How small can the difference among competitors be for coexistence to occur

Closely related competitors comprising ofEscherichia coli strains having the same metabolic system and differing only with a few bases on the glutamine synthetase gene in the plasmid pKGN were previously shown to coexist in a chemostat. The differences among these closely related competitors can be considered large enough to allow coexistence as the level of enzyme activity is different. To bring the difference among competitors to the slightest possible, the mutation was introduced on the noncoding region of the plasmid pKGN harbored in the wild-type strain (strain W). The new strain, strain W’, carries the plasmid pKGN’ with a 4-base insertion at theHind III site in the polycloning site of pKGN. As the noncoding region is a nucleotide segment that is not translated into amino acids, the relatedness between strains W and W’ is the closest possible from the genetic point of view. Interestingly, though both strains are almost identical, they can coexist stably in a chemostat irrespective of the initial population size. These experimental results suggest that in the natural ecosystem, no matter how akin competitors are, coexistence is not impossible.     

The effects of intransitive competition on coexistence

Coexistence theory has been developed with an almost exclusive focus on interactions between two species, often ignoring more complex and indirect interactions, such as intransitive loops, that can emerge in competition networks. In fact, intransitive competition has typically been studied in isolation from other pairwise stabilising processes, and thus little is known about how intransitivity interacts with more traditional drivers of species coexistence such as niche partitioning. To integrate intransitivity into traditional coexistence theory, we developed a metric of growth rate when rare, , to identify and quantify the impact of intransitive competition against a backdrop of pairwise stabilising niche differences. Using this index with simulations of community dynamics, we demonstrate that intransitive loops can both stabilise or destabilise species coexistence, but the strength and importance of intransitive interactions are significantly affected by the length and the topology of these loops. We conclude by showing how can be used to evaluate effects of intransitivity in empirical studies. Our results emphasise the need to integrate complex mechanisms emerging from diverse interactions into our understanding of species coexistence.     

Heteromyopia and the spatial coexistence of similar competitors

Most spatial models of competing species assume symmetries in the spatial scales of dispersal and interactions. This makes analysis tractable, and has led to the conclusion that segregation of species in space does not promote coexistence. However, these symmetries leave parts of the parameter space uninvestigated. Using a moment‐approximation method, we present a spatial version of the Lotka–Volterra competition equations to investigate effects of removing symmetries in the distances over which individuals disperse and interact. Some spatial segregation of the species always comes about due to competition, and such segregation does not necessarily lead to coexistence. But, if interspecific competition occurs over shorter distances than intraspecific competition (heteromyopia), spatial segregation becomes strong enough to promote coexistence. Such coexistence is most likely when the species have similar dynamics, in contrast to the competition–colonization trade‐off that requires large competitive differences between species.     

Predator selectivity alters the effect of dispersal on coexistence among apparent competitors

While the majority of studies on dispersal effects on patterns of coexistence among species in a metacommunity have focused on resource competitors, dispersal in systems with predator–prey interactions may provide very different results. Here, we use an analytical model to study the effect of dispersal rates on coexistence of two prey species sharing a predator (apparent competition), when the traits of that predator vary. Specifically, we explore the range in immigration rates where apparent competitors are able to coexist, and how that range changes with predator selectivity and efficiency. We find that if the inferior apparent competitor has a higher probability of being consumed, it will require less immigration to invade and to exclude the superior prey as the predator becomes more opportunistic. However, if the inferior apparent competitor has a lower probability of being consumed (and lower growth rates), higher immigration is required for the inferior prey to invade and exclude the superior prey as the predator becomes more opportunistic. We further find that the largest range of immigration rates where prey coexist occurs when predator selectivity is intermediate (i.e. they do not show much bias towards consuming one species or the other). Increasing predator efficiency generally reduces the immigration rates necessary for the inferior apparent competitor to invade and exclude the superior apparent competitor, but also reduces the range of immigration rates where the two apparent competitors can coexist. However, when the superior apparent competitor has a higher probability of being consumed, increased predator efficiency can increase the range of parameters where the species can coexist. Our results are consistent with some of the variation observed in the effect of dispersal on prey species richness in empirical systems with top predators.     

Sex and space destabilize intransitive competition within and between species

Organisms ranging from bacteria and corals to plants and vertebrates can form intransitive competitive networks, in which coexistence can be maintained because no one species or genotype is superior to all others. However, in the simplest case with three competing types, the long-term outcome may not be so clear if two of the three represent the ends of a continuous heritable trait distribution within one species, as has been recently demonstrated empirically in a short-term experiment with plants. Using simulation models of this scenario, results with asexual reproduction confirm previous studies which showed that local interactions promote coexistence. However, with sexual reproduction, genetic variance is reduced because selection fluctuates between favouring the two extremes during population cycles, while sex continually produces intermediates. Sex thus slows the response to selection when it is the strongest and therefore slows the recovery from extreme abundances, creating larger abundance fluctuations. Local interactions do not stabilize dynamics with sex because the resultant spatial patches of one species are genetically heterogeneous, such that particular phenotypes do not benefit from spatial refuges. In sharp contrast to previous models suggesting that sex or local interactions stabilize population dynamics, here sex and local interactions destabilize dynamics and increase extinction risk.     

Patchiness, aggregation, and the coexistence of competitors for ephemeral resources

Abstract. 1. A laboratory experiment was conducted to determine the outcome of competition between two species of carrion-breeding Diptera for food at two levels of resource patchiness.
2. Adults of Fannia howardi Malloch (Muscidae) and Megaselia scalaris Loew (Phoridae) were released into a large population cage and were given access to three 2g sections of pork kidney in one treatment (low patchiness) and to twelve 0.5g sections in the other (high patchiness).
3. Increasing the patchiness of the resource resulted in reduced overlap in the distribution of the two species, and therefore less interspecific competition. However, this did not result in improved survival of the poorer competitor, F.howardi. Both species aggregated their eggs at high patchiness, and therefore suffered from increased levels of intraspecific competition.
4. The experiment shows that increasing patchiness in this way could lead to prolonged coexistence, as has been predicted in other studies.     

Hutchinson revisited: Patterns of density regulation and the coexistence of strong competitors

Ecologists have long been searching for mechanisms of species coexistence, particularly since G.E. Hutchinson raised the ‘paradox of the plankton’. A promising approach to solve this paradox and to explain the coexistence of many species with strong niche overlap is to consider over-compensatory density regulation with its ability to generate endogenous population fluctuations.Previous work has analysed the role of over-compensation in coexistence based on analytical approaches. Using a spatially explicit time-discrete simulation model, we systematically explore the dynamics and conditions for coexistence of two species. We go beyond the analytically accessible range of models by studying the whole range of density regulation from under- to very strong over-compensation and consider the impact of spatial structure and temporal disturbances. In particular, we investigate how coexistence can emerge in different types of population growth models.We show that two strong competitors are able to coexist if at least one species exhibits over-compensation. Analysing the time series of population dynamics reveals how the differential responses to density fluctuations of the two competitors lead to coexistence: The over-compensator generates density fluctuations but is the inferior competitor at strong amplitudes of those fluctuations; the competitor, therefore, becomes frequent and dampens the over-compensator's amplitudes, but it becomes inferior under dampened fluctuations.These species interactions cause a dynamic alternation of community states with long-term persistence of both species. We show that a variety of population growth models is able to reproduce this coexistence although the particular parameter ranges differ among the models. Spatial structure influences the probability of coexistence but coexistence is maintained for a broad range of dispersal parameters.The flexibility and robustness of coexistence through over-compensation emphasize the importance of nonlinear density dependence for species interactions, and they also highlight the potential of applying more flexible models than the classical Lotka-Volterra equations in community ecology.     

Positive interactions among competitors can produce species-rich communities

Although positive interactions between species are well documented, most ecological theory for investigating multispecies coexistence remains rooted in antagonistic interactions such as competition and predation. Standard resource-competition models from this theory predict that the number of coexisting species should not exceed the number of factors that limit population growth. Here I show that positive interactions among resource competitors can produce species-rich model communities supported by a single limiting resource. Simulations show that when resource competitors reduce each others' per capita mortality rate (e.g. by ameliorating an abiotic stress), stable multispecies coexistence with a single resource may be common, even while the net interspecific interaction remains negative. These results demonstrate that positive interactions may provide an important mechanism for generating species-rich communities in nature. They also show that focusing on the net interaction between species may conceal important coexistence mechanisms when species simultaneously engage in both antagonistic and positive interactions.     

Aggregation, intraguild interactions and the coexistence of competitors on small ephemeral patches

It is well established that intraspecific aggregation has the potential to promote coexistence in communities of species competing for patchy ephemeral resources. We developed a simulation model to explore the influence of aggregation on coexistence in such communities when an important assumption of previous studies – that interspecific interactions have only negative effects on the species involved – is relaxed. The model describes a community of competing insect larvae in which an interaction that is equivalent to intraguild predation (IGP) can occur, and is unusual in that it considers species exploiting very small resource patches (carrying capacity=1). Model simulations show that, in the absence of any intraspecific aggregation, variation between species in the way that resource heterogeneity affects survival increases the likelihood of species coexistence. Simulations also show that intraspecific aggregation of the dominant competitor's eggs across resource patches can promote coexistence by reducing the importance of interspecific competition relative to that of intraspecific competition. Crucially, however, this effect is altered if one competitor indulges in IGP. In general, coexistence is only possible when the species that is capable of IGP is less effective at exploiting the shared resource than its competitor. Because it reduces the relative importance of interspecific interactions, intraspecific aggregation of the eggs of a species that is the victim of IGP actually reduces the likelihood of coexistence in parts of parameter space in which the persistence of the other species is dependent on its ability to exploit its competitor. Since resource heterogeneity, intraspecific aggregation and IGP are all common phenomena, these findings shed light on mechanisms that are likely to influence diversity in communities exploiting patchy resources.     

Allelopathic adaptation can cause competitive coexistence

The maintenance of plant diversity is often explained by the ecological and evolutionary consequences of resource competition. Recently, the importance of allelopathy for competitive interactions has been recognized. In spite of such interest in allelopathy, we have few theories for understanding how the allelopathy influences the ecological and evolutionary dynamics of competing species. Here, I study the coevolutionary dynamics of two competing species with allelopathy in an interspecific competition system, and show that adaptive trait dynamics can cause cyclic coexistence. In addition, very fast adaptation such as phenotypic plasticity is likely to stabilize the population cycles. The results suggest that adaptive changes in allelopathy can lead to cyclic coexistence of plant species even when their ecological characters are very similar and interspecific competition is stronger than intraspecific competition, which should destroy competitive coexistence in the absence of adaptation.     

The effect of density-independent mortality on the coexistence of exploitative competitors for renewing resources

Many ecologists believe that higher mortality imposed on competing species increases the probability that they will coexist. This belief has persisted in spite of many theoretical counterarguments. However, few of those counterarguments have been based on models having explicit representation of the resources for which competition is occurring. This article analyzes a series of consumer-resource models of competition for nutritionally substitutable renewable resources and determines the range of relative resource requirements that allow coexistence. In most cases, if consumers are initially efficient at reducing resource densities, increasing density-independent mortality widens the range of resource requirements of the consumers that allow coexistence, provided the increase in mortality is not too great. The coexistence-promoting effects of mortality occur because a very efficient consumer species usually reduces the diversity of the set of resources it consumes. This lessens the extent to which resource utilization differences between consumer species can be expressed. Mortality, in this case, increases the diversity of resource types, widening the conditions for coexistence. However, sufficiently high mortality will usually reduce the range of parameters allowing coexistence, in agreement with much previous theory. The results presented here also predict maximal diversity at intermediate levels of productivity. Previous empirical studies and theory are reviewed in light of the theory developed here.     

Enigmatic adult overwintering in damselflies: coexistence as weaker intraguild competitors due to niche separation in time

Odonata, like most freshwater invertebrates, tend to overwinter in water due to the thermal properties of a water environment. Winter damselflies (genus Sympecma), however, hibernate as adults in terrestrial habitats. The strategy of adult overwintering combined with high mortality is associated with several unique adaptations to semiarid conditions, but winter damselflies maintain this unique life history throughout almost the entire Palaearctic. We assume that the unique strategy of adult overwintering in temperate zones is indirectly maintained by niche separation in time. We used phenological data from the Czech Republic to compare the seasonal phenology of Sympecma spp. with other coexisting odonate species. Seasonal population growth patterns between S. fusca and other coexisting species representing different life histories were compared using GLMMs and LME. The models showed negative non-linear dependence between the population growth of S. fusca and the estimated abundance of compared species. We found that the specific strategy of adult overwintering makes it possible to avoid seasonal maxima of competition and predation in adult and larval stages. Adults may benefit from free niches during spring while larvae may benefit from size advantage among intraguild competitors and optimal conditions for development.     

Evolutionary rescue and the coexistence of generalist and specialist competitors: an experimental test

Competition for resources is thought to play a critical role in both the origins and maintenance of biodiversity. Although numerous laboratory evolution experiments have confirmed that competition can be a key driver of adaptive diversification, few have demonstrated its role in the maintenance of the resulting diversity. We investigate the conditions that favour the origin and maintenance of alternative generalist and specialist resource-use phenotypes within the same population. Previously, we confirmed that competition for hosts among φ6 bacteriophage in a mixed novel (non-permissive) and ancestral (permissive) host microcosm triggered the evolution of a generalist phenotype capable of infecting both hosts. However, because the newly evolved generalists tended to competitively exclude the ancestral specialists, coexistence between the two phenotypes was rare. Here, we show that reducing the relative abundance of the novel host slowed the increase in frequency of the generalist phenotype, allowing sufficient time for the specialist to further adapt to the ancestral host. This adaptation resulted in ‘evolutionary rescue’ of the specialists, preventing their competitive exclusion by the generalists. Thus, our results suggest that competition promotes both the origin and maintenance of biodiversity when it is strong enough to favour a novel resource-use phenotype, but weak enough to allow adaptation of both the novel and ancestral phenotypes to their respective niches.     

Impossibility of coexistence of three pure and simple competitors in configurations of three interconnected chemostats

It is well established that pure and simple microbial competitors cannot coexist at a steady state if their environment is homogeneous. For the case of two microbial populations competing purely and simply in two interconnected chemostats having time-invariant input(s), it is known from the literature that a stable steady state of coexistence arises in domains of the operating parameters space and is attributed to the spatial heterogeneities of the system, which allow a different species to have the competitive advantage in each one of the two sub-environments. This article investigates whether the aforementioned result can be extended to the case of three species competing in three interconnected vessels. By studying all possible alternate configurations of the three-chemostat system, it is shown that coexistence of the three species is impossible, except possibly for some discrete values of the operating parameters. Some potential explanations for the results are discussed.     

Mutualism can mediate competition and promote coexistence

Mutualistic interactions are not believed to promote coexistence of competitors because mutualisms produce positive feedbacks on abundances whereas coexistence requires negative feedbacks. Here we show that a mutualism between an anemonefish (Amphiprion) and its sea anemone host mediates the effect of asymmetrical competition for space between the anemonefish and another damselfish (Dascyllus) in a manner that fosters their coexistence. Amphiprion stimulates increases in host area, the shared resource, but social interactions cap the number of anemonefish to two adults per host. Space generated by the mutualism becomes differentially available to Dascyllus because the effectiveness of an anemonefish in excluding its competitor declines with increases in the area it defends. This alters Amphiprion's ratio of per capita intra‐ to interspecific effects and thus facilitates coexistence of the fishes. This mechanism may be prevalent in nature, adding another major pathway by which mutualism can enhance diversity.