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The end of the Pleistocene was marked by the extinction of almost all large land mammals worldwide except in Africa. Although the debate on Pleistocene extinctions has focused on the roles of climate change and humans, the impact of perturbations depends on properties of ecological communities, such as species composition and the organization of ecological interactions. Here, we combined palaeoecological and ecological data, food-web models and community stability analysis to investigate if differences between Pleistocene and modern mammalian assemblages help us understand why the megafauna died out in the Americas while persisting in Africa. We show Pleistocene and modern assemblages share similar network topology, but differences in richness and body size distributions made Pleistocene communities significantly more vulnerable to the effects of human arrival. The structural changes promoted by humans in Pleistocene networks would have increased the likelihood of unstable dynamics, which may favour extinction cascades in communities facing extrinsic perturbations. Our findings suggest that the basic aspects of the organization of ecological communities may have played an important role in major extinction events in the past. Knowledge of community-level properties and their consequences to dynamics may be critical to understand past and future extinctions.

Community coalescence, the mixing of different communities, is widespread throughout microbial ecology. Coalescence can result in approximately equal contributions from the founding communities or dominance of one community over another. These different outcomes have ramifications for community structure and function in natural communities, and the use of microbial communities in biotechnology and medicine. However, we have little understanding of when a particular outcome might be expected. Here, we integrate existing theory and data to speculate on how a crucial characteristic of microbial communities—the type of species interaction that dominates the community—might affect the outcome of microbial community coalescence. Given the often comparable timescales of microbial ecology and microevolution, we explicitly consider ecological and evolutionary dynamics, and their interplay, in determining coalescence outcomes. This article is part of the theme issue ‘Conceptual challenges in microbial community ecology’.

Foundation species structure communities, promote biodiversity, and stabilize ecosystem processes by creating locally stable environmental conditions. Despite their critical importance, the role of foundation species in stabilizing natural communities has seldom been quantified. In theory, the stability of a foundation species should promote community stability by enhancing species richness, altering the population fluctuations of individual species, or both. Here we tested the hypothesis that the stability of a marine foundation species, the giant kelp Macrocystis pyrifera, increased the stability of the aggregate biomass of a phylogenetically diverse assemblage of understory algae and sessile invertebrates that compete for space beneath the giant kelp canopy. To achieve this goal, we analyzed an 18-yr time series of the biomass of giant kelp and its associated benthic community collected from 32 plots distributed among nine shallow reefs in the Santa Barbara Channel, USA. We showed that the stability of understory algae and sessile invertebrates was positively and indirectly related to the stability of giant kelp, which primarily resulted from giant kelp’s direct positive association with species richness. The stability of all community types was positively related to species richness via increased species stability and species asynchrony. The stabilizing effects of richness were three to four times stronger when algae and invertebrates were considered separately rather than in combination. Our finding that diversity–stability relationships were stronger in communities consisting of species with similar resource requirements suggests that competition for shared resources rather than differential responses to environmental conditions played a more important role in stabilizing the community. Increasing threats to structure-forming foundation species worldwide necessitates a detailed understanding of how they influence their associated community. This study is among the first to show that dampened temporal fluctuations in the biomass of a foundation species is an important determinant of the stability of the complex communities it supports.

Habitat complexity is one of the most important factors structuring biotic assemblages, yet we still lack basic understanding of the underlying mechanisms. Although it is one of the primary targets in conservation management, no methods are available for comparing complexity across ecosystems, and system-specific qualitative assessment predominates. Despite its overwhelming importance for faunal diversity and abundance, there has been surprisingly little interest in examining its effects on other community and ecosystem attributes. We discuss possibilities of such effects, outlining potentially fruitful areas for future research, and argue that complexity may be implicated in community persistence and ecosystem stability by acting as a decoupling mechanism in predator–prey interactions. We provide a brief overview of methods used to quantify complexity in different ecosystems, highlighting contributions of the current issue of Hydrobiologia, and discuss potential application of these approaches for cross-ecosystem comparisons. Better understanding of the role of habitat complexity resulting from such comparisons is critically important for preservation of biodiversity and ecosystem function in an era of unprecedented habitat loss.

Summary Global reactive nitrogen (N) is projected to further increase in the coming years. Previous studies have demonstrated that N enrichment weakens the temporal stability of the ecosystem and the primary productivity through decreased biodiversity and species asynchrony. Mowing is a globally common practise in grasslands; and infrequent mowing can maintain or increase plant diversity under N enrichment conditions. However, it is unclear how infrequent mowing affects ecosystem stability in the face of N enrichment. By independently manipulating the frequency (twice vs. monthly additions per year) and rate (i.e. 0, 1, 2, 3, 5, 10, 15, 20, and 50 g N m−2 year−1) of NH4NO3 inputs and mowing (unmown vs. mown) over 3 years (2011–2013) in a temperate grassland of northern China, we aimed to examine the interactive effects of N enrichment and mowing on ecosystem stability. The results show that mowing maintained a positive relationship between species richness and ecosystem stability despite N addition, but that it exacerbated the negative effects of N addition on ecosystem stability. Mowing increased mean primary productivity and plant species richness, but it also increased the synchrony of population fluctuations and the variability of primary productivity under N enrichment, thereby contributing to a decline in the ecosystem stability. Thus, our study reveals that infrequent mowing can buffer the negative effects of N enrichment on biodiversity to some extent and further increase the primary productivity, but it exacerbates the loss of ecosystem stability with N enrichment, thereby threatening local and/or semiarid regional food security. Lay Summary

Mixing of entire microbial communities represents a frequent, yet understudied phenomenon. Here, we mimicked estuarine condition in a microcosm experiment by mixing a freshwater river community with a brackish sea community and assessed the effects of both environmental and community coalescences induced by varying mixing processes on microeukaryotic communities. Signs of shifted community composition of coalesced communities towards the sea parent community suggest asymmetrical community coalescence outcome, which, in addition, was generally less impacted by environmental coalescence. Community stability, inferred from community cohesion, differed among river and sea parent communities, and increased following coalescence treatments. Generally, community coalescence increased alpha diversity and promoted competition from the introduction (or emergence) of additional (or rare) species. These competitive interactions in turn had community stabilizing effect as evidenced by the increased proportion of negative cohesion. The fate of microeukaryotes was influenced by mixing ratios and frequencies (i.e. one-time versus repeated coalescence). Namely, diatoms were negatively impacted by coalescence, while fungi, ciliates, and cercozoans were promoted to varying extents, depending on the mixing ratios of the parent communities. Our study suggests that the predictability of coalescence outcomes was greater when the sea parent community dominated the final community, and this predictability was further enhanced when communities collided repeatedly.

Background Microbiome dynamics are both crucial indicators and potential drivers of human health, agricultural output, and industrial bio-applications. However, predicting microbiome dynamics is notoriously difficult because communities often show abrupt structural changes, such as “dysbiosis” in human microbiomes. Methods We integrated theoretical frameworks and empirical analyses with the aim of anticipating drastic shifts of microbial communities. We monitored 48 experimental microbiomes for 110 days and observed that various community-level events, including collapse and gradual compositional changes, occurred according to a defined set of environmental conditions. We analyzed the time-series data based on statistical physics and non-linear mechanics to describe the characteristics of the microbiome dynamics and to examine the predictability of major shifts in microbial community structure. Results We confirmed that the abrupt community changes observed through the time-series could be described as shifts between “alternative stable states“ or dynamics around complex attractors. Furthermore, collapses of microbiome structure were successfully anticipated by means of the diagnostic threshold defined with the “energy landscape” analysis of statistical physics or that of a stability index of nonlinear mechanics. Conclusions The results indicate that abrupt microbiome events in complex microbial communities can be forecasted by extending classic ecological concepts to the scale of species-rich microbial systems. BZtjfqfgm5MWSae9EkdSwR Video Abstract

Multispecies mutualistic interactions are ubiquitous and essential in nature, yet they face several threats, many of which have been exacerbated in the Anthropocene era. Understanding the factors that drive the stability and persistence of mutualism has become increasingly important in light of global change. Although dispersal is widely recognized as a crucial spatially explicit process in maintaining biodiversity and community structure, knowledge about how the dispersal of mutualists contributes to the persistence of mutualistic systems remains limited. In this study, we used a synthetic mutualism formed by genetically modified budding yeast to investigate the effect of dispersal on the persistence and stability of mutualisms under exploitation. We found that dispersal increased the persistence of exploited mutualisms by 80% compared to the isolated systems. Furthermore, our results showed that dispersal increased local diversity, decreased beta diversity among local communities, and stabilized community structure at the regional scale. Our results indicate that dispersal can allow mutualisms to persist in meta-communities by reintroducing species that are locally competitively excluded by exploiters. With limited dispersal, e.g. due to increased fragmentation of meta-communities, mutualisms might be more prone to breakdown. Taken together, our results highlight the critical role of dispersal in facilitating the persistence of mutualism.

Understanding the stability dynamics of naturally occurring grassland succession under nutrient enrichment is crucial for effective ecosystem management. We carried out an 11‐year field experiment to examine how grassland community stability responds to nitrogen (N) enrichment across the three successional stages (early‐, mid‐ and late‐successional stages). Our results showed that stability did not increase linearly over time; instead, natural successional grasslands became less stable in later stages due to a loss of species diversity. However, N enrichment helped slow this decline by strengthening population stability. We also found that in the early successional stage, species asynchrony driven by compensatory effects supports community stability, while in the mid‐ and late‐successional stages, it is maintained by the conservative growth strategies of dominant species. These results highlight the importance of stage‐specific nitrogen management, where reducing inputs during early stages maintains diversity‐driven asynchrony, and moderate applications in later stages promote population stability.

Adding nutrients to nutrient-limited ecosystems typically lowers plant diversity and decreases species asynchrony. Both, in turn, decrease the stability of productivity in the response to negative climate fluctuations such as droughts. However, most classic studies examining stability have been done in relatively wet grasslands dominated by perennial grasses. We examined how nutrient additions influence the stability of productivity to rainfall variability in an arid grassland with a mix of perennial and annual species. Of the nutrients, only nitrogen increased productivity, and only in wet years. In addition, only nitrogen decreased the stability of productivity. Thus, nutrient addition makes ecosystem productivity less stable in both wet and arid grasslands. However, the mechanism is very different. In contrast to wet grasslands, adding nitrogen to an arid grassland did not decrease diversity. Rather, stability decreased with nitrogen addition due to an increase in annual species that increased productivity. In other words, in our arid grassland, nitrogen addition decreased ecosystem stability because of increased ecosystem responsiveness to positive climate fluctuations. These climate fluctuations were facilitated by annual species that take advantage of wet years and can escape dry years as seeds. Our data support the conclusion that nutrient additions decrease the stability of productivity in both wet and arid grasslands. Nutrient enrichment increases the sensitivity of productivity to low rainfall years in wet grasslands, whereas nutrient enrichment in arid grasslands increases the sensitivity of productivity to high rainfall years.