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。     

Effects of nitrogen availability on competition between Bromus tectorum and Bouteloua gracilis

Exotic plant invasions are a serious concern for land managers and conservationists. There is evidence that increased nitrogen availability favors exotic species and decreased nitrogen availability favors non-weedy native species. This study was conducted to test the effect of nitrogen availability on competition between two grass species with contrasting life histories, cheatgrass (Bromus tectorum), a North American exotic, and blue grama (Bouteloua gracilis), a North American native. We investigated the effects of nitrogen availability and competition on aboveground biomass, belowground biomass, height, and % nitrogen tissue concentrations by growing the two species in the greenhouse under five levels of nitrogen and six levels of competition. Nitrogen availability affected competition between Bromus tectorum and Bouteloua gracilis. At the lowest level of N availability, neither species was affected by competition. As N availability increased, aboveground biomass gain of Bromus was more negatively affected by intraspecific competition relative to interspecific competition while the opposite occurred for Bouteloua. At the competition level at which each species gained the most aboveground biomass, Bromus had a linear response to increasing N availability while the response of Bouteloua was asymptotic. Our results do provide some support for the theory that fast growing exotic species have a rapid response to nutrient enrichment while native non-weedy species do not, and that low N levels can reduce competitive pressure from the exotic on the native.     

A competition model for a seasonally fluctuating nutrient

A model of two species consuming a single, limited, periodically added resource is discussed. The model is based on chemostat-type equations, which differ from the classical models of Lotka and Volterra. The model incorporates nonlinear functional response curves of the Holling or Michaelis-Menten type to describe the dependence of the resource-exploitation rate on the amount of resource. Coexistence of two species due to seasonal variation is indicated by numerical studies.     

An experimental design and a statistical analysis separating interference from exploitation competition

Previous experimental studies of competition among foragers rarely distinguished between exploitation and interference competition. In many systems this separation is experimentally impossible without interfering with the natural behavior of the animals. Consequently, these studies can only demonstrate the combined effect of interference and exploitation on the forager’s feeding rate, namely, it usually decreases in a decelerating rate as a function of density. We suggest here a simple experimental and statistical procedure that facilitates the separation of the effects of interference from those of exploitation. This procedure includes manipulation of both predator density and the foraging experiment duration. The statistical analysis is based on multiple linear regression. The working assumption is that exploitation can be neglected at the beginning of the foraging experiment because, initially, predators do not experience diminishing returns in prey capture rates. Using both the results of an individual-based simulation and a field experiment dataset of gerbils foraging for seeds in an artificial food patch located in the field, we demonstrate that our procedure can successfully detect and separate the effect of interference from the combined overall effect of competition (i.e., interference plus exploitation). Inon Scharf and Ido Filin contributed equally to this paper.     

Size and density mediate transitions between competition and facilitation

Species simultaneously compete with and facilitate one another. Size can mediate transitions along this competition–facilitation continuum, but the consequences for demography are unclear. We orthogonally manipulated the size of a focal species, and the size and density of a heterospecific neighbour, in the field using a model marine system. We then parameterised a size‐structured population model with our experimental data. We found that heterospecific size and density interactively altered the population dynamics of the focal species. Size determined whether heterospecifics facilitated (when small) or competed with (when large) the focal species, while density strengthened these interactions. Such size‐mediated interactions also altered the pace of the focal’s life history. We provide the first demonstration that size and density mediate competition and facilitation from a population dynamical perspective. We suspect such effects are ubiquitous, but currently underappreciated. We reiterate classic cautions against inferences about competitive hierarchies made in the absence of size‐specific data.     

Within plant interspecific competition does not limit the highly invasive thrips,Frankliniella occidentalis in Florida

1. Species invasions are often linked to reductions in biodiversity, and competitive superiority is often cited as the main reason for the success of an invasive species. Although invaded ecosystems are often examined, few have studied areas in which an invasive species has failed to successfully invade. 2. The western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is a damaging pest and tospovirus vector that has invaded most of the world, and competitive superiority is considered one of the main reasons for this species' success. 3. However, a recent study demonstrated that competition between larval F. occidentalis and a native thrips species may be limiting F. occidentalis abundance in much of the eastern United States. Frankliniella occidentalis also has a limited abundance in central and southern Florida, which is dominated by the endemic F. bispinosa (Morgan). The potential for interspecific competition to limit F. occidentalis abundance in Florida was assessed. 4. The effects of competition between F. occidentalis and F. bispinosa on adult reproduction on a common host (Capsicum annuum L.) were quantified, using a response surface experimental design and a combination of linear and non‐linear competition models. 5. Evidence of symmetric competition between these thrips species was found, but contrary to expectations, F. occidentalis reproduced more in dense interspecific populations than F. bispinosa. These results suggest that, unlike most of the eastern US, interspecific competition is not important in limiting F. occidentalis abundance in central and southern Florida.     

Genetic variation in tolerance of competition and neighbour suppression in Arabidopsis thaliana

Intraspecific competitive interactions can profoundly influence phenotypic evolution. However, prior studies have rarely evaluated the evolutionary potential of the two components of competitive ability, tolerance of competition and suppression of neighbours. Here, we grow a set of 20 Arabidopsis thaliana recombinant inbred lines in three competitive treatments (noncompetitive, intra‐genotypic competition and inter‐genotypic competition) to examine if there is genetic variation for the components of competitive ability and whether neighbour relatedness has an effect on fitness. We find evidence for genetic variation in tolerance of competition and neighbour suppression and that these two competitive strategies are correlated, such that genotypes that tolerate competition will also strongly suppress neighbours. We further observe that the effect of neighbour relatedness on fitness of target individuals depends on neighbour identity, i.e. whether target individuals perform better when competing against self vs. nonself individuals depends on the genotypic identity of the nonself neighbour. The results are particularly relevant to evolutionary responses under multi‐level selection.     

The effect of elevated CO2 on growth and competition in experimental phytoplankton communities

We report an experiment designed to identify the effect of elevated CO₂ on species of phytoplankton in a simple laboratory system. Major taxa of phytoplankton differ in their ability to take up CO₂, which might lead to predictable changes in the growth rate of species and thereby shifts in the composition of phytoplankton communities in response to rising CO₂. Six species of phytoplankton belonging to three major taxa (cyanobacteria, diatoms and chlorophytes) were cultured in atmospheres whose CO₂ concentration was gradually increased from ambient levels to 1000 parts per million over about 100 generations and then maintained for a further 200 generations at elevated CO₂. The experimental design allowed us to trace a predictive sequence, from physiological features to the growth response of species to elevated CO₂ in pure culture, from the growth response in pure culture to competitive ability in pairwise mixtures and from pairwise competitive ability to shifts in the relative abundance of species in the full community of all six species. CO₂ altered the dynamics of growth in a fashion consistent with known differences among major taxa in their ability to take up and use CO₂. This pure‐culture response was partly successful in predicting the outcome of competition in pairwise mixtures, especially the enhanced competitive ability of chlorophytes relative to cyanobacteria, although generally statistical support was weak. The competitive response in pairwise mixtures was a good predictor of changes in competitive ability in the full community. Hence, there is a potential for forging a logical chain of inferences for predicting how phytoplankton communities will respond to elevated CO₂. Clearly further extensive experiments will be required to validate this approach in the greater complexity found in diverse communities and environments of natural systems.     

Predation of whiting and haddock on sandeel: aggregative response, competition and diel periodicity

The predation of haddock Melanogrammus aeglefinus and whiting Merlangius merlangus on sandeel Ammodytes marinus was investigated based on intensive sampling [performed with a bottom trawl (GOV) equipped with a small meshed codend cover; sediment samples were taken with a van Veen grab] in a restricted area of c . 15 × 20 nautical miles in the northern North Sea during a 5 day period in July 1996. The analysis of the spatial distribution of predators revealed a pronounced aggregation of whiting in the south‐west part of the area, where sandeel catches were also highest. This pattern was thought to be the result of an aggregative behaviour of whiting. The sandeel concentration most likely reflected a restricted patch of coarse sediment of the preferred grain size for sandeels to bury in. In haddock the aggregation was less obvious. Both predators fed almost exclusively on sandeel in the south‐west part of the area with haddock stomachs containing more sandeel than whiting stomachs. The stomach contents in both predators increased rapidly during the night, indicating that the predators were targeting burying sandeels. This would explain the competitive advantage of the benthivorous haddock.     

Interspecific competition affects growth and herbivore damage of Brassica napus in the field

Resource competition can influence plant fitness either directly, or indirectly by influencing the amount of herbivore damage received by plants in the field. We previously found that competition could constrain the constitutive and woundinduced expression of defensive trypsin inhibitors in pot-grown Brassica napus seedlings in the greenhouse, suggesting that the ability of a plant to chemically defend itself could be constrained by competition in the field. Guided by these results, we investigated whether competition would affect growth and the presence of herbivores and herbivore damage on B. napus plants in the field. We established sixteen 1 m 2 plots in the field in a 7 x7 mgrid. Nine two-week-old B. napus seedlings were transplanted from the greenhouse into each 1 m 2 plot. Half of the plots were kept weed-free and half were left to develop interspecific weed competi-tors.After six weeks, three randomly chosen plants in each plot were measured for height, number of leaves, leaf area removed by herbivores, and the presence of aphids, leaf miners, and eggs of ladybird beetles. Consistent with the induction of the shade-avoidance response, plants in plots with weed competitors were significantly taller and had half as many leaves as plants in weed-free plots. Competing plants also had 60% more leaf arearemoved by herbivores, an 80% higher proportion of leaves with aphids, and an equal proportion of leaves with leaf miners. In this study, weed competition had dramatic effects on growth, leaf area removal by herbivores, and the presence of aphids on B. napus plants in the field. Together with our demonstration that competition can constrain the expression of trypsin inhibitor activity, these results suggest that resource competition may limit theability of a plant to defend itself from natural enemies, leading to greater herbivory. In turn, increased herbivory on competing plants could exacerbate the direct effects of competition on plant fitness.     

Visual competition

Binocular rivalry--the alternations in perception that occur when different images are presented to the two eyes--has been the subject of intensive investigation for more than 160 years. The psychophysical properties of binocular rivalry have been well described, but newer imaging and electrophysiological techniques have not resolved the issue of where in the brain rivalry occurs. The most recent evidence supports a view of rivalry as a series of processes, each of which is implemented by neural mechanisms at different levels of the visual hierarchy. Although unanswered questions remain, this view of rivalry might allow us to resolve some of the controversies and apparent contradictions that have emerged from its study.