Mesoscale vortices and the paradox of the plankton

Coexistence of competitive species is severely limited by the availability of resources and the characteristics of the environment. In particular, the so-called 'competitive exclusion principle' states that, at equilibrium, the number of coexisting species cannot be larger than the number of resources for which they compete. However, many in situ observations have revealed prolonged coexistence of a large number of competitive plankton species, a phenomenon known as 'the paradox of the plankton'. Here we investigate this problem and show that ocean mesoscale vortices generate transport barriers and incomplete horizontal mixing, allowing for a prolonged survival of the less-fit species, even for fully homogeneous resource distributions. In such a situation, the temporarily less-fit plankton species are protected from competition by the action of the vortices.     

Can environmental fluctuation prevent competitive exclusion in sympatric flycatchers?

Ecology has been characterized by a central controversy for decades: namely, whether the distribution and abundance of organisms are determined by species interactions, such as competitive exclusion, or by environmental conditions. In part, this is because competitive exclusion has not been convincingly demonstrated in open, natural systems. In addition, traditional theoretical models cannot predict the outcome of competitive interactions in the presence of environmental variability. In this paper we document the limiting influence of strong interspecific competition on population dynamics and nestling mortality in a mixed population of pied flycatchers (Ficedula hypoleuca) and collared flycatchers (F. albicollis in a narrow zone of sympatry. Whereas the former species was limited mainly by interspecific competition, the latter species was limited by the concerted influences of intraspecific competition and climate. The analysis suggests a march towards competitive exclusion of the pied flycatcher during warm periods. However, competitive exclusion is apparently prohibited on a local scale because intraspecific competition among individual collared flycatchers intensifies when they are forced to cope with severe environmental conditions, promoting the temporary and local presence of pied flycatchers.     

Coexistence introducing regulation of environmental conditions

Interactions between environmental conditions and environment-affecting species have not been investigated extensively. In this study, the population dynamics of species yielding regulative feedback between temperature (a representative of environmental condition) and species with a temperature-altering trait was examined. We considered a simple closed model that described the population of two species (at least one of them had a temperature-altering trait) competing for one resource. The long-term outcomes of the competition and changes of temperature were explored against increasing background temperature. As a result of simulations, the regulation of temperature was accompanied by the coexistence of two species, which was contrary to the 'Gause's exclusion principle'. The steady-state analysis showed that (i) the temperature-altering trait allowed species to coexist and (ii) the coexistence of species with the trait could introduce the regulation of temperature. A 'trade-off' in their ability to utilize a resource plays a key role in this coexistence and homeostasis of temperature. This may imply that actual environmental conditions can be automatically stabilized by resource competition among species in natural ecosystems.     

Analysis of species coexistence co-mediated by resource competition and reproductive interference

Reproductive interference is any interspecific sexual interaction that reduces the reproductive success of females through promiscuous reproductive activities of heterospecific individuals. This phenomenon is ubiquitous in nature in both plants and animals, and is frequently observed in biological invasions. However, its effects on interspecific competition remain incompletely understood despite growing concern. To study the interactive effects of resource competition and reproductive interference on species coexistence and exclusion, we analyzed a unified competition model including both processes in symmetric and asymmetric scenarios. The results of our model showed that resource competition and reproductive interference act synergistically to promote competitive exclusion. We also found that when the two processes are asymmetric, the species that is superior in reproductive interference can coexist with or even exclude the species that is superior in resource competition. Therefore, coexistence is possible via an unbalanced trade-off between resource use and reproduction. Our results suggest that integration of reproductive interference and resource competition will contribute to a better understanding of interspecific competition and to more effective biodiversity conservation against management of biological invasions.     

Interspecific competition in small rodents: from populations to individuals

The role of interspecific competition in shaping animal and plant communities has formed one of the major issues in ecology for decades. Small mammals, mainly rodents, have been among the model systems used for research on interspecific competition. Most studies within small mammal systems in the past have examined effects of competition on population attributes such as on population size, habitat use, or population dynamics. Population-level responses are the cumulative effects of individual responses, however, the influence of competition on individual life-history traits has rarely been studied. Research on life-histories may bridge gaps between population biology and effects of competition on individual behaviour. In this paper, we review recent research approaches to interspecific competition in rodents based on census data and species assemblages, that use regression analysis, time series analysis, removal and exclusion experiments, and showcase our own experimental research on the effects of interspecific competition on individual life-history traits in boreal voles.     

Coexistence of protected avian predators: does a recovering population of White-tailed Eagle threaten to exclude other avian predators?

The processes of competition and predation determine the degree to which species can coexist; the importance of competition in particular has been emphasized at high trophic levels. Competition exclusion will occur when habitat overlap between sympatric species is high. In this study, we investigated nesting habitat overlap between internationally protected diurnal tree-nesting avian predators of central Europe, namely, White-tailed Eagle (Haliaeetus albicilla), Lesser Spotted Eagle (Aquila pomarina), Black Stork (Ciconia nigra), and Osprey (Pandion haliaetus). We found significantly different nesting habitats among the study species and suggest that this could be a consequence of the resource-based segregation, but not a consequence of asymmetrical interspecific competition. The results also show that habitat of the recovering populations of White-tailed Eagle overlapped with the habitat used by the Lesser Spotted Eagle, Black Stork, and Osprey to varying extents with a niche overlap values being below the competition exclusion threshold. Nevertheless, we suggest that competition by White-tailed Eagle at a population level may limit Osprey, though not Lesser Spotted Eagle or Black Stork.     

Species matter: the role of competition in the assembly of congeneric bacteria

Interspecific competition is an important driver of community assembly in plants and animals, but phylogenetic evidence for interspecific competition in bacterial communities has been elusive. This could indicate that other processes such as habitat filtering or neutral processes are more important in bacterial community assembly. Alternatively, this could be a consequence of the lack of a consistent and meaningful species definition in bacteria. We hypothesize that competition in bacterial community assembly has gone undetected at least partly because overly broad measures of bacterial diversity units were used in previous studies. First, we tested our hypothesis in a simulation where we showed that how species are defined can dramatically affect whether phylogenetic overdispersion (a signal consistent with competitive exclusion) will be detected. Second, we demonstrated that using finer-scale Operational Taxonomic Units (OTUs) (with more stringent 16S rRNA sequence identity cutoffs or based on fast-evolving protein coding genes) in natural populations revealed previously undetected overdispersion. Finally, we argue that bacterial ecotypes, diversity units incorporating ecological and evolutionary theory, are superior to OTUs for the purpose of studying community assembly.     

Effects of competition on lifetime estimates of inbreeding depression in the outcrossing plant Crepis sancta (Asteraceae)

Inbreeding depression was studied in two populations of a Mediterranean allogamous colonizing species Crepis sancta. In order to test the hypothesis that the magnitude of inbreeding depression can be modified by successional processes, the growth and survival of individuals resulting from two generations of inbred crosses including selfing were analysed with interspecific competition (in natural vegetation) and without interspecific competition (by removing natural vegetation). Inbreeding depression was weak for seed production. Germination was little affected by inbreeding but mortality and the number of capitula showed inbreeding depression, especially in the presence of competition. This suggests that inbreeding depression is very sensitive to variations in environmental conditions such as interspecific competition. As a consequence, inbreeding depression cannot be considered as constant in natural conditions.     

Heterogeneous interspecific interactions in a host-parasite system

Macroparasites of vertebrates usually occur in multi-species communities, producing infections whose outcome in individual hosts or host populations may depend on the dynamics of interactions amongst the different component species. Within a single co-infection, competition can occur between conspecific and heterospecific parasite individuals, either directly or via the host's physiological and immune responses. We studied a natural single-host, multi-parasite model infection system (polystomes in the anuran Xenopus laevis victorianus) in which the parasite species show total interspecific competitive exclusion as adults in host individuals. Multi-species infection experiments indicated that competitive outcomes were dependent on infection species composition and strongly influenced by the intraspecific genetic identity of the interacting organisms. Our results also demonstrate the special importance of temporal heterogeneity (the sequence of infection by different species) in competition and co-existence between parasite species and predict that developmental plasticity in inferior competitors, and the induction of species-specific host resistance, will partition the within-host-individual habitat over time. We emphasise that such local (within-host) context-dependent processes are likely to be a fundamental determinant of population dynamics in multi-species parasite assemblages.     

Competition theory,evolution, and the concept of an ecological niche

This article examines some of the main tenets of competition theory in light of the theory of evolution and the concept of an ecological niche. The principle of competitive exclusion and the related assumption that communities exist at competitive equilibrium - fundamental parts of many competition theories and models - may be violated if non-equilibrium conditions exist in natural communities or are incorporated into competition models. Furthermore, these two basic tenets of competition theory are not compatible with the theory of evolution. Variation in ecologically significant environmental factors and non-equilibrium in population numbers should occur in most natural communities, and such changes have important effects on community relations, niche overlap, and the evolution of ecosystems. Ecologists should view competition as a process occurring within a complexdynamic system, and should be wary of theoretical positions built upon simple laboratory experiments or simplistic mathematical models.In considering the relationship between niche overlap and competition, niche overlap should not be taken as a sufficient condition for competition; many factors may prevent or diminish competition between populations with similar resource utilization patterns. The typically opposing forces of intraspecific and interspecific competition need to be simultaneously considered, for it is the balance between them that in large part determines niche boundaries.     

Interspecific pollen transfer and competition between co-occurring plant species

Summary Computer simulations of a pollinator foraging in a mixture of two species were used to explore how plant reproduction can be influenced by interspecific pollination movements. Interspecific pollen transfer led to strong competitive effects when availabilities of pollen, receptive stigma surfaces, or pollinator movements were limited relative to the total number of fertilizations possible in the mixed population. Results from simulations suggest that competition for pollination through interspecific pollen transfer can result in rapid exclusion of one of two species, and that such competition represents a selective force promoting stable divergence of potential competitors in habitat affinity, flowering time, or other characteristics related to pollinator sharing.     

Evidence for competition between mudsnails (Hydrodiidae): a field experiment

The paper describes an experimental investigation of competition between Hydrobia ulvae and H. ventrosa using enclosed populations at a site at which the species coexist naturally. lntraspecific competition is more intense than interspecific competition and may have a regulatory influence on snail densities. Other experimental studies of competition between mudsnails are reviewed and found to infer strongly the occurrence of competition in natural field populations. However, features of the species' field distributions previously attributed to interspecific competition can be more convincingly explained by other processes.     

Effect of Interspecific Competition and Herbivory on the Recruitment of an Invasive Alien Plant: Conyza sumatrensis

Most studies on the spread of a species have been carried out retrospectively. Conyza sumatrensis provides an exception; it has only been in Great Britain since 1984 and thereby provides the opportunity to follow an invasion from its early stages. Many different factors could lead to the exclusion of invading species from new habitats. Here we report results of a field experiment to determine the recruitment ability and population dynamics of C. sumatrensis within treatments of different levels of interspecific competition and herbivory over three years. Interspecific competition was manipulated using three cultivation treatments giving low, medium and high intensity competition. Factorial combinations of mollusc, insect and vertebrate (rabbit) herbivory were achieved using both chemical and physical exclusion techniques. We show that although C. sumatrensis recruitment occurs in areas where disturbance has caused low interspecific competition, maintenance of a population during secondary succession is unlikely. Effects of herbivory were dependent on the type of herbivory and the plant stage grazed. Rabbit grazing had strong negative effects on recruitment and survival of C. sumatrensis throughout the experiment. Insect herbivory, however, had no effect on any life stage. Mollusc herbivory showed a significant interaction with interspecific competitors, reducing recruitment only where competition was experimentally reduced. This revised version was published online in July 2006 with corrections to the Cover Date.     

Active dispersal is differentially affected by inter‐ and intraspecific competition in closely related nematode species

Competition is one of the main drivers of dispersal, which can be an important mechanism to achieve permanent or temporal coexistence of multiple species. This coexistence can be achieved by a dispersal‐competition tradeoff, spatial store effects or neutral dynamics. Here we test the effect of inter‐ and intraspecific competition on dispersal of four species of the marine nematode species complex Litoditis marina. A previous study in closed microcosms without a possibility for dispersal had demonstrated pronounced interspecific competition, leading to the exclusion of one species. We now investigated whether 1) the dispersal is affected by interspecific interactions, by intraspecific competition (density) or by food availability, 2) the dispersal dynamics influence assemblage composition and can lead to co‐occurrence of the species, and 3) the abiotic environment (here salinity) can affect these dynamics. We show that density is the main driver for dispersal in two of the four species. Dispersal of a third species always started at the same time irrespective of density, whereas in the fourth species interspecific interactions accelerated dispersal. Remarkably, this fourth species was not a strong competitor, suggesting that a dispersal–competition tradeoff does not explain the observed coexistence. Salinity did not alter the timing of dispersal when interspecific interactions were present but did affect assemblage composition. Consequently, spatial store effects may influence coexistence. All four species co‐occurred in fairly stable abundances throughout the present experiment indicating the importance of species specific dispersal strategies for coexistence. Co‐occurrence can be facilitated because competition is postponed or avoided by dispersal. Neutral dynamics also played a role as intra‐ and interspecific competition were of similar importance in three of the four species. We conclude that dispersal is a driver of the coexistence of closely related nematode species, and that population density and interspecific interactions shape these dynamics.     

Apparent competition through facilitation between Melanoides tuberculata and Biomphalaria glabrata and the control of schistosomiasis

Interactions between two species that result in reduced growth rates for both and extinction of one of the species are generally considered cases of asymmetric interspecific competition. Exploitative or interference competition is the usual mechanism invoked. Here we describe another mechanism producing the same result, named apparent competition through facilitation (ACF), observed between Melanoides tuberculata and Biomphalaria glabrata populations. The superior competitor actually gives some benefit to the other species, whose population becomes unstable with progressively increasing oscillations, leading to extinction. A model of ACF using difference equations suggests initial dynamics distinct from traditional interspecific competition. The dynamics of two freshwater snails in the field and in laboratory experiments suggest ACF, and these relations should be considered in studies of schistosomiasis control. ACF could occur in natural populations, but might have gone undetected because the final result is similar to traditional interspecific competition.     

The significance of ratios of detritus types and micro-organism productivity to competitive interactions between aquatic insect detritivores

Investigations of competitive interactions emphasize non-detrital resources, even though detritus is a major component of most food webs. Studies of competing species focus usually on single resource types, although consumers in nature are likely to encounter mixtures of resource types that may affect whether competition results in exclusion or coexistence. The invasive mosquito Aedes albopictus is capable of excluding the native mosquito Ochlerotatus triseriatus in competition for single detritus types in laboratory and field microcosms. In this study, we used nine ratios of two detritus types (animal and leaf) common in natural containers to test whether detritus ratios affect the outcome of competition. Under intraspecific and interspecific competition, A. albopictus attained higher survival and estimated population growth rate than did O. triseriatus. Unlike past studies, both species had positive growth and high adult survival, with little evidence of competitive effects, under one resource ratio (10:1 ratio of leaf : animal detritus) regardless of mosquito densities, suggesting potential coexistence. Path analysis showed that densities of larvae had negative effects on population growth for O. triseriatus but not for A. albopictus, indicating competitive superiority of A. albopictus. Population growth of both species was affected strongly by the direct paths from animal (positive) and leaf (negative) detritus, and the indirect effect of leaf detritus via bacterial production (positive). Field sampling established that detritus entered real tree holes in ratios similar to those in our experiment, suggesting that natural variation in detritus ratios may influence local coexistence of these species. Seasonal variation in ratios of plant and animal detritus indicated that temporal as well as spatial variation in inputs may be important for potential coexistence.     

Interspecific Competition and Speciation in Endoparasitoids

Ecological speciation occurs when inherent reproductive barriers to gene flow evolve between populations as a result of divergent natural selection. Frequency dependent effects associated with intraspecific resource competition are thought to be one important source of divergent selection facilitating ecological speciation. Interspecific competition may also play an important role in promoting population divergence. Although evidence for interspecific competition in nature is ubiquitous, there is currently little empirical data supporting its role in the speciation process. Here, we discuss two general models in which interspecific competition among species can promote ecological speciation among populations within a species. In both models, interspecific competition is the source of divergent selection driving adaption to different portions of the resource distribution, generating ecological reproductive isolation from other conspecific populations. We propose that the biology of endoparasitoids that attack phytophagous insects make model systems for studying the role of interspecific competition in ecological speciation. We describe details for one such system, the community of endoparasitic braconid wasps attacking Rhagoletis fruit flies, as a potential model for investigating competitive speciation. We conclude by hypothesizing that a model in which interspecific competition forces an inferior competitor to alternative fly hosts may be a common theme contributing to parasitoid diversification in the Rhagoletis-parasitoid system.     

Competition among the pioneers in a seasonal soft-bottom benthic succession: field experiments and analysis of the Gilpin-Ayala competition model

Summary Controlled experiments, designed to assess the effects of pioneers on succession on an intertidal sandflat, provided evidence for interspecific competition between juvenile Hobsonia florida (Polychaeta, Ampharetidae) and oligochaetes. The field data were fitted to both the linear Volterra and non-linear Gilpin-Ayala competition equations. With its greater number of parameters, the Gilpin-Ayala model must provide a better fit to observed population abundances. The Gilpin-Ayala model is flawed as an explanation of the population trajectories of the H. florida and oligochaetes, because its non-linearity parameter affects only intraspecific competion. With either model our field data demonstrate a solution to Hutchinson's paradox. With competition coefficients near unity and similar carrying capacities, the predicted population trajectories are heavily dependent on initial conditions. The predicted times to competitive exclusion are long and can easily exceed the typical period of environmental constancy. Our study offers evidence for Neill's competitive bottleneck: competition acts primarily on the developmental stages of one of a pair of competing species. The permanent meiofauna may act as a competitive bottleneck for the population growth of benthic macrofauna. The mechanism of this competitive interaction probably involves exploitative interspecific competition for benthic diatoms.     

Diet quality determines interspecific parasite interactions in host populations

The widespread occurrence of multiple infections and the often vast range of nutritional resources for their hosts allow that interspecific parasite interactions in natural host populations might be determined by host diet quality. Nevertheless, the role of diet quality with respect to multispecies parasite interactions on host population level is not clear. We here tested the effect of host population diet quality on the parasite community in an experimental study using Daphnia populations. We studied the effect of diet quality on Daphnia population demography and the interactions in multispecies parasite infections of this freshwater crustacean host. The results of our experiment show that the fitness of a low‐virulent microsporidian parasite decreased in low, but not in high‐host‐diet quality conditions. Interestingly, infections with the microsporidium protected Daphnia populations against a more virulent bacterial parasite. The observed interspecific parasite interactions are discussed with respect to the role of diet quality‐dependent changes in host fecundity. This study reflects that exploitation competition in multispecies parasite infections is environmentally dependent, more in particular it shows that diet quality affects interspecific parasite competition within a single host and that this can be mediated by host population‐level effects.