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Speciation is frequently initiated but rarely completed, a phenomenon hypothesized to arise due to the failure of nascent lineages to persist. Although a failure to persist often has ecological causes, key gaps exist between ecological and evolutionary theories that, if filled, would clarify when and why speciation succeeds or fails. Here, we apply ecological coexistence theory to show how the alignment between different forms of niche opportunity and niche use shape the initiation, progression, and completion of speciation. Niche evolution may drive coexistence or competitive exclusion, and an ability to coexist ecologically may help or hinder speciation. Our perspective allows progress towards unifying the origin and maintenance of species diversity across the tree of life.

In this study, five types of CaF2 CaO Al2O3 slags with varying CaO/Al2O3 mass ratio and CaF2 content were selected for investigating the vaporization behavior of slag bearing high CaF2 content by thermo gravimetry and ion and molecule coexistence theory. The results show that the volatile product of slag is the AlF3. The vaporization rate is affected by the vapor pressure of AlF3, mole number of complex molecules and the viscosity of slag. For all slags, the vaporization rate of slag bearing approximately 70 wt% CaF2 and 30 wt% Al2O3 is the fastest due to the ultra-high vapor pressure of AlF3. In the test of slags containing 70 wt% CaF2, vaporization rate firstly decreases, then increases with CaO/Al2O3 mass ratio increasing from 0.05 to 1.10 and 2.00. In the case of slags containing same CaO/Al2O3 mass ratio, vaporization rate firstly increases, then decreases with CaF2 content decreasing from 68.9 wt% to 48.1 wt% and 28.7 wt%. Slag T5 bearing approximately 30 wt% CaF2, 33.9 wt% CaO and 36.1 wt % Al2O3 has lower electrical conductivity, slower vaporization rate and larger sulfide capacity, which will be economical, environmentally friendly and suitable for vacuum electroslag remelting.

The results of predicting thermodynamic properties in the full composition range of Ti-Al binary melts in a temperature range from 1973 to 2273 K were obtained by coupling with the developed thermodynamic model for calculating mass action concentration Ni of structural units in Ti-Al system based on the atom and molecule coexistence theory (AMCT). Temperature dependence of the activity coefficients of Ti and Al in natural logarithmic form in the infinitely dilute solution (0>xTi

Character displacement is one of the most studied phenomena in evolutionary biology, yet research has narrowly focused on demonstrating whether or not displacement has occurred. We propose a new experimental approach, adopted from the coexistence literature, that directly measures interspecific competition among sympatric and allopatric populations of species. Doing so allows increased ability to (i) test predictions of character displacement without biases inherent to character-centric tests, (ii) quantify its effect on the stability of coexistence, (iii) resolve the phenotypic pathways through which competitive divergence is achieved, and (iv) perform comparative tests. Our approach extends research to forms of character displacement not readily identified by past methods and will lead to a broader understanding of its consequences for community structure.

Based on the contemporary coexistence theory developed from macroecology, the regional species pool, environmental processes, dispersal assembly and biotic interactions are considered to determine the assembly of a local community. However, we do not fully understand whether these kinds of mechanisms are all-important for the whole microbial community or for specific functional groups. Here we studied the underlying processes driving the β-diversity of soil-based diazotrophic communities across a large geographical distance in Chinese grasslands by considering the roles of species pools and ecological assembly processes. Results indicated that the species pool (γ-diversity) showed significant and positive correlations to the observed β-diversity of the diazotrophic community; that is, diazotrophic community turnover increased with the size of the species pool. Environmental filtering played a greater role than dispersal effects or biotic interactions on β-diversity. Biotic interactions drove the assemblage of diazotrophs by influencing the co-occurrence pattern of species. Overall, these results demonstrated the importance of the species pool and local community assembly processes in shaping diazotrophic β-diversity patterns in grassland soils.

Despite the general acknowledgment of the role of niche and stochastic process in community dynamics, the role of species relative abundances according to both perspectives may have different effects regarding coexistence patterns. In this study, we explore a minimum probabilistic stochastic model to determine the relationship of populations relative and total abundances with species chances to outcompete each other and their persistence in time (i.e., unstable coexistence). Our model is focused on the effects drift (i.e., random sampling of recruitment) under different scenarios of selection (i.e., fitness differences between species). Our results show that taking into account the stochasticity in demographic properties and conservation of individuals in closed communities (zero-sum assumption), initial population abundance can strongly influence species chances to outcompete each other, despite fitness inequalities between populations, and also, influence the period of coexistence of these species in a particular time interval. Systems carrying capacity can have an important role in species coexistence by exacerbating fitness inequalities and affecting the size of the period of coexistence. Overall, the simple stochastic formulation used in this study demonstrated that populations initial abundances could act as an equalizing mechanism, reducing fitness inequalities, which can favor species coexistence and even make less fitted species to be more likely to outcompete better-fitted species, and thus to dominate ecological communities in the absence of niche mechanisms. Although our model is restricted to a pair of interacting species, and overall conclusions are already predicted by the Neutral Theory of Biodiversity, our main objective was to derive a model that can explicitly show the functional relationship between population densities and community mono-dominance odds. Overall, our study provides a straightforward understanding of how a stochastic process (i.e., drift) may affect the expected outcome based on species selection (i.e., fitness inequalities among species) and the resulting outcome regarding unstable coexistence among species.

In order to explore the slag with less fluoride vaporization for vacuum electroslag remelting of Ce-bearing steel, the fluoride vaporization behavior of CaF2–CaO–Al2O3-(Ce2O3) slag was investigated by thermo gravimetry analysis at 1673 K. For the purpose of figuring out the mechanism of the Ce2O3 effect on fluoride vaporization in slags, the viscosity of slags was measured, and the activity of components in molten slag was calculated based on the ion and molecule coexistence theory. The fluoride vaporization of slag under vacuum condition at 1823 K was also studied. The results show that the fluoride vaporization products are CaF2 and AlF3. The weight loss caused by fluoride vaporization reduces with increasing Ce2O3 content. The Ce2O3 addition lowers the breaking temperature of viscosity, which leads to the decrease of viscosity at 1673 K with the increase in Ce2O3 content. It implies the viscosity change is not the reason inhibiting fluoride vaporization because a lower viscosity will promote the fluoride vaporization. The vapor pressure of CaF2 and AlF3 decreases with increasing Ce2O3 content due to activity change, which contributes to the suppression effect on the fluoride vaporization in investigated slags. The fluoride vaporization of slag under vacuum is inhibited because of Ce2O3 addition as well.

Understanding the processes that govern species distributions is a fundamental goal of ecology. Species differ in patterns of resource-use due to differences in response to biotic and environmental factors. Some species dominate favourable habitat through competitively superiority, while others have evolved to exploit niches of abiotic stress where competition is low. We observed interspecific differences in the use of horizontal and vertical habitats by Antarctic bryozoa. The most spatially dominant species, Inversiula nutrix , mainly occupied horizontal surfaces, while nine other bryozoan species mainly occupied vertical surfaces. We hypothesized that this spatial segregation may be due to either competitive displacement of other species by I. nutrix , or tolerance of I. nutrix to an environmental stress associated with horizontal orientation. To investigate this we quantified competitive interactions between all species, and conducted a manipulative field experiment to test effects of sedimentation on bryozoan growth. We found that I. nutrix was not a superior competitor, but was more tolerant of sedimentation than were other species. Despite mediocre competitive ability, tolerance to a stressor has allowed I. nutrix to occupy resources unavailable to the other species, and thereby gain spatial dominance within the community.

Environmental fluctuations have important consequences in the organization of ecological communities, and understanding how such a variability influences the biodiversity of an ecosystem is a major question in ecology. In this paper, we analyze the case of two species competing for the resources within the framework of the neutral theory in the presence of environmental noise, devoting special attention on how such a variability modulates species fitness. The environment is dichotomous and stochastically alternates between periods favoring one of the species while disfavoring the other one, preserving neutrality on the long term. We study two different scenarios: in the first one species fitness varies linearly with the environment, and in the second one the effective fitness is re-scaled by the total fitness of the individuals competing for the same resource. We find that, in the former case environmental fluctuations always reduce the time of species coexistence, whereas such a time can be enhanced or reduced in the latter case, depending on the correlation time of the environment. This phenomenon can be understood as a direct consequence of Chesson's storage effect., 11 pages, 4 figures

Spatial interactions are widely acknowledged to play a significant role in sustaining diversity in ecological communities. However, theoretical work on this topic has focused on how spatial processes affect coexistence of species that differ in their strategies, with less attention to how spatial processes matter when competitors are equivalent. Furthermore, though it is recognized that models with local dispersal and local competition may sustain higher diversities of equivalent competitors than models in which these are not both localized, there is debate as to whether this reflects merely equalizing effects or whether there is also a stabilizing component. In this study, we explore how dispersal limitation and nonspecific local competition influence the outcome of species coexistence in communities driven by stochastic drift. We demonstrate that space alone acts as a stabilizing factor in a continuous space model with local dispersal and competition, as individuals of rare species on average experience lower total neighborhood densities, causing per capita reproductive rates to decrease systematically with increasing abundance. These effects prolong time to extinction in a closed system and enhance species diversity in an open system with constant immigration. Fundamentally, these stabilizing effects are obtained when dispersal limitation interacts with local competition to generate fluctuations in population growth rates. Thus this effect can be considered a fluctuating mechanism similar to spatial or temporal storage effects, but generated purely endogenously without requiring any exogenous environmental variability or species dissimilarities.

Ecologists are increasingly interested in predicting how intraspecific variation and changing trait values impact species interactions and community composition. For many traits, much of this variation is caused by phenotypic plasticity, and thus the impact of plasticity on species coexistence deserves robust quantification. Partly due to a lack of sound theoretical expectations, empirical studies make contradictory claims regarding plasticity effects on coexistence. Our critical review of this literature, framed in modern coexistence theory, reveals that plasticity affects species interactions in ways that could impact stabilizing niche differences and competitive asymmetries. However, almost no study integrates these measures to quantify the net effect of plasticity on species coexistence. To address this challenge, we outline novel empirical approaches grounded in modern theory.

Understanding the mechanisms determining population density of species in communities and ecological networks is an important task in ecological studies. Interactions and carrying capacity largely determine population density of species and then community structure. However, their impacts on population density have not been fully investigated in ecological networks. In this study, we examined the associations of interspecific interaction strength and carrying capacity with population density in three kinds of theoretical and empirical ecological networks with different complexity. We firstly demonstrated both the net direct and indirect interaction strength of a species received from the other species showed positive associations with population density of the species in all ecological networks (except for in predation networks), particularly in more complex ecological networks. Direct interaction was more important than indirect interaction in determining population density. Carrying capacity showed a positive association with population density, particularly in less complex ecological networks. Our results suggest that interspecific interaction strength is more important than carrying capacity in determining species dominance in more complex networks.

Global change continues to push individual species towards extinction and in many cases these species cannot be expected to disappear silently, without a trace, from the communities in which they reside. Importantly, the loss of certain species can trigger a cascade of secondary extinctions, resulting in further degradation of a system with potential effects on ecosystem functioning. It is thus crucial to better our understanding of the types of species which, if lost, will affect community structure the most, because in this context they are analogous to keystone species. Here, using a dynamical approach and simulating species loss in model communities, we study whether the importance of species changes as the structure of the food web changes by analyzing the potential importance of 11 species traits during food web collapse following simulated extinctions. We find that the keystone status of species traits is context dependent, with the identity of the most important trait changing during food web collapse. The relative importance of two trait categories tend to be inversely related, with body size based traits being important in intact to moderately degraded webs and interaction strength based species traits being important in highly collapsed food webs. The results of our study furthermore suggest that the response of communities to small and large perturbations may be related in the sense that the same kinds of species are important in both situations.

How does the stress-gradient hypothesis affect coexistence in relation to established theory? For two orthogonal stress gradients, a spatially explicit agent based simulation is used to project diversity for simple competitive and facilitative interactions and for three variations of the stress-gradient hypothesis: intraspecific and interspecific competitive and facilitative interactions are a function of the abiotic environment; interactions are relative to species-specific fitness along gradients; or interaction is fixed by species regardless of the abiotic environment. Simulations are run with two orthogonal environmental gradients for two representations of niche. Facilitation can increase diversity by maintaining larger source populations and thus higher establishment rates and sink populations. With species hierarchically related in niche space, the simulations show that positive interactions and changing interactions along a stress gradient maintain greater diversity through intraspecific competition that is effective where dominance would occur and through facilitation where stress is high. A changing environment that favors some species and harms others decreases diversity in the hierarchical cases, where poor competitors most likely subject to interspecific interaction respond most strongly. Diversity outcomes differ among the three stress gradient variations because the intensity of interactions differs across the environmental gradients, not because of change in the environment.

Species-rich communities, in which many competing species coexist in a single trophic level, are quite frequent in nature, but pose a formidable theoretical challenge. In particular, it is known that complex competitive systems become unstable and unfeasible when the number of species is large. Recently, many studies have attributed the stability of natural communities to the structure of the interspecific interaction network, yet the nature of such structures and the underlying mechanisms responsible for them remain open questions. Here we introduce an evolutionary model, based on the generic Lotka-Volterra competitive framework, from which a stable, structured, diverse community emerges spontaneously. The modular structure of the competition matrix reflects the phylogeny of the community, in agreement with the hierarchial taxonomic classification. Closely related species tend to have stronger niche overlap and weaker fitness differences, as opposed to pairs of species from different modules. The competitive-relatedness hypothesis and the idea of emergent neutrality are discussed in the context of this evolutionary model.

According to the ion and molecule coexistence theory, a thermodynamic model of lead oxide activity in PbO–CaO–SiO2–FeO–Fe2O3 slag system was established at the temperature of 1273–1733 K. The activities of PbO in slag were calculated, and their equal activity curves were plotted. The influences of slag basicity Q, iron oxide rate R and temperature T on activity NPbO and activity coefficient γPbO were also investigated. Results show that the calculated values of γPbO are in good agreement with the reported experimental data, showing that the model can wholly embody the slag structural characteristics. NPbO departures positively from Raoult values, and increases with increasing PbO content in slag but changes little with T. γPbO increases with increasing Q, and goes through the maximum with increasing R for basic slag (Q

From the viewpoint of competitor-mediated coexistence, it is important to study the influence of an exotic species on other native species in terms of exogenous effects, in general, exotic species are weaker than native ones because they have evolved in a different environment. Even if an exotic species is weaker, however, it might cooperate with a native species, after which the competitive relations among native species may have reversed. This motivates us to consider the ecological situation whereby one exotic competing species invades the native system of two strongly competing species. Therefore, we discuss the problem of competitive exclusion or competitor-mediated coexistence using a three-species competition–diffusion system.

Using a model of resource acquisition, we studied species competition in a case where resources limit population growth. Our model is based on calculations of the distribution of individuals of single or multiple species over consumed resources. Calculations show that, as equilibrium is reached in purely resource competitive systems, the density of resources is lowered to the lowest sustainable level, directly leading to low levels of fitness among species. In the case of competition between species with different lowest sustainable levels, the density of the more successful must be limited by some cause other than the resource in question for all species to coexist. We explore two cases of such coexistence.

In this paper, we study the law of survival for species in interspecific competition in the cyclic and predator–prey system. In our model, the successful rate for a predator to prey depends on the individual ability to prey and the two interacting clusters sizes, and the size of a cluster is determined by the aggregation degree between individuals. Experimental results show that only one species can survive when competition occurs on one niche. And which species can survive ultimately depends on the relative relationship between the average individual ability to prey and the aggregation degree between it and its competing species. If competing species have identical values for the average individual ability to prey and the aggregation degree, the species that can survive is determined at random. Therefore, Gause’s Competitive Exclusion Principle is correct, but the causes of competing species to survive are different.

The thermodynamic properties of the full composition range of liquid Ni-Ti, Ni-Al, and Ti-Al alloys were determined using the atom and molecule coexistence theory (AMCT), which is an established thermodynamic model that was used to calculate the mass action concentration (activity) of the structural units in the abovementioned three alloy systems. The temperature dependence of the activity coefficients γ i θ of Ti and Al dissolved in molten nickel to form dilute solutions (0 ≤ xi ≤ 0.01) relative to pure liquid were estimated from the calculated activity coefficients of Ti and Al at different temperatures. The standard Gibbs free energy changes Δsol G m , i θ of dissolving Ti and Al in nickel and the enthalpies of mixing ΔmixHm, i of liquid Ni-Ti, Ni-Al, and Ti-Al were also obtained. The results indicated that the determined thermodynamic properties, such as the Δsol G m , i θ , and ΔmixHm, i of the liquid binary alloys reasonably agreed with the experimental data previously reported in the literatures. Thermodynamic properties were predicted for three important subsystems, which included the Ni-Ti, Ni-Al, and Ti-Al alloys, and therefore, the activity ai and ΔmixHm, Ni‐Al‐Ti of the nickel-rich liquid Ni-Al-Ti ternary alloys were also obtained at 1873 K. Furthermore, the values calculated using the classical geometric models, including the Toop, Kohler, and Muggianu models, were compared with those determined by the GSM in this study.

Ecological theory predicts that species coexisting locally may either be functionally more dissimilar than expected by chance due to niche differentiation ('trait divergence'), or rather be functionally similar given that species bearing traits associated with low competitive ability are likely to be excluded ('trait convergence'). How the importance of trait divergence and convergence varies with productivity and depending on different plant traits still remains controversial. However assessing the effect of site productivity on meadow species assembly can help defining proper management practice for these ecosystems. We sought evidences for trait convergence and divergence across a productivity gradient over 21 wet meadows in Železné hory Mts. (Czech Republic), considering 13 plant traits. In each meadow, we recorded species composition in four 1m2 plots divided into 100 subplots, and different environmental parameters and site characteristics linked to productivity (standing biomass, soil water table and soil nutrients content). Species spatial aggregation within plots (calculated by the V-score metric on the 100 subplots per plot averaged across all plots in each meadow) was correlated with trait dissimilarity between species in a rather limited number of cases. Significant divergence was observed when traits were considered in combination or when using phylogeny as a proxy of functional similarity. Moreover, plot level patterns of trait divergence and convergence depended on local environmental characteristics. Specifically, for several of the traits studied (e.g. lifespan, combined traits) there was evidence that functional divergence of species decreased with increasing productivity. Overall from these results at fine scale we cannot rule out neutrality, but, where significant, patterns suggest stronger effects of niche differentiation between coexisting species (trait divergence) rather than convergence of functionally similar species. Nevertheless our results also support the view that increased productivity, coupled with intense competition for resources (mainly for light), can lead to the coexistence of more functionally similar species. This is relevant because biodiversity enhancement is generally achieved by maintaining low productivity in these meadows. In general our study gives insights on how species coexistence is related to life history traits and environmental variation, which is also relevant for the conservation and management of species-rich meadows. © 2013 Elsevier B.V.

A traditional question in community ecology is whether species' traits are distributed as more-or-less regularly spaced clusters. Interspecific competition has been suggested to play a role in such structuring of communities. The seminal theoretical work on limiting similarity and species packing, presented four decades ago by Robert MacArthur, Richard Levins and Robert May, has recently been extended. There is now a deeper understanding of how competitive interactions influence community structure, for instance, how the shape of competition kernels can determine the clustering of species' traits. Competition is typically weaker for greater phenotypic difference, and the shape of the dependence defines a competition kernel. The clustering tendencies of kernels interact with other effects, such as variation in resource availability along a niche axis, but the kernel shape can have a decisive influence on community structure. Here we review and further extend the recent developments and evaluate their importance.

a b s t r a c t To understand species abundance distributions effectively, it is essential to consider the forces which create the overwhelmingly long 'tail' of rare species. Both neutral and niche theory have been invoked to explain the rarity of certain species in ecological assemblages: niche theory stresses the importance of the resource division process, while neutral theory focuses on stochastic drift in densities of compet- itively identical species. Here, we show how migration and niche partitioning can act simultaneously to increase both the number of rare species and the total species richness. By simulating the dynam- ics of a metacommunity using an interconnected network of local communities, we incorporate niche partitioning into the otherwise per capita equivalent processes of birth and death. Niche partitioning is defined by the species niche fitness function, which is measured by the species optimal environmental conditions, the fundamental niche breadths of each species, and by the breadth of the environmental gradient of each local community. For a given niche breadth at the local community scale, unimodal rela- tionships appear between the rate of migration, the number of rare species, and the total species richness. At the metacommunity scale, species richness was at first unaffected by increasing migration, but then crashed. At wide niche breadth, both the number of rare species and the total number of species started to decline at a low rate of migration. In contrast, when niche breadth was narrow, neither the number of rare species nor the total number of species started to decline until migration rates were much higher. This is because, for a given migration rate, niche breadth can affect the proportions of individuals coming from adjacent local communities. The integration of niche partitioning and migration therefore provides a rational explanation for the widespread rarity of species in ecological communities. © 2013 Elsevier B.V. All rights reserved.

It is well established theoretically that competing species may coexist by having different responses to variation over time in the physical environment. Whereas previous theory has focused mostly on year-to-year environmental variation, we investigate how within-year variation can be the basis of species coexistence. We ask also the important but often neglected question of whether the species differences that allow coexistence are compatible with evolutionary processes. We seek the simplest circumstances that permit coexistence based on within-year environmental variation, and then evaluate the robustness of coexistence in the face of evolutionary forces. Our focus is on coexistence of annual plant species living in arid regions. We first consider environmental variation of a very simple structure where a single pulse of rain occurs, and different species have different patterns of growth activity following the rain pulse. We show that coexistence of two species is possible based on the storage effect coexistence mechanism in this simplest of varying environments. We find an exact expression for the magnitude of the storage effect that allows the functioning of the coexistence mechanism to be analyzed. However, in these simplest of circumstances, coexistence in our models is not evolutionarily stable. Increasing the complexity of the environment to two rain pulses leads to evolutionarily stable species coexistence, and a route to diversity via evolutionary branching. This demonstration of the compatibility of a coexistence mechanism with evolutionary processes is an important step in assessing the likely importance of a mechanism in nature.

Understanding the mechanisms of species diversity maintenance within plant communities has become a fundamental issue in ecology over the past decades. While some models have tried to explore these mechanisms, few studies have integrated the dynamic interactions with neighbours in a spatially explicit way. The present model uses Voronoi polygons to dynamically partition a landscape patch into areas occupied by individual plants. It thus incorporates neighbourhood competition for space, unlike grid-based models with nearest-neighbour competition. In closed two-species communities, dynamic Voronoi partitioning promoted species coexistence, especially under local dispersal. This suggests that grid-based models overestimate species extinction rates. Likewise, multispecies communities without immigration had substantially greater species richness in the space partitioning model than in the grid-based model but only under distance-limited dispersal. In contrast, richness levels were similar in both models under global dispersal or with immigration from the metacommunity. Trait variation among species reduced species richness, but more so for traits associated with competition for space. This suggests that some traits are more important than others in governing species richness. Overall, our study demonstrates that combining species identity (traits) with partitioning of physical space can improve understanding of diversity regulation.

A common situation observed in fragmented habitats is that species densities diminish within smaller fragments. Some species, however, do show an opposite tendency. We argue that release of competition between a strong competitor and a weak one is a plausible explanation for these different sensitivities to area reduction. We provide a quantitative model for competitive release caused by habitat limitation, solely based on the balance of diffusion and growth of the species. We show that in small habitat patches the stronger competitor has its density diminished, as opposite to the weaker competitor who, in a certain area range, has its density increased. We examine field data from an ecological experiment in Amazonia (BDFFP) which measured densities of two Amazonian rodents who showed opposite sensitivity to area reduction and we argue that our model explains the observations accurately. This implies that (i) area reduction is a strong factor determining densities of species in patches of habitat, regardless of considerations on degradation or edge effects and (ii) that species interactions have to be taken into account to explain sensitivity to size of the patches in ecological communities. We also discuss alternative explanations, such as predator release and effects due to imperfect isolation. Moreover, we stress the conceptual and mathematical simplicity of our model, which, nevertheless explains a phenomenon not yet well understood.

Understanding biodiversity maintenance when species compete for shared limiting resources remains an outstanding ecological problem. For a half century, the competition–colonization trade-off has been invoked to explain species coexistence. More recently, asymmetric dispersal has been shown to alter the conditions for species coexistence under pure scramble competition for space, even in the absence of spatial heterogeneity. Therefore, in this study, we investigated why and how asymmetric dispersal may alter the effect of competition–colonization trade-off on species coexistence. Our main findings are that: (1) self-recruitment of a superior competitor facilitates persistence of an inferior competitor; (2) the degree to which a community is recruitment-limited influences the threshold dispersal asymmetry required for persistence of a species with a birth or survival disadvantage; (3) an inferior competitor with a high recruitment strategy is better able to benefit from spatial asymmetries in dispersal than one with a high survival strategy, and (4) an inferior competitor may persist, and even exclude the superior competitor, when dispersal is highly asymmetric (i.e., the competition–colonization trade-off can be violated). These results underscore the importance of dispersal patterns promoting species coexistence and their synergies with species' colonization and competition abilities, and highlight the need to assess dispersal-mediated coexistence within a framework that incorporates the interactive effects of dispersal patterns, demographic and competitive strategies.

The aim of this work is to investigate the effect of density-dependent dispersal on the outcome of competition in a heterogeneous environment. We present a classical spatial interspecific competition model with two patches connected by fast dispersal. We assume local pre-emptive competition, i.e. on each isolated patch, the species having the largest initial density would exclude the other one. We assume that individuals of species 1 disperse at constant dispersal rates, while species 2 dispersal is density dependent. Species 2 individuals are more likely to disperse when there are many species 1 competitors in a patch. We investigate if a dispersal strategy that aims at avoiding its competitor can be beneficial. Dispersal between patches is assumed to be faster than local population dynamics on each patch. We take advantage of these time scales in order to reduce the complete model into an aggregated model governing total densities of both species at a slow time scale. The analysis of this global model shows that using a density dependent strategy for dispersal is beneficial for species 2 in the sense that it prevents its own extinction and even it allows species 2 to exclude species 1 under some conditions.

One of the most detrimental impacts of invasive species is the exclusion of native species, which reduces biodiversity and can alter community structure. Coexistence between invaders and native species across large scales, however, might be promoted by niche partitioning and/or stochastic processes, even when one species is excluded in some habitats. Here, we examined the effects of species traits, stochastic processes, and niche partitioning on coexistence of two morphocryptic whitefly species in the Bemisia tabaci complex: the invasive Mediterranean (MED) species and the native Middle East-Asia Minor 1 (MEAM1) species. These species engage in intense reproductive interference, which can result in the exclusion of one species or the other in shared habitats. Both species, however, have coexisted in sympatry in Israel for many years, where MED is invasive and MEAM1 is native. Using a spatially explicit model, we show that both stochastic processes and niche partitioning can promote coexistence between MEAM1 and MED, although predicted community structure differs drastically in each scenario. Comparison of field observations with model results indicated that variation in habitat use leading to niche partitioning was a primary factor driving coexistence between MEAM1 and MED across landscapes, although stochastic processes affected the establishment of rare species within habitats. In many systems, combining models with field surveys can be used to isolate and test mechanisms underlying patterns of community structure following invasions.

Forests that are composed of two or more tree species with similar ecological strategies appear to contradict the competitive exclusion principle. Beech-maple communities are a well-known example of such a system. On a local scale, a number of mechanisms have been proposed to explain the coexistence of these two species. These are reciprocal replacement, external factors that favour alternatively one or the other species and demographic stochasticity. This paper presents and analyses a simple mathematical model that shows that external factors are not an essential requirement for coexistence. Rather, coexistence requires interspecific differences in light transmissivity through the crowns of adult trees. However, all the three mechanisms mentioned above can be interpreted within the framework of the model. Furthermore, many models of forest dynamics make use of shade tolerance as a key feature in describing successional dynamics. Despite its importance, however, shade tolerance does not have a commonly accepted quantitative definition. Here, a simple scheme is proposed where the relationship between shade tolerance, individual traits (growth and survival) and successional status is defined. This might have important implications in understanding the overall dynamics. Theoretical results have been compared with a number of studies carried out in North American forests. In particular, coexistence in beech-maple communities and the relation between shade tolerance and successional status in a beech-hemlock-birch community have been discussed.

Local extinction of native species and colonization of non-native species are commonly invoked as responsible for changes in species similarity among biotas of different regions. In this study we used a model of species similarity between islands to assess the emergent, and unexplored, effects of changes in colonization by native species, extinction of non-native species, and propagule pressure on species similarity among insular communities. The model predicts that extinction probability of endemic species has a positive but asymptotic effect on species similarity, which is exacerbated by increasing colonization and reducing extinction of non-native species. Species similarity tends to increase with increasing colonization probability by non-native species, however this effect may be reduced, or even reverted, when the islands are exposed to an elevated number of non-native species that are prone to extinction, high levels of endemic species loss, and an initially large number of native species shared between islands. Species similarity was positively affected by the propagule pressure rate of non-native species only when their colonization and extinction probabilities were large and small enough, respectively. A negative effect of propagule pressure rate can be caused by an increase in the pool size of non-native species, which involves the introduction of different species into different islands, promoting biotic differentiation between islands. Our results indicate that the interactions between colonization, extinction and species pool lead to nonlinear responses and unexpected scenarios of biotic change. In order to validate model predictions, future research programs should focus on understanding the dynamics on such complex meta-communities where coexist native, non-native and endemic species.

Theories for species coexistence often emphasize niche differentiation and temporal segregation of recruitment to avoid competition. Recent work on mutualism suggested that plant species sharing pollinators provide mutual facilitation when exhibit synchronized reproduction. The facilitation on reproduction may enhance species persistence and coexistence. Theoretical ecologists paid little attention to such indirect mutualistic systems by far. We propose a new model for a two-species system using difference equations. The model focuses on adult plants and assumes no resource competition between these well-established individuals. Our formulas include demographic parameters, such as mortality and recruitment rates, and functions of reproductive facilitation. Both recruitment and facilitation effects reach saturation levels when flower production is at high levels. We conduct mathematical analyses to assess conditions of coexistence. We establish demographical conditions permitting species coexistence. Our analyses suggest a ‘‘rescue’’ effect from a ‘‘superior’’ species to a ‘‘weaker’’ species under strong recruitment enhancement effect when the later is not selfsustainable. The facilitation on rare species may help to overcome Allee effect.

A universal thermodynamic model of calculating mass action concentrations for structural units or ion couples in ternary and binary strong electrolyte aqueous solution was developed based on the ion and molecule coexistence theory and verified in four kinds of binary aqueous solutions and two kinds of ternary aqueous solutions. The calculated mass action concentrations of structural units or ion couples in four binary aqueous solutions and two ternary solutions at 298.15 K have good agreement with the reported activity data from literatures after shifting the standard state and concentration unit. Therefore, the calculated mass action concentrations of structural units or ion couples from the developed universal thermodynamic model for ternary and binary aqueous solutions can be applied to predict reaction ability of components in ternary and binary strong electrolyte aqueous solutions. It is also proved that the assumptions applied in the developed thermodynamic model are correct and reasonable, i.e., strong electrolyte aqueous solution is composed of cations and anions as simple ions, H2O as simple molecule and other hydrous salt compounds as complex molecules. The calculated mass action concentrations of structural units or ion couples in ternary and binary strong electrolyte aqueous solutions strictly follow the mass action law.

It is widely accepted that niche differentiation plays a key role in coexistence on relatively small scales. With regard to a large community scale, the recently propounded neutral theory suggests that species abundances are more influenced by history and chance than they are by interspecies competition. This inference is mainly based on the probability that competitive exclusion is largely slowed by recruitment limitation, which may be common in species rich communities. In this respect, a theoretical study conducted by Hurtt and Pacala (1995) for a niche differentiated community has been frequently cited to support neutral coexistence. In this paper, we focused on the effect of symmetric recruitment limitation on delaying species competitive exclusion caused by both symmetric and asymmetric competition in a large homogeneous habitat. By removing niche differentiation in space, we found that recruitment limitation could delay competitive exclusion to some extent, but the effect was rather limited compared to that predicted by Hurtt and Pacala's model for a niche differentiated community. Our results imply that niche differentiation may be important for species coexistence even on large scales and this has already been confirmed in some species rich communities.

Competition effects on community development are difficult to quantify in species-rich plant communities due to the complexity of possible interactions. We used multispecies mixtures to investigate how species identity and competitive interactions influence the development of plant communities. Given the same set of species with differing initial abundance in various communities, we tested whether communities would become more similar (converge) or dissimilar (diverge) over time depending on the relative importance of species identity and competition. Twenty-four experimental communities were established by planting seedlings of twelve wetland species at different relative abundances and absolute densities. The development of the communities was monitored over three years, and yearly changes in biomass were modelled as a linear function of the species biomass at the start of each period. After three years, a clear dominance structure had developed, with four species making up 80% of the aboveground biomass. In all three years, community dynamics was driven by differences in relative growth rates among the species (i.e. an effect of species identity). However, in the second and third years negative density dependence was also important, with changes in the relative abundance of the most abundant species being negatively related to their biomass at the start of the period. Multiple species interactions – though generally weaker than effects of species identity and intraspecific competition – became increasingly important and also contributed to the dominance pattern. It is concluded that species identity and negative density dependence of the dominant species were the most important factors causing the experimental plant communities to converge. We suggest that model systems composed of several species offer a useful method for investigating the influence of functional traits upon community dynamics.

Competition theory has developed separately for direct competition and for exploitative competition. However, the combined effects of the two types of competition on species coexistence remain unclear. To examine how intraspecific and interspecific direct competition contributes to the coexistence of species competing for a single resource, we constructed a chemostat-type resource competition model. With general functions for intraspecific and interspecific direct competition, we derived necessary and sufficient conditions (except for a critical case that rarely occurs in a biological sense) that determine the number of stably coexisting species. From these conditions, we found that the number of coexisting species is determined just by the invasibility of each species into subcommunities with a smaller number of species. In addition, using a combination of rigorous mathematical theory and a simple graphical method, we can demonstrate how the stronger intraspecific direct competition facilitates species invasion, leading to a larger number of coexisting species.

Plant species create aggregations of conspecifics as a consequence of limited seed dispersal, clonal growth and heterogeneous environment. Such intraspecific aggregation increases the importance of intraspecific competition relative to interspecific competition which may slow down competitive exclusion and promote species coexistence. To examine how spatial aggregation impacts the functioning of experimental assemblages of varying species richness, eight perennial grassland species of different growth form were grown in random and aggregated patterns in monocultures, two-, four-, and eight-species mixtures. In mixtures with an aggregated pattern, monospecific clumps were interspecifically segregated. Mixed model ANOVA was used to test (i) how the total productivity and productivity of individual species is affected by the number of species in a mixture, and (ii) how these relationships are affected by spatial pattern of sown plants. The main patterns of productivity response to species richness conform to other studies: non-transgressive overyielding is omnipresent (the productivity of mixtures is higher than the average of its constituent species so that the net diversity, selection and complementarity effects are positive), whereas transgressive overyielding is found only in a minority of cases (average of log(overyielding) being close to zero or negative). The theoretical prediction that plants in a random pattern should produce more than in an aggregated pattern (the distances to neighbours are smaller and consequently the competition among neighbours stronger) was confirmed in monocultures of all the eight species. The situation is more complicated in mixtures, probably as a consequence of complicated interplay between interspecific and intraspecific competition. The most productive species (Achillea, Holcus, Plantago) were competitively superior and increased their relative productivity with mixture richness. The intraspecific competition of these species is stronger than that of most other species. The aggregated pattern in the full mixture increased the survival of subordinate species, and consequently, we conclude that an aggregated pattern can promote species coexistence (or at least postpone competitive exclusion), particularly in comparison with homogeneously sown mixtures.

Network analyses of mutualistic or antagonistic interactions between species are very popular, but their biological interpretations are often unclear and incautious. Here I propose to distinguish two possible implications of network patterns in conjunction with solutions to avoid misinterpretations. Interpretations can be either (1) niche-based, describing specialisation, trait (mis-)matching between species, niche breadth and niche overlap and their relationship to interspecific competition and species coexistence, or (2) impact-based, focusing on frequencies of interactions between species such as predation or infection rates and mutualistic services, aiming to quantify each species’ relative contribution to an ecological effect. For niche-based implications, it is crucial to acknowledge the sampling limitations of a network and thus control for the number of observations of each species. This is particularly important for those kinds of networks that summarise observed interactions in communities (e.g. bipartite host–parasitoid or plant–animal networks), rather than compile information from different sources or experiments (as in many food webs). Variation in total observation frequencies may alone explain network patterns that have often been interpreted as ‘specialisation asymmetries’ (nestedness, dependence asymmetries). I show analytically that ‘dependence asymmetries’ between two species (or two guilds) only reflect variation in their total observation frequencies. To depict true asymmetries in niche breadth, independent data are required for both species. Moreover, simulated co-extinction scenarios assume that each species ‘depends’ on its associated partners in the network (again niche-based), but species that appear most endangered are simply those with one or very few observations and are not necessarily specialised. Distinguishing niche-based and impact-based interpretations may help to bridge terminological and conceptual gaps between network pattern analyses and traditional community ecology.

The population and community level consequences of positive interactions between plants remain poorly explored. In this study we incorporate positive resource-mediated interactions in classic resource competition theory and investigate the main consequences for plant population dynamics and species coexistence. We focus on plant communities for which water infiltration rates exhibit positive dependency on plant biomass and where plant responses can be improved by shading, particularly under water limiting conditions. We show that the effects of these two resource-mediated positive interactions are similar and additive. We predict that positive interactions shift the transition points between different species compositions along environmental gradients and that realized niche widths will expand or shrink. Furthermore, continuous transitions between different community compositions can become discontinuous and bistability or tristability can occur. Moreover, increased infiltration rates may give rise to a new potential coexistence mechanism that we call controlled facilitation. © 2009 Elsevier Inc. All rights reserved.

Weakly density-dependent effects, characterized by fractional scaling exponents close to one, are rarely studied in the ecological literature. Here, we consider the effect of an additional weakly density-dependent term on a simple competition model. Our investigation reveals that weak density-dependence opens up an "invisible niche". This niche does not constitute a new mechanism for coexistence, but is a previously unexplored consequence of known mechanisms. In the invisible niche a weaker competitor can survive at very low density. Coexistence thus requires large habitat size. Such niches, if found in nature, would have a direct impact on species-area laws and species-abundance curves and should therefore receive more attention.

A universal thermodynamic model of calculating the mass action concentrations of components in a ternary strong electrolyte aqueous solution has been developed based on the ion and molecule coexistence theory, and verified in the NaCl-KCl-H2O ternary system at 298.15 K. To compare the difference of the thermodynamic model in binary and ternary strong electrolyte aqueous solutions, the mass action concentrations of components in the NaCl-H,0 binary strong electrolyte aqueous solution were also computed at 298.15K. A transformation coefficient was required to compare the calculated mass action concentration and reported activity because they were obtained at different standard states and concentration units. The results show that the transformation coefficients between calculated mass action concentrations and reported activities of the same components change in a very narrow range, The calculated mass action concentrations of components in the NaCl-H,0 and NaCl-KCl-H2O systems are in good agreement with the reported activities. This indicates that the developed thermodynamic model can reflect the structural characteristics of solutions, and the mass action concentration also strictly follows the mass action law. (C) 2008 University of Science and Technology Beijing. All rights reserved.

Trade-offs between competitive ability and the other life-history traits are considered to be a major mechanism of competitive coexistence. Many theoretical studies have demonstrated the robustness of such a coexistence mechanism ecologically; however, it is unknown whether the coexistence is robust evolutionarily. Here, we report that evolution of life-history traits not directly related to competition, such as longevity, and predator avoidance, easily collapses competitive coexistence in several competition systems: spatially structured, and predator-mediated two-species competition systems. In addition, we found that a superior competitor can be excluded by an inferior one by common mechanisms among the models. Our results suggest that ecological competitive coexistence due to a life-history trait trade-off balance may not be balanced on an evolutionary timescale, that is, it may be evolutionarily fragile.

The calculating models of mass action concentrations for electrolyte aqueous solutions NaBr-H2O, LiNO3-H2O, HNO3-H2O, and KF-H2O have been developed at 298.15 K and their molalities ranging from 0.1 mol/kg to saturation according to the ion and molecule coexistence theory as well as mass action law. The calculated mass action concentration is based on pure species as the standard state and the mole fraction as the concentration unit, and the reported activities are usually based on infinite dilution as the standard state and molality as the concentration unit. Hence, the calculated mass action concentration must be transformed to the same standard state and concentration unit. The transformation coefficients between calculated mass action concentrations and reported activities of the same component fluctuate in a very narrow range. Thus, the transformed mass action concentrations not only agree well with reported activities, but also strictly obey mass action law. The calculated results show that the new developed models can embody the intrinsic structure of investigated four electrolyte aqueous solutions. The results also indicate that mass action law has its widespread applicability to electrolyte binary aqueous solutions.

The introduction and spread of non-native species is now recognized as one of the most significant threats to natural ecosystems world-wide. Land degradation due to such invasion has been responsible for the displacement of native species and for the destruction of the quality of wildlife habitats. The success or failure of such invasion depends upon a number of factors, perhaps two of the most important being the relative growth rates of the invasive and native species and their different ability to survive the competition for limited resources. For an invasive species with greater fecundity and rate-of-growth than the native species, all other things being equal, the invasive species will ultimately prevail driving the native species to extinction. If, on the other hand, the native species is characterized by greater survivability within habitats with limited resources, all other things being equal, the invasive species will not be able to penetrate the native habitat and will be driven to extinction where there may have been initial coexistence. The intermediate situation is therefore of interest where the difference or trade-off between growth and survival is not that extreme and the details of which may lead to either extinction or coexistence. To investigate the effect of such trade-off at an invasive wave-front, a cellular “ceiling model” of population growth below a fixed carrying capacity has examined the diffusive invasion of a native habitat. This is accomplished by modifying the “ceiling model” of population growth and limitation, to mirror differences in species survivability by a simple zero-sum partitioning of the demographic fixed carrying capacity. Calculations are performed for a single deme and for a 200 × 200 cellular grid of demes. For the cellular grid, the relative values of such trade-off are shown to yield either regions of species coexistence or extinction and this effect on the topological features of the wave-front is illustrated. This new model, motivated by paradigmatic considerations and consequently idealistic, addresses formal issues and may be considered a template for future investigations motivated by more pragmatic considerations.

We investigated compositional and demographical characteristics of major tree species in an old-growth subalpine forest in central Japan, based on a 25-year study. The stand is minimally disturbed and thus canopy structure is fairly stable. During the census, species dominance ranks in terms of both number and basal area were maintained. But, proportion of population size of major species within community significantly differed, and reflecting limited or no recruitment, declines in population size were found for all species including highly shade-tolerant Abies, suggesting demographic non-equilibrium in this forest community. This is because, according to each species’ regeneration attributes, disturbances fluctuating in space and time are required for persistence of their populations. So, competitive exclusion of other dominants by Abies spp., proposed as a mechanism of succession in several subalpine forests, seems unlikely. In addition to such non-equilibrium process, life-history differences among dominants, expressed as the equilibrium concept, are also greatly involved in species persistence because trade-offs of mortality and abundance/recruitment were observed between species. However, juvenile recruitment is closely related to canopy dynamics and disturbances, so that species persistence and resultant coexistence cannot be fully explained only by such differential demography. Here, it is specifically notable that not only current disturbance regimes but also ancient disturbance histories are both fundamentally of paramount importance for community maintenance and species coexistence. Populations and communities are fluctuating on the template shaped by both ancient and recent disturbances, and hereby, even in the late-successional forest, they are demographically non-equilibrium. Thus, further careful consideration is needed to evaluate complexity of the role and variability of natural disturbances in old-growth forest ecosystems.

The role that interspecific interactions play in shaping parasite communities is uncertain. To date, models of competition between helminth species have assumed that interaction occurs through parasite-induced host death. To our knowledge, there has been no theoretical exploration of other forms of competition. We examine models in which competition acts at the point of establishment within the host, and at the time of egg production by the adult worm. The models used are stochastic and we allow hosts to vary in their rate of exposure to infective larvae. We derive the Lotka-Volterra model of competition when exposure is homogenous and thus demonstrate that two helminth species cannot coexist on a single limiting resource. We show that coexistence of species is promoted by heterogeneity in host exposure provided that the rates of exposure to the two species are not perfectly correlated, and, if they are positively correlated, provided that the degree of heterogeneity in host exposure is similar for the two competing helminth species. These results are robust to the mechanism of competition.

Current threats of invasive species have significant implications for ecological systems. Given their potential impacts, invasive species have been the subject of extensive empirical and theoretical studies. However, these studies have tended to focus on species that produce highly visible ecological and economic impacts. In our study, we take a step back from focusing on these high-impact invasive species, and examine the general colonization (invasion) process of exotic species that have various “competitive abilities” against the native species. Using a two-species cellular automaton model, we demonstrate that: (1) a threshold level of competitive ability is required for the exotic species to successfully establish in a new landscape and (2) an exotic species with superior competitive ability does not necessarily become dominant in a landscape (alternatively, a species that has inferior competitive ability may successfully colonize a new system). Our findings have significant implications for the study of species invasions and also provide clues to how species assemble in ecological communities.

Most theoretical studies on character displacement and the coexistence of competing species have focused attention on the evolution of competitive traits driven by inter-specific competition. We investigated the evolution of the maturation rate which is not directly related to competition and trades off with the birth rate and how it influences competitive outcomes. Evolution may result in the superior competitor becoming extinct if, initially, the inferior competitor has a lower, and the superior one a higher, maturation rate at the coexistence equilibrium. This counterintuitive result is explained by an explosive increase in the adult population of the inferior competitor as a result of the more rapid evolution of its maturation rate, which is caused by differences in the intensity and direction of selection on the maturation rates of the two species and in their adult densities, which are related to differences in their life histories. Thus, a life history trait trade-off with a competitive trait may cause a competitive ecological coexistence to collapse.

Understanding coexistence is a central concern for community ecologists. The limited combination of parameter values over which coexistence can occur in most models of competing species suggests that coexistence may not be a general phenomenon, yet observations of communities demonstrate that coexistence of competitors is common. In this paper, I use a well-studied host–parasitoid system, California red scale and its parasitoids, to explore whether intraguild predation (IGP) is a likely coexistence mechanism for these species and to motivate an exploration of how the level of biological detail in theoretical models influences conclusions about the likelihood of coexistence via IGP. Although prior work has shown that parasitoid and host densities in the field follow the patterns predicted by simple IGP theory, in this detailed model parameterized with empirically derived values, coexistence is not predicted. Thus, in spite of the concordance between field observations and simple model predictions, the current model demonstrates that IGP is unlikely to be the sole coexistence mechanism for the parasitoids of California red scale. Furthermore, IGP is an equalizing coexistence mechanism, and these results demonstrate that adding biological detail to this type of mechanism does not substantially increase the predicted coexistence region. This important result adds to the growing literature showing that simple models often can capture the fundamental processes operating in more complex models and may be sufficient to capture underlying ecological patterns. Because models lacking system-specific details are better able to produce general conclusions, the current result suggests that general theory may be used to gain insights into a diversity of ecological systems.

Simple patch-occupancy models of competitive metacommunities have shown that coexistence is possible as long as there is a competition-colonization tradeoff such as that of superior competitors and dispersers. In this paper, we present a model of competition between three species in a dynamic landscape, where patches are being created and destroyed at a different rate. In our model, species interact according to a linear non-transitive hierarchy, such that species Y(3) outcompetes and can invade patches occupied by species Y(2) and this species in turn can outcompete and invade patches occupied by the inferior competitor Y(1). In this hierarchy, inferior competitors cannot invade patches of species with higher competitive ability. Analytical results show that there are regions in the parameter space where coexistence can occur, as well as regions where each of the species exists in isolation depending on species' life-history traits associated with their colonization abilities and extinction proneness as well as with the dynamics of habitat patches. In our model, the condition for coexistence depends explicitly on patch dynamics, which in turn modulate the limiting similarity for species coexistence. Coexistence in metacommunities inhabiting dynamic landscapes although possible is harder to attain than in static ones.