Changes In Competitive Ability Research Articles

Changes in the herbicide sensitivity and competitive ability of Abutilon theophrasti over 28 years: Implications for hormesis and weed evolution.

The potential of weed species to respond to selection forces affecting the evolution of weedy traits such as competitive ability is poorly understood. This research characterized evolutionary growth changes in a single Abutilon theophrasti Medik. population comparing multiple generations collected from 1988 to 2016. A competition study was performed to understand changes in competitive ability, and a herbicide dose-response study was carried out to assess changes in sensitivity to acetolactate synthase-inhibiting herbicides and glyphosate over time. When grown in monoculture, A. theophrasti biomass production per plant increased steadily across year-lines while leaf number decreased. In replacement experiments, A. theophrasti plants from newer year-lines were more competitive and produced more biomass and leaf area than the oldest year-line. No clear differences in sensitivity to imazamox were observed among year-lines. However, starting in 1995, this A. theophrasti population exhibited a progressive increase in growth in response to a sublethal dose of glyphosate (52 g a.e. ha-1 ), with the 2009 and 2016 year-lines having more than 50% higher biomass than the nontreated control. This study demonstrates that weeds can rapidly evolve increased competitive ability. Furthermore, the results indicate the possibility of changes in glyphosate hormesis over time. These results highlight the importance of the role that rapid (i.e., subdecadal) evolution of growth traits might have on the sustainability of weed management strategies. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Trends in geographic and gender balance among authors

Trends in geographic and gender balance among authors

Using ecological coexistence theory to understand antibiotic resistance and microbial competition.

Tackling antibiotic resistance necessitates deep understanding of how resource competition within and between species modulates the fitness of resistant microbes. Recent advances in ecological coexistence theory offer a powerful framework to probe the mechanisms regulating intra- and interspecific competition, but the significance of this body of theory to the problem of antibiotic resistance has been largely overlooked. In this Perspective, we draw on emerging ecological theory to illustrate how changes in resource niche overlap can be equally important as changes in competitive ability for understanding costs of resistance and the persistence of resistant pathogens in microbial communities. We then show how different temporal patterns of resource and antibiotic supply, alongside trade-offs in competitive ability at high and low resource concentrations, can have diametrically opposing consequences for the coexistence and exclusion of resistant and susceptible strains. These insights highlight numerous opportunities for innovative experimental and theoretical research into the ecological dimensions of antibiotic resistance.

Evolution of size-dependent intraspecific competition predicts body size scaling of metabolic rate.

Growth in body size is accompanied by changes in foraging capacity and metabolic costs, which lead to changes in competitive ability during ontogeny. The resulting size‐dependent competitive asymmetry influences population dynamics and community structure, but it is not clear whether natural selection leads to asymmetry in intraspecific competition.We address this question by using a size‐structured consumer–resource model, in which the strength and direction of competitive asymmetry between different consumer individuals depends on the scaling of maximum ingestion and maintenance metabolism with consumer body size. We use adaptive dynamics to study selection on the scaling exponents of these processes.Selection leads to an identical scaling of maximum ingestion and maintenance metabolism with consumer body size. Equal scaling exponents neutralize strong competitive differences within the consumer population, because all consumer individuals require the same amount of resources to cover maintenance requirements. Furthermore, the scaling exponents respond adaptively to changes in mortality such that biomass production through growth or reproduction increases in the life stage that is subject to increased mortality. Also, decreasing size at birth leads to increased investment in juvenile growth, while increasing maximum size leads to increased investment in post‐maturation growth and reproduction.These results provide an explanation for observed variation in the ontogenetic scaling of metabolic rate with body size. Data of teleost fish are presented that support these predictions. However, selection towards equal scaling exponents is contradicted by empirical findings, which suggests that additional ecological complexity beyond this basic consumer–resource interaction is required to understand the evolution of size‐dependent asymmetry in intraspecific competition. A plain language summary is available for this article.

Changes in competitive ability between safflower-weeds affected by timing of nitrogen fertilizers

Nitrogen (N) effects on crop-weed competition and its information may help to improved integrated weed management systems. A two-year field experiment (2015 and 2016) was conducted on a silty clay loam soil in semi-arid climatic conditions to determine the combined effects of N sources (ammonium nitrate (AN), ammonium sulfate (AS), sulfur coated urea (SCU), urea (U)) and timing fertilization (sowing, stem elongation, flowering) on weed-safflower (Carthamus tinctorius L.) competition and safflower yield at Shiraz University, Iran. The experimental design was a split split-plot with three replications. Results showed that weeds significantly reduced safflower yield components approximately 39% and caused a decrease in seed yield up to 70%. In weed free, the highest seed yield (3303.52 kg ha-1) and oil yield (753.09 kg ha-1) were achieved by AN and U fertilizers, respectively, when N fertilization was applied half (50 kg N ha-1) at sowing and other half (50 kg N ha-1) at stem elongation and zero N application at flowering stage (T1N1/2, T2N1/2, and T3N0). However, ANor U fertilizer timing of T1N1/2, T2N1/2, and T3N0 increased relative competition intensity (RCI) and ability to withstand competition (AWC) indices approximately 80%. On the contrary, U fertilizer timing of T1N1/2, T2N1/2, and T3N0 increased ability to compete (AC) index up to 20% as compared to the AN fertilizer. Our research has shown that U fertilizer timing of one third of the N (33.5 kg N ha-1) fertilization at sowing, two thirds of the N (66.5 kg N ha-1) fertilization at stem elongation and zero N application at flowering (T1N1/3, T2N2/3, and T3N0) can be used to advise farmers of the importance of strategic fertilizer management in terms of both weed management and safflower yield.

Mechanical Constraints on Flight at High Elevation Decrease Maneuvering Performance of Hummingbirds

High-elevation habitats offer ecological advantages including reduced competition, predation, and parasitism [1]. However, flying organisms at high elevation also face physiological challenges due to lower air density and oxygen availability [2]. These constraints are expected to affect the flight maneuvers that are required to compete with rivals, capture prey, and evade threats [3-5]. To test how individual maneuvering performance is affected by elevation, we measured the free-flight maneuvers of male Anna's hummingbirds in a large chamber translocated to a high-elevation site and then measured their performance at low elevation. We used a multi-camera tracking system to identify thousands of maneuvers based on body position and orientation [6]. At high elevation, the birds' translational velocities, accelerations, and rotational velocities were reduced, and they used less demanding turns. To determine how mechanical and metabolic constraints independently affect performance, we performed a second experiment to evaluate flight maneuvers in an airtight chamber infused with either normoxic heliox, to lower air density, or nitrogen, to lower oxygen availability. The hypodense treatment caused the birds to reduce their accelerations and rotational velocities, whereas the hypoxic treatment had no significant effect on maneuvering performance. Collectively, these experiments reveal how aerial maneuvering performance changes with elevation, demonstrating that as birds move up in elevation, air density constrains their maneuverability prior to any influence of oxygen availability. Our results support the hypothesis that changes in competitive ability at high elevations are the result of mechanical limits to flight performance [7].

Contemporary Evolution of Plant Growth Rate Following Experimental Removal of Herbivores

Herbivores are credited with driving the evolutionary diversification of plant defensive strategies over macroevolutionary time. For this to be true, herbivores must also cause short-term evolution within plant populations, but few studies have experimentally tested this prediction. We addressed this gap using a long-term manipulative field experiment where exclosures protected 22 plant populations from natural rabbit herbivory for <1 to 26 years. We collected seeds of Rumex acetosa L. (Polygonaceae) from our plots and grew them in a common greenhouse environment to quantify evolved differences among populations in individual plant growth rate, tolerance to herbivory, competitive ability, and the concentration of secondary metabolites (tannins and oxalate) implicated in defense against herbivores. In 26 years without rabbit herbivory, plant growth rate decreased linearly by 30%. We argue that plant growth rate has evolved as a defense against intense rabbit herbivory. In contrast, we found no change in tolerance to herbivory or concentrations of secondary metabolites. We also found no change in competitive ability, suggesting that contemporary evolution may not feed back to alter ecological interactions within this plant community. Our results combined with those of other studies show that the evolution of gross morphological traits such as growth rate in response to herbivory may be common, which calls into question assumptions about some of the most popular theories of plant defense.

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.

Fire increases aboveground biomass, seed production and recruitment success of Molinia caerulea in dry heathland

During the last decades, the perennial tussock grass Molinia caerulea has shown an increased abundance in European heathlands, most likely as a result of increased nitrogen deposition and altered management schemes. Because of its deciduous nature, Molinia produces large amounts of litter each year, which may affect the intensity and frequency of accidental fires in heathlands. These fires may influence plant population dynamics and heathland community organization through their effects on plant vital attributes and competitive interactions. In this study, fire-induced changes in competitive ability and invasiveness of Molinia through changes in biomass production, seed set and seed germination under both natural and laboratory conditions were investigated. We found that fire significantly increased aboveground biomass, seed set and germination of Molinia. Seed set was twice as high in burned compared to unburned heathland. Two years after fire, seedling densities in natural conditions were on average six times higher in burned than in unburned heathland, which resulted in increased abundance of Molinia after burning. The seed germination experiment indicated that seeds harvested from plants in burned heathland showed higher germination rates than those from unburned heathland. Hence, our results clearly demonstrate increased invasive spread of Molinia after large and intense fires. Active management guidelines are required to prevent further encroachment of Molinia and to lower the probability of large fires altering the heathland community in the future.

EVOLUTIONARY CHANGE IN PARASITOID RESISTANCE UNDER CROWDED CONDITIONS IN DROSOPHILA MELANOGASTER

Patterns of investment of limiting resources in such processes as competing for food and defense against natural enemies are shaped by trade-offs and constraints. In Drosophila melanogaster artificial selection for increased resistance to parasitoids results in a correlated decrease in larval competitive ability. Here we ask whether selection for competitive ability leads to a correlated reduction in parasitoid resistance. Replicated lines of D. melanogaster were maintained under crowded or uncrowded conditions for eight generations. As expected, the crowded lines evolved higher competitive ability (when tested against a common strain of fly). But instead of parasitoid resistance decreasing, we found a significant increase, and that this was associated with elevated densities of haemocytes in second-instar larvae. To understand these results we measured a variety of life-history traits in the two sets of lines. We find evidence that directly and indirectly selected changes in competitive ability are due to different mechanisms. We also ask why crowded conditions should select for increased resistance to parasitism, and conclude that it is unlikely to be due to correlated selection for resistance to other natural enemies, but might be due to correlated selection for better wound responses.

EVOLUTIONARY CHANGE IN PARASITOID RESISTANCE UNDER CROWDED CONDITIONS IN DROSOPHILA MELANOGASTER

Patterns of investment of limiting resources in such processes as competing for food and defense against natural enemies are shaped by trade-offs and constraints. In Drosophila melanogaster artificial selection for increased resistance to parasitoids results in a correlated decrease in larval competitive ability. Here we ask whether selection for competitive ability leads to a correlated reduction in parasitoid resistance. Replicated lines of D. melanogaster were maintained under crowded or uncrowded conditions for eight generations. As expected, the crowded lines evolved higher competitive ability (when tested against a common strain of fly). But instead of parasitoid resistance decreasing, we found a significant increase, and that this was associated with elevated densities of haemocytes in second-instar larvae. To understand these results we measured a variety of life-history traits in the two sets of lines. We find evidence that directly and indirectly selected changes in competitive ability are due to different mechanisms. We also ask why crowded conditions should select for increased resistance to parasitism, and conclude that it is unlikely to be due to correlated selection for resistance to other natural enemies, but might be due to correlated selection for better wound responses.

Changes in competitive ability between a C4crop and a C3weed with elevated carbon dioxide

Abstract Using climate-controlled glasshouses, the growth of grain sorghum was evaluated with and without the presence of common cocklebur at current and projected future atmospheric concentrations of carbon dioxide [CO2]. Single-leaf photosynthetic rates declined for both species in competition; however, elevated CO2 reduced the percentage decline in common cocklebur and increased it in sorghum by 35 d after sowing (DAS) relative to ambient CO2. In monoculture, elevated CO2 significantly stimulated leaf photosynthetic rate, leaf area, and aboveground dry weight of common cocklebur more than that of sorghum. However, the stimulation of aboveground biomass or leaf area for monocultures of sorghum and common cocklebur at elevated CO2 did not adequately predict the CO2 response of aboveground biomass or leaf area for sorghum and common cocklebur grown in competitive mixtures. Overall, by 41 DAS, plant relative yield (PRY), in terms of aboveground biomass and leaf area, increased significantly for common cock...

Relative competitive ability does not change over time in blackbirds

Summary Individual variation in feeding rate in the absence of competitors (absolute feeding rate) and decline in feeding rate in the presence of competitors (susceptibility to interference) are frequently used as measures of competitive ability or fitness in short‐term foraging studies. They are also crucial parameters of individual‐based population models that predict the distribution of animals across patches. There are, however, few data on whether absolute feeding rate and susceptibility to interference competition remain constant for individuals between years. The hypothesis that absolute feeding rate and susceptibility to interference were similar between years was tested by observing 25 European blackbirdsTurdus merulafeeding in seminatural experimental patches, during January to March over a 2–4‐year period 1995–98. Absolute feeding rate was measured as the feeding rates of solitary blackbirds. Susceptibility to interference was measured as the change in feeding rate of a focal bird when it fed in the presence of different numbers of competitors. Individuals changed significantly in their feeding rate between years, but most individuals did not change in their feeding rate relative to others in the population. The absolute feeding rate of an individual was significantly positively correlated with its feeding rate in the subsequent year. There was no significant variation in susceptibility to interference for individuals between years. Only two of 27 blackbirds showed a significant change in susceptibility to interference between years. Relative absolute foraging rate and susceptibility to interference competition were reasonably similar between years in blackbirds. Relative fitness measures derived from short‐term measures of foraging ability may therefore be valid over long periods. In ideal free models that incorporate individual competitive ability, rather than population averages, temporal changes in competitive ability can possibly be ignored.

Predator-mediated coexistence of epiphytic grass shrimps that compete for refuges

Epiphytic grass shrimps, Latreutes acicularis and Heptacarpus geniculatus, are often found to coexist in seagrass beds in Japanese shallow seas. The seagrass provides effective refuges by its complex vegetation. A laboratory experiment showed that the competitive interactions between the two shrimps were qualitatively altered in the presence of a predator. H. geniculatus was competitively superior in the absence of the predator, while the presence of the predator acted to lower activities of both species, resulting in a reduced intra- and interspecific interference competition. Based on the experimental data on the behavioral changes, we constructed a model for population dynamics of the shrimp community, and examined the conditions for coexistence of the prey species in terms of their recruitment rates and death rates in the presence and absence of predator. The results show that predator-induced behavioral changes in competitive ability make coexistence significantly easier to achieve.

Behavioral response to predators reverses the outcome of competition between prey species

In a laboratory experiment it was shown that piscivorous predators reversed the outcome of competitive interactions between two fish prey species, juveniles of roach (Rutilus rutilus) and perch (Perca fluviatilis), by behaviorally affecting their use of two available habitats, an open water habitat and a structurally complex refuge. The shift in the competitive relationship was the result of predators forcing the juvenile fishes into a prey refuge with high structural complexity. While roach was competitively superior in the unstructured habitat, perch was superior in the structurally complex prey refuge. The reversal in competitive relationship was demonstrated both with respect to foraging rate and growth rate and resulted from the high structural complexity in the prey refuge interfering with the roach's swimming performance. Because survival and growth patterns through the juvenile stages have profound effects on the population/community dynamics of size-structured populations such as those of fish, behaviorally induced changes in competitive ability should have significant implications also at the population and community levels.

Seasonal Changes in Competitive Ability of Contrasting Populations of Dactylis glomerata

Seasonal Changes in Competitive Ability of Contrasting Populations of Dactylis glomerata

The sterile release method with unequal male competitive ability

A model is presented which is a generalization of a model analyzed previously. The present model examines the condition in which the released sterile males do not compete equally with the wild fertile males for mates or incur unequal mortality. It is found that the general qualitative behavior of the system does not change with changes in competitive ability or mortality of the sterile males; the quantitative behavior does change, however. Predictably, eradication and control are made more difficult by a reduction in competitive ability of the sterile males or by increased mortality of steriles. The effects of density and frequency dependent changes in competitive ability are briefly considered.

EVOLUTIONARY CHANGES IN EGG-EATING BEHAVIOR OF FLOUR BEETLES IN MIXED-SPECIES POPULATIONS.

Theoretical studies in many areas of evolutionary biology have far outpaced experimental work designed to test assumptions and/or even models themselves. One of these areas is that of foraging strategies of predators. There is a vast literature on optimal strategies for searching, pursuing and capturing food (e.g., MacArthur and Pianka, 1966; Emlen, 1966, 1968; Royama, 1970; Rapport, 1971; Schoener, 1971; Katz, 1974; Pulliam, 1974; Estabrook and Dunham, 1976). Natural selection is viewed as a (somewhat as a computer is viewed by many workers)-predators, prey and environments (inputs) are fed into box and adaptations (outputs) come out. Natural selection is supposed to assimilate information contained in inputs and generate optimal solutions to problem of obtaining food. Experimental studies to support theory are meager by comparison (but see Charnov, 1976; Zach and Falls, 1976; Cowie, 1977). To my knowledge there are no studies that involve black box component. What one would like is an experimental system where there is sufficient knowledge to enable one to make predictions about changes in output from planned alterations in input. Even though natural selection would still be treated as a black box, predictions would be based on some notion of optimal solutions to changes in input. Such a system is available in form of the Tribolium model, long used by population ecologists and more recently by geneticists and by population biologists (King and Dawson, 1972). Two species of flour beetles, Tribolium castaneum and T. confusum, are ecologically similar but reproductively isolated, thus permitting maintenance of mixed-species cultures. The two species form a mutual predatorprey system; cannibalism is one of primary regulators of population size and along with predation serves as an important determinant of outcome of interspecific competition (Park et al., 1964, 1965; King and Dawson, 1972). Both larval and adult stages prey on eggs and pupae. I have suggested that cannibalistic behavior in flour beetles evolved through individual selection (Dawson, 1968a). Others prefer arguments based on group selection (e.g., Brereton, 1962). It has even been argued that cannibalism is a laboratory artifact (Colinvaux, 1973). That is, through combination of artificially high densities and prevention of dispersal, aberrant behavior of cannibalism is elicited. Several years ago I established a series of cultures designed to permit changes in competitive ability and foraging behavior through time. The rationale for starting these cultures was as follows: In a mixedspecies culture, both species act as predators, prey, cannibals and victims. Predatory behavior is not random; both adults and larvae of two species have different rates and patterns of eating (Park et al.,-1965, 1968, 1970, 1974). Therefore, if this behavior is subject to natural selection, we should be able to observe changes in rates and patterns of eating in cultures where we artificially maintain different amounts of intraspecific and interspecific association. In our cultures there are populations of both species, in varying proportions, along with an alternative food (the flour-yeast medium) that is perfectly adequate to sustain populations. This paper is one of several on evolu-

Ecological studies on cocco1d bacteria in a pine forest soil—III. competitive interactions between bacterial strains in soil

Strains of bacteria with differing degrees of overall (phenetic) similarity have been paired and grown together and separately in both acid and alkaline soils from a pine forest. A decrease in yield of an organism when grown in the presence of a second organism was taken as evidence of competition. It was shown that, contrary to expectations, pairs of organisms with different degrees of similarity might have similar competitive interactions. It is postulated that this is because one key property could result in a marked change in competitive ability while not markedly altering the taxonomic relationships of the organisms. The possibility that spatial separation of organisms can result in two or more organisms with identical functions becoming established in soil is discussed.

FACTORS RESPONSIBLE FOR A CHANGE IN INTERSPECIFIC COMPETITIVE ABILITY IN DROSOPHILA.

Classical interspecific competition theory assumes all individuals of a particular species are genetically alike in their ability to compete with another species. Recently, a number of studies have demonstrated the importance of the genotypes of competing species on the outcome of interspecific competition. A study by Lerner and Ho (1961) in Tribolium first showed that the genetic constitution of competing species was an important factor in determining competitive outcome. Other studies by Moore (1952), Pimentel et al. (1965) and Ayala (1966) have observed reversals of numerical dominance in competing species and suggested that genetic changes in competitive ability were responsible. If there is genetic variation for competitive ability, natural selection for increased competitive ability should occur when two competitors are artificially maintained in association. Such experiments for both intraand interspecific competition have had mixed results, although they appear to have been more successful when selection was for intraspecific competitive ability. Seaton and Antonovics (1967) in Drosophila and Bryant and Turner (1972) in Musca found significant increases in intraspecific competitive ability while van Delden (1970) observed increased interspecific competitive ability only after 65 generations in Drosophila. Selection for intraspecific competitive ability did not lead to a demonstrable change in experiments by Sokal et al. (1970) in Tribolium and Musca and for interspecific competitive ability in studies by Park and Lloyd (1955) in Triboliumn and by Futuyma (1970) and Barker (unpubl.) in Drosophila. Selection pressure for interspecific competitive ability for both competitors should be greatest when the two species are nearly equal in competitive ability and maintain similar numbers since at this point interspecific competition is maximized for both species relative to intraspecific competition. In an effort to find a strain of Drosophila melanogaster which could maintain nearly equal numbers to a v (vermilion eye) strain of D. siinulans, in 1968 J. S. F. Barker tested a number of mutant Drosophila melanogaster strains maintained by W. G. Baker. In a one generation test, he found that a yw (yellow body, white eye) strain of melanogaster produced a similar number of progeny to his simulans strain. Specifically, in mixed culture the melanogaster strain produced 5 9.0% of the progeny when 50% of the parents were melanogaster (Barker, 1971). The present study was initiated to examine interspecific competition between these two strains over an extended period of time using a serial transfer approach (Buzzati-Traverso, 1955; Ayala, 1966). Unexpectedly, melanogaster quickly eliminated simulans in interspecific competition. As a result the focus of this work became to (1) demonstrate whether or not there was a genetic change in the competitive ability of the melanogaster strain and then upon finding that there was, (2) determine in what respects the melanogaster strain changed such that its competitive ability was dramatically altered.