for early-arriving species to preempt or modify niches before other species arrive. Organizing current knowledge this way reveals an outstanding key question: How are regional species pools that yield priority effects generated and maintained? 1

Assembly of seed-associated microbial communities within and across successive plant generations

“…Indeed, most fungal spores can disperse from centimeters to meters (Norros et al 2012). Taken together, the predicted importance of dispersal for seed fungal community assembly and the observed differences in fungal diversity and composition within the same experimental plot are indicative of historical contingency (Fukami 2015). In historical contingency, the order of species arrival impacts the final composition of the community.…”

Effects of dispersal and selection on stochastic assembly in microbial communities

“…Despite this possibility, there is no support that this priority effect would be stronger under suboptimal conditions or strong environmental selection (here, higher lignin/N). In general, priority effects are stronger in communities with high productivity or turnover (Fukami, 2015), or after experiencing a disturbance or environmental shift (Mergeay et al, 2011), none of which describe high lignin/N scenarios.…”

“…We tested for priority effects in all levels of selection by examining the relationship between a taxon's final abundance and its starting abundance or year of introduction, but they were not significantly correlated in any scenario (P40.05, data not shown). A positive correlation might provide evidence for a 'niche preemption' priority effect (sensu Fukami, 2015), whereby earlier colonizers fill niche space from which later colonizers are excluded, but the test could miss 'environmental modification' priority effects that occur as early colonizers modify the environment in a way that favors certain taxa. The latter type of priority effect could have occurred in DEMENT; in the model, a microbial cell's effect on grid cell substrate chemistry lingers until a new leaf is introduced (annually).…”

“…Inclusion of feedbacks between regional and local pools might have resulted in greater homogenization of community composition across treatments in our model, or increased within-group distance by reinforcing priority effects (Fukami, 2015).…”

“…Because microbes have high dispersal rates, large population sizes, fast growth rates and a propensity for dormancy, communities were traditionally thought to assemble deterministically. However, recent characterizations of microbial communities provide evidence that in addition to environmental selection, stochastic processes can be important drivers of microbial community assembly (Ferrenberg et al, 2013;Nemergut et al, 2013;Zhou et al, 2013;Fukami, 2015).…”