Fluctuations In Abundance Of Species Research Articles

The relationship of microbial biodiversity and clinical forms of oral lichen planus: analysis based on 16S rRNA sequencing

BACKGROUND: The composition and changes of microbiota have a significant impact on overall health and the development of various diseases. Of particular relevance is the problem of changes in the oral microbiota in patients with lichen planus of the oral mucosa. Studying the relationship between the composition of the oral microbiota and the pathogenesis of oral lichen planus will improve the understanding of the mechanisms of this disease. Thus, this topic is of considerable interest to a wide range of specialists in the field of medicine and biology. AIM: Detailed analysis of oral cavity microbiota and establishment of potential pathogenetic microbial associations with oral lichen planus. MATERIALS AND METHODS: The study included samples from patients diagnosed with various forms of oral red squamous lichen planus (lichen planus erosive-ulcerative) and a control group. The investigation was based on analyzing microbial diversity metrics (alpha and beta diversity), relative abundance of bacterial taxa, and identification of unique bacterial taxa in the oral red squamous lichen planus patients. This analysis utilized the 16S rRNA sequencing method. RESULTS: The analysis revealed a rich bacterial composition in patients with oral lichen planus, which was significantly different from that in the control group. Differences were also observed between the subgroups, especially between the typical and erosive-ulcerative forms of the disease. Notably, beta diversity did not show significant differences between the groups, indicating a similar overall microbiota composition despite fluctuations in the relative abundance of species. Nevertheless, the typical clinical form of the disease demonstrated more significant differences in the microbiota structure compared to the hyperkeratotic and erosive-ulcerative forms. Furthermore, analysis of the study groups revealed the presence of 50% shared microbial species, while the other half was represented by unique species associated with oral lichen planus. Regarding the subgroups, it was found that unique microorganisms correlated with the typical and erosive-ulcerative forms, respectively, providing a deeper understanding of the specific microbiological profile in the context of this disease. CONCLUSION: The study confirmed the hypothesis of an association between the microbiota composition and oral lichen planus, which may be of importance for the development of novel therapeutic approaches.

Temporal variation and spatial factors affecting zooplankton communities in freshwater rock pools

Most ecological studies have focused on one snapshot in a single location and the same taxonomic group. However, relatively few studies have explored different taxonomic groups over multiple periods, particularly in ephemeral Neotropical ecosystems. This study investigates the seasonal dynamics of zooplankton communities and their responses to environmental variables in temporary pools in the Seasonally Dry Forest Ecosystem, Pernambuco, Brazil. We examined the seasonal fluctuations in species abundance, diversity, and evenness of Cladocera, Copepoda, and Rotifera in five temporary rock pools. Although these pools are meters apart, they vary considerably in size and drought sensitivity, leading to distinct seasonal dynamics mediated by water availability. We asked (i) how do species abundance, diversity, and evenness vary seasonally? (ii) Do these seasonal shifts differ between Cladocera, Copepoda, and Rotifera? And (iii) How do water environmental variables and spatial distance of pools influence zooplankton species composition? We hypothesized that Cladocera and Copepoda are positively affected by the rainy season, but Rotifera is mostly common in the dry season. Our findings underscore seasonality as the primary driver influencing the abundance and composition of zooplankton communities. We observed higher abundance of Cladocera and Rotifera during the post-rainy season, but Copepoda was dominant in the rainy season, leading to a strong seasonal variation in species evenness. These differences among groups highlight the importance of considering biological differences when understanding the ecological drivers of temporary freshwater ecosystems.

Chaotic turnover of rare and abundant species in a strongly interacting model community

The composition of ecological communities varies not only between different locations but also in time. Understanding the fundamental processes that drive species toward rarity or abundance is crucial to assessing ecosystem resilience and adaptation to changing environmental conditions. In plankton communities in particular, large temporal fluctuations in species abundances have been associated with chaotic dynamics. On the other hand, microbial diversity is overwhelmingly sustained by a "rare biosphere" of species with very low abundances. We consider here the possibility that interactions within a species-rich community can relate both phenomena. We use a Lotka-Volterra model with weak immigration and strong, disordered, and mostly competitive interactions between hundreds of species to bridge single-species temporal fluctuations and abundance distribution patterns. We highlight a generic chaotic regime where a few species at a time achieve dominance but are continuously overturned by the invasion of formerly rare species. We derive a focal-species model that captures the intermittent boom-and-bust dynamics that every species undergoes. Although species cannot be treated as effectively uncorrelated in their abundances, the community's effect on a focal species can nonetheless be described by a time-correlated noise characterized by a few effective parameters that can be estimated from time series. The model predicts a nonunitary exponent of the power-law abundance decay, which varies weakly with ecological parameters, consistent with observation in marine protist communities. The chaotic turnover regime is thus poised to capture relevant ecological features of species-rich microbial communities.

Stabilizing effects of spatially heterogeneous disturbance via reduced spatial synchrony on a rocky shore community.

Understanding how synchronous species fluctuations affect community stability is a main research topic in ecology. Yet experimental studies evaluating how changes in disturbance regimes affect the synchrony and stability of populations and communities remain rare. We hypothesized that spatially heterogeneous disturbances of moderate intensity would promote metacommunity stability by decreasing the spatial synchrony of species fluctuations. To test this hypothesis, we exposed rocky shore communities of algae and invertebrates to homogeneous and gradient-like spatial patterns of disturbance at two levels of intensity for 4 years and used synchrony networks to characterize community responses to these disturbances. The gradient-like disturbance at low intensity enhanced spatial β diversity compared to the other treatments and produced the most heterogeneous and least synchronized network, which was also the most stable in terms of population and community fluctuations. In contrast, homogeneous disturbance destabilized the community, enhancing spatial synchronization. Intense disturbances always reduced spatial β diversity, indicating that strong perturbations could destabilize communities via biotic homogenization regardless of their spatial structure. Our findings corroborated theoretical predictions, emphasizing the importance of spatially heterogeneous disturbances in promoting stability by amplifying asynchronous spatial and temporal fluctuations in population and community abundance. In contrast to other networks, synchrony networks are vulnerable to the removal of most peripheral nodes, which are less synchronized, but may contribute more to stability than other nodes by dampening large fluctuations in species abundance. Our findings suggest that climate change and direct anthropogenic disturbance can compromise the stability of ecological communities through combined effects on diversity and synchrony, as well as further affecting ecosystems through habitat loss.

Pollinator asynchrony drives the temporal stability of flower visitation rates, but not of plant reproductive success

Abstract The stability of ecological communities over time, often quantified using their aggregated abundance or biomass, is thought to be influenced by species richness and the asynchrony of species abundance fluctuations. However, the relationship between community stability and the stability of ecosystem functioning provided by this community has been little explored, especially for functions involving several trophic levels such as plant pollination. Understanding the mechanisms driving community stability and how this should be related to the stability of ecosystem functions is important to predict the resilience of communities in front of the increasing threats to biodiversity. Using a 5‐year dataset across 13 Mediterranean shrublands and 12 plant species, we assessed the effect of pollinator richness and asynchrony as drivers of stability in plant visitation rates and plant reproductive success. Additionally, we analysed whether the effects of species richness and visitation rate on plant reproductive success were consistent over the years. The stability of plant visitation rates is driven by the asynchrony of pollinator species abundances, but the effect of pollinator species richness is only indirect, as richer communities also tend to have more asynchronous populations. In contrast, the stability of plant reproductive success is not related to the stability of pollinator visitation rates. A year‐by‐year analysis reveals that plants in the studied system are unlikely to be limited by pollinator abundance except in specific years, blurring any potential effect of visitation rate stability on reproductive success stability. Synthesis: We highlight the importance of asynchronous fluctuating species that are redundant in their function as an important factor for the stability of ecosystems. This is particularly important for pollinators with diverse responses to environmental fluctuations, as their asynchrony between years ensures stable levels of flower visitation rates. However, we warn that final ecosystem functions such as plant reproductive success, which depends not only on pollinators but on plant competition or resource availability, might be decoupled from first order ecosystem functions (i.e. visitation rates and pollen deposition) when pollination is not a limiting factor.

Environmental fluctuations explain the universal decay of species-abundance correlations with phylogenetic distance

Multiple ecological forces act together to shape the composition of microbial communities. Phyloecology approaches-which combine phylogenetic relationships between species with community ecology-have the potential to disentangle such forces but are often hard to connect with quantitative predictions from theoretical models. On the other hand, macroecology, which focuses on statistical patterns of abundance and diversity, provides natural connections with theoretical models but often neglects interspecific correlations and interactions. Here, we propose a unified framework combining both such approaches to analyze microbial communities. In particular, by using both cross-sectional and longitudinal metagenomic data for species abundances, we reveal the existence of an empirical macroecological law establishing that correlations in species-abundance fluctuations across communities decay from positive to null values as a function of phylogenetic dissimilarity in a consistent manner across ecologically distinct microbiomes. We formulate three variants of a mechanistic model-each relying on alternative ecological forces-that lead to radically different predictions. From these analyses, we conclude that the empirically observed macroecological pattern can be quantitatively explained as a result of shared population-independent fluctuating resources, i.e., environmental filtering and not as a consequence of, e.g., species competition. Finally, we show that the macroecological law is also valid for temporal data of a single community and that the properties of delayed temporal correlations can be reproduced as well by the model with environmental filtering.

Eccentricity signal in the nannofossil time-series across the Mid-Pleistocene Transition in the northwestern Pacific Ocean (ODP Site 1209)

The Mid-Pleistocene Transition (MPT; 1.25–0.6 million years ago, Ma) is one of the most important and still debated climate reorganizations during which the glacial/interglacial cycles switched from a 41-thousand years (kyr) cycle (i.e. obliquity) to a quasi-periodic 100-kyr cycle (associated with orbital eccentricity). Variations in the orbital geometry can affect the abundance and distribution of certain marine biota such as the coccolithophores, a group of unicellular calcifying phytoplankton, whose skeletal remains – called nannofossils – represent a valid tool within the geological archives to infer change in surface water conditions and/or coccolithophore productivity and how orbital variations may have impacted them. Here, we apply for the first time various time series analytical techniques to the nannofossil dataset from mid-latitudinal Ocean Drilling Program (ODP) Site 1209 in the northwest Pacific Ocean for the interval spanning the last 1.6 Myr. To better interpret the orbital signal recorded by different nannofossil species we used time series analyses (i.e. wavelet, autocorrelation and cross correlation) to identify the main periodicities by single nannofossil species during the MPT, and to investigate further their response timings to those orbital drivers. In addition, we investigated how the recorded periodicities can improve understanding of the paleoecological preferences of particular species. The combination of multiple time series analyses allowed identification of the 100-kyr periodicity as the main cyclicity recorded in most analyzed species at Site 1209, documenting the predominance of the eccentricity-related signal at mid-latitudes and a reduced or absent influence of the obliquity response. Thus, our data highlight how orbital influence varies by latitude impacting the nannofossil species. The lag between eccentricity and species abundance fluctuations was also investigated, identifying a fast response ranging between 20 and 40 kyr for the taxa Calcidiscus leptoporus subspecies leptoporus, Gephyrocapsa caribbeanica small, and Reticulofenestra spp. (>5 μm). This study corroborates the potential of nannofossils to deepen understanding of the dynamics and effects of variations in orbital geometry through time. It also underlines the need to extend the study of the responses of specific species through the use of different time series analysis techniques in order to return complementary information and detect clearer orbital signals.

Six-Year Study of a Nocturnal Flying Coleoptera Community in Southern Québec, Canada

The aim of this study is the quantitative assessment of temporal variations in the composition and structure of a nocturnal flying beetle community in northeastern North America. In southern Québec (Canada), we collected by blacklight trapping a total of 33,382 individuals representing 625 Coleoptera species or morphospecies belonging to 63 families from a grassland at Scotstown during six years (2007–2012). Carabidae and Staphylinidae were the most abundant families and had the highest species richness. This blacklight trapping throughout the April–September period over many years provides critical information for assessments of beetle diversity in southern Québec, based on 1) the large proportion of species collected in small numbers (1–2 adults), 2) nearly 60% of species flying for only 1–3 months over six years, and 3) the large proportion of dominant/subdominant species in spite of strong year-to-year fluctuations in the relative abundance of species. Consequently, light trapping investigations in the future should be extended across many years to improve the inventory of species in a region, aid in the evaluation of the rarity of some Nearctic species, and study long-term effects of certain environmental factors (e.g., anthropogenic disturbance, climatic changes). We also studied the seasonal flight pattern for the 57 most abundant species (at least 100 individuals) including two eudominant species, Bembidion versicolor (LeConte) (Carabidae) (7.5% of the total catches) and Ectopria nervosa (Melsheimer) (Psephenidae) (6.8%); the flight peak of many species was during mid-summer. We determined the sex ratio for 23 abundant species; 12 species presented a female-biased sex ratio.

Dynamics of an aquatic diffusive predator–prey model with double Allee effect and [formula omitted]-dependent capture rate

To investigate ocean acidification and Allee effects on the dynamics of a marine predator–prey system, an aquatic diffusive predator–prey model with double Allee effect on prey and pH-dependent capture rate is considered. First, we study the stability of constant steady state solutions using linearized theory. Second, the nonexistence of nonconstant positive steady state solutions is shown for appropriate ranges of parameters. Furthermore, we show the existence of a Hopf bifurcation and derive the direction and stability of the bifurcating periodic solutions. Both theoretical analysis and numerical simulation show that changing predator–prey interaction strengths, due to changing environmental conditions, can fundamentally change the system dynamics, even for apparently small changes in interaction strength. As the interaction strength decreases due to decreasing ocean pH, the system dynamics transition from persistent fluctuations in species abundances (periodic solutions), to stable coexistence, to predator extinct (with stable non-zero prey abundance), suggesting the potential for ocean acidification to decrease the abundance and diversity of marine species by weakening predation rates. Moreover, double Allee effect parameters together determine the stability of periodic solutions when the spatially homogeneous bifurcating periodic solutions exist, and the wavelength becomes longer as the Allee effect increases.

Emergent phases of ecological diversity and dynamics mapped in microcosms.

From tropical forests to gut microbiomes, ecological communities host notably high numbers of coexisting species. Beyond high biodiversity, communities exhibit a range of complex dynamics that are difficult to explain under a unified framework. Using bacterial microcosms, we performed a direct test of theory predicting that simple community-level features dictate emergent behaviors of communities. As either the number of species or the strength of interactions increases, we show that microbial ecosystems transition between three distinct dynamical phases, from a stable equilibrium in which all species coexist to partial coexistence to emergence of persistent fluctuations in species abundances, in the order predicted by theory. Under fixed conditions, high biodiversity and fluctuations reinforce each other. Our results demonstrate predictable emergent patterns of diversity and dynamics in ecological communities.

Community stability is related to animal diversity change

AbstractUnderstanding the drivers of community stability in times of increasing anthropogenic pressure is an urgent issue. Biodiversity is known to promote community stability, but studies of the biodiversity–stability relationship rarely consider the full complexity of biodiversity change. Furthermore, finding generalities that hold across taxonomic groups and spatial and temporal scales remains challenging because most investigations have narrow taxonomic, spatial, and temporal scopes. We used organismal data collected through the National Ecological Observatory Network (NEON) at sites across the contiguous United States to evaluate linkages between community stability and biodiversity change for four taxonomic groups: small mammals, ground beetles, fish, and freshwater macroinvertebrates. We defined community stability as constancy of aggregate species' abundance. We quantified change in biodiversity as (1) dissimilarity in community taxonomic and functional composition and species replacement and richness change components of that dissimilarity and (2) change in species' abundance distributions as captured by change in species rank, richness, and evenness. We found that community stability increased with species replacement and with contribution of species replacement to overall dissimilarity for all taxonomic groups, but declined with increasing change in species richness and evenness. This is consistent with the notion that temporal fluctuations in species abundance can help stabilize community properties. We also found that community stability was highest when change in community functional composition was either lower or higher than expected given reshuffling of each community's taxonomic composition. This suggests that long‐term community stability can result from fluctuations of functionally similar species in assemblages with high taxonomic reshuffling. On the contrary, the functional uniqueness of fluctuating species compensates for lower taxonomic reshuffling to drive stabilization of community properties. Our study provides an initial assessment of the relationship between community stability and biodiversity change and illustrates the utility of fine temporal resolution data collected across ecosystems and biomes to understand the general mechanisms underlying biodiversity–stability relationships.

Long‐term resilience in microcrustacean communities despite environmental changes

AbstractThe ability of communities to withstand stress or maintain their species composition over extended periods of environmental changes is a matter of major concern. Here, we utilize a dataset where microcrustacean communities were related to water chemistry in 82 boreal‐alpine lakes over a 50‐yr time span (1968 and 2016), with the same person involved in sampling both times ensuring identical sites and sampling protocols. Over this time span, there has been a 1–2°C increase in mean annual temperatures and prolonged growing seasons. Also, the strong changes in SO4 deposition and acidification, peaking in the early 1980s, followed by a recovery period, have caused not only an increase in pH but also a pronounced browning (elevated DOC) and reduced levels of Ca over the past three decades. Despite this, the microcrustacean diversity and community composition have remained remarkably stable, and this holds both for the species‐rich larger, boreal sites and the species‐poor alpine sites. While likely there are interannual fluctuations in species abundance, and perhaps species may also be missing for certain periods, a combination of local refugia, resting egg seed‐banks and a regional recolonization that may occur at the meta‐population level, aims at maintaining a strong stability in community composition.

Drought effects on invertebrate metapopulation dynamics and quasi-extinction risk in an intermittent river network.

Ecological communities can remain stable in the face of disturbance if their constituent species have different resistance and resilience strategies. In turn, local stability scales up regionally if heterogeneous landscapes maintain spatial asynchrony across discrete populations-but not if large-scale stressors synchronize environmental conditions and biological responses. Here, we hypothesized that droughts could drastically decrease the stability of invertebrate metapopulations both by filtering out poorly adapted species locally, and by synchronizing their dynamics across a river network. We tested this hypothesis via multivariate autoregressive state-space (MARSS) models on spatially replicated, long-term data describing aquatic invertebrate communities and hydrological conditions in a set of temperate, lowland streams subject to seasonal and supraseasonal drying events. This quantitative approach allowed us to assess the influence of local (flow magnitude) and network-scale (hydrological connectivity) drivers on invertebrate long-term trajectories, and to simulate near-future responses to a range of drought scenarios. We found that fluctuations in species abundances were heterogeneous across communities and driven by a combination of hydrological and stochastic drivers. Among metapopulations, increasing extent of dry reaches reduced the abundance of functional groups with low resistance or resilience capacities (i.e. low ability to persist in situ or recolonize from elsewhere, respectively). Our simulations revealed that metapopulation quasi-extinction risk for taxa vulnerable to drought increased exponentially as flowing habitats contracted within the river network, whereas the risk for taxa with resistance and resilience traits remained stable. Our results suggest that drought can be a synchronizing agent in riverscapes, potentially leading to regional quasi-extinction of species with lower resistance and resilience abilities. Better recognition of drought-driven synchronization may increase realism in species extinction forecasts as hydroclimatic extremes continue to intensify worldwide.

Temporal consistency in interactions among birds, ants, and plants in a neotropical savanna

Interactions among animals and plants are key to understanding seed dispersal, plant regeneration and plant community patterns. These interactions can be dynamic, with changes of species and functional roles across space and time. Despite fluctuations in species abundances and resources over time being the rule in natural communities, most studies approach plant–frugivore interactions as temporally static. We documented changes in bird and ant species in interactions with plants producing carbohydrate‐ (Miconia rubiginosa) or lipid‐rich diaspores (Xylopia aromatica) comparing two fruiting seasons 15 years apart in a savanna in southeastern Brazil. We tested if abundance and body size of dispersal vectors (birds and ants) affected the frequency of interactions and the quantitative effectiveness of seed dispersal (QTC). Rich bird (26 species) and ant (18 genera) assemblages interacted with the diaspores. A core of bird and ant taxa was temporally consistent, and responsible for most seed removal across the two years for both plants. Temporal transience was more common for sporadic partners of interactions, and for species with similar functional roles. Abundance and body size of birds affected the interspecific contributions to QTC. Large‐bodied birds dispersed large quantities of seeds of our relatively small‐seeded plants, even if their visits were sporadic. For ants, variations in temperature and rainfall across time seems more important in driving variations in the contributions to QTC. The combined effect of birds and ants in the same system helps to stabilize the temporal fluctuations in the amount of seeds dispersed. However, ants alone are unlikely to replace the functional roles of vertebrate dispersers. Even though species in the assemblage change either their quantitative contribution to seed dispersal or their presence or absence in the interaction network, species persisting in interaction over time are the most important quantitative partners for plants.

Species abundance fluctuations over 31 years are associated with plant–soil feedback in a species‐rich mountain meadow

Abstract Increasing evidence suggest that plant–soil interactions play an essential role in plant community assembly processes. Empirical investigations show that plant species abundance in the field is often related to plant–soil biota interactions; however, the direction of these relations have yielded inconsistent results. We combined unique 31‐year long field data on species abundances from a species‐rich mountain meadow with single time point plant–soil feedback greenhouse experiments of 24 co‐occurring plant species. We tested whether these relations were dynamic in time, whether coupled increases and decreases in abundance between years were related to plant–soil feedback and whether these changes were underlain by years in which manuring was applied. The prevailingly negative relationship between plant–soil feedback and plant relative abundance in the field was significantly time‐dependent, which may reconcile the contrasting results in literature. Furthermore, significantly coupled oscillations appeared between species relative abundance changes and plant–soil feedback, which were likely moderated by years in which manuring was applied. Our results are consistent with the notion that the more abundant species are stabilised by negative plant–soil feedback, and the less abundant species co‐vary with the fluctuations of these more competitive species. Synthesis. Our results project plant–soil feedback as an important regulatory mechanism in plant communities, operating in conjunction with species' competitive ability and soil nutrient availability. We suggest that negative feedback is particularly prominent in more abundant plant species that profit from more readily available soil nutrients than less abundant species with positive feedback. Negative plant–soil feedback may thus prevent more abundant plant species from out‐competing less abundant plant species, facilitating stable species co‐existence.

Macroecological laws describe variation and diversity in microbial communities

How the coexistence of many species is maintained is a fundamental and unresolved question in ecology. Coexistence is a puzzle because we lack a mechanistic understanding of the variation in species presence and abundance. Whether variation in ecological communities is driven by deterministic or random processes is one of the most controversial issues in ecology. Here, I study the variation of species presence and abundance in microbial communities from a macroecological standpoint. I identify three macroecological laws that quantitatively characterize the fluctuation of species abundance across communities and over time. Using these three laws, one can predict species’ presence and absence, diversity, and commonly studied macroecological patterns. I show that a mathematical model based on environmental stochasticity, the stochastic logistic model, quantitatively predicts the three macroecological laws, as well as non-stationary properties of community dynamics.

Forecasting unprecedented ecological fluctuations.

Forecasting 'Black Swan' events in ecosystems is an important but challenging task. Many ecosystems display aperiodic fluctuations in species abundance spanning orders of magnitude in scale, which have vast environmental and economic impact. Empirical evidence and theoretical analyses suggest that these dynamics are in a regime where system nonlinearities limit accurate forecasting of unprecedented events due to poor extrapolation of historical data to unsampled states. Leveraging increasingly available long-term high-frequency ecological tracking data, we analyze multiple natural and experimental ecosystems (marine plankton, intertidal mollusks, and deciduous forest), and recover hidden linearity embedded in universal 'scaling laws' of species dynamics. We then develop a method using these scaling laws to reduce data dependence in ecological forecasting and accurately predict extreme events beyond the span of historical observations in diverse ecosystems.

Assessing spatio-temporal changes in marine communities along the Portuguese continental shelf and upper slope based on 25 years of bottom trawl surveys

The Portuguese continental coast is influenced by several oceanographic processes and is located near the confluence of three biogeographic realms (from the North Atlantic, South Atlantic and Mediterranean). Given these features, the topography of the Portuguese coast, possible variations in fishing effort and reported increasing sea surface temperature in the last decades, we hypothesized that changes in marine communities in space and time occurred since 1990. In this study, research survey data collected yearly along the Portuguese continental slope (20–500 m deep) from 1990 to 2016 were analysed with the objective of identifying spatio-temporal changes. Latitude and depth were found to play a major role in communities’ spatial differentiation, probably associated to temperature, and three ecological areas were defined (north, southwest and south). In the studied period, Macroramphosus spp. abundance showed a marked decrease in all areas whereas Sparids increased in abundance in the south. Despite these major changes and fluctuations in species abundance over time no major trends in communities were observed. Fishing activity, environmental conditions (including climate change) and biotic factors are all drivers possibly responsible for those changes.

Stability of A Coevolving Host-parasite System Peaks at Intermediate Productivity.

Habitat productivity may affect the stability of consumer-resource systems, through both ecological and evolutionary mechanisms. We hypothesize that coevolving consumer-resource systems show more stable dynamics at intermediate resource availability, while very low-level resource supply cannot support sufficiently large populations of resource and consumer species to avoid stochastic extinction, and extremely resource-rich environments may promote escalatory arms-race-like coevolution that can cause strong fluctuations in species abundance and even extinction of one or both trophic levels. We tested these ideas by carrying out an experimental evolution study with a model bacterium-phage system (Pseudomonas fluorescens SBW25 and its phage SBW25Φ2). Consistent with our hypothesis, this system was most stable at intermediate resource supply (fewer extinction events and smaller magnitude of population fluctuation). In our experiment, the rate of coevolution between bacterial resistance and phage infectivity was correlated with the magnitude of population fluctuation, which may explain the different in stability between levels of resource supply. Crucially, our results are consistent with a suggestion that, among the two major modes of antagonistic coevolution, arms race is more likely than fluctuation selection dynamics to cause extinction events in consumer-resource systems. This study suggests an important role of environment-dependent coevolutionary dynamics for the stability of consumer-resource species systems, therefore highlights the importance to consider contemporaneous evolutionary dynamics when studying the stability of ecosystems, particularly those under environmental changes.

Chaotic attractor in two-prey one-predator system originates from interplay of limit cycles

We investigate the appearance of chaos in a microbial 3-species model motivated by a potentially chaotic real world system (as characterized by positive Lyapunov exponents (Becks et al., Nature 435, 2005). This is the first quantitative model that simulates characteristic population dynamics in the system. A striking feature of the experiment was three consecutive regimes of limit cycles, chaotic dynamics and a fixed point. Our model reproduces this pattern. Numerical simulations of the system reveal the presence of a chaotic attractor in the intermediate parameter window between two regimes of periodic coexistence (stable limit cycles). In particular, this intermediate structure can be explained by competition between the two distinct periodic dynamics. It provides the basis for stable coexistence of all three species: environmental perturbations may result in huge fluctuations in species abundances, however, the system at large tolerates those perturbations in the sense that the population abundances quickly fall back onto the chaotic attractor manifold and the system remains. This mechanism explains how chaos helps the system to persist and stabilize against migration. In discrete populations, fluctuations can push the system towards extinction of one or more species. The chaotic attractor protects the system and extinction times scale exponentially with system size in the same way as with limit cycles or in a stable situation.