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.

1. Novel communities that result from exposure to contaminants and other anthropogenic stressors often persist in ecosystems that have experienced regime shifts.Because these systems may not return to pre-disturbance conditions after removal of a stressor, understanding the ecological consequences of regime shifts has important implications for how restoration success is defined. Long-term observational studies can potentially identify regime shifts in disturbed ecosystems; however, experimental approaches may be necessary to demonstrate their ecological consequences.2. We report results of a long-term (28 years) observational study and a series of stream mesocosm experiments that investigated a regime shift in a mining-contaminated watershed. We tested the hypothesis that establishment of a novel, metal-tolerant macroinvertebrate community in a previously contaminated stream impeded recovery of sensitive species, despite significant improvements in water quality over the past two decades.3. We observed significant improvements in abundance and species richness, but differences in community composition and trophic structure persisted downstream from a former source of metal contamination. These downstream communities were dominated by large, metal-tolerant caddisflies that likely impeded colonization by metal-sensitive groups (e.g. grazing mayflies). Mesocosm experiments conducted with reference and downstream communities demonstrated that novel communities retained their tolerance to metals, but were significantly more sensitive to other stressors. We suggest that the failure of downstream communities to recover represented a contaminant-induced regime shift that resulted from the sustained dominance of metal-tolerant species. 4. Synthesis and applications. Restoration ecologists generally consider increased species diversity or abundance of recreationally important species as indicators of restoration success. However, few stream restoration projects have quantified ecological resilience, and none have included a significant experimental component. Our research is the first to experimentally demonstrate that novel communities in a stream recovering from historical metal pollution have lower resilience | 2699 Journal of Applied Ecology WOLFF et aL. S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section at the end of the article. How to cite this article: Wolff BA, Duggan SB, Clements WH. Resilience and regime shifts: Do novel communities impede ecological recovery in a historically metal-contaminated stream? J Appl Ecol. 2019;56:2698-2709. https ://doi.

We integrated the results of field surveys with a mesocosm experiment to quantify the ecological impacts of a gasoline and diesel spill on a third-order stream in western Colorado (United States). The spill caused a massive fish-kill of brown trout (Salmo trutta) and mottled sculpin (Cottus bairdii), which extended several kilometers downstream. Despite significant decreases in petroleum hydrocarbon concentrations, subsequent surveys indicated little recovery of fish populations 4 years after the spill. Benthic macroinvertebrate communities were also affected, yet some metrics commonly used to assess stream ecological integrity (e.g., total abundance and species richness) showed no difference between impacted and reference sites. The failure of some groups to recover 2.5 years after the spill was likely a result of their comparatively slow reproduction and recolonization rates. To support our hypothesis that effects observed in the field resulted from petroleum exposure, we conducted a mesocosm experiment in which benthic macroinvertebrate communities were exposed to a simulated diesel spill. We observed significant decreases in the abundance of most macroinvertebrate groups at the lowest exposure concentration (75 mg/L diesel) and a strong concentration-dependent drift response across all groups. Our study suggests that relatively small petroleum spills can significantly affect stream communities, and these effects may persist several years after sediment concentrations return to background levels.

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