Relative Competition Intensity Research Articles

Channel Dynamics Under Price and Service Competition

This paper studies a distribution system in which a manufacturer supplies a common product to two independent retailers, who in turn use service as well as retail price to directly compete for end customers. We examine the drivers of each firm's strategy, and the consequences for total sales, market share, and profitability. We show that the relative intensity of competition with respect to each competitive dimension plays a key role, as does the degree of cooperation between the retailers. We discover a number of insights concerning the preferences of each party regarding competition. For instance, there will be circumstances under which both retailers would prefer an increase in competitive intensity. Our analysis generalizes existing knowledge about manufacturer wholesale pricing strategies, and rationalizes behaviors that would not be evident without both price and service competition. Finally, we characterize the structure of wholesale pricing mechanisms that can coordinate the system, and show that the most commonly used formats (those that are linear in the order quantity) can achieve coordination only under very limiting conditions.

Indirect effects in a desert plant community: is competition among annuals more intense under shrub canopies?

An unresolved discussion in contemporary ecology deals with the relative importance of competition along environmental gradients. In deserts, local-scale differences in environmental productivity may be caused by the presence of shrubs, which represent a favorable habitat for annual populations within a nutrient-poor matrix. In this study, we attempted to test the hypothesis that facilitation of desert annuals by shrubs increase the intensity of competition among the annual plants. Such negative indirect effects have so far been ignored in studies about plant-plant interactions. We tested our hypothesis by measuring seedling survival and fecundity of four abundant annual plant species with and without neighbors in open areas and under shrub canopies in a sandy desert area. Our findings did not indicate indirect negative effects of shrubs on their understory annuals. Sensitivity to the presence of neighbors varied between species and surprisingly, the species with the smallest seeds was the only one which was not negatively affected by the presence of neighbors. In contrast to our hypothesis, there was no difference between the habitat types shrubs and openings in absolute and relative competition intensity. Our overall results suggest that negative indirect effects of shrubs are unimportant in determining demographic response of understory annual plants.

Competition intensity and its importance: results of field experiments with Anthoxanthum odoratum

The effect of community productivity on competition was studied in 82 permanent plots using two removal experiments with the rhizomatous perennial grass Anthoxanthum odoratum. The removal of neighbouring plants had a positive effect on the number of shoots and total above-ground biomass of Anthoxanthum but no significant effect on mean shoot biomass. The relative competition intensity coefficient (RCI) calculated from these data showed that competition intensity increased with increasing community productivity. Similarly, the importance of competition and the difference between local maximum and local average population density increased with increasing community productivity. We concluded that for Anthoxanthum the impact of competition is greater in high-productivity areas and that competition reduces population density. No evidence was found supporting the importance of positive interactions between plants in tundra areas.

Clonal growth and storage in relation to competition in genets of the rhizomatous perennial Amphibromus scabrivalvis

The grass Amphibromus scabrivalvis Trin. Swallen produces new ramets from the base of established ramets (to produce a phalanx growth form) and along rhizomes (to produce a guerrilla growth form). It stores resources in stem bases. This research examined genotypic variation in clonal growth and storage and its relation to the performance of replicated genets under competition. It was hypothesized that (i) genets would vary significantly in traits important to clonal growth and storage during competition, (ii) competition would reduce clonal growth capability, (iii) guerrilla-like spacing would enhance clonal growth under competition at low competitor density, and (iv) allocation to storage organs would increase under competition. Fourteen genets of A. scabrivalvis were divided into ramets and planted into pots in the glasshouse alone or in competition with four equally distributed seedlings of Lolium perenne L., a phalanx competitor. Leaf area and mass, as well as the distance of new ramets from the parental ramet, were measured at 9 and 18 weeks. Dry mass of leaves, stem bases, rhizomes, and roots was determined at 18 weeks. Relative competition intensity, based on the difference in total mass per genet between control and competition treatments relative to the control, varied between 0.69 and 0.83. Ramet production, the proportion of the total leaf area in new ramets, and ramet distances varied significantly among genets and were reduced by competition. Mean ramet distance was positively correlated with total mass. Allocation of dry mass to storage organs was significantly increased under competition and may allow genet persistence within an occupied area. The relative ranking of genets in the control and competition treatment was not different, and there were no differences in the relative degree to which the total mass of genets was reduced by competition, despite significant variation in total mass accumulated in the absence of competition.Key words: Amphibromus scabrivalvis, clonal grass, competition, genotypic variation, guerrilla strategies, phalanx strategies, rhizomatous perennial.

Empirical Approaches to Quantifying Interaction Intensity: Competition and Facilitation along Productivity Gradients

Quantitative synthesis across studies requires consistent measures of effect size among studies. In community ecology, these measures of effect size will often be some measure of the strength of interactions between taxa. However, indices of interaction strength vary greatly among both theoretical and empirical studies, and the connection between hypotheses about interaction strength and the metrics that are used to test these hypotheses are often not explicit. We describe criteria for choosing appropriate metrics and methods for comparing them among studies at three stages of designing a meta-analysis to test hypotheses about variation in interaction intensity: (1) the choice of response variable; (2) how effect size is calculated using the response in two treatments; and (3) whether there is a consistent quantitative effect across all taxa and systems studied or only qualitatively similar effects within each taxon–system combination. The consequences of different choices at each of these stages are illustrated with a meta-analysis to examine the relationship between competition/facilitation intensity and productivity in plants. The analysis used a database of 296 cases in 14 studies. The results were unexpected and largely inconsistent with existing theory: competition intensity often significantly declined (rather than increased) with productivity, and facilitation was sometimes restricted to more productive (rather than less productive) sites. However, there was considerable variation in the pattern among response variables and measures of effect size. For example, on average, competitive effects on final biomass and survival decreased with standing crop, but competitive effects on growth rate did not. On the other hand, facilitative interactions were more common at low standing crop for final biomass and growth rate, but more common at high standing crop for survival. Results were more likely to be significant using the log response ratio (ln[removal/control]) as the effect size than using the relative competition intensity ([removal − control]/removal), although the trends for these conceptually similar indices did not differ. When all studies were grouped in a single meta-regression of interaction intensity on standing crop to test quantitative similarity among studies, survival showed the clearest negative relationship. However, when the same regressions were done for each unique combination of taxon and site within each study to test for qualitative similarity among studies, the slopes averaged over studies tended to be negative for biomass and growth rate, but not different from zero for survival. These results are subject to a number of caveats because of the limitations of the available data—most notably, the extension of effects of interactions on individual growth or survival to effects on population distribution and abundance or community structure is highly problematic. Nevertheless, the fact that none of the meta-analyses demonstrated a significant positive relationship between competition and standing crop but that we frequently found negative relationships is an important pattern that has not been apparent from qualitative surveys of individual studies, and it demonstrates the potential power of meta-analysis in ecology. We conclude with recommendations to overcome some of the limitations of the currently available data and meta-analytical procedures.

Responses of Early Successional Northern Hardwood Forests to Changes in Nutrient Availability

In many mesic forests the dominant trees are limited concurrently by light and soil resources, and understanding the mechanisms of competition and predicting outcomes of competition are especially difficult when co-limitation exists. We altered soil resource availability during the early stages of stand development after clearcutting of northern hardwood forests to examine the mechanism of competition. Specifically, we sought empirical evidence about the role of various physiological, morphological, allocational, and architectural responses in regulating plant competition. We expected the competitive ability of the extreme pioneer species, Prunus pensylvanica (pin cherry), to be enhanced by increased nutrient supply, with consequent effects at the community and ecosystem levels of organization. Nutrient availability was increased by about three-fold by monthly fertilization for 6 yr in nine even-aged northern hardwood stands dominated by pin cherry, three each of three ages (6, 12, and 18 yr at initiation of the experiment). Measurements in the control plots indicated that the interval of stand development from age 6 to 23 yr was marked by a peak in basal area and leaf area of pin cherry at about age 17 yr, followed by a steady decline in P. pensylvanica dominance thereafter. Fertilization increased and prolonged the dominance of P. pensylvanica, indicating that nutrient limitation accelerates the demise of this species during the second and third decades of stand development. All species in the plots responded to fertilization with increased foliar nutrient (N, P, and K) concentrations and often higher specific leaf area (area:mass ratio), and these responses were most pronounced for P. pensylvanica. Although the light-response curve for photosynthesis of P. pensylvanica was altered by fertilization, with higher rates at low light levels, photosynthesis of its principal competitor, Betula papyrifera, was not affected. The marked growth response of P. pensylvanica was accompanied by changes in its canopy architecture, as the trees had more leaf area per unit stem basal area, and proportionally more of this leaf area was in the upper canopy. In contrast, height and leaf area of B. papyrifera were similar in the control and fertilized plots. Seed deposition of P. pensylvanica also increased in the fertilized plots during one year of high seed production. Thus, the performance in competition of P. pensylvanica was improved by the removal of apparent nutrient limitations on its physiological performance, canopy growth, and ability to compete for light. Leaf area index of the fertilized plots was only slightly higher than the control plots, and the same was true for stand basal area. The removal of nutrient limitation increased the intensity of one-sided competition for light by concentrating the dominance among the largest trees; consequently, very high mortality of suppressed stems of all species occurred. The increased dominance of the fast-growing P. pensylvanica contributed to increases in aboveground net primary productivity (ANPP) in the fertilized plots. Some of this ANPP response was probably associated with reduced C allocation to roots in some of the fertilized plots, but this pattern was not consistent across all the stands. The results indicate that the outcome of interspecific competition in mesic forests, where co-limitation by light and soil resources prevails, depends upon the effect of site quality upon the relative intensity of one-sided competition (for light).

RESPONSES OF EARLY SUCCESSIONAL NORTHERN HARDWOOD FORESTS TO CHANGES IN NUTRIENT AVAILABILITY

In many mesic forests the dominant trees are limited concurrently by light and soil resources, and understanding the mechanisms of competition and predicting outcomes of competition are especially difficult when co-limitation exists. We altered soil resource availability during the early stages of stand development after clearcutting of northern hardwood forests to examine the mechanism of competition. Specifically, we sought empirical evidence about the role of various physiological, morphological, allocational, and architectural responses in regulating plant competition. We expected the competitive ability of the extreme pioneer species, Prunus pensylvanica (pin cherry), to be enhanced by increased nutrient supply, with consequent effects at the community and ecosystem levels of organization. Nutrient availability was increased by about three-fold by monthly fertilization for 6 yr in nine even-aged northern hardwood stands dominated by pin cherry, three each of three ages (6, 12, and 18 yr at initiation of the experiment). Measurements in the control plots indicated that the interval of stand development from age 6 to 23 yr was marked by a peak in basal area and leaf area of pin cherry at about age 17 yr, followed by a steady decline in P. pensylvanica dominance thereafter. Fertilization increased and prolonged the dominance of P. pensylvanica, indicating that nutrient limitation accelerates the demise of this species during the second and third decades of stand development. All species in the plots responded to fertilization with increased foliar nutrient (N, P, and K) concentrations and often higher specific leaf area (area:mass ratio), and these responses were most pronounced for P. pensylvanica. Although the light-response curve for photosynthesis of P. pensylvanica was altered by fertilization, with higher rates at low light levels, photosynthesis of its principal competitor, Betula papyrifera, was not affected. The marked growth response of P. pensylvanica was accompanied by changes in its canopy architecture, as the trees had more leaf area per unit stem basal area, and proportionally more of this leaf area was in the upper canopy. In contrast, height and leaf area of B. papyrifera were similar in the control and fertilized plots. Seed deposition of P. pensylvanica also increased in the fertilized plots during one year of high seed production. Thus, the performance in competition of P. pensylvanica was improved by the removal of apparent nutrient limitations on its physiological performance, canopy growth, and ability to compete for light. Leaf area index of the fertilized plots was only slightly higher than the control plots, and the same was true for stand basal area. The removal of nutrient limitation increased the intensity of one-sided competition for light by concentrating the dominance among the largest trees; consequently, very high mortality of suppressed stems of all species occurred. The increased dominance of the fast-growing P. pensylvanica contributed to increases in aboveground net primary productivity (ANPP) in the fertilized plots. Some of this ANPP response was probably associated with reduced C allocation to roots in some of the fertilized plots, but this pattern was not consistent across all the stands. The results indicate that the outcome of interspecific competition in mesic forests, where co-limitation by light and soil resources prevails, depends upon the effect of site quality upon the relative intensity of one-sided competition (for light).

Responses to Severe Competitive Stress in a Clonal Plant: Differences between Genotypes

The rhizomatous perennial grass (Amphibromus scabrivalvis) has the ability to be phalanx-like and guerrilla-like in its growth form, producing new ramets both from the base of established ramets (phalanx) and spread out along elongated rhizomes (guerrilla). Changes in clonal growth and allocation patterns of A. scabrivalvis in response to severe competitive stress induced by a background of Lolium perenne, a phalanx species, were studied. Eleven genotypes of A. scabrivalvis were divided into ramets and planted into pots in the glasshouse alone or into a background of dense L. perenne. Plants were harvested after 27 wk. Competitive stress was severe, with relative competition intensity for the genotypes ranging from 0.89 to 0.97. Both ramet production and mean interramet distances were significantly depressed by competition. Under competition, A. scabrivalvis showed a much higher allocation to roots and rhizomes compared to the control. This may serve to increase storage under competitive conditions. Genotypes producing a large number of ramets with high interramet distances in the control were more strongly affected by competition than genotypes producing fewer ramets with low interramet distances. This suggests that a more phalanx-like growth strategy may be selectively favored under severe competitive stress from a phalanx species in A. scabrivalvis. In communities dominated by perennial plants, general growth strategy, as exemplified by clone architecture, may be a critical determinant of population development and maintenance under highly competitive conditions.

Sperm Competition in Fishes: The Evolution of Testis Size and Ejaculate Characteristics

Fishes show one of the widest ranges of sperm competition intensity of any animal group. Here we present a comparative study whose aim is to investigate the effect of relative intensity of sperm competition on investment in spermatogenesis and the number and size of sperm produced. We find that both the gonadosomatic index (GSI = [gonad weight/body weight] x 100) and sperm numbers increase with intensity of sperm competition across species but that sperm length decreases. These new findings are consistent with a raffle-based mode of sperm competition in fishes. Most of these results (positive correlation of the GSI and sperm number with sperm competition intensity) concur with the predictions of current sperm competition theory. However, we also find that sperm longevity decreases with sperm length across species. Current models for continuous fertilization suggest that if length increases a sperm's speed but decreases its longevity, sperm length should increase with sperm competition intensity, whereas models for instant fertilization suggest that sperm length should remain constant. The negative relationship found between sperm competition and sperm length therefore does not fit predictions of either model.

Relationships between a tropical grass and lucerne on a solodic soil in a subhumid, summer - winter rainfall environment

Summary. Relationships between a tropical grass (Digitaria eriantha) and lucerne (Medicago sativa cv. Hunter River) grown in mixture were studied in a field experiment on a solodic soil on the Far North-West Slopes of New South Wales and a glasshouse experiment in simulated solodic profiles. Temperature and moisture conditions covering the range encountered at the field site were incorporated into the glasshouse study, together with high and low levels of available nitrogen. The relative yield ratio and its components, and the relative competition intensity for each species were used to investigate competitive trends with time. Digitaria growing with lucerne commonly showed a trend to smaller reductions of its yield with time, and eventually higher yields in mixture than in monoculture. This indicated a transfer of nitrogen from legume to grass. When such benefits to the grass coincided with only moderate reductions in lucerne yield, the mixture outyielded both monocultures. This occurred principally in the absence of applied nitrogen, at summer temperatures and at adequate levels of soil moisture. Lucerne was least competitive under very wet or dry conditions at summer temperatures, a situation aggravated by the dominance of digitaria in summer. Except in extreme cases, lucerne recovered its competitive strength when conditions were more favourable, especially in the autumn–spring period. When the time trend in the relative competition intensity of lucerne was near parallel to that of digitaria, the relative yield ratio was near constant, a condition of competitive equilibrium. Such a situation occurred under moderate temperature and moisture conditions most favourable to lucerne growth. This study indicates the importance of a high level of adaptation of a legume to climate and soil, for achieving competitive balance with an associated grass. It also suggests that management should include appropriate means to rectify any imbalance caused by deficiencies in legume adaptation, or summer dominance by the grass.

On Quantifying the Intensity of Competition Across Gradients

Several current theories about population and community ecology make predictions as to how the of competition will change in different environments, for example along gradients in resource availability (e.g., Grime 1979, Tilman 1982, 1988, Grubb 1985, Keddy 1989) or in predation (e.g., Harper 1969, Menge and Sutherland 1976). Testing such theories requires that of competition be quantified in a manner that will allow comparisons among different species or conditions. In this note I critique several recently published measures of and discuss more appropriate measures. A requirement for any general index of is that it standardize the growth or performance of individuals in competition to account for environmental differences that are not related to competition. For example, plants grown in competition at low nitrogen levels will generally be smaller than those grown at the same density at high nitrogen levels; this difference in growth or performance may be largely due to differences in nitrogen availability and not any inherent differences in the of competition. Recently, both Campbell and Grime (1992) and Turkington et al. (1993) have proposed and used competitive along soil-fertility and disturbance gradients. Construction and implications of the indices used in these studies have been discussed by Grace (1993, 1995). The two studies use different indices and come to somewhat different conclusions, but each uses a measure in which plant performance in interspecific competition is standardized by performance in intraspecific competition (monocultures). I suggest that such measures cannot be clearly interpreted and that previous studies using such measures (e.g., Campbell and Grime 1992, Turkington et al. 1993) should be evaluated cautiously. Campbell and Grime (1992) quantified competition by growing each of seven species in monocultures at 750 seeds/M2 (monoculture performance) and growing all seven species together in an additive design at a total density of 5250 seeds/M2 (mixture performance), then replicating each monoculture and the mixture across nutrient and disturbance gradients. Performance of each species was determined as total biomass per square metre. Their index was calculated as a relative competition intensity or RCI (Grace 1995):

On the Measurement of Plant Competition Intensity

Assumptions about how competition relates to environmental conditions lie at the heart of several models of plant community organization. According to the R-C-S (Ruderal-Competitor-Stress tolerator) model (Grime 1979), competition is presumed to increase with increasing habitat productivity. In contrast, the Resource Ratio model (Tilman 1982, 1988) assumes that total competition is unaffected by habitat productivity though the ratio of aboveground to belowground competition varies. Finally, the Habitat Templet model of plant communities (Taylor et al. 1990) assumes that competition is unrelated to habitat productivity but, instead, is controlled by the disturbance rate. It has been recently pointed out that the exact method used to quantify competition can fundamentally affect the interpretation (Grace 1993). In this paper I argue that different expressions of competition are not equally meaningful. Specifically, I show that the relativized index of competition accurately reflects mixture dynamics while the absolute index does not. There has been considerable previous discussion about the R-C-S and Resource Ratio models (Thompson 1987, Tilman 1987, Thompson and Grime 1988, Tilman 1989). Several experimental studies have recently been conducted to test between the R-C-S and Resource Ratio models. Campbell and Grime (1992) conducted greenhouse studies subjecting monocultures and mixtures of species to gradients of soil fertility and disturbance. They found that competition did not vary with productivity when expressed as the proportional decrease in growth due to competition (relative competition intensity or RCI). However, when the results were expressed as the absolute reduction in growth due to competition (absolute competition intensity or ACI), competition was observed to increase with increasing productivity. Campbell and Grime went on to argue that the expression of competition in relative form . . obscures the

Competitive Effects of Three Tropical Tree Species on Two Species of Piper

Owing to differences in morphology and growth rates among tree species, monospecific tree stands may differ in their ability to suppress the growth of understory plants through shading and root competition. I assessed relative understory suppression and intensity of root competition in year-old stands of Cordia alliodora, Hyeronima alchorneoides, and Cedrela odorata by measuring the growth of seedlings of the pioneer tree Piper auritum and cuttings of the shadetolerant shrub P. phytolaccaefolium that were planted into trenched and untrenched sites under the trees. After 5 mo, growth of P. auritum was significantly less under Cordia and Hyeronima than under Cedrela or in the open. The most severe growth reductions occurred under Hyeronima, the species with the highest leaf area index (LAI). Lack of a significant trenching response under Hyeronima suggests the competitive effects of this species were largely the result of shading. In contrast, the competitive effects of Cordia, a species with relatively high fine root density and open canopy, were almost entirely below ground. Nonetheless, the effects of Cordia and Hyeronima on the growth of P. auritum seedlings were equivalent when compared on a per-unit of tree canopy LAI basis. Lack of P. auritum growth suppression under Cedrela is likely the result of the open canopies and relatively low fine root densities of the Cedrela stands. In contrast to the growth of P. auritum, P. phytolaccaefolium growth did not differ among tree overstories. These results suggest that, among tree species, differences in morphology can lead to differences in understory suppression, but these differences depend upon the understory species.

POSTDISPERSAL SIBLING COMPETITION AND THE EVOLUTION OF SINGLE-SEEDEDNESS IN CRYPTANTHA FLAVA.

The evolution of indehiscent (unopening) single-seeded fruits dehiscent, multiseeded fruits has apparently occurred many times among flowering plants (Stebbins 1967; Cronquist 1968; Casper and Wiens 1981; Uma Shaanker et al. 1988). In some taxa, single-seededness is achieved through a reduction in ovule number, whereas in others more than one ovule is present, but only one matures into a seed. The repeated evolution of single-seededness together with indehiscence suggests strong selection pressure for single-seeded dispersal units (Cronquist 1968; Casper et al. 1992). Single-seeded dispersal units could have several advantages. For wind-dispersed species in which the entire fruit serves as the diaspore, a reduction in seed number can increase dispersal distance (Augspurger and Hogan 1983; Augspurger 1986), which is important when the probability of postdispersal seed predation and/or seedling mortality increases with local seed densities or proximity to the parent plant (Janzen 1970; Connell 1971; Howe and Smallwood 1982). However, because the association of single-seededness and indehiscence also occurs among species dispersed by animals (Casper et al. 1992), additional, more-general fitness advantages are likely. Packaging only one seed per fruit could minimize predispersal seed predation (Herrera 1984) or compensate for increases in the number of fruits or seed size (Adams 1967; Marshall et al. 1985). Uma Shaanker et al. (1988) suggest that the abortion of all but one ovule results from competition among the developing offspring and may not be in the best interest of the maternal plant. This study tests another possibility-that sibling competition arising when more than one seed germinates in the same location is an important factor selecting for single-seeded dispersal units (Casper and Wiens 1981; Casper et al. 1992). Sibling competition is used here to mean a density-dependent reduction in performance among interacting siblings relative to the performance of siblings when they are not interacting, the definition favored by Cheplick (1992). Sibling competition has also been applied to the relative intensity of competition among genetically similar individuals compared with competition among individuals less closely related (Williams and Mitton 1973; Williams 1975; Maynard Smith 1978). Although both forms of sibling competition may be relevant to some evolutionary problems, the first usage more often is considered important in the evolution of some dispersal and seed dormancy traits (Hamilton and May 1977; Venable and Lawlor 1980; Schoen and Lloyd 1984; Ellner 1986; Quinn and Engel 1986), whereas the latter form is more often hypothesized to be a factor in the evolution of sexual breeding systems and outcrossing (Williams and Mitton 1973; Williams 1975; Maynard Smith 1978; Taylor 1979; Bulmer 1980). Empirical studies, which have addressed mostly the second hypothesis (e.g., Ellstrand and Antonovics 1985; Schmitt and Antonovics 1986; Willson et al. 1987; Kelley et al. 1988; McCall et al. 1989; Tonsor 1989), differ widely in their findings (Cheplick 1992). In this study, competition among seedlings from the same dispersal unit is examined in the semidesert, herbaceous perennial Cryptantha flava. Like most members of the Boraginaceae, C. flava has four ovules; although all four ovules often initiate embryos, usually only one (sometimes two) develops into a mature nutlet (seed). At least eight congeneric species exhibit this fixed reduction in seed number, and in all cases, the nutlet is persistent within a highly pubescent calyx, which serves as the dispersal unit and is functionally analogous to an indehiscent fruit. The unit is dispersed by wind, but oneand two-

Potential Reproductive Rates and the Operation of Sexual Selection

In most animals, members of one sex compete more intensely for mates than members of the other sex, and show a greater development of secondary sexual traits. The relative intensity of mating competition in the two sexes depends on the operational sex ratio (OSR) (the ratio of males that are ready to mate to females that are ready to mate) at the site and times when mating occurs. The extent and direction of biases in the OSR is closely related to the potential rates of reproduction that individual males and females can achieve, although the distribution of the two sexes in space and time, sex differences in development time or life expectancy, and biases in the sex ratio at birth or hatching can also be important. The potential rates of reproduction in the two sexes are, in turn, affected by the proportion of time and energy expended by male and female parents on their progeny, though other factors may constrain reproductive rate in one or both sexes. We outline a simple model of the factors affecting the OSR, where relative parental expenditure by the two sexes and the adult sex ratio are fixed, and a more complex model where the adult sex ratio varies in relation to the reproductive activity of the two sexes. This framework for relating sex differences in mating competition to the OSR, potential reproductive rates, and parental expenditure differs from Triver's concept of the relation between parental investment and mating competition in three ways: first, it identifies the OSR as the immediate factor determining which sex completes most intensely for mates; second, it recognizes that factors other than the future fitness costs of rearing offspring can affect the potential reproductive rate of the two sexes; third, it suggests an empirical measure (potential reproductive rate) that can be estimated in natural populations and used to predict the distribution of mating competition.

Interactions between Larval Salamanders: A Case of Competitive Equality

We used a replicated field experiment to determine the relative intensity of intraspecific and interspecific competition between larvae of Siren intermedia and Notophthalmus viridescens. These two salamanders possess markedly different morphologies and life histories and belong to distantly related salamander families, but feed on similar prey. Each species was raised in artificial ponds, alone at either low density (3 larvae per pond, 1.95 m-2) or high density (6 larvae per pond, 3.90 m-2) or together in a 3:3 ratio for a combined density of 6 larvae per pond. These 5 treatments were each replicated 4 times in a total of 20 ponds. The experimental design allowed us to explicitly compare the relative intensities of intra- and interspecific competition under realistic conditions. Competition affected different components of fitness in each species. Both intraspecific and interspecific competition caused a decline in survival of Notophthalmus, but had no detectable effect on body size or length of larval period. Negative densitydependent mortality may regulate Notophthalmus populations in the larval stage of their complex life cycle. Intraspecific competition caused a decline in both the survival and growth of larval Siren, but interspecific competition caused only growth to decline. Decreased growth rates may increase the vulnerability of larval Siren to desiccation and to predation. We found no evidence to reject the null hypothesis that larvae of these two species are equal competitors at the densities we employed. Our results indicate that, despite dissimilarities in life history and morphology, and a distant phylogenetic relationship, larval Siren and larval Notophthalmus compete as equals. J.E. Fauth, W.J. Resetarits, Jr. and H.M. Wilbur, Dept of Zoology, Duke Univ., Durham, NC 27706, USA.

Effect of timing and rate of hexazinone application on weed control and early growth of Pinus radiata on two contrasting sites

Summary The effects of timing (1–12 months after planting) and rate (1–8 kg ha −1) of hexazinone application on weed control and the growth of Pinus radiata two years after planting were studied in two field trials established on contrasting sites at Longford and Neerim in Gippsland, Victoria. Hexazinone applications were compared with a routine treatment of amitrole + atrazine in mixture (1 + 4 kg ha −1), also applied from 1 to 12 months after planting. Growth of P. radiata was closely correlated with live-weed cover. At Longford, maximum weed control and growth response occurred with 4 kg ha −1 hexazinone, whereas at Neerim, growth of P. radiata continued to increase to 8 kg ha −1 hexazinone. Maximum tree growth at Longford was achieved for hexazinone application in August-September, 1–2 months after planting. At Neerim, there appeared to be greater flexibility in time of application than at Longford, with best responses being recorded for applications in the September to December period. Amitrole + atrazine generally resulted in less weed control and growth of P. radiata than hexazinone, and provided less flexibility with respect to time of application. It is hypothesised that differences between sites in the optimum time of hexazinone application are related to differences in rainfall, soil moisture storage, and the relative intensity of competition for moisture in the first summer as determined by the rate of establishment and species of weeds.

Predicting Plant Reproductive Success from Models of Competition for Pollination

Models of reproduction in plants competing for pollination were constructed on the basis of mechanisms of interaction. I first developed an analytical model that predicts influences of interspecific pollen movement and pollinator visit rate on fruit set. When the only form of competition is loss of pollen due to interspecific pollen movement, the relative intensity of competition diminishes as pollinator visit rate increases. Stigmatic interference from foreign pollen can markedly change this relationship, and with certain modes of interference fruit set may even drop as pollinators become very abundant. Application of the model using field measurements indicated that interspecific pollen loss alone can account for the lowered fruit set of the understory herb Stellaria pubera when competing with Claytonia virginica. Movements and pollen carryover of the observed insect pollinators of S. pubera were simulated on a computer. The program then utilized the observed relationship between seed set and pollen receipt to predict seed production. The simulations suggested that the effect of interspecific pollen movement is greatly reduced when plants are grouped into single-species patches, and that seed production depends on pollinator movement patterns and pollen carryover.