Including competitive asymmetry in measures of local interference in plant populations

Summary Although considerable evidence exists that plant competition is generally asymmetric or one-sided, with larger plants having a disproportionate competitive effect on smaller plants, currently employed measures of local interference generally assume that competition is two-sided. We describe a simple measure of competitive asymmetry in which the effects of neighbors smaller than a focal individual are discounted by a constant factor, and include this variable in a composite measure of local interference. In this model competition varies between complete asymmetry (the effects of smaller plants are entirely discounted) and complete symmetry (the competitive effect of a neighbor is proportional to its size). The proposed method is applied to two natural populations and one experimental monoculture. In all cases an asymmetric model provides the best fit to the data. Completely two-sided models account for 26–39% of the variance in relative growth rate, while relatively one-sided models account for 44–57%. The increases in r ² values resulting from the inclusion of asymmetry are significant in the two cases in which the data permit randomization tests. Our results suggest that interference is completely asymmetric in a population of Impatiens pallida, a species with very low root allocation and a shallow crown, and somewhat less asymmetric in an experimental monoculture of Ambrosia artemisiifolia and a natural stand of Pinus rigida, cases in which competition for water and nutrient resources is likely to be of greater importance.     

A correction for including competitive asymmetry in measures of local interference in plant populations

In 1989, Thomas and Weiner described a method to measure the degree of competitive symmetry in plant populations in which the performance of a focal plant is explained as a function of the size and number of its neighbors. However, if plant size is correlated with plant performance, then Thomas and Weiner's method may give misleading results. Statistically controlling for the effect of plant size on performance corrects the problem and highlights the observation that size is an important determinant of plant growth rate.     

Negative density dependence can offset the effect of species competitive asymmetry: a niche-based mechanism for neutral-like patterns

The debate on the role of species differences in shaping biodiversity patterns, with its two extremes of pure niche theory and neutral theory, is still ongoing. It has been demonstrated that a slight difference in competitive ability of species severely affects the predictions of the neutral model. At the same time, neutral patterns seem to be ubiquitous. Here, we model both negative density dependence (NDD) and competitive asymmetry (CA) simultaneously. Our simulation results show that an appropriate intensity of NDD can offset the negative effect of CA (modeled as fecundity difference) on species coexistence and produce a neutral-like species abundance distribution. Therefore, our model provides a plausible mechanistic explanation of neutral-like patterns, but contrary to the neutral model, a species' relative abundance is positively related to its competitive ability in our model.     

Effects of short-term hunger and competitive asymmetry on facultative aggression in nestling black guillemots Cepphus grylle

Siblings in a diversity of species are facultatively aggressive,yet the proximate control of the aggressive response and theecological conditions selecting for such systems are poorlyunderstood. In this study, we investigated the effects of foodamount (food amount hypothesis) and competitive asymmetry onsibling aggression in black guillemot broods. Parental provisioningrates were experimentally manipulated in broods comprisinga range of hatching intervals over a 12-h period. Aggressionbecame evident only after parental provisioning rates wereexperimentally reduced. When parental provisioning resumed,adults did not increase their feeding rate to compensate forthe induced food deficit, and the result of sibling rivalry was a change in the allocation of parental deliveries from oneof equality to one in favor of the dominant chick. Food-deprivedchicks from synchronous broods were more aggressive than thosefrom asynchronous broods, suggesting that one benefit of hatchingasynchrony in the black guillemot is to establish an efficientcompetitive hierarchy among siblings which minimizes the needfor costly aggressive interactions. On the following day, siblingaggression ceased, and chicks regained an equal share of parentalfeeds. Our results provide the first evidence that short-termfood shortage per se acts as an initial trigger for aggressionand also show that the aggressive response is complicated byfactors associated with hatching and laying order.     

Effect of local spatial plant distribution and conspecific density on bumble bee foraging behaviour

1. Size variations in pollinator populations may modify competitive interactions among foragers. Competition among pollinators has been shown to lead to one of two contrasting behaviours: either specialisation to the most profitable plant species or generalisation to several species. When foraging, pollinators are also confronted with heterogeneity in the spatial distribution of plant resources. Because variations in both the forager density and plant spatial distribution can affect pollinator behaviour, focus was on the interactive effect of these two factors. 2. Bumble bee (Bombus terrestris L.) individuals were trained on a focal species (Lotus corniculatus L.) and experimentally tested whether variations in the forager density (two or six bumble bees foraging together), plant community spatial distribution (two plant species: L. corniculatus and Medicago sativa, which were either patchily or randomly distributed), and their interaction modified bumble bee foraging behaviour. 3. It was shown that when confronted with a high forager density, bumble bees focused their visits towards the most familiar species, especially when foraging under a random plant distribution. These modifications affected the fruiting of the focal plant species, with a significantly lower reproductive success under low density/patchy conditions. 4. This study demonstrates that the foraging decisions of bumble bees are influenced by variations in both the conspecific density and plant spatial distribution. Given the increasing impact of human activities on plant‐pollinator communities, this raises the question of the potential implications of these results for plant communities in natural conditions when confronted with variations in the pollinator density and spatial distribution of plants.     

Effects of shading and plant density on leaf growth of Viciafaba

Faba beans (Viciafaba cv. Maris Bead) were grown in the field at densities of 20 and 60 plants m^-2 and under zero, 34 and 56% reduction in incoming solar radiation. Measurements of the area of individual leaves were made every two days. Rates of leaf appearance and the duration of expansion of a leaf were virtually unaffected by the experimental treatments. Leaves required approximately 200 ^oC days (above 1 ^oC) from appearance to full expansion. Shading consistently increased specific leaf area but decreased leaf weight only at the greater shading level. At low density leaves were largest under 34% shade and smallest under 56% shade, but at high density there were no significant differences in leaf size.     

Effects of plant density on the survival rate of cabbage pests

The population density of herbivores depends on the spatial scale as well as the temporal scale. In a small-scale, short-term experiment, the number of individuals entering from the surrounding area will be most influential in determining the herbivore density. In large-scale, long-term experiments, however, the density of herbivores will rather be influenced by the survival rate of individuals inside the field because most of the herbivorous population derives from the parents that developed inside the field. If we want to predict the large-scale long-term density of herbivores, therefore, emphasis should be placed on the estimation of survival rate. To elucidate the effects of plant density on the large-scale long-term abundance of cabbage pests, we examined the survival rates of three lepidopterous pests, the small white butterfly Pieris rapae crucivora Boisduval (Pieridae), the beet semi-looper Autographa nigrisigna (Walker) (Noctuidae), and the diamondback moth Plutella xylostella (Linnaeus) (Yponomeutidae) under two levels of plant spacing (sparse plot, 2 m × 2 m interval; dense plot, 0.5 m × 0.5 m interval). The experiment with four blocks was repeated in two seasons. The number of eggs per plant was larger in the sparse plots than in the dense plots for all species. The survival rate of eggs and larvae, on the contrary, was lower in the sparse plots than in the dense plots. The lower survival rate of eggs in the sparse plots was mainly caused by the density dependency, while the lower survival rate of larvae in the sparse plots was mainly caused by the direct effects of plant density. It was thus suggested that the density of herbivores may become lower in the sparsely planted field in the long run because of the higher mortality of larvae. Received: September 16, 1998 / Accepted: March 22, 1999     

Effects of local density on fecundity and mating speed for periodical cicadas

Summary This paper catalogues several different dichotomies that have all been termed r- and K-selection. The status of the concept of r- and K-selection is discussed and a more restricted usage of the terms is recommended.     

The effects of density, spatial pattern, and competitive symmetry on size variation in simulated plant populations

Patterns of size inequality in crowded plant populations are often taken to be indicative of the degree of size asymmetry of competition, but recent research suggests that some of the patterns attributed to size-asymmetric competition could be due to spatial structure. To investigate the theoretical relationships between plant density, spatial pattern, and competitive size asymmetry in determining size variation in crowded plant populations, we developed a spatially explicit, individual-based plant competition model based on overlapping zones of influence. The zone of influence of each plant is modeled as a circle, growing in two dimensions, and is allometrically related to plant biomass. The area of the circle represents resources potentially available to the plant, and plants compete for resources in areas in which they overlap. The size asymmetry of competition is reflected in the rules for dividing up the overlapping areas. Theoretical plant populations were grown in random and in perfectly uniform spatial patterns at four densities under size-asymmetric and size-symmetric competition. Both spatial pattern and size asymmetry contributed to size variation, but their relative importance varied greatly over density and over time. Early in stand development, spatial pattern was more important than the symmetry of competition in determining the degree of size variation within the population, but after plants grew and competition intensified, the size asymmetry of competition became a much more important source of size variation. Size variability was slightly higher at higher densities when competition was symmetric and plants were distributed nonuniformly in space. In a uniform spatial pattern, size variation increased with density only when competition was size asymmetric. Our results suggest that when competition is size asymmetric and intense, it will be more important in generating size variation than is local variation in density. Our results and the available data are consistent with the hypothesis that high levels of size inequality commonly observed within crowded plant populations are largely due to size-asymmetric competition, not to variation in local density.     

氮肥和种植密度对紫茎泽兰生长和竞争的影响

紫茎泽兰（Ageratina adenophora）是我国西南热带亚热带地区危害最严重的外来入侵植物之一。本研究在野外设置不同氮肥供给水平和种植密度下的紫茎泽兰、拔毒散（Sida szechuensis）、伏生臂形草（Brachiaria decumbens）和非洲狗尾草（Setaria anceps）的单种和紫茎泽兰与其他3种植物混种的人工群落,从植被-土壤相互作用的角度分析紫茎泽兰的入侵机制。结果表明：单种时4种受试植物均表现了对氮营养的响应,随氮肥水平的升高,根冠比降低,比叶面积升高;通过比较总生物量增幅,氮响应能力非洲狗尾草>伏生臂形草>紫茎泽兰>拔毒散。混种时受试植物与紫茎泽兰的生物量比和株高也表明非洲狗尾草的竞争能力>伏生臂形草>紫茎泽兰>拔毒散;与拔毒散混种的紫茎泽兰以及与 紫茎泽兰混种的伏生臂形草和非洲狗尾草的种内竞争随种植密度的增大而增大,导致种间竞争能力下降,在高种植密度下对混种物种的抑制作用减弱。紫茎泽兰在氮响应能力、竞争能力和表型可塑性方面均强于本地种,可能与其入侵机制有关;牧草的氮响应能力和竞争能力强于紫茎泽兰,其中非洲狗尾草又强于伏生臂形草,可能更适宜于入侵地的替代控制。     

The effect of recycling on plant competitive hierarchies

Evidence from field studies suggests that some plant species enhance their persistence by reinforcing patterns of N availability through differences in litter quality. Using mathematical models of nutrient flow, we explore whether and how recycling affects plant growth, competition, and coexistence and whether it leads to positive feedbacks. Two mechanisms are considered: the ability of plants to access two forms of soil N, complex (e.g., organic) and simple (e.g., nitrate), and the effect of density-dependent limitation of growth. Except in the trivial case of limitation by N in one form without density dependence, differences in litter quality can prevent the establishment of competitors. Feedback can, conversely, facilitate the invasion of competitors. At equilibrium, the rate of decomposition does not affect the outcome of competition. Species affect their long-term persistence if they alter the fraction of nitrogen that is returned to the soil and becomes available for plant uptake. Increasing the fraction of N that is recycled favors specialists in complex nitrogen and species that suppress the growth of others at high nitrogen availability. Increasing the rate of microbial decomposition of complex nitrogen favors specialists in simple nitrogen.     

Effects of solar radiation on bacterial and fungal density on aquatic plant detritus

1. The effects of solar radiation on bacterial and fungal growth on aquatic macrophyte detritus were studied in a microcosm experiment. Senescent leaves of Phragmites australis were incubated for 63 days in shallow water in the shade under photosynthetically active radiation (PAR) together with ultraviolet radiation, or under filters removing either ultraviolet B (UVB) or both UVB and ultraviolet A (UVA). 2. Bacterial abundance and bacterial ³H-leucine incorporation in the water were measured, together with α- and β-D-glucosidase activity. In addition, bacterial abundance and fungal biomass associated with the litter were measured. 3. The results indicate that both PAR and UVA affect the micro-organisms involved in the decomposition of leaf litter. The α/β-D-glucosidase activity ratio was less than one in irradiated and more than one in shaded microcosms, suggesting a change in the substrate dissolved organic matter composition towards more β- than α-glycosidic linkages as a result of solar radiation. 4. Microcosms receiving UVB displayed a significantly higher β-D-glucosidase activity than shaded microcosms, and those exposed to PAR or PAR + UVA, demonstrating the potential importance of UVB radiation. 5. The free-living bacteria tended to be dominated by filamentous forms in microcosms subject to solar radiation, especially PAR, and attached microbial communities showed a greater tendency to be dominated by bacteria in irradiated microcosms than in shaded microcosms.     

No impact of a native beetle on exotic plant performance and competitive ability due to plant compensation

The novel associations between invasive plants and their natural enemies in the introduced range have recently received increasing attention; however, the effects of novel enemies on exotic plant performance and competition with native species remain poorly explored. Here, we tested the impact of herbivory by a native beetle, Cassida piperata, on the performance of the exotic species Alternanthera philoxeroides and competition with a native congener, Alternanthera sessilis, using common garden experiments in central China. We found A. philoxeroides was able to fully compensate for intense herbivory by C. piperata. Herbivory by C. piperata that released at the average density in this region had no impact on competition between the native and exotic plant species. Our results indicate that herbivory by novel enemies may not reduce exotic plant performance due to plant compensation. However, high tolerance to herbivory may not confer a competitive advantage for exotic species compared to less tolerant native competitors if the herbivore damage is below a certain threshold. Thus, it is necessary to assess the impact of novel enemies on exotic plant performance and competition with native plants along gradients of insect densities. This may lead to a better understanding of how best to exploit the role of native herbivores in facilitating or slowing plant invasions.     

Effects of regional pool size on local diversity in small-scale annual plant communities

The shape of the relationship between local and regional richness has been used to infer the relative strength of local vs. regional processes in structuring communities. While most empirical work has demonstrated linear relationships, recent reviews suggest that prior work has contained biases by selecting communities from different biological provinces and by defining large-scale local communities. We evaluate the utility of inferring interaction strength from local–regional relationships by examining 11 years of data from small-scale, annual plant communities at one site with strong local interactions. Both winter and summer annual plant assemblages exhibited linear rather than saturating relationships over a wide range of regional pool sizes. Our empirical work supports recent theory suggesting that local–regional relationships cannot be used to infer the strength of interactions in communities.     

Effects of cooperation on competitive systems

A persistence-extinction theory of coupled cooperative-competitive systems is developed for three dimension Lotka-Volterra systems. Model conclusions indicate, as intuitively expected, that protocooperation can be beneficial for all species in the community. An outcome that is, perhaps, unexpected is that a protocooperative species can drive a stable competitive community to extinction. The models are successively modified to include a hierarchy of obligatory interactions. Obligatory cooperation generally leads to an increased degree of extinction from the perspective that it is generally attained under weaker conditions and generally involves more species.     

Effects of plant density and nutrient levels on fruit polymorphism in Atriplex sagittata

Atriplex sagittata is a heterocarpic species producing three types of fruits which differ in morphology and ecological properties. This study focused on variation in biomass allocation into particular fruit types under different density and fertilization levels. The reduction in total weight under stressful conditions was accompanied by a reduction of reproductive structures in terms of both total fruit production and mean fruit weight. Allocation of biomass to particular fruit types under different density-fertilization treatments showed considerable variation. The non-dormant, bracteolate fruit type (termed C) contributed 80–90% to the total fecundity and its production was not affected by density-fertilization treatments. The production of this fruit type is higher in the upper part of the maternal plant stem. Production of the very dormant ebracteate fruit type (termed A) increased under favourable conditions and was greater on the lower part of the stem, whereas more of the so-called type B fruit (covered by bracteoles and exhibiting dispersal and dormancy characteristics intermediate between types A and C) was produced under suboptimal conditions and was concentrated in the middle part of the plant body. This pattern contradicts to some extent the theory that deeply dormant and less dispersible fruits will be produced under stressful conditions and may be considered a new model for the behaviour of plants with more than two heterocarpic fruits. The type B fruit, intermediate in both morphology and ecological behaviour, provides a continuum between extreme strategies. Received: 8 February 1998 / Accepted: 19 November 1998