Spatial heterogeneity and interspecific competition

A model of two competing species is presented in which each species is able to disperse over a single spatial axis. The spatial axis is composed of two intervals with different carrying capacities. We ask the question: If species one is alone and at population dynamic equilibrium, then when can species two successfully invade when rare? We say that an interval is “suitable” if the interval can be invaded by species two in the absence of dispersal by both species, and we say an interval is “unsuitable” if the interval cannot be invaded by species two in the absence of dispersal by both species. We offer three findings: (I) If one interval is suitable and the other is unsuitable, then the success of invasion depends upon the length of the suitable interval. Invasion succeeds if the suitable interval is larger than a threshold minimum and fails otherwise. (II) It is possible for species two to invade even though both intervals are unsuitable. (III) It is possible for species two to fail to invade even though both intervals are suitable.     

Detection of interspecific competition in parasite communities

Matrices of correlation coefficients between the abundances or intensities of all pairs of helminth species, across all individual hosts in a sample, are regularly used to detect possible cases of interspecific competition in parasite communities. In these matrices, however, the range of possible values that any correlation coefficient can take is not -1 to 1, contrary to what is generally assumed. The number and magnitude of other correlation coefficients in a matrix will constrain the values that any given correlation can achieve. This property of matrices, and of inter-related natural variables, is explained and illustrated with 2 examples from real helminth communities. As a rule, the presence of many negative correlations in a matrix raises the lower value that any of them can possibly achieve. This has important but previously overlooked implications for the interpretation of correlation coefficients, and the detection of competition in natural parasite communities.     

Host defence mediates interspecific competition in ectoparasites

1. Interspecific competition influences which, how many and where species coexist in biological communities. Interactions between species in different trophic levels can mediate interspecific competition; e.g. predators are known to reduce competition between prey species by suppressing their population sizes. A parallel phenomenon may take place in host-parasite systems, with host defence mediating competition between parasite species. 2. We experimentally investigated the impact of host defence (preening) on competitive interactions between two species of feather-feeding lice: 'wing' lice Columbicola columbae and 'body' lice Campanulotes compar. Both species are host-specific parasites that co-occur on rock pigeons Columba livia. 3. We show that wing lice and body lice compete and that host defence mediates the magnitude of this competitive interaction. 4. Competition is asymmetrical; wing louse populations are negatively impacted by body lice, but not vice versa. This competitive asymmetry is consistent with the fact that body lice predominate in microhabitats on the host's body that offer the most food and the most space. 5. Our results indicate that host-defence-mediated competition can influence the structure of parasite communities and may play a part in the evolution of parasite diversity.     

Replacement interaction diagrams in interspecific competition experiments

Replacement interaction diagrams offer a new approach in displaying results of interspecific competition experiments. They allow visual assessment of both actual and relative responses of two species in a replacement series to treatments arranged in a factorial array. Yield data are plotted on a semi-logarithmic scale along the vertical axis. Treatments are arranged on the horizontal axis. The degree of interspecific interference or competition in mixed populations is best assessed by comparing the species response in a mixture with that in the monoculture. In competition experiments involving a number of treatment variables, replacement interaction diagrams highlight trends in treatment responses and allow some comparisions of responses between plant frequences.     

Web-mediated interspecific competition among spider mites

Some spider mites, such as Tetranychus spp. and Amphitetranychus spp., create complicated webs (CWs), whereas others, such as Panonychus spp., produce little webs (LWs). We verified whether interspecific competition occurred between CW and LW mites via habitat arrangement under laboratory conditions. The complicated webs produced by CW mites clearly inhibited juvenile development in LW mites, whereas there was no effect of LW mites on CW mites. In oviposition site choice tests, both CW and LW females preferred the lower surface of leaves to the upper surface. The preference of LW mites for the lower leaf surface, even in the presence of CW mite webs, suggests that the costs of amensalism are outweighed by the possible benefits, such as avoiding rain. These findings show that the shift in mite species composition from LW to CW mites can occur as a consequence of the interspecific association between spider mites via their webs, without pesticide applications or the presence of natural enemies.     

种间竞争的一个新的数学模型—对经典的Lotka—Vol—terra竞争方程的扩充

本文根据营养动力学理论,建立了一类种间竞争的新的数学模型:它是单种群增长的Cui-Lawson模型,在种间竞争上的推广。新的种间竞争模型克服了经典的种间竞争的Lotka-Volteira方程的局限与不足,具有更广泛和复杂的行为,并在特殊条件下以Lotka-Volterra竞争方程为其特例。因此,新的种间竞争的数学模型是更一般的解释性模型,是对经典的Lotka-Voterra竞争方程的扩充。     

Reproductive interference in laboratory experiments of interspecific competition

Many studies that have researched interspecific competition in Callosobruchus (bean beetles), Drosophila (fruit flies), and Tribolium (flour beetles) have considered the major drivers of interspecific competition to be interspecific resource competition and intraguild cannibalism. These competition drivers have a density-dependent effect on the population dynamics. However, some studies have also detected a relative-frequency-dependent effect in the observed population dynamics. The most likely causal mechanism of this relative frequency dependence is reproductive interference, defined as any interspecific sexual interaction that damages female reproductive success. Reproductive interference has been overlooked by most laboratory studies in spite of the critical effect on the competition outcome. In this paper, I review laboratory studies of these insect genera from the perspective of reproductive interference and show that the reported results can be more reasonably interpreted by the joint action of reproductive interference and resource competition, including intraguild cannibalism. In addition, on the basis of results reported by a small number of related studies, I discuss the behavioral and evolutionary changes induced in those genera by reproductive interference.     

Evolutionary branching under asymmetric competition

I investigate the evolution of a continuous trait, such as body size or arms level, which affects the outcome of competitive contests such that the contestant with the larger trait value has a higher probability of winning. I show that a polymorphism of distinctly different strategies can evolve in an initially monomorphic population even if mutations have only small phenotypic effect. In a simple Lotka-Volterra-type model of asymmetric competition, I derive the conditions under which two strategies can gradually evolve from a single ancestral strategy; the evolution of higher level polymorphisms is studied by numerical analysis and computer simulations of specific examples. High levels of polymorphism may build up during evolution. The coevolution of strategies in polymorphic populations, however, may also lead to extinction, which decreases the level of polymorphism. I discuss whether the evolution of several haploid strategies from a single initial strategy may correspond to the evolution of several sympatric species in a diploid outbreeding population.     

Evolutionary dynamics through multispecies competition

Disruptive selection, emerging from frequency-dependent intraspecific competition can have very exciting evolutionary outcomes. One such outcome is the origin of new species through an evolutionary branching event. Literature on theoretical models investigating the emergence of disruptive selection is vast, with some investigating the sensitivity of the models on assumptions of the competition and carrying capacity functions’ shapes. What is seldom modeled is what happens once the population escapes its effect via increase phenotypic or genotypic variance. The expectation is mixed: disruptive selection could diminish and ultimately disappear or it could still exist leading to further speciation events through multiple evolutionary branching events. Here, we derive the conditions under which disruptive selection drives two subpopulations that originated at a branching point to other points in trait space where each subpopulation again experiences disruptive selection. We show that the general pattern for further branchings require that the competition function to be even narrower than what is required for the first evolutionary branching. However, we also show that the existence of disruptive selection in higher dimensional systems is also sensitive to the shapes of the functions used.     

Seasonal dynamics and interspecific competition in Oneida Lake Daphnia

I investigated the population dynamics and competitive interactions of two species of the suspension-feeding crustacean Daphnia in Oneida Lake, N.Y. Both species have persisted in the lake for decades, but their water-column densities are negatively correlated. The larger Daphnia pulicaria dominates in some years, the smaller D. galeata mendotae in others, and in some years one species replaces the other seasonally. Although this pattern results in part from annual variation in vertebrate predation pressure, predation alone cannot explain the irregular daphnid dynamics. In 1992–1995, I examined the water-column abundances, birth and death rates of both species. D. pulicaria dominated in two years, D. galeata mendotae was replaced by D. pulicaria in one year and in 1994, both species persisted in low numbers. To test the effect of temporal changes on the strength of intra- and interspecific competition on both juvenile and adult daphnids, I manipulated a series of field enclosures in 1994 and 1995. The outcome of competition varied within and between years, and its effects were most evident at the highest densities and lowest resource levels. For adults of both species, the effects of interspecific competition were detected more often than those of intraspecific competition. Lipid reserves (a metric of fitness) among juveniles were generally low, with those of D. galeata mendotae often being less than those of D. pulicaria. Contrary to the results of other studies examining competition in daphnids, spatial segregation and predictable within-year reversals in competitive dominance most likely do not play a large role in fostering coexistence of the Oneida Lake daphnids. Instead, coexistence of these competitors is promoted by interannual variation and long-lived diapausing eggs. Received: 20 July 1997 / Accepted: 21 November 1997     

Competition and coexistence in plant communities: intraspecific competition is stronger than interspecific competition

Theory predicts that intraspecific competition should be stronger than interspecific competition for any pair of stably coexisting species, yet previous literature reviews found little support for this pattern. We screened over 5400 publications and identified 39 studies that quantified phenomenological intraspecific and interspecific interactions in terrestrial plant communities. Of the 67% of species pairs in which both intra‐ and interspecific effects were negative (competitive), intraspecific competition was, on average, four to five‐fold stronger than interspecific competition. Of the remaining pairs, 93% featured intraspecific competition and interspecific facilitation, a situation that stabilises coexistence. The difference between intra‐ and interspecific effects tended to be larger in observational than experimental data sets, in field than greenhouse studies, and in studies that quantified population growth over the full life cycle rather than single fitness components. Our results imply that processes promoting stable coexistence at local scales are common and consequential across terrestrial plant communities.     

Species coexistence under resource competition with intraspecific and interspecific direct competition in a chemostat

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.     

Metabolic evolution in response to interspecific competition in a eukaryote

1. Download : Download high-res image (203KB)
2. Download : Download full-size image

     

濒危植物宝华玉兰种内与种间竞争

利用Hegyi单木竞争指数模型对江苏镇江宝华山国家森林公园特有濒危植物宝华玉兰种内、种间竞争强度进行定量分析。结果表明:宝华玉兰竞争压力主要来自种间,占总竞争的90.7%。宝华玉兰种内竞争、种间竞争强度大小顺序为:紫楠>野核桃>建始槭>宝华玉兰种内>毛竹>化香>枫香>皂荚>朴树。整个林分竞争木对对象木的竞争强度与对象木的胸径大小近似服从幂函数关系,而伴生树种竞争木对对象木的竞争强度与对象木的胸径大小近似服从对数函数关系。种间竞争强度和对象木的胸径大小呈显著负相关。利用模型预测,当宝华玉兰的胸径达到30cm后,竞争强度几乎没有变化。研究认为,将无性系分株换算为一株与其胸高断面积之和相等的个体更为合理。     

Intra- and interspecific competition among coexisting lotic snails

The competitive interactions of two lotic snails, Elimia cahawbensis and Elimia carinifera , were examined in a second-order spring-fed stream. We first demonstrated food limitation in laboratory microcosms where snails grew faster when exposed to enhanced periphyton levels. We then tested the magnitude and relative strengths of intra- and interspecific competition in similar stream and laboratory mesocosm experiments. Treatments were maintained in Plexiglas enclosures over a 7-week period with 0, 1×, 2× and 4× ambient biomass of each species alone, as well as mixed species treatments at 2× and 4× ambient. Snail responses to treatments were almost identical in field and laboratory experiments. Growth rates of both species were reduced by increased density of snails indicating strong intra- and interspecific competition among E. cahawbensis and E. carinifera . An analysis of the strengths of intra- and interspecific competition indicated minimal differences for either species, implying a lack of competitive dominance. Although periphyton biomass was generally highest without snails, there was little difference in periphyton biomass and snail production over the four-fold density range, regardless of species composition. These results suggest that E. cahawbensis and E. carinifera are functionally redundant with density-dependent responses in growth rate resulting in similar grazing pressure across a density gradient. This clearly demonstrates that species impact is not necessarily reflected by measures of abundance or biomass, and that secondary production should be considered.     

Maternal effects and the outcome of interspecific competition

1. Maternal environmental effects create lagged population responses to past environments. Although they are ubiquitous and vary in expression across taxa, it remains unclear if and how their presence alters competitive interactions in ecological communities.
2. Here, we use a discrete‐time competition model to simulate how maternal effects alter competitive dynamics in fluctuating and constant environments. Further, we explore how omitting maternal effects alter estimates of known model parameters from observational time series data.
3. Our simulations demonstrate that (i) maternal effects change competitive outcomes, regardless of whether competitors otherwise interact neutrally or exhibit non‐neutral competitive differences, (ii) the consequences of maternal effects for competitive outcomes are mediated by the temporal structure of environmental variation, (iii) even in constant conditions, competitive outcomes are influenced by species'' maternal effects strategies, and (iv) in observational time series data, omitting maternal effects reduces variation explained by models and biases parameter estimates, including competition coefficients.
4. Our findings demonstrate that the ecological consequences of maternal effects hinge on the competitive environment. Evolutionary biologists have long recognized that maternal effects can be an important but often overlooked strategy buffering populations from environmental change. We suggest that maternal effects are similarly critical to ecology and call for research into maternal effects as drivers of dynamics in populations and communities.

     

Historical factors mediate the effects of interspecific competition

Biotic interactions, such as interspecific competition, are potentially important in determining whether introduced species succeed or fail to establish wild populations. Such effects may be difficult to detect, however, because the outcome of interspecific competition may depend on historical and largely unpredictable circumstances such as the timing of introductions and the number of individuals of each species introduced. I used a stochastic birth-death model to explore the effects of interspecific competition, the timing of introductions and the numbers of individuals of each species introduced, on invasion success in a two-species competitive system. I then compared the model predictions with actual data on establishment outcomes for passerine birds introduced to New Zealand, for which we have data on the timing of introductions, the size of release populations, and a measure of the strength of per capita competition (the degree of morphological similarity among species). The model and data agree well, suggesting that interspecific competition was an important determinant of invasion success in this assemblage, but that the outcome of competition depended critically on circumstances such as the timing of introductions and number of individuals released. Hence, while there is a deterministic component to invasion success in this assemblage (morphologically similar species are less likely to establish), historical circumstances played a critical role in mediating the outcomes.