Fluctuation Induces Evolutionary Branching in a Mathematical Model of Ecosystems

Ecological systems are always subjected to various environmental fluctuations. They evolve under these fluctuations and the resulting systems are robust against them. The diversity in ecological systems is also acquired through the evolution. How do the fluctuations affect the evolutionary processes? Do the fluctuations have direct impact on the species diversity in ecological systems? In the present paper, we investigate the relation between the environmental fluctuation and the evolution of species diversity with a mathematical model of evolutionary ecology. In the model, individual organisms compete for a single restricted resource and the temporal fluctuation in the resource supply is introduced as the environmental fluctuation. The evolutionary process is represented by the mutational change of genotypes which determines their resource utilization strategies. We found that when the environmental state is switched form static to fluctuating conditions, the initial closely related population distributed around the genotype adapted for the static environment is destabilized and divided into two groups in the genotype space; i.e., the evolutionary branching is induced by the environmental fluctuation. The consequent multiple species structures is evolutionary stable at the presence of the fluctuation. We perform the evolutionary invasion analysis for the phenomena and illustrate the mechanisms of the branchings. The results indicate a novel process of increasing the species diversity via evolutionary branching, and the analysis reveals the mechanisims of the branching preocess as the response to the environmental fluctuation. The robustness of the evolutionary process is also discussed.     

Effects of foundation species genotypic diversity on subordinate species richness in an assembling community

Foundation (dominant or matrix) species play a key role in structuring plant communities, influencing processes from population to ecosystem scales. However, the effects of genotypic diversity of foundation species on these processes have not been thoroughly assessed in the context of assembling plant communities. We modified the classical filter model of community assembly to include genotypic diversity as part of the biotic filter. We hypothesized that the proportion of fit genotypes (i.e. competitively superior and dominant) affects niche space availability for subordinate species to establish with consequence for species diversity. To test this hypothesis, we used an individual‐based simulation model where a foundation species of varying genotypic diversity (number of genotypes and variability among genotypes) competes for space with subordinate species on a spatially heterogeneous lattice. Our model addresses a real and practical problem in restoration ecology: choosing the level of genetic diversity of re‐introduced foundation and subordinate species. Genotypic diversity of foundation species significantly affected equilibrium community diversity, measured as species richness, either positively or negatively, depending upon environmental heterogeneity. Increases in genotypic diversity gave the foundation species a wider niche breadth. Under conditions of high environmental heterogeneity, this wider niche breadth decreased niche space for other species, lowering species richness with increased genotypic diversity until the genotypes of the foundation species saturated the landscape. With a low level of environmental heterogeneity, increasing genotypic diversity caused the foundation species niche breadth to be overdispersed, resulting in a weak positive relationship with species richness. Under these conditions, some genotypes are maladapted to the environment lowering fitness of the foundation species. These effects of genotypic diversity were secondary to the larger effects of overall foundation species fitness and environmental heterogeneity. The novel aspect of incorporating genotype diversity in combination with environmental heterogeneity in community assembly models include predictions of either positive or negative relationships between species diversity and genotypic diversity depending on environmental heterogeneity, and the conditions under which these factors are potentially relevant. Mechanistically, differential niche availability is imposed by the foundation species.     

Limiting similarity and functional diversity along environmental gradients

Recent developments in community models emphasize the importance of incorporating stochastic processes (e.g. ecological drift) in models of niche‐structured community assembly. We constructed a finite, spatially explicit, lottery model to simulate the distribution of species in a one‐dimensional landscape with an underlying gradient in environmental conditions. Our framework combines the potential for ecological drift with environmentally‐mediated competition for space in a heterogeneous environment. We examined the influence of niche breadth, dispersal distances, community size (total number of individuals) and the breadth of the environmental gradient on levels of species and functional trait diversity (i.e. differences in niche optima). Three novel results emerge from this model: (1) niche differences between adjacent species (e.g. limiting similarity) increase in smaller communities, because of the interaction of competitive effects and finite population sizes; (2) immigration from a regional species pool, stochasticity and niche‐assembly generate a bimodal distribution of species residence times (‘transient’ and ‘resident’) under a heterogeneous environment; and (3) the magnitude of environmental heterogeneity has a U‐shaped effect on diversity, because of shifts in species richness of resident vs. transient species. These predictions illustrate the potential importance of stochastic (although not necessarily neutral) processes in community assembly.     

Temporal variation in spatial organization of a rodent community in the southwestern Kyzylkum desert (Middle Asia)

The distribution of niches in resource space and the niche patterns of a 14-species community of Middle Asian desert rodents were studied during two years - at low and high rodent density - using discriminant function analysis Nineteen quantitative environmental parameters (soil structure and vegetation), measured in 550 plots within 22 1 -ha grids, were considered The first three canonical axes of resource space account for 72% of the variance The first two axes represent complex environmental gradients the first axis represents a general landscape gradient from sand to clay soils, the second axis reflects a gradient of in creasing productivity The third axis reflects with in-habitat environmental variation All community parameters, as well as parameters of individual species niches, were unstable between years At the same time, different parameters vary in different extent Position of niche centroids along macro-habitat axes, as. well as macrohabitat niche breadth, were relatively stable between years, but these parameters for microhabitat axis and values of niche overlap were much more variable A strong correlation between changes m relative between-habitat niche breadth and differences in average niche overlap with relative changes in species abundances indicate density dependence of these parameters Changes in niche overlap is a consequence of between-year differences in guild patterns Guild structure was pronounced at high density when the level of niche overlap was intermediate At low density, when the level of niche overlap decreased, guild structure was incon-spicous Different levels of diversity differed in their sensitivity to density changes α-diversity was relatively constant as a result of between-year stability of niche centroid positions However level of ß-diversity varied significantly between years reflecting changes in the level of niche overlap, because a decrease in niche overlap leads to an increase in the rate of species turnover     

Microclimate Fluctuation Correlated with Beta Diversity of Epiphyllous Bryophyte Communities

Site‐to‐site variation in community composition, or beta diversity, is a major component of regional diversity. While many mechanisms, such as dispersal limitation and habitat heterogeneity, have been shown to affect beta diversity, interactions between habitat heterogeneity and environmental fluctuation have not been thoroughly investigated. This study uses leaf‐colonizing (epiphyllous) bryophyte communities as a model system to investigate the effects of microclimate fluctuation on beta diversity. I hypothesized that beta diversity would increase with increasing microclimate fluctuation, as niche breadth of species was reduced with increasing fluctuation. A total of 354 leaf‐colonizing bryophyte communities from 18 sites on the island of Moorea, French Polynesia were collected and identified. At each site, temperature and relative humidity were measured and converted to vapor pressure deficit (VPD). My analyses showed that beta diversity among communities on different host types tended to increase with the increasing daily range of VPD at a given site. It is possible that high fluctuation in microclimate conditions augments the differences in habitat quality among host types, resulting in greater dissimilarities among epiphyllous communities. However, host niche breadths of major epiphyllous species did not decrease with increasing VPD range. Overall, the results suggest that beta diversity may increase with environmental fluctuation, but it is not likely to be the results of reduced niche breadth as theoretically suspected.     

Does hydrological fluctuation alter impacts of species richness on biomass in wetland plant communities?

Aims The diversity–productivity relationship is one of the most critical questions in ecology and can be altered by environmental factors. Hydrological fluctuation affects growth of wetland plants, and such effects vary with plant species. Therefore, we hypothesized that hydrological fluctuation changes effects of species richness on productivity of wetland plant communities.Methods We constructed wetland plant communities consisting of three or six wetland plant species and subjected them to hydrological fluctuation (i.e. gradually changing water level) of two frequencies and two ranges, with unchanged water level as the control. We measured height, root and shoot dry mass of each plant at harvest.Important findings Hydrological fluctuation significantly decreased biomass of wetland plant communities, which was due to impacts of fluctuation range, but not those of fluctuation frequency. Community biomass was significantly higher when species richness was higher, and such an effect did not depend on hydrological fluctuation. Therefore, hydrological fluctuation can decrease the productivity of wetland plant communities but may not alter the diversity–productivity relationship.     

Mechanisms inducing spatially extended synchrony in mast seeding: The role of pollen coupling and environmental fluctuation

Trees in mature forests often show intermittent reproduction. Intensive flowering and seed production occur only once in several years (mast seeding), often synchronized over a long distance. Recently, coupled map models for the dynamics of individual energy reserves have been adopted to explain the phenomena. Even in a constant environment, the trees show a large between-year fluctuation in seed crops and the reproduction can be synchronized over the whole forest if the fruit production is limited by the availability of outcross pollen (pollen coupling). The model with local coupling in which trees are coupled by pollen exchange only with the neighbors shows that a strong synchronization of tree reproduction can develop over the whole forest that may be orders of magnitude larger than the distance of direct pollen exchange between trees. However, their fluctuation is close to the period-two oscillation, and is unable to explain observed intermittent reproduction of a longer interval between mast years. Finally the effect of common environmental fluctuation experienced by different individuals is studied, when the annual productivity and the reproductive threshold of trees fluctuate between years. In the absence of pollen limitation, environmental fluctuation correlated strongly between individuals (Moran effect) failed to produce a high positive correlation in seed production between individuals. If both pollen limitation and correlated environmental fluctuation are at work, a significantly large correlation was maintained. Hence, both pollen coupling and common environmental fluctuation are needed to explain synchronized reproduction with intervals longer than 2years.     

Quantifying the role of deterministic assembly and stochastic drift in a natural community of Arctic mosses

The interpretation of natural plant communities frequently invokes species‐sorting controlled by niche differences along spatial environmental gradients. This process of niche structuring can be explained by reference to functional traits, which provide a mechanistic explanation for community structure. In contrast, models explaining species coexistence obviate the limiting effect of niche difference, by invoking processes which cause species‐level drift, e.g. demographic stochasticity. This paper investigates a simple habitat with strong gradients (moss communities in a patterned arctic wetland) to identify signature‐patterns under‐pinning the relative importance of deterministic assembly and stochastic drift in a natural community. First, ordination analysis was used to confirm community composition structured by a range of nine carefully selected functional traits. Second, to determine whether traits explaining community composition might also explain species richness, local species richness (sR) was compared to (1) observed trait diversity and (2) expected trait diversity based on permutation tests, which are used to simulate null community assembly for different values of sR. Traits explaining species composition, consistent with deterministic niche structuring, do not appear to maintain sR. This surprising result was explained by decomposing the community into individual pair‐wise comparisons, i.e. species niche‐differences and association (χ²). Results support deterministic processes via the sorting of species with similar and contrasting niches, at opposite ends of a composite environmental gradient. Nevertheless, stochastic drift is apparent in the random structure of a majority of pair‐wise associations; in addition, a species’ abundance was in general not related to environmental distance from response‐optima. We suggest therefore that spatial pattern in the moss community is a balance between deterministic forces with respect to species traits and controlling environmental gradients, and stochastic drift, which weakens this deterministic structure.     

Mutators in space: the dynamics of high-mutability clones in a two-patch model

Clones of bacteria possessing high-mutability rates (or mutators) are being observed in an increasing number of species. In a constant environment most mutations are deleterious, and hence the spontaneous mutation rate is generally low. However, mutators may play an important role in the adaptation of organisms to changing environments. To date, theoretical work has focused on temporal variability in the environment, implicitly assuming that environmental conditions are constant through space. Here, we develop a two-patch model to investigate how spatiotemporal environmental variability and dispersal might influence mutator dynamics. Environmental conditions in each patch fluctuate between two states; the rate of fluctuation varies in each patch at differing phase angles. We find that at low and intermediate rates of fluctuation, an increase in dispersal results in a decrease in the density of mutators. However, at high rates of environmental change, dispersal causes an increase in mutator density. For all frequencies of environmental fluctuation these trends are enhanced as the phase angle approaches 180 degrees. We argue that future work, both empirical and theoretical, is needed to improve our understanding of how spatiotemporal variability impacts on mutator densities and dynamics.     

Niche width and niche overlap: a method based on type-2 fuzzy sets

Complicated ecosystems and the high non-linearity of evolution has made biology more adaptable to a variety of environments. The relationship between life and environment demands a dynamic definition of niche and its measurement. In this paper we propose a model of niche with dynamic character based on the “broad band” effect in type-2 fuzzy sets. The niche in this definition is an interval in each ecological dimension which is dynamic in character and depends on the actual environment. We also give formulas for niche width and niche overlap. We compute the niche width and overlap for plants and animals and then compare these results with previous results. The results for niche width in this paper reflect the diversity of resources used by species or communities. The results for niche overlap demonstrate overlap under different environmental conditions. The results are, moreover, intervals, which could provide more information. The model in this paper could therefore be used to describe the state of every resource comprehensively, reflecting the interaction between species and environment.     

Evidence that niche specialization explains species-energy relationships in lake fish communities

1. Interspecific niche differences have long been identified as a major explanation for the occurrence of species-rich communities. However, much fieldwork studying variation in local species richness has focused upon physical habitat attributes or regional factors, such as the size of the regional species pool. 2. We applied indices of functional diversity and niche overlap to data on the species niche to examine the importance of interspecific niche differentiation for species richness in French lake fish communities. We combined this information with environmental data to test generalizations of the physiological tolerance and niche specialization hypotheses for species-energy relationships. 3. We found evidence for a largely non-saturating relationship (relative to random expectation) between species richness and functional evenness (evenness of spacing between species in niche space), while functional richness (volume of niche space occupied) peaked at moderate levels of species richness and niche overlap showed an initial decrease followed by saturation. This suggests that increased niche specialization may have allowed species to coexist in the most species-rich communities. 4. We tested for evidence that increased temperature, local habitat area, local habitat diversity and immigration affected species richness via increased niche specialization. Temperature explained by far the largest amount of variation in species richness, functional diversity and niche overlap. These results, combined with the largely non-saturating species richness-functional evenness relationship, suggest that increased temperature may have permitted increased species richness by allowing increased niche specialization. 5. These results emphasize the importance of niche differences for species coexistence in species-rich communities, and indicate that the conservation of functional diversity may be vital for the maintenance of species diversity in biological communities. Our approach may be applied readily to many types of community, and at any scale, thus providing a flexible means of testing niche-based hypotheses for species richness gradients.     

Trait plasticity in species interactions: a driving force of community dynamics

Evolutionary community ecology is an emerging field of study that includes evolutionary principles such as individual trait variation and plasticity of traits to provide a more mechanistic insight as to how species diversity is maintained and community processes are shaped across time and space. In this review we explore phenotypic plasticity in functional traits and its consequences at the community level. We argue that resource requirement and resource uptake are plastic traits that can alter fundamental and realised niches of species in the community if environmental conditions change. We conceptually add to niche models by including phenotypic plasticity in traits involved in resource allocation under stress. Two qualitative predictions that we derive are: (1) plasticity in resource requirement induced by availability of resources enlarges the fundamental niche of species and causes a reduction of vacant niches for other species and (2) plasticity in the proportional resource uptake results in expansion of the realized niche, causing a reduction in the possibility for coexistence with other species. We illustrate these predictions with data on the competitive impact of invasive species. Furthermore, we review the quickly increasing number of empirical studies on evolutionary community ecology and demonstrate the impact of phenotypic plasticity on community composition. Among others, we give examples that show that differences in the level of phenotypic plasticity can disrupt species interactions when environmental conditions change, due to effects on realized niches. Finally, we indicate several promising directions for future phenotypic plasticity research in a community context. We need an integrative, trait-based approach that has its roots in community and evolutionary ecology in order to face fast changing environmental conditions such as global warming and urbanization that pose ecological as well as evolutionary challenges.     

The impact of environmental variability and species composition on the stability of experimental microbial populations and communities

Understanding how environmental fluctuations affect the stability of populations and communities is complex, for example, because direct effects of environmental variability on populations may be modified and propagated across communities by species interactions. One way to explore and further understand these complexities is via a factorial manipulation of community composition and environmental conditions. Using laboratory based aquatic microcosms we manipulated environmental fluctuation by creating two environments; one with variable light and one with constant light. Within these environments, community composition was manipulated by constructing communities from all possible combinations of three species that vary in their reliance on light for growth (an autotroph: a diatom completely reliant on light, a heterotroph: a Paramecium species not reliant on light, and a mixotroph: a Paramecium species somewhat reliant on light). Community composition was predicted to affect populations and communities by introducing and altering competitive interactions between species and affecting the degree of niche differentiation between species. We found that population stability was predominantly influenced by an interaction between community composition and environmental variability, whereby the effect of environmental variability synergistically combined with effects of community composition to reduce population stability. Covariance of populations was determined by an interaction between community composition and environmental variability, though this did not result from the effect of niche differentiation between species. Species interactions drove correlations between population biomass and the environment which otherwise did not exist. Our results demonstrate the complex and interrelated effects of abiotic and biotic factors on population and community stability, and suggest the need to consider aspects of community composition when predicting the impact of environmental fluctuations.     

Low levels of climate niche conservatism may explain clade diversity patterns in the South African genus Pelargonium (Geraniaceae)

? Premise of the study: Sharp climatic gradients in South Africa and in particular in the Cape Floristic Region (CFR) provide a diversity of niches over short distances that may have promoted ecological diversification in local clades. Here we measured the extent to which closely related species occupy divergent climates and test whether niche lability is correlated with higher species diversity in the genus. ? Method: We integrated phylogenetic information and environmental niche models (ENM) to assess the levels of climate niche conservatism. ENMs for 113 species of Pelargonium were calculated using maximum entropy. We used two tests, one assessing climate niche equivalency and the other testing niche similarity between sister species and within sections. We also examined whether niche similarity was correlated with phylogenetic relatedness across the genus. ? Key results: Niche similarity was mostly independent of phylogenetic relationships. Compared to random expectations, 23% of closely related species pairs had climate niches that were more similar, and only 6.5% were more disparate; the remaining 70% of comparisons had similarities that fell within random expectations. Similar trends were observed when analyses were restricted to only sister species pairs. Although the overall proportion of niche divergence was low, this was significantly related to sectional diversity. We also found a negative relationship between diversity and the proportion of random niches. ? Conclusions: Lack of widespread niche conservatism in a highly heterogeneous landscape and few instances of significant climate niche lability suggest that an adaptive divergence process was implicated in the Pelargonium radiation.     

Environmental heterogeneity,species diversity and co‐existence at different spatial scales

The positive relationship between spatial environmental heterogeneity and species diversity is a widely accepted concept, generally associated with niche limitation. However, niche limitation cannot account for negative heterogeneity–diversity relationships (HDR) revealed in several case studies. Here we explore how HDR varies at different spatial scales and provide novel theories for small‐scale species co‐existence that explain both positive and negative HDR. At large spatial scales of heterogeneity (e.g. landscape level), different communities co‐exist, promoting large regional species pool size and resulting in positive HDR. At smaller scales within communities, species co‐existence can be enhanced by increasing the number of different patches, as predicted by the niche limitation theory, or alternatively, restrained by heterogeneity. We conducted meta‐regressions for experimental and observational HDR studies, and found that negative HDRs are significantly more common at smaller spatial scales. We propose three theories to account for niche limitation at small spatial scales. (1) Microfragmentation theory: with increasing spatial heterogeneity, large homogeneous patches lose area and become isolated, which in turn restrains the establishment of new plant individuals and populations, thus reducing species richness. (2) Heterogeneity confounded by mean: when heterogeneity occurs at spatial scales smaller than the size of individual plants, which forage through the patches, species diversity can be either positively or negatively affected by a change in the mean of an environmental factor. (3) Heterogeneity as a separate niche axis: the ability of species to tolerate heterogeneity at spatial scales smaller than plant size varies, affecting HDR. We conclude that processes other than niche limitation can affect the relationship between heterogeneity and diversity.     

Ecotone formation through ecological niche construction: the role of biodiversity and species interactions

Rapid changes in species composition, also known as ecotones, can result from various causes including rapid changes in environmental conditions, or physiological thresholds. The possibility that ecotones arise from ecological niche construction by ecosystem engineers has received little attention. In this study, we investigate how the diversity of ecosystem engineers, and their interactions, can give rise to ecotones. We build a spatially explicit dynamical model that couples a multispecies community and its abiotic environment. We use numerical simulations and analytical techniques to determine the biotic and abiotic conditions under which ecotone emergence is expected to occur, and the role of biodiversity therein. We show that the diversity of ecosystem engineers can lead to indirect interactions through the modification of their shared environment. These interactions, which can be either competitive or mutualistic, can lead to the emergence of discrete communities in space, separated by sharp ecotones where a high species turnover is observed. Considering biodiversity is thus critical when studying the influence of species–environment interactions on the emergence of ecotones. This is especially true for the wide range of species that have small to moderate effects on their environment. Our work highlights new mechanisms by which biodiversity loss could cause significant changes in spatial community patterns in changing environments.     

Evolutionary and ecological causes of the latitudinal diversity gradient in hylid frogs: treefrog trees unearth the roots of high tropical diversity

Why are there more species in the tropics than in temperate regions? In recent years, this long-standing question has been addressed primarily by seeking environmental correlates of diversity. But to understand the ultimate causes of diversity patterns, we must also examine the evolutionary and biogeographic processes that directly change species numbers (i.e., speciation, extinction, and dispersal). With this perspective, we dissect the latitudinal diversity gradient in hylid frogs. We reconstruct a phylogeny for 124 hylid species, estimate divergence times and diversification rates for major clades, reconstruct biogeographic changes, and use ecological niche modeling to identify climatic variables that potentially limit dispersal. We find that hylids originated in tropical South America and spread to temperate regions only recently (leaving limited time for speciation). There is a strong relationship between the species richness of each region and when that region was colonized but not between the latitudinal positions of clades and their rates of diversification. Temperature seasonality seemingly limits dispersal of many tropical clades into temperate regions and shows significant phylogenetic conservatism. Overall, our study illustrates how two general principles (niche conservatism and the time-for-speciation effect) may help explain the latitudinal diversity gradient as well as many other diversity patterns across taxa and regions.     

Consequences of the genetic threshold model for observing partial migration under climate change scenarios

Migration is a widespread phenomenon across the animal kingdom as a response to seasonality in environmental conditions. Partially migratory populations are populations that consist of both migratory and residential individuals. Such populations are very common, yet their stability has long been debated. The inheritance of migratory activity is currently best described by the threshold model of quantitative genetics. The inclusion of such a genetic threshold model for migratory behavior leads to a stable zone in time and space of partially migratory populations under a wide range of demographic parameter values, when assuming stable environmental conditions and unlimited genetic diversity. Migratory species are expected to be particularly sensitive to global warming, as arrival at the breeding grounds might be increasingly mistimed as a result of the uncoupling of long‐used cues and actual environmental conditions, with decreasing reproduction as a consequence. Here, we investigate the consequences for migratory behavior and the stability of partially migratory populations under five climate change scenarios and the assumption of a genetic threshold value for migratory behavior in an individual‐based model. The results show a spatially and temporally stable zone of partially migratory populations after different lengths of time in all scenarios. In the scenarios in which the species expands its range from a particular set of starting populations, the genetic diversity and location at initialization determine the species’ colonization speed across the zone of partial migration and therefore across the entire landscape. Abruptly changing environmental conditions after model initialization never caused a qualitative change in phenotype distributions, or complete extinction. This suggests that climate change‐induced shifts in species’ ranges as well as changes in survival probabilities and reproductive success can be met with flexibility in migratory behavior at the species level, which will reduce the risk of extinction.     

Legacy effects of soil homogenization on tallgrass prairie restoration: toward resolved understanding of the relationship between soil heterogeneity and plant species diversity

Within plant communities, niche‐based species sorting can occur among distinct soil patches (microsites), increasing coexistence and diversity. Microsite edges (microedges) may also offer additional niche space. Therefore, in recently abandoned croplands, which often have uniform soils caused by a legacy of tillage (soil homogenization), the plant species diversity of future restoration efforts may be reduced. We conducted an experiment during the early establishment phase (3 years) of a tallgrass prairie restoration on former cropland to determine if soil homogenization decreases species diversity and alters community composition, and if microedges offer additional niche space. Heterogeneous plots with sand‐ or woodchip‐enriched patches were compared to plots made up of the same components, but distributed homogeneously, and pits and mounds were compared to flat topsoil. Homogenization decreased diversity in flat topsoil plots relative to pit plots and increased diversity in woodchip plots. In both cases, the treatments with the lowest canopy cover and greatest plant density had the greatest diversity. Sand and topographic homogenization decreased diversity, but when a drought occurred in year two, the effect was suppressed in the sand treatment and magnified in the pit plots. Microedges had properties unique from adjacent patches. Overall, variability in heterogeneity–diversity relationships was affected by interactions with plant growth patterns and environmental conditions. Our results indicate that while the addition of contrasting soil microsites has the potential to promote increased diversity in grassland restoration on former cropland, the patch components and design must be optimized to achieve this management goal.     

Effect of landscape context on fish metacommunity structuring in stream networks

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