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Competition for resources can lead to species exclusion. However, this exclusion may be avoided if species show differential adaptation to physical environment. Empirical studies on competition are difficult when species are phylogenetically close and have complex life cycles. This is the case of B. plicatilis and B. manjavacas, two cryptic rotifer species differing in their salinity niches and in life-history traits related to sex and diapause. These differences have been suggested to promote the stable co-occurrence observed in natural populations of these species. However, in a previous empirical study, the outcome of competition between both species was always exclusion. Here, we theoreticallyexplored the effect of complex life-history traits and salinity fluctuations on the long-term competitive outcome of B. plicatilis and B. manjavacas. We developed a model and simulated ecological scenarios combining different growing period lengths, levels of crossed induction of sex between species and salinity regimes. Results show that a fluctuating salinity regime, an intermediate length of growing season and a low level of crossed induction of sex are essent ial conditionsto take into account to explain coexistence.

Two species of the genus Helix are widespread in Georgia: H. lucorum has a Mediterranean distribution whereas H. buchii is a Caucasian endemic typically associated with broadleafed forests. In spite of their sympatry within Georgia, they are never syntopic. Furthermore, in contrast with H. buchii, H. lucorum is mainly found in areas subject to human disturbance. Another large helicoid species, Caucasotachea calligera, is widespread in Georgia and usually co-occurs with either Helix. The distribution patterns of these species suggest that interspecific competition might play an important role in shaping the distribution of the two Helix species. In order to see whether their ecological niches were different enough to provide such a distribution pattern, I used predictive ecological niche models (ENM) based on the Maximum Entropy algorithm. ENMs showed that the niches of these species in Georgia were significantly different but not fully separated (� 15–36% overlap). The distributional pattern of H. lucorum should not be considered truly natural in Georgia and may be anthropogenic. The fact that the two Helix species never co-occur may result from factors other than ecological niche differentiation at any macro scale. Since competition remains the most useful and informative assumption to explain the distributional pattern of these congeneric species, microhabitat requirements also need to be tested as a potential driver.

The concept of a varying evolutionary tempo that is regulated by energy was first suggested in the 1950s. It was based on the observation that energy-rich habitats appear to be the centres of evolutionary change, producing novel characters more frequently and having greater speciation rates. Subsequently, a number of studies have found positive relationships between evolutionary rate and energy. Gradients of energy occur across a range of scales and these have been invoked to explain, for example, higher tropical species richness. Precipitation has also been found to influence evolutionary rate, suggesting that biologically available energy and productivity are the important variables in the relationship rather than solar energy alone. Here, we take the theoretical step of investigating at smaller scales the implications of faster evolutionary tempo where productivity is greater: first at the level of the species population and, subsequently, for the same-guild co-habitants in a community. To facilitate, this we begin by applying the concept of gradients of available energy, from more productive to less productive sectors within the species niche, in the context of source-sink theory. We then propose that a species population will have its highest rate of evolution, for changes that confer a positive selection coefficient throughout the niche, in that sector where it has best adaptive fit and greatest per capita energy flux. We also posit that, where it conferred a shift in fitness, an evolutionary change at niche optimum could subsequently affect conspecific populations occupying lower energy niche sectors via the selection-mediated dispersal of the apomorphy into the more marginal components of the niche. We then infer that this type of change in a species with a generally higher per capita energy flux might negatively affect adjacent more slowly evolving species (living in less productive peripheral niches) in situations where the increased fitness conferred by a particular apomorphy was relevant to conditions occurring beyond the limits of the progenitor niche hypervolume. We therefore suggest a directional component to evolution at the compressed scales of niche and community level microevolution whereby populations and species occupying more productive conditions have greater tempos of change with attendant enhancements of both their competitive influence and their evolutionary potential. In this manner, previously recorded macro-evolutionary patterns indicating faster evolution with increased energy at larger scales are interpreted in the context of proposed micro-evolutionary relationships at intraspecific and locally interspecific scales. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110, 696–714.

Aims The process of facilitation, where a species increases the survival, growth, and fitness of another species, is becoming increasingly recognized as a critical factor in shaping the structure of plant communities. This process is particularly important in stressful environments. Yet few studies have attempted to incorporate positive interactions into community ecological theories such as the neutral theory of biodiversity. Here, we use an equalizing trade-off model as a foundation to study the potential impact of facilitation on species richness and community temporal turnover. Methods Based on a spatially explicit birth–death trade-off model, we assume that the occurrence of facilitation is dependent on the presence of interspecific neighbours. We further propose that the realized birth rate for a given individual subject to facilitation is proportional to the number of interspecific neighbours within its neighbourhood. Thus, in our model, the individuals of rare species will benefit more from the existence of heterospecific individuals than common species. Important Findings As the facilitative coefficient increased, the species richness for simulated communities at the dynamically stochastic equilibrium was also increasing. Simulations also demonstrated that facilitation could increase the replacement of species through time: communities with facilitation become more dissimilar (i.e. have smaller Bray–Curtis similarity values) than communities without or with a lower degree of facilitation after the same time interval. Facilitation from interspecific neighbours on rare species increased their population sizes and consequently made them less prone to extinction, thus enhancing species richness. Meanwhile, in a saturated community, with the increase of species richness, mean population size of entire communities decreased, making species more prone to extinction on average, and thus increased the community temporal turnover. Our results suggest that future experimental work on the effect of facilitation on community-level properties should focus not just on species richness but also on other descriptors of community dynamics such as the temporal species turnover.

Aims Much recent theory has focused on the role of neutral processes in assembling communities, but the basic assumption that all species are demographically identical has found little empirical support. Here, we show that the framework of the current neutral theory can easily be generalized to incorporate species differences so long as fitness equivalence among individuals is maintained through trade-offs between birth and death. Methods Our theory development is based on a careful reformulation of the Moran model of metacommunity dynamics in terms of a non-linear one-step stochastic process,which is describedbyamaster equation. Important Findings We demonstrate how fitness equalization through demographic trade-offs can generate significant macroecological diversity patterns,leadingtoaverydifferentinterpretation oftherelationbetween Fisher’s a and Hubbell’s fundamental biodiversity number. Our model shows that equal fitness (not equal demographics) significantly promotes species diversity through strong selective sieving of community membership against high-mortality species, resulting in a positive association between species abundance and per capita death rate. An important implication of demographic trade-off is that it can partly explain the excessively high speciation rates predicted by the neutral theory of the stronger symmetry. Fitness equalization through demographic trade-offs generalizes neutral theory by considering heterospecific demographic difference, thus representing a significant step toward integrating the neutral and niche paradigms of biodiversity.

Aims According to conventional theory, larger plant species are likely to inflict more intense competition on other (smaller) species. We tested a deducible prediction from this: that a larger species should generally be expected to impose greater limits on the number of species that can coexist with it. Methods Species richness was sampled under plant canopies for a selection of woody species (‘host’ species) that display a wide range of adult sizes (from small shrubs to large trees), growing within natural vegetation of the Interior Douglas-fir zone of southern British Columbia, Canada. These data were compared with species richness levels sampled within randomly placed plots within the host species habitat. Important Findings A prominent host species size effect on species richness was detected but only narrowly at the small end of the species size range. Across most (90%) of the increasing size range of host species, the number of species residing under the host canopy showed no significant decrease relative to the number expected by random assembly, based on species richness within randomly defined equivalent areas within the habitat of the host species. This apparent ‘null effect’, we suggest, is explained not because these larger species have no effect on community assembly. We postulate that larger species are indeed likely to be more effective in causing competitive exclusion of some smaller species (as expected from conventional theory), but that any potential limitation effect of this on resident species richness is offset for two reasons: (i) larger species also generate niche spaces that they cannot exploit under their own canopies and so have minimal impact (as competitors) on smaller species that can occupy these niches and (ii) certain other small species—despite small size—have effective competitive abilities under the severe competition that occurs within host neighbourhoods of larger species. These and other recent studies call for re-evaluation of traditional views on the role of plant size in affecting competitive ability and community assembly.

Large-scale ecological patterns emerge from multiple biological processes that interact on several scales and are affected by the heterogeneity of the environment. I use a landscape simulation model to explore and analyze how interactions among various ecological processes affect patterns of mammalian species diversity according to body size. The model incorporates ecological realism by using mammalian allometric relationships with realistic values taken from field data. It uses energy as a common currency to model and compare how species of different sizes use their environment. Components of the model include landscape, habitat, patch, community, species, and population. Processes include continuous-time population growth, saturation effect, species-habitat matching, demographic stochasticity (local-scale), dispersal, and catastrophic stochasticity (landscape-scale). I simulated a 16-patch landscape, with each patch having a unique habitat, and a 10-species pool in which species differ only in body size. Summed over the landscape, a log-normal relationship between body size and species abundance emerges. Additionally, geographic range increases nonlinearly with the log of body size. These patterns are qualitatively consistent with patterns of mammalian species diversity observed in natural systems, suggesting that they can emerge from scaling of habitat use with body size. I also simulated a simple 4-patch landscape, with each patch having a unique habitat, and a 26-species pool to draw from in which species differ only in body size. The results show that interspecific competition by itself reduces species diversity in each habitat and in the entire landscape. Stochasticity depresses mean population sizes but opens opportunities for species to avoid competitive exclusion. It also allows different habitats to have different communities that are characterized by the consequences of which large species become locally extinct at random. Demographic and catastrophic stochasticities differ, however, in their characteristic effects on species diversity. These characteristic effects can be used as fingerprints to identify processes acting in nature. Dispersing individuals move between habitats and reestablish the local populations of their species. Overall, the model provides us with predictions of how process-interactions in heterogeneous landscape may affect species composition and community structure.

Many of our concepts about species coexistence are rooted firmly in the analysis of competitor isoclines whose slopes estimate the average magnitude of competition. Realistically, competition will vary among habitats, and habitat selection will be a major contributor to coexistence. Competition will vanish to zero (the ghost of competition) when species occupy completely separate habitats, even though the potential for competition remains high. Competitive potential can be estimated from slopes of absolute isolegs that define boundaries between specialized use of a single habitat and the joint occupation of one or more additional habitats. Interpretations of current theories suggest, however, that we may seldom be able to plot isolegs because they represent a wall of competition that species seldom cross. If so, isoclines bend sharply at the isoleg, and population dynamics are restricted to the ghost region. But when competition is resolved by habitat selection, isoclines bend gradually, and the wall of competition disappears. The isolegs become visible through analysis of habitat isodars, lines that represent the set of each species' density such that expected fitness is equivalent in each occupied habitat. Preliminary analyses of rodent isodars agree with theory, reveal the ghost, and confirm a central role for density-dependent habitat selection in competitor coexistence.

Metapopulation diversity patterns depend on the relations among the timescales of local biological interactions (predation, competition), the rates of dispersal among local populations and the patterns of disturbance. We investigate these relationships using a family of simple non-linear Markov chain models. We consider three models for interspecific competition; if the species are identified with early and late successional species, the models describe the facilitation, inhibition and tolerance models of ecological succession. By adding a third competing species we also compare transitive competitive hierarchies and intransitive competitive networks. Finally, we examine the effects of predation in mediating coexistence among competing prey species. In each model we find circumstances in which biotic or abiotic disturbance can increase both local and regional diversity, but those circumstances depend on the various timescales in the model in ways that arc neither obvious nor trivial.

The assumptions and predictions of metapopulation models for competing species are discussed in relation to empirical studies of colonization and extinction in metapopulations. In three species of Daphnia in rockpools, interspecific competition increased local extinction rates, while no effects on colonization rates were detected. Distributional patterns were consistent with several predictions of the competition model; for example, the number of species on an island increased with the number of pools and the proportion of pools occupied by each species decreased with increasing species number. It is concluded that interspecific competition is important for the distributional dynamics of Daphnia species in rockpools, but the question whether the coexistence of these species depends on metapopulation dynamics is still unresolved. Other studies on the effects of interspecific competition on colonization and extinction rates are discussed.

An analysis of pattern can reveal a great deal about the processes that regulate the species structure of a community, especially in those cases where experiments are not possible. It is predicted that differing mechanisms of regulation should result in different patterns in the distribution and abundance of potentially competing species and in the ways in which they utilize limiting resources. In this paper we describe these patterns in the zooplankton community of the North Pacific central gyre. The gyre community is old, stable and diverse and past studies have suggested that it is regulated by in situ processes. An analysis of samples collected over many years shows that the species are close to equilibrium. There are not large spatial, seasonal or year to year changes in abundance and disturbances do not seem to be large enough to perturb the species structure and prevent competitive exclusion. Food and predation are the two main limiting factors. Competition, by inference, appears to be strong, even between relatively dissimilar species. In spite of this the species do not seem to be specialists and niche separation is not apparent in either space or food utilization. This is surprising considering the age, stability and high diversity of the system. It is concluded that either some other mechanism, such as frequency dependent predation, regulates the species structure or that the mechanisms of niche separation are much more subtle than is suspected. Some of the implications of regulation through predation are discussed.

Four sympatric species of Fundulus (F. heteroclitus, F. majalis, F. diaphanus , and F. luciae ) are distributed along tidal height and salinity gradients such that F. heteroclitus co-occurs with each species, while the remaining species rarely occur together. Feeding habitats of all the species are similar, and food has been shown to limit population size of F. heteroclitus . This paper examines whether competition is an important structuring force within this guild by addressing two questions: 1) is the spatial separation exhibited by three of the species due to physiological barriers or due to competitive exlusion? and 2) when F. heteroclitus occurs with other Fundulus species does competition for resources take place? Laboratory studies indicate that all four species are tolerant of a wide range of environmental conditions; available evidence suggests that physiological barriers are an unlikely explanation for spatial segregation among some members of this guild. Competitive exclusion seems a likely alternate explanation, but is supported only by studies of diet overlap. Similarly, only inference from diet overlap is available to answer the second question. A field experiment is presented here in which F. heteroclitus and F. majalis were placed in enclosures separately and together. Competition between these species appears to be important, and similar field experiments are recommended to investigate competitive interactions among other species pairs within the genus

The idea that biological interactions between species restrict their distributions beyond limits set by the inorganic environment was established by plant ecologists well before the work of Volterra, Gause, and the development of modern niche theory. Mechanisms of competition between plant species, and the influence of predation and environmental factors on the outcome of interspecific competition, were demonstrated by extensive field experiments. These early botanical achievements were ignored by zoologists whose subsequent, independently derived ideas about competition paralleled those of plant ecologists to a remarkable degree. Application of niche theory to the real world was closely tied to the advent of the “modern synthesis.” The primary innovation of niche theory was not simply a new appreciation of the possible importance of competition between animals, but the incorporation of an evolutionary perspective to distributional problems largely absent among ecologists since Darwin.

synopsis. Ectothermic vertebrates respond to the temperature of their habitat in a manner that is remarkably similar to their response to more traditional ecological resources such as food. We review the response to temperature primarily from literature on fishes in terms of ecological concepts related to niche theory and competition. The width of the fundamental thermal niche is about 4?C when measured by a mean plus and minus one standard deviation of the distribution of temperature occupied in a laboratory gradient. Fish of temperate freshwater appear to fail into three thermal guilds along the temperature re? source axis ?cold, cool, and warm water fishes. Realized thermal niches are similar in central tendency to fundamental niches, but niche width appears to be more narrow for the realized niche in limited sample data. The success of interference competition for space with preferred temperature is tied to social dominance in a manner analogous to food competition. Thermal niche shifts in the face of interspecific competition for preferred temperature appear supported by one laboratory study. Exploitation competition in re? spect to temperature seems nebulous. If animals successfully compete for their thermal niche, growth and perhaps other measures of fitness are maximized. Cost/benefit models for thermal resources and food resources lead to similar predictions about resource use. We suggest that viewing temperature and other niche axes in the way ecologists have viewed food resources would be useful.

Invasion-structured communities have more species than do coevolution-structured communities assembled using the same resource distribution. Species in invasion-structured communities are tightly packed, occupying the upper portion of the resource axis; species in coevolution-structured communities are more widely spaced, and most are located in the lower portion of the resource axis. As a consequence, coevolution-structured communities tend to be more stable than comparable invasion-structured communities, but more open to invasion. Both invasion-structured and coevolution-structured communities have niche separations that are significantly different than would be expected if species were assorted at random. Two-species communities formed by the invasion-only algorithm under asymmetric competition had the majority of their niche separations in the range 0-0.5. All other communities had niche separations that were greater than expected. The most common separations were in the range 1.0-3.5. Thus, while not a common feature of many communities, nicheseparation patterns similar to those described by Hutchinson (1959) appear as an "ensemble" property of many communities. The faunal-buildup graphs formed by the coevolutionary algorithm differ from those formed by the invasion-only algorithm, showing community cycling whenever asymmetric competition is present. Through this cycling behavior the coevolutionary faunal-buildup algorithm provides both a theoretical basis for Wilson's (1959) taxon cycle and a hypothesis explaining the distribution of Anolis lizards in the Lesser Antilles.