Effects Of Intraspecific Competition Research Articles

Demography, competitive interactions and grazing effects of intertidal limpets in southern New Zealand

Population dynamics and the effects of intraspecific competition on limpet growth and maintenance of bare patches were investigated for the intertidal limpet Cellana ornata (Dillwyn) at a boulder-dominated site and on a rocky platform near Kaikoura (South Island), New Zealand. Distribution and abundance patterns of C. ornata were described in relation to other biota and tidal height. C. ornata occurred almost exclusively in patches devoid of macroalgae, particularly in the mid-tidal zone. Both adult and juvenile limpets were most abundant on the tops of boulders, where their numbers were positively correlated with barnacle cover, which averaged 77%. The size structure and growth patterns of C. ornata were different between populations. Mark-recapture studies showed that the slopes of annual growth increments regressed on initial sizes were similar at both sites but that the annual increments on the platform were about 6 mm greater than on boulders. Growth virtually ceased at 27 mm for limpets at the barnacle-dominated boulder site and at 40 mm at the platform site. Recruiting cohorts had 20% survival on boulders and 37% on the platform during their first year. The largest size classes at both sites had around 57% annual survival. To test the effects of varying limpet densities on the growth and mortality of limpets and the maintenance of bare patches, densities of C. ornata were experimentally increased at both sites. Beyond a density of 4 per 0.25 m 2, sizes and survival of limpets were reduced at both sites, but the effect was more pronounced at the boulder site. Limpets at the boulder site were more effective at maintaining bare space than those on the reef platform. Enclosing limpets in plots with and without barnacles showed that C. ornata and a co-occurring species ( Cellana denticulata (Martyn)) grazed more effectively and had greater growth in cleared plots. Overall, there was considerable variation in the demographics of C. ornata between populations driven by site-specific characteristics.

Neighbourhood analysis in the savanna palm Borassus aethiopum: interplay of intraspecific competition and soil patchiness

A previous study of the spatial patterns of Borassus aethiopum (a humid savanna palm tree) led to the following predictions: H I. Trees and termite mounds positively influence all developmental stages; H2. There is intense competition between B. aethiopum juveniles; H3. Juvenile and seedling survival and growth rates are higher away from mother plants; H4. Mound positive effect overwhelms the effect of intraspecific competition for very young B. aethiopum individuals, the reverse applies to older, non-reproductive individuals. To test for these hypotheses the potential positive and negative neighbourhood effects were analysed. Progression of seedlings into the juvenile stage and juvenile growth were analysed through logistic regression models including the effects of four neighbourhood indices designed to model the respective influence of B. aethiopum juveniles and adults, trees and termite mounds. Statistical results explain the observed spatial pattern and reveal two main neighbourhood effects that influence both B. aethiopum juveniles and seedlings: a positive effect of mounds due to the fact that they constitute nutrient-rich soil patches, and a negative effect of juveniles due to intraspecific competition. Seedlings would be mostly affected by the mound positive effect, whereas juveniles would be mostly affected by competition.

Relative effects of exophytic predation, endophytic predation, and intraspecific competition on a subcortical herbivore: consequences to the reproduction of Ips pini and Thanasimus dubius.

We used a laboratory assay to partition the effects of predation and intraspecific competition on the establishment, mating success, and brood development of an endophytic herbivore. We selected a system in which the same predator feeds both exophytically and endophytically on the same prey, to evaluate the role of herbivore feeding guild on predator numerical and functional responses. The bark beetle, Ips pini (Coleoptera: Scolytidae) reproduces within the stems of conifers. Males establish mating chambers under the bark, produce aggregation pheromones, and are subsequently joined by females that construct ovipositional galleries. Thanasimus dubius (Coleoptera: Cleridae) adults prey on adults alighting on the bark surface. T. dubius females then oviposit at the bark beetles' entrance sites, and their larvae prey on developing bark beetle larvae within the tree. We imposed a controlled 3×3 factorial design of prey and predator adult densities on red pine logs. Both predation and competition decreased I. pini reproduction. However, the per capita effect of predation was greater than competition, with one adult T. dubius reducing herbivore reproduction by an equivalent amount as four to five competing males and their harems. Increased densities of adult T. dubius on the plant surface reduced the number of prey captured per predator. Total predation on adults and larvae was similar. However, adult T. dubius on the plant surface ate approximately 18-35times more I. pini per day than did their endophytic larvae. Within the plant, cannibalism among T. dubius, low herbivore densities, limited feeding times, and presumably the complex gallery architecture of I. pini reduced the number of predator progeny. The progeny of I. pini showed even sex ratios in the absence of predators, but were female biased when predators were present. We quantified a relatively narrow set of predator and prey densities that can generate replacement rates greater than one for this predator that specializes on endophytic herbivores. We attribute some of the benefits of an endophytic lifestyle not only to escape from generalist predators, but also to relatively low functional and numerical responses of adapted predators.

The neighborhood matters: effects of neighbor number and sibling (or kin) competition on floral traits in Spergularia marina (Caryophyllaceae).

Effects of abiotic factors on the expression of floral and gender traits have been well documented in wild plant species; by contrast, little is known of the effect(s) on an individual's floral phenotype of the genetic composition of its neighboring conspecifics. Here we report the results of a greenhouse experiment conducted to detect the effects of genetic attributes of an individual's local environment on the expression of floral traits in the selfing annual, Spergularia marina (Caryophyllaceae). First, to test the hypothesis that negative effects of intraspecific competition are stronger when an individual competes with genetically similar individuals than when it competes with unrelated genotypes, we evaluated the effects of the number of nearest-neighbor kin (vs. unrelated individuals) on floral traits. Plants adjacent to two kin produced significantly fewer stamens per flower than plants adjacent to one or no siblings, indicating that kin competition reduced allocation to male function. Second, to test the hypothesis that the genetic diversity of a neighborhood influences the phenotype of a focal plant, we determined whether the number of maternal families represented among an individual's nearest neighbors influences its phenotype. The number of maternal families surrounding a focal plant did not affect floral trait expression. These results suggest that in S. marina, male function is more sensitive than female function to the genetic environment and that as an individual's genetic similarity to its neighbors increases, so do the negative effects of competition for limited resources.

Effect of water deficits on within-plot variability in growth and grain yield of spring wheat in northwest China

For adapted cultivars under normal crop densities, biological yield is largely determined by the pool of available resources, e.g. water, nutrients and photosynthetically active radiation, while the nature and intensity of intraspecific competition plays an important role in determining the magnitude of harvest index (HI). Water deficits can drastically reduce the HI from its genetic potential to zero. This study was conducted to determine the effect of drought-weighted intraspecific competition on the HI and, consequently, the grain yield in spring wheat populations along a natural moisture gradient in northwestern China. Along the natural moisture gradient (annual mean rainfall decreased 328→204→185 mm per year, supplemented with 70 mm of irrigation), culm size inequality (as measured by the Gini coefficient of above-ground biomass per culm) always increased, and Lorenz curves were more concave. HI decreased significantly in 1999 (0.364→0.345→0.307) and 2000 (0.341→0.303→0.251). There was a significant negative correlation between the Gini coefficient and the HI of spring wheat along the moisture gradient ( R 2=0.92, P<0.01). These results suggested that size hierarchies in spring wheat populations are closely correlated with the water regime in the field, and that under greater drought stress there are relatively more smaller plants with lower HI (size-dependent reproductive allocation). Size inequality is an index of competitive status in plant populations under stress environments. Agriculturally, greater size inequality may result in a competitive cost for energy and photosynthetic products, in other words, growth redundancy, which is detrimental to reproductive allocation and consequently, grain yield. The results support the view that stand uniformity in field crops is an important mechanism for increasing grain yield.

A Model of Simultaneous Evolution of Competitive Ability and Herbivore Resistance in a Perennial Plant

Plant populations often experience the joint effects of intraspecific competition and herbivory, yet the impact of the interaction of these two factors on the outcome of evolution is largely unknown. Here, we develop a spatially explicit simulation model to examine interactions between the evolution of herbivore resistance and competitive ability in the goldenrod Solidago altissima. We define competitive ability as either competitive effect, the ability of a plant to deplete resources and make them unavailable to competitors, or competitive response, the ability to grow, survive, and reproduce despite depletion of resources by neighboring competitors. We considered symmetric and asymmetric modes of competition and explored the following questions: (1) Does the selective effect of competition differ for the two components of competitive ability? (2) What are the effects of the evolution of competitive ability and resistance on each other? (3) Can trade-offs between competitive ability and resistance emerge, given no relationship between these two traits prior to selection? Our results showed that competitive response evolved quickly regardless of the mode of competition, but self-suppression hindered the evolution of competitive effect. The evolution of resistance appeared to be independent of the evolution of competitive ability. Intraspecific competition was the major selective force in our model. At natural levels of herbivory, selection for resistance played a secondary role in structuring the population. Resistant genotypes were only favored at very low resistance costs. At high cost levels, the costs of maintaining resistance far outweighed the benefits. The selective forces of competition and herbivory resulted in trade-offs between competitive response and herbivore resistance, but only at low costs of resistance. Vigorous growth associated with a high competitive response might translate into trade-offs between herbivore tolerance and resistance. The strong selective effects of competitors, coupled with the weaker selection from herbivores, suggest that plant traits directly associated with growth that confer tolerance to both competitors and consumers may be the targets of selection.

Temperature-Dependent Interactions between Juvenile Steelhead and Sacramento Pikeminnow in Laboratory Streams

We examined the temperature dependence of interactions between juvenile steelhead Oncorhynchus mykiss and juvenile Sacramento pikeminnow Ptychocheilus grandis in laboratory streams. Growth of dominant steelhead in water 20-23°C was reduced by more than 50% in trials with Sacramento pikeminnow compared with trials with steelhead alone. Comparison of the two-species treatment with one with an equal density of fish comprised of steelhead alone indicated that the per capita effect of Sacramento pikeminnow was similar to the intraspecific effect of steelhead in water 20-23°C. In contrast, the growth of dominant steelhead in water 15-18°C was unaffected by Sacramento pikeminnow. The temperature-dependence of the effect of Sacramento pikeminnow on dominant steelhead was also reflected in the size of the area defended by steelhead and in the behavior of Sacramento pikeminnow under the two thermal regimes. Compared with all other treatment combinations, dominant steelhead defended the smallest areas when accompanied by Sacramento pikeminnow in warm water. Behavioral interactions with steelhead initiated by Sacramento pikeminnow were about 50 times more frequent in warm water. For the growth of subdominant steelhead, the effect of intraspecific competition exceeded the interspecific effect of Sacramento pikeminnow regardless of water temperature. While thermal regime affects the distributions of these two species, this experiment suggests that it also affects the outcome of competition between them.

SIZE-DEPENDENT INTERACTIONS BETWEEN TWO TERRESTRIAL AMPHIBIANS, PLETHODON CINEREUS AND PLETHODON GLUTINOSUS

We used two field experiments to examine the nature and effects of size-dependent interactions between two terrestrial salamanders, Plethodon cinereus and Plethodon glutinosus. We reared juveniles of P. cinereus in an enclosure alone, with a similarly-sized conspecific, with a similarly-sized P. glutinosus, and with a larger P. glutinosus. We also reared juveniles of Plethodon glutinosus alone. We found that both intraspecific and interspecific competition reduced growth in juveniles of P. cinereus, but the effect of intraspecific competition did not differ from that of interspecific competition. Interspecific competition reduced growth in juveniles of P. glutinosus. Size asymmetry did not appear to affect competition between juveniles. Size, not species identity, determined the use of the space beneath wooden cover objects placed in the enclosures.In the second experiment, we housed adults of P. cinereus and juveniles of P. glutinosus with similarly-sized conspecific individuals to simulate intraspecific competition. Adults of P. cinereus were also paired with similarly-sized, smaller, and larger individuals of P. glutinosus. Interspecific competition did not affect growth differently than intraspecific competition for P. cinereus, and the size of the competitor did not affect the growth of this species. Interspecific competition reduced the growth of P. glutinosus significantly more than intraspecific competition and had the greatest effect when competing individuals were similarly-sized.

Effects of intraspecific competition and food deprivation on the immature phase of Ascia monuste orseis (Lepidoptera, Pieridae)

The effect of intraspecific competition for food on larvae and of food deprivation for 24 h on 2nd and 4th instars of Ascia monuste orseis (Godart, 1819) was investigated. Intraspecific competition for food during the immature phase leads to long pupation time, high larval mortality, reduced adult weight, and reduced number of eggs per female. In food deprivation experiments, the major differences in A. monuste orseis performance were long pupation time in the group that was deprived during the 2nd instar; and a negative effect on reproduction in the group that was deprived during the 4th instar, with reduced adult weight. Both food deprived periods tested are critical, and deprivation during the 2nd instar seems to have an effect as drastic as during the 4th instar because it directly affects larvae survival. Immatures can resist food deprivation for 24 h during the 2nd and 4th instars (low mortality), have a compensatory behaviour (high ingestion and biomass gain) during the 5th instar, and do not demonstrate cannibalistic behaviour during food deprivation.

Connecting geographical distributions with population processes

The geographical distribution of a species is determined by a large number of complex processes operating over spatial scales spanning 10 orders of magnitude. Patterns in population processes have been described at numerous scales. We show that two patterns, measured at different scales, jointly allow us to infer heretofore unknown patterns in the distribution of demographic patterns across the geographical range of a species. The resulting model describes three fundamentally different modes of geographical variation in vital rates of populations. One mode is characterized by a positive nonlinear relationship between the maximum rate of population growth and the intensity of intraspecific competition across a geographical range. That is, populations that grow rapidly are also those where individuals experience the greatest per capita negative effect of the presence of other individuals. The second mode of behaviour is described by a negative nonlinear relationship between maximum growth rate and density dependence. Under this scenario, populations with low capacity to grow rapidly have highest intensities of intraspecific competitive effects. A third mode of behaviour is characterized by a weak positive relationship between growth rate and intraspecific competition, with very little geographical variation in maximum growth rate. A survey of studies relating temporal means and variances in population abundance for a variety of species indicate that the second mode of geographical variation in population dynamics across species ranges is the most common, though a few species appear to be characterized by the third mode.

Inequalities in size and intensity-dependent growth in a mermithid nematode parasitic in beach hoppers.

Inequality in body sizes is a common feature in populations of helminth parasites, with potential consequences for egg production and population genetics. Inequalities in body lengths and the effects of intraspecific competition on worm length were studied in a species of mermithid nematode parasitic in the crustacean Talorchestia quoyana (Amphipoda: Talitridae). The majority of the 753 worms recovered were relatively small, and an analysis using a Lorenz curve and Gini coefficient suggested that there were no marked inequalities in body lengths among the worms. Total worm length in the 356 infected amphipods (i.e. the sum of the lengths of all the worms in a host) increased steadily as a function of the number of worms per amphipod, whereas the length of the longest worm per amphipod peaked in amphipods harbouring intermediate numbers of worms. This last result was not significantly accounted for by the observed increase in host size with increasing intensity of infection, but resulted from a correlation between worm length and host size. As the number of worms per amphipod increased, the relative sizes of the second-, third-, and fourth-longest worms per host increased markedly. This means that relative inequalities in sizes become less pronounced, i.e. subordinate worms get closer in size to the longest worm, as the number of worms per host increases. The main consequence of this phenomenon is that worm sizes in the mermithid population are more homogeneous than they would be if intraspecific competition had stronger effects on worm growth.

Interactive effects of initial size, stocking density, and type of predator deterrent netting on survival and growth of cultured juveniles of the soft-shell clam, Myaarenaria L., in eastern Maine

Recent declines in commercial harvests of soft-shell clams, Mya arenaria L., in Maine, USA, have prompted state and local officials to consider enhancing wild stocks with hatchery-reared seed. We conducted two manipulative field experiments in the soft-bottom intertidal zone during 1990–1991 in eastern Maine to assess effects of predation, intraspecific competition, and initial planting size on the survival and growth of cultured individuals of Mya. Experiment I (23 June 1990 to 13 June 1991) tested interactive effects of two planting sizes (small=8.5 mm shell length (SL); large=11.8 mm SL) and protective netting on fate and growth of clams. Animals of each size were added to separate experimental units within each of 60 1-m 2 areas delimited by a wooden box. To deter predators, 50 boxes were covered with a specific type of plastic netting that differed in aperture size (4.2, 6.4 and 12.8 mm) and degree of rigidity (flexible vs. extruded) while 10 boxes served as controls (without netting). Small clams grew at a faster rate than large clams, but both added approximately 18 mm of new shell by the end of the study. Growth was unaffected by netting size and rigidity, but 13% more clams were recovered alive after a year in protected vs. unprotected treatments (84% vs. 71%). Survival was independent of netting type. The presence of netting resulted in nearly a 3× enhancement of wild spat (<15 mm SL) compared to unprotected controls (568.8±24.4 vs. 199.6±22.8 m −2). This result suggests that the decline of wild stocks in eastern Maine may not be related to recruitment failure, but to post-settlement events, such as predation, which remove clams from the intertidal. In Experiment II (15 April to 6 October 1991), clam (14.6±0.2 mm SL) density was manipulated across four levels from 333 to 2664 m −2 in protected (extruded netting, 12.8 mm aperture) and unprotected 1-m 2 boxes. Survival within unprotected boxes was independent of stocking density (79%), but was inversely density-dependent in protected boxes (77% in the lowest density treatment increasing to a mean of 88% in the other three treatments). A negative cubic relationship explained the effect of density on growth. We present the first mariculture strategy for public stock enhancement or private entrepreneurs interested in rearing M. arenaria in Maine and the northeast US. Hatchery-reared juveniles 8–10 mm SL should be planted in the spring near or below mid tide levels at densities between 333 and 666 m −2 and protected with flexible netting (6.4 mm aperture) raised several centimeters above the sediment surface. Netting should be removed from mud flats in the late fall before the threat of ice and severe winter storms. Animals should attain sizes between 25 and 30 mm SL during this time and reach a size refuge from burrowing and other predators. Growth to legal, commercial size (50.8 mm SL) should take another 2–4 years depending on geographic location and mean seawater temperature.

Effects of intraspecific competition and scavenging on growth of the periwinkle Littorina littorea

The effects of density of conspecifics, size of experimental cages, and scavenging of ani- mal tissue on the growth rate of Littorina littorea (L.) were tested during the summers of 1991, 1992, and 1994. In 1991, the effects of intraspecific competition were examined by holding individually marked periwinkles in cages of 5 different sizes and at 4 densities. Cages were 25, 50, 100, 200, and 400 cm 2 in area, and were of different sizes to test for caging artifacts. Experimental densities were 50, 100, 200, and 400 m -2 and encompassed normal field densities (x = 299 m -2 ; SD = 205). Growth was measured as final shell height and the amount of shell added at the lip of the aperture. The experiment showed evidence for intraspecific competition and no effect of cage size. Periwinkles, which were between 12 and 16 mm in shell height, added an average of 6.8 mm per 6 wk at the lip when held at a density of 50 m -2 and 1.6 mm per 6 wk when held at 400 m -2 . The results were con- firmed with a 1992 experiment, which used 100 cm 2 cages and densities of 100, 200, and 400 m -2 . Field observations suggested that L. littorea may also scavenge animal tissue, and this was tested in a 1994 experiment in which periwinkles were provided mussel (Mytilus edulis) tissue. Growth rates were nearly doubled when snails were given mussel tissue. While L. littorea is one of the most stud- ied gastropods, this study is the first experimental test of intraspecific competition, and the first demonstration that scavenging can enhance growth rates of periwinkles.

HABITAT DE LA TRUITE COMMUNE (SALMO TRUTTA L.) PENDANT LA PÉRIODE JUVÉNILE EN RUISSEAU : PRÉFÉRENCES, MOUVEMENTS, VARIATIONS JOURNALIÈRES ET SAISONNIÈRES.

This paper summarizes our recent investigations on habitat requirements for juvenile brown trout (Salmo trutta) in small tributaries of a lowland river system, the Scorff River (Brittany, France). Preferences for water velocity, depth, substrate and shelter, temporal variations in habitat use, and effects of predation and intra-specific competition on habitat choice, were studied both at field and laboratory. Using results from the literature, we propose a synthesis of the diel and seasonal shifts in habitat use by juvenile brown trout, Bull. Fr. Peche Piscic. (2002) 365/366 : 435-454 — 435 —

The effects of ultraviolet‐B radiation and intraspecific competition on growth, pollination success, and lifetime female fitness in Phacelia campanularia and P. purshii (Hydrophyllaceae)

While a considerable amount of attention has been devoted to the effects that increased ultraviolet-B (UV-B) radiation has on vegetative plant growth and physiological function, the impact that UV-B may have on plant fitness has been the focus of fewer studies, with attention given primarily to a few crop species. Further, the possible interactions between UV-B and additional potential stresses found in natural environments have rarely been studied experimentally. Because the reported effects of increased UV-B on plant growth and fitness have been highly variable, studies that focus on factors that may lead to these differences in results are important for the formulation of accurate predictions about future plant success under varying UV-B levels. We examined the effects of UV-B dose and intraspecific competition on growth, phenology, pollen production, pollination success, fruit and seed production, and offspring quality in two species of Phacelia. Increased UV-B was neutral or beneficial for all traits, while competition was neutral or detrimental. There were no significant interactions between UV-B and competition in the parental generation. Phacelia campanularia offspring were unaffected by parental competition, but derived indirect beneficial effects on germination, growth, and fitness traits from parental enhanced UV-B.

THE NEIGHBORHOOD MATTERS: EFFECTS OF NEIGHBOR NUMBER AND SIBLING (OR KIN) COMPETITION ON FLORAL TRAITS IN SPERGULARIA MARINA (CARYOPHYLLACEAE)

Effects of abiotic factors on the expression of floral and gender traits have been well documented in wild plant species; by contrast, little is known of the effect(s) on an individual's floral phenotype of the genetic composition of its neighboring conspecifics. Here we report the results of a greenhouse experiment conducted to detect the effects of genetic attributes of an individual's local environment on the expression of floral traits in the selfing annual, Spergularia marina (Caryophyllaceae). First, to test the hypothesis that negative effects of intraspecific competition are stronger when an individual competes with genetically similar individuals than when it competes with unrelated genotypes, we evaluated the effects of the number of nearest-neighbor kin (vs. unrelated individuals) on floral traits. Plants adjacent to two kin produced significantly fewer stamens per flower than plants adjacent to one or no siblings, indicating that kin competition reduced allocation to male function. Second, to test the hypothesis that the genetic diversity of a neighborhood influences the phenotype of a focal plant, we determined whether the number of maternal families represented among an individual's nearest neighbors influences its phenotype. The number of maternal families surrounding a focal plant did not affect floral trait expression. These results suggest that in S. marina, male function is more sensitive than female function to the genetic environment and that as an individual's genetic similarity to its neighbors increases, so do the negative effects of competition for limited resources.

Response Surface Experimental Designs for Investigating Interspecific Competition

Competition is an important interaction in ecology, and many experiments have been done to examine the effects of intraspecific and interspecific competition. Unfortunately, most of these experiments have been done using either substitution or additive experimental designs. Substitution designs, and to a lesser extent additive designs, severely limit the useful inferences that ecologists can draw from the resulting data. Response surface experimental designs, which vary the densities of two competing species independently, offer the advantage of being able to compare the fits of alternative competition models and to estimate model parameters. This kind of experimental design has been relatively little used in ecology to date. I consider the use of response surface designs from two perspectives. The first is the optimal allocation of experimental effort among density treatments, with the goal of distinguishing among alternative models. Second, I compare the ability of six response surface designs to estimate known competition coefficients correctly, using simulated data. Low densities and densities near equilibrium in one generation tend to return similar densities the following generation and thus provide little or no information for comparing alternative population models. Experimental designs that span a wide range of densities, including high densities above carrying capacities, provide more accurate parameter estimates in general, but low densities are helpful for estimating population growth rates. Designs that are not fully factorial are one compromise solution that can yield accurate parameter estimates for models and yet require less experimental effort than full factorial surfaces. In general, response surfaces are a powerful class of experimental design and offer potentially stronger connections between empirical and theoretical approaches than traditional experimental designs.

RESPONSE SURFACE EXPERIMENTAL DESIGNS FOR INVESTIGATING INTERSPECIFIC COMPETITION

Competition is an important interaction in ecology, and many experiments have been done to examine the effects of intraspecific and interspecific competition. Un- fortunately, most of these experiments have been done using either substitution or additive experimental designs. Substitution designs, and to a lesser extent additive designs, severely limit the useful inferences that ecologists can draw from the resulting data. Response surface experimental designs, which vary the densities of two competing species independently, offer the advantage of being able to compare the fits of alternative competition models and to estimate model parameters. This kind of experimental design has been relatively little used in ecology to date. I consider the use of response surface designs from two perspectives. The first is the optimal allocation of experimental effort among density treatments, with the goal of distinguishing among alternative models. Second, I compare the ability of six response surface designs to estimate known competition coefficients correctly, using sim- ulated data. Low densities and densities near equilibrium in one generation tend to return similar densities the following generation and thus provide little or no information for comparing alternative population models. Experimental designs that span a wide range of densities, including high densities above carrying capacities, provide more accurate pa- rameter estimates in general, but low densities are helpful for estimating population growth rates. Designs that are not fully factorial are one compromise solution that can yield accurate parameter estimates for models and yet require less experimental effort than full factorial surfaces. In general, response surfaces are a powerful class of experimental design and offer potentially stronger connections between empirical and theoretical approaches than traditional experimental designs.

Effects of intraspecific competition on size variation and reproductive allocation in a clonal plant

Clonal plants grow in diameter rather than height, and therefore competition among genets is likely to be symmetric and to result in smaller variation in size of genets than in non‐clonal plants. Moreover, clonal plants can reproduce both sexually and vegetatively. We studied the effects of density on the size of rosettes and of clones, variation in the size of rosettes and of clones, and allocation to sexual and vegetative reproduction in the clonal herb Ranunculus reptans. We grew plants from an artificial population of R. reptans in 32 trays at two densities. After four months, differences in density were still apparent, although clones in the low‐density treatment had on average 155% more rosettes and 227% more rooted rosettes than clones in the high‐density treatment. The coefficient of variation of these measures of clone size was 15% and 83% higher, respectively, in the low‐density treatment. This indicates that intraspecific competition among clones of R. reptans is symmetric and increases the effective population size. Rooted rosettes were larger and varied more in size in the low‐density treatment. The relative allocation of the populations to sexual and to vegetative reproduction was 19% and 13% higher, respectively, in the high‐density treatment. Moreover, seeds produced in the high‐density treatment had a 24% higher mass and a 7% higher germination percentage. This suggests that with increasing density, allocation to sexual reproduction increases more than allocation to vegetative reproduction in R. reptans, which corresponds to the response of some other species with a spreading growth form but not of species with a compact growth form. We conclude that intraspecific competition is an important factor in the life‐history evolution of R. reptans because intraspecific competition affects its clonal life‐history traits and may affect evolutionary processes such as genetic drift and selection through its effect on the effective population size.

Inbreeding depression under intraspecific competition in a highly outcrossing population of Crepis sancta (Asteraceae): evidence for frequency‐dependent variation

Inbreeding depression is a major selective factor acting to maintain outcrossing in hermaphroditic plants. Recently it has been shown that environmental conditions may greatly affect the levels of inbreeding depression. In this study, the effects of intraspecific competition, from either crossed or inbred progeny, and plant density on the expression of inbreeding depression were estimated for the allogamous colonizing plant Crepis sancta (Asteraceae). The population used in this experiment showed a very high outcrossing rate (t = 0.99). Inbreeding depression was measured for germination, survival to reproduction, number of capitula, and a multiplicative fitness estimate. We found that inbreeding depression on survival, the number of capitula, and total fitness was the greatest when inbred plants compete with outbred plants. The effect of density on inbreeding depression was less evident. The major implication of our study is that the relative fitness of selfed progeny is strongly influenced by the type of competitors (outbred or inbred). These results support the hypothesis that inbreeding depression varies according to the density and frequency of outbred plants and suggest that it could be considered as a density- and frequency-dependent phenomenon.