Let's start from the end. Our research suggests that patterns of community assembly during succession under disturbance can act as an early warning of upcoming patterns in diversity. What does this mean? Why is it important?
Human activity is producing an extensive and persistent effect on Earth . The accumulating evidence of changing disturbance regimes linked to human activity is alarming. Large shifts in features of individual disturbances and disturbance regimes are occurring, with further changes predicted for the future . Indeed, climate change and the increasing frequency of extreme weather events, urban air pollution, and contamination of oceans by plastic waste have dramatically raised awareness that both biodiversity and the human civilization face an existential environmental crisis . For example, wildfires have increased in area, intensity and frequency over the last two decades, impacting human lives, crops, and biodiversity. Twenty of the hottest years in history have occurred in the past 22 years, and extreme events like heat waves are projected to increase in frequency by more than an order of magnitude as climate change continues . All these changes in disturbance regimes are occurring concurrently with anthropogenic alternations of the global ecosystem such as global rises in temperatures, increased mass pollution events, deforestation and defaunation of ecosystems, and more wildland conversion for human use. Some, if not all, of these trends are expected to continue, while it is also likely that new disturbance regimes will arise, including the possibility of new types of disturbances that involve plastics, toxins, and agricultural chemicals . Although these trends seem inevitable in the short term, the design of mitigations strategies or policies for conservation could benefit from frameworks that can anticipate biodiversity changes under disturbed regimes.
What is disturbance? Why should we study it?
In ecology, disturbance is an event in time that disrupts the structure of a community by changing resources, substrate availability, or the physical environment . It is considered a major factor influencing biodiversity. While a disturbance may result in inhibition, injury, or death for some individuals in a community, it also creates opportunities for other individuals to grow or reproduce. Further, disturbances can profoundly alter trajectories of ecosystem dynamics and lead to unpredictable or undesired ecosystem responses. Indeed, how disturbance relates with stochastic and deterministic assembly mechanisms remains largely unknown, particularly under fluctuating disturbances. Given the growing human population and its impact on natural and engineered ecosystems , management and conservation practices are faced with increasing frequencies and magnitudes of various disturbances that occur on different scales. However, despite increases in the frequency, duration, and scale of disturbance events, predicting the outcome of disturbance remains a challenge . Thus, understanding how or why disturbances might enhance or reduce ecosystem vulnerability is an important area of concern as ecosystems are faced with rapid-paced environmental changes.
Microbes and bioreactors: a model system to study disturbance ecology
During my Ph.D. and first years of postdoctoral research at the Stefan Wuertz lab at the Singapore Centre for Environmental Life Sciences Engineering (SCELSE), we have worked to integrate the concepts of disturbance-diversity-performance using sludge bioreactors from an interdisciplinary perspective involving the fields of ecology, engineering, microbiology, and molecular biology [7-13]. Why microbes? Because microbes typically exist as diverse, complex and dynamic communities  and are involved in all biogeochemical cycles . Why sludge bioreactors? Such bioreactors constitute model systems for microbial ecology , as they harbor microorganisms from all domains of life and provide a measurable and controllable ecosystem function that plays a key role for human health and sustainable development.
Fig. 1 - Disturbance is multidimensional. Understanding its effect is tricky, as it can vary in type, intensity, frequency and its effects can be scale-dependent. Thus, so far at SCELSE we worked with two types of environmentally relevant disturbances using activated sludge reactors: 3-chloroaniline which is a xenobiotic compound and organic loading shocks in the form of double chemical oxygen demand. We did experiments at a microcosm scale with replicated sequencing batch reactors subjected to different frequencies of disturbance, either pulses or continuous for periods of 5-6 weeks. We further experimented at a two-orders of magnitude bigger scale of volume and longer periods of operation, around 5 to 6 months.
The need for disturbance ecology theory
Ecological theory plays an important role in aiding our understanding the effect of disturbance in biodiversity by providing organization, structure and predictive power . A long-standing concept of ecology that can be used to explore possible outcomes is the intermediate disturbance hypothesis (IDH), which predicts a diversity peak at intermediate levels of disturbance due to competition-colonization trade-offs faced by organisms .
Fig. 2 - The intermediate disturbance hypothesis (IDH) was defined for species richness, which is just one aspect of the overall structure of the community, leaving gaps related to the other aspects of biodiversity and role of assembly mechanisms and the ultimate impact (if any) on ecosystem function. Through the intermediate stochasticity hypothesis (ISH, right panel), we hypothesized about the role of stochastic and deterministic assembly mechanisms as shapers of community structure.
We recently expanded the IDH into the intermediate stochasticity hypothesis (ISH) , which poses that when intermediate disturbance frequencies gave rise to unpredictable environments for organisms rendering their specialized traits less advantageous, stochastic equalization of competitive advantages across the overall pool of organisms would lead to a higher α-diversity. In contrast, either no disturbance or press disturbance conditions at the extreme ends of a disturbance range would allow fewer adapted organisms to dominate, thus lowering the α-diversity. The ISH could also be framed as an intermediate disturbance-maximum stochasticity-and-diversity hypothesis (poster image). Unlike the IDH, the ISH incorporates assembly mechanisms (deterministic and stochastic) that shape community structure (α- and β- diversity) across a disturbance gradient. Further, it predicts not only a pattern in species richness, as originally conceived in the IDH, but also in higher-order α-diversity indices since variations in the underlying assembly mechanisms also affect the abundance distributions of taxa. The ISH further considers that the output of a stochastic process is affected by some uncertainty, which in this case means there are several possible paths for the evolution of the structure and function of a community. In this regard, stochasticity operating at intermediate levels of disturbance in replicated systems could lead to similar high α-diversity (local, e.g., within a reactor), but not necessarily to similar β-diversity (compositional variation across sites, e.g., between reactors) and community function. Yet, more research is needed to test the broad validity of the ISH since disturbance is a multidimensional phenomenon, as it can be of different types and have different frequencies, intensities, and extents.
Testing the intermediate stochasticity hypothesis (ISH) in an experimental microcosm
Given the aforementioned, the objective of this work was to test the central tenet of the ISH that intermediate disturbance frequencies promote stochastic assembly processes, resulting in increased α-diversity and variable β-diversity. We resorted to our model system, an experimental setup comprised of thirty activated sludge sequencing batch reactors of 25-mL each, representing a microcosm scale, all harboring complex microbial communities collected from a full-scale wastewater treatment plant. These were subjected to six different frequencies of alteration in the feeding scheme of the substrate by doubling the organic carbon content in the feed and keeping the nitrogen content constant. Such alteration represents a disturbance for microbes in activated sludge systems due to changes in competition for oxygen, substrate, and biofilm space . Thus, organic loading shocks were shown to affect relevant functions in activated sludge systems, like carbon removal, sludge settleability, and nitrification, as well as the overall structure and assembly of the microbial community . An interesting aspect of this setup is that most relevant bacteria in activated sludge have generation times of less than 24 hours. Hence, the 42-day length of our study represented around tens to hundreds of generations of many different taxa, allowing the detection of significant patterns in assembly and structure. In other words, a similar study on communities of larger organisms (i.e., plants and animals) would have required considerably larger scales of space and time.
The main findings
In a nutshell, we found stochastic assembly processes to be more important at intermediate disturbance frequencies where the highest α-diversity was also observed, together with high β-diversity dispersion across within-treatment replicates as predicted by the ISH. Furthermore, we observed that a peak in the relative contribution of stochasticity preceded the formation of a peak in α-diversity across a disturbance frequency range. This means that community assembly patterns during succession under disturbance can act as an early warning of upcoming patterns in diversity. Plus, stochastic assembly operating at intermediate levels of disturbance could be the reason why higher diversity does not necessarily mean better function. While these findings are encouraging, further research in a variety of ecosystems and scales is needed to validate the broad applicability of the ISH, which is why we encourage the scientific community to explore this framework.
How could the ISH be useful?
We believe that the ISH framework can help to anticipate changes in biodiversity on an increasingly disturbed planet, by tracking the changes in the assembly mechanisms of communities due to disturbance. Further, the predictions of the ISH could help to identify cases when disturbance-induced stochastic assembly promotes alternative states of community structure that compromise or enhance ecosystem function, to design mitigation or intensification strategies. Furthermore, it could be used to promote community resistance and resilience to future disturbances via increased α-diversity and functional-gene diversity. Alternatively, this theoretical framework could help in the design of functionally resilient communities that do not occur naturally, through the stochastic mechanisms that are initially elicited at intermediate frequencies of disturbance and provide an advantage to rare or low-abundance taxa. Therefore, we posit that the ISH may provide a general understanding of disturbance-induced changes in community structure and function during succession, by integrating the influence of the underlying assembly processes over time.
Fig. 3 – Conceptual representation of the intermediate stochasticity hypothesis (ISH), a theoretical framework towards a general understanding of disturbance-induced changes in community structure during succession, by integrating the influence of the underlying assembly processes. Copied from Santillan and Wuertz (2022). NPJ Biofilms Microbiomes, 8(1): p. 1-11. A detailed explanation of the figure here.
If you want to find out more, the Open Access publication is available here: Ezequiel Santillan and Stefan Wuertz. Microbiome assembly predictably shapes diversity across a range of disturbance frequencies in experimental microcosms. NPJ Biofilms Microbiomes (2022) DOI: 10.1038/s41522-022-00301-3.
And finally, here is a short YouTube video where I present the background and main findings of this work.
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