Changes in assembly processes in soil bacterial communities following a wildfire disturbance

Abstract

Although recent work has shown that both deterministic and stochastic processes are important in structuring microbial communities, the factors that affect the relative contributions of niche and neutral processes are poorly understood. The macrobiological literature indicates that ecological disturbances can influence assembly processes. Thus, we sampled bacterial communities at 4 and 16 weeks following a wildfire and used null deviation analysis to examine the role that time since disturbance has in community assembly. Fire dramatically altered bacterial community structure and diversity as well as soil chemistry for both time-points. Community structure shifted between 4 and 16 weeks for both burned and unburned communities. Community assembly in burned sites 4 weeks after fire was significantly more stochastic than in unburned sites. After 16 weeks, however, burned communities were significantly less stochastic than unburned communities. Thus, we propose a three-phase model featuring shifts in the relative importance of niche and neutral processes as a function of time since disturbance. Because neutral processes are characterized by a decoupling between environmental parameters and community structure, we hypothesize that a better understanding of community assembly may be important in determining where and when detailed studies of community composition are valuable for predicting ecosystem function.

Figure 1

Non-metric multidimensional scaling ordination based on Bray-Curtis distances showing the change in bacterial community composition and increase in β diversity in burned soil bacterial communities (4 week communities=gray squares; 16 week communities=black squares) compared with unburned soil bacterial communities (4 week=open triangles; 16 week=solid triangles) 4 and 16 weeks after the fire.

Figure 2

Plot showing the null deviation (Chase and Myers, 2011) of burned and unburned communities 4 and 16 weeks after the fire. A null deviation close to zero suggests that neutral processes are more important in structuring the community, whereas larger positive or negative null deviations suggest that niche-based processes are more important. Different letters indicate significant differences between sample dates based on permutation tests (P<0.05).

Figure 3

The three hypothesized phases of community assembly following disturbance. Phase 1 is characterized by more neutral assembly processes; Phase 2 is more niche-based and Phase 3 is increasingly more neutral.