Low Competitive Ability Research Articles

Influência de Brachiaria brizantha nas características fisiológicas relacionadas a fotossíntese e uso eficiente da água em cana-de-açúcar

O aumento da população das plantas daninhas eleva a habilidade competitiva dessas com as plantas cultivadas, podendo causar interferência nos aspectos fisiológicos das culturas. Objetivou-se com o trabalho avaliar a interferência de populações de Brachiaria brizantha nas características relacionadas a atividade fotossintética e o uso eficiente da água das cultivares de cana-de-açúcar RB72454, RB867515 e SP80-1816. Foi conduzido um experimento a campo, onde os tratamentos foram constituídos por 12 populações de plantas de B. brizantha que emergiram juntamente com a cultura. Aos 120 dias após a emergência da cana-de-açúcar foram realizadas as avaliações da concentração de CO2 subestomático (Ci - µmol mol-1), a atividade fotossintética (A - µmol m-2 s-1), a condutância estomática (Gs - mol m-1 s-1), a taxa de transpiração (E), a eficiência do uso da água (EUA) e a massa seca da parte aérea (SDM - g por planta) da cultura. Todas as variáveis fisiológicas e a massa seca da cultivar RB72454 foram afetadas pelo aumento da densidade das plantas de B. brizantha, bem como a transpiração da cultivar RB867515. A cultivar RB72454 apresenta menor habilidade competitiva com as plantas daninhas e a atividade fotossintética é limitada por fatores que reduzem o influxo de CO2 para o interior da folha.

Habilidade competitiva entre biótipos de Cyperus difformis L. resistente ou suscetível a herbicidas inibidores de ALS e destes com arroz irrigado

Em biótipos de plantas daninhas resistentes a herbicidas pode haver menor habilidade competitiva, comparativamente aos biótipos suscetíveis. O objetivo da pesquisa foi investigar a habilidade competitiva entre biótipos de Cyperus difformis L. resistente ou suscetível a herbicidas e destes com arroz irrigado. Foram realizados quatro experimentos em casa de vegetação, em 2007 e 2008, em delineamento inteiramente casualizado, com quatro repetições, sendo os tratamentos do primeiro arranjado em série aditiva e dos demais arranjados em série de substituição. Em cada experimento, as proporções entre biótipos de C. difformis resistente e suscetível e destes com arroz foram de 100:0, 75:25, 50:50, 25:75 e 100:0%, com população total de 1060 plantas m-2. As variáveis avaliadas foram área foliar, estatura e matéria seca da parte aérea. A análise da competitividade foi realizada por aplicação de diagramas e interpretações dos índices de competitividade. Biótipos de C. difformis resistente e suscetível a herbicidas inibidores de ALS, em geral, possuem habilidade competitiva equivalente; a cultura do arroz, com a cultivar BRS Querência, tem habilidade competitiva superior a C. difformis resistente ou suscetível a herbicidas inibidores de ALS; para a cultura do arroz, a competição intraespecífica é mais importante, enquanto para C. difformis resistente ou suscetível a herbicidas inibidores de ALS, a mais importante é a competição interespecífica.

Waterlogging and canopy interact to control species recruitment in floodplains

Summary 1. The extent to which seedling recruitment contributes to local functional diversity depends on the environmental filters operating in a plant community. Classical community assembly models assume that habitat constraints and competition act like hierarchical filters with habitat filtering as the dominant one. Alternative models assume a synergic interaction since responses to environmental stress and competition may impose physiological trade‐offs in plants. 2. River floodplains are an ideal system to test the relationship between habitat and competition filtering in community (re)assembly, as flooding causes changes in both habitat stress (waterlogging, resulting in anoxia and toxicity) and competition (dieback of vegetation) on one hand and acts as an effective seed dispersal vector on the other hand. 3. We conducted a mesocosm experiment on early community assembly from a pool of 34 floodplain species covering a wetness gradient. Seed mixtures were sown in a full factorial design with water level, canopy and mowing as controlling factors. We measured the biomass of all species after one growing season and determined germination and seedling growth traits, both outside (response to waterlogging/no waterlogging) and in a growth‐chamber (response to light/darkness). 4. Species recruitment was analysed in relation to the controlling factors and measured functional traits using co‐inertia analysis. Furthermore we analysed the effects of the controlling factors on several aspects of functional diversity. 5. There was no establishment in grass sward, unless mowing was applied. Species‐rich communities only developed when germination and early establishment phases occurred on waterlogged bare soil. High water level did not suppress establishment but reduced the total biomass and lowered inter‐specific competition. The effect of mowing on species richness depended upon the interplay between waterlogging and canopy. 6. Establishment success under canopy required seedling strategies to tolerate shade. The elimination of typical wetland specialists from oxic mesocosms was clearly an effect of their poorer and/or slower germination and lower competitive abilities in comparison to non‐wetland plants, leading to their disappearance in this low‐stress environment. 7. Our results indicate that single stress factors can enhance species richness and functional diversity through limiting competition but a synergic interaction of different stresses can lead to reduced richness.

Does relative time of emergence affect stand composition and yield in a grass–legume mixture? Kura clover (Trifolium ambiguum)–meadow bromegrass (Bromus biebersteinii) and Kura clover–orchardgrass (Dactylis glomerata) mixtures

AbstractEstablishing Kura clover (Trifolium ambiguum) in mixtures with grass species is challenging, because slow growth of clover seedlings results in low competitive ability. This study examined establishment success by altering time of seeding of the grass component to reduce competition with Kura clover seedlings. Two trials, one of Kura clover–meadow bromegrass (Bromus biebersteinii) and the other Kura clover–orchardgrass (Dactylis glomerata) mixtures were planted in Edmonton, Alberta. Grasses were seeded at the same time as the clover, or introduced when the clover reached one true leaf or three true leaves, in the autumn of the planting year or the following spring. Species composition varied significantly between treatments. When sown at the same time, Kura clover contributed 31 and 14% of yield in the establishment year when sown with meadow bromegrass and orchard grass, respectively. Delaying grass sowing until Kura clover had one or three leaves gave a higher percentage of Kura clover compared with planting at the same time. Autumn and spring grass sowing resulted in stands of 78 and 80% clover with meadow bromegrass, and 74 and 67% clover with orchardgrass. Altering the competitive advantage of the grass species to produce a more balanced mixture was successfully achieved by delaying seeding of the grass relative to Kura clover. A long interval before introducing the grass (autumn or following spring), was not successful as established Kura clover seedlings have an increased competitive ability.

Forage and seed yield response of lucerne cultivars to chemically weeded and non-weeded managements and implications for germplasm choice in organic farming

Organic livestock production is steadily increasing in Europe. Lucerne ( Medicago sativa L.) can play a key role in organic crop-livestock systems of southern Europe, owing to its positive attributes of adaptation, forage quality and nitrogen provision. This study tackled the unprecedented question on whether specific variety recommendation and, possibly, specific breeding are necessary for lucerne in organic systems. In lowland of northern Italy, where competition against weeds is expected to be the main determinant of success in organic farming, eight cultivars were evaluated in one environment under a chemically weeded management and an ‘organic’ management mimicked by no weed control, with the objectives of: (i) assessing the extent of cultivar × management interaction for dry matter (DM) and seed yield traits; and (ii) comparing different germplasm types for competitive ability and suitability to organic farming. The experiment was designed as a split-plot holding managements on main plots and cultivars on sub-plots. The test germplasm encompassed varieties bred under conventional or organic management and landraces from northern and central Italy, as well as germplasm with semi-erect or semi-prostrate habit. The recorded traits were lucerne DM yield over 3 years (13 harvests), DM yield over the first harvest of both the fourth and fifth year (as a measure of persistence), seed yield and its components assessed in the third year, and competitive ability against weeds as expressed by lucerne proportion under ‘organic’ management. Weed competition caused a severe reduction of DM yield and lucerne proportion in the ‘organic’ management but just a slight, non-significant reduction of seed yield, for which the greater stand thinning in this management relative to the chemically weeded one was largely compensated by increases in racemes per stem and seed weight. No cultivar × management interaction was observed for 3-year DM yield, seed yield and persistence ( P > 0.05), suggesting that specific variety testing under organic management is not necessary. Greater relative growth rate, implying both better DM yield in the absence of severe weed competition and greater competitive ability under organic management, was inferred as the main reason for the consistency between managements for response of best-performing germplasm. The low competitive ability of semi-prostrate germplasm reflected its low vigour. The results did not suggest any specific adaptation to organic conditions of landraces or material bred under organic management. Specific adaptation to the region of evaluation was the main driving factor for cultivar adaptation to both managements.

Bi‐parental vs. cooperative breeding in a passerine: fitness‐maximizing strategies of males in response to risk of extra‐pair paternity?

In socially monogamous species, males that risk cuckoldry more than others might gain inclusive fitness by yielding paternity to relatives. We tested this prediction in the Tibetan ground tit Pseudopodoces humilis, an unusual facultative cooperative breeder wherein most helpers (87% males) join a mated pair shortly before clutch completion. Extra-pair paternity among bi-parental broods occurred less often (26% of broods, 9% of young) compared with cooperative broods (68%, 25%). In the former, most extra-pair sires (88%) were pair breeders unrelated to the cuckolded males, whereas in the latter, sires (87%) were mainly helpers related to the dominant males. Brood productivity did not differ between the bi-parental and cooperative breeders, but helpers' partitioning over group paternity reduced the realized reproductive success of helped males. After taking inclusive fitness into account, however, there was no difference in success of dominant males between the two social systems. One possible explanation for the differences in the rates of cuckoldry in the two systems was body size, because pair-bond males in bi-parental situations were significantly larger than those in cooperative ones. We propose two alternative strategies for males to maximize fitness: breed as a pair if large to avoid cuckoldry from helpers, or breed cooperatively if small but compromise some paternity to relatives. Our results provide an unusual route to the incidence of cooperative groups, based on constraints imposed by low competitive ability of breeding males rather than some external ecological or demographic factors.

Specific Features of Mixed Culture of Invertebrates Maintained under Laboratory Conditions

~ ~ ~ Competition among invertebrates was studied in mixed culture (the ciliates Paramecium caudatum and Paramecium bursaria and the rotifer Brachionis plicatilis) under laboratory conditions. The invertebrates were fed on various food sources: 1) Chlorella biomass and 2) biomasses of bacteria and yeast. The P.bursaria - Chlorella condutrix endosymbiosis was used a natural model of a simple biotic cycle. P.caudatum was found to be a poor competitor: even when the initial percentage of P.caudatum cells was large and there was no food limitation, this ciliate had been expelled by the end of the experiment. The main reason for the low competitive ability of P.caudatum in the studied system was likely the inhibiting effect of invertebrates' metabolites. The outcome of the competition between the ciliate P. bursaria and the rotifer B. plicatilis was different depending upon the food source: 1) Chlorella or 2) bacteria and yeast. When fed on Chlorella, P. bursaria was a better competitor than B. plicatilis. With bacteria and yeast used as food, the opposite outcome of competition was observed: the competitive ability of B. plicatilis was better than that of P. bursaria. It was assumed that due to turbidity of the culture solution in experiments with bacteria and yeast used as food, the zoochlorella photosynthesis rate decreased and, hence, the P. bursaria - zoochlorella symbiosis became less competitive. Analysis of experimental results of P. bursaria and P. caudatum single species cultures in the light and in the dark, with bacteria used as a food source, confirmed that assumption. The results of the experiments confirmed the beneficial role of symbiotic relationships of the ciliate P. bursaria and zoochlorella. The number of symbiotic units in the light reached 220 units/ml, while in the dark it was much smaller. The P. caudatum number remained practically the same in the light and in the dark.

Improvement of direct tree seeding with cover crops in afforestation: Microclimate and resource availability induced by vegetation composition

The low cost of direct tree seeding makes it increasingly useful for afforestation. However, the success of the technique is often unpredictable due to a series of different adverse biotic and abiotic factors. A plant cover with low competitive ability towards resources could offer a natural alternative to the herbicides currently used to control weeds. The effects of cover plants on availability of resources and microclimate were studied in a field experiment in the central France during three years. An experiment design crossing three mixtures of tree seeds with four vegetation compositions (a bare soil, flora of the meadow, and two mixtures of cover plants) was established and repeated randomly in three blocks in a meadow. Vegetation composition, tree seedling emergence, light availability, soil water content, and temperatures under each treatment were measured. The vegetation composition of the meadow stayed stable, with grass species dominating. The cover plants sown disappeared rapidly after one or two years. Outcome of direct seeding of trees depended on the vegetation treatment. Bare soil gave the highest emergence rate while meadow vegetation gave the lowest, with the mixture of cover plants being intermediate. Light availability and soil water content were very low under the meadow vegetation but were very high on the bare soil, again with cover plant mixtures being intermediate. This study confirms that a bare soil obtained by herbicide is a secure way to ensure tree seedling establishment in afforestation by direct seeding. However, with the onus currently on reducing herbicide use, the results suggest that sowing a mixture of cover plants could be an acceptable alternative to herbicides. The cover plants could offer protection against frost, scorching temperatures, or the water run-off encountered on bare soil. However, there are also a number of unresolved issues that need be addressed before the cover plants technique can be recommended.

The evolution of growth rate, resource allocation and competitive ability in seeder and resprouter tree seedlings

Many woody plant species in fire disturbed communities survive disturbance events by resprouting. The resprouting life history is predicted to be costly to plants as resources are diverted into storage for post-fire regrowth rather than allocated to current growth, and resprouting species typically grow more slowly than seeder species (species that do not resprout after disturbance events). Differences in allocation to current growth are also predicted to make resprouter species poorer competitors compared to seeder species. We tested the predictions that the evolution of a resprouter life history is associated with slow growth, increased allocation to storage, and low competitive ability in woody plant seedlings. We grew eight phylogenetically independent pairs of seeder and resprouter species in competition and no competition treatments in a field experiment near Sydney, Australia. The presence of competitors reduced plant growth rates across taxa and fire response life histories. However, relative to seeder species, resprouter species were not slower growing, they did not allocate more resources to storage, and they did not have lower competitive abilities. We propose that differences in resource allocation to storage are not responsible for differences in growth rate and competitive ability. Rather, growth rate and competitive ability in seedlings are associated with key aspects of plant life history such as life-span and body size at maturity. These traits that are sometimes, but not always, related to fire response life histories.

Chemosensory assessment of rival competitive ability and scent-mark function in a lizard, Podarcis hispanica

Recent studies have stressed the role of scent marks as signals potentially mediating competitor assessment. According to this view, receivers may use scent marks to derive information about the costs of exploiting a given area, but few studies have directly addressed this hypothesis. One of its main predictions is that scent marks should reflect a signaller's competitive ability. We simulated the situation faced by an intruding male when entering the scent-marked territories of rival males of varying competitive ability to test predictions about the role of scent marks in a lizard, Podarcis hispanica . We report that males were attracted to areas marked by males of similar or higher competitive ability (i.e. larger size), but not to areas scent marked by males of lower competitive ability, and that this preference disappeared towards the end of the breeding season. Our results show that (1) male lizards can assess rival competitive ability (i.e. rival size) on the basis of scent marks alone, (2) scent marks do not function as chemical barriers to deter intruders, and (3) male response to marked areas varies throughout the breeding season, suggesting a shift in the cost-to-benefit balance of entering a scent-marked area. We propose that male assessment of rival competitive ability may function as an indirect assessment mechanism of territory resource quality in this species, and thus that scent marks may convey information not only about costs but also about the benefits of exploiting a scent-marked area.

Hierarchical competition in pond-breeding anuran larvae in a Mediterranean area

AbstractThe anuran larval guild is frequently characterised by the co-occurrence, with high niche overlap, of distinct species in the same pond at variables densities during development. Anuran larvae have therefore been widely studied as a model system for competition. Body size and activity level are considered the most important factors that influence the outcome of competition between tadpoles. As species from temporary ponds normally show higher activity levels in order to achieve rapid growth and thus reduce the risk of desiccation, these species are often considered superior competitors. We designed several laboratory experiments to examine the intra- and interspecific effects on growth rate, mass at metamorphosis and survival to metamorphosis of six species in a Mediterranean area. Body size and activity level were used as explanatory covariables to determine competitive ability among species. An asymmetric and hierarchical relationship was found among the six species. Larger tadpole species were more successful in competitive interactions than smaller ones, but no relationship was found between activity level and competition effects. Species typically found in temporary ponds (Pelodytes punctatus and Bufo calamita) were considered poor competitors in contrast with other communities studied. Species with low competitive ability can persist by using refuges in which competition is reduced (e.g. ephemeral ponds).

Effect of Different Sowing Patterns of Oil Linseed (Linum usitatissimum L.) Crop on Brassica sp. as a Strategy for Sustainable Weed Management

ABSTRACT The low competitive ability of linseed is one of the main factors that negatively affects its seed yield. In a sustainable agricultural context, it is necessary to look for weed management alternatives to the use of herbicides. The effect of seed density and plant arrangement on the competitiveness of linseed and on its relationship with seed yield, dry matter, and N accumulation and partition, was evaluated in a field experiment carried out in La Plata, Argentina. Two densities of linseed and two plant arrangements were tested both in the presence and the absence of weeds (canola, Brassica napus ssp. oleifera forma annua, was used as a ‘weed simulator’). The presence of weeds generated a reduction in total aerial biomass and linseed yield, together with a lower production of seeds per m and a lower accumulation of N in seeds. Total aerial biomass at harvest increased at high density, while dry matter partition to seed (harvest index) was reduced, and N accumulation was increased. The effect of linseed on Brassica biomass was higher at high density. These results suggest that changes in linseed density can improve its competitive ability and can thus be utilized as a strategy for weed management for more sustainable agricultural systems.

A functional group approach to the management of UK arable weeds to support biological diversity

SummaryWeeds have an important role in maintaining farmland biodiversity. This needs to be balanced with their potential negative impact on crop yield and quality. Mechanistic models of crop–weed competition are an important tool in striking this balance. A range of common UK annual weeds were screened for the eco‐physiological traits required by the models. Using multivariate techniques, a number of functional groups with a similar pattern of productivity and competition were identified, based on trade‐offs between traits. A scheme was developed to assign species outside of the data set to one of the groups, based on life cycle, seed mass, maximum height and time of first flowering. As well as having a similar competitive ability, species within a group also appeared to have a similar ecosystem function, in terms of supporting higher trophic groups. Two beneficial groups of species were identified that combined a relatively low competitive ability with a high importance for invertebrates and birds. The identification of functional groups in the UK arable flora is a useful tool for assessing a weed community in the context of reconciling biodiversity provision with crop production. Preserving beneficial plant functional types within the crop would complement non‐cropped wildlife refuges, such as field margins.

An investigation of dispersal of the introduced brown alga Undaria pinnatifida (Harvey) Suringar and its competition with some species on the man-made structures of Torquay Marina (Devon, UK)

As part of a study into the ecology of the recently introduced macroalga Undaria pinnatifida in the British Isles, the competitive ability of this adventive kelp with native species was investigated in Torquay Marina (Devon, UK). Sites of introduction were surveyed for the presence of native species filling a similar ecological niche and long-term competition experiments set up over 6 years. Experiments involving clearance, removal and monitoring showed that Undaria has a low competitive ability in the infralittoral and sublittoral of a rocky breakwater. Undaria did not manage to colonise the nearby (200 m) shoreline for at least 7 years from the date of its original introduction to the floating pontoons of Torquay Marina. Undaria did out-compete native species in a marina environment: two kelp species, Laminaria digitata and Laminaria saccharina, and a sea squirt, Styela clava.

Consequences of differing competitive abilities between juvenile and adult plants

The competitive ability of perennial plants can change with life‐stage, but whether these changes have fitness consequences is unknown. We present a simple model of two components of fitness, mortality and flowering rates, for two grassland species with very different patterns of competitive ability and life‐stage. Achillea millefolium seedlings are poor competitors while the adults are good competitors. In contrast, Solidago missouriensis seedlings and adults have similar competitive ability. Models of the two species show that the overall effects of competition on growth are more important than interspecific differences in competitive ability in determining mortality and flowering rates, though the higher seedling competitive ability of S. missouriensis relative to A. millefolium seedlings does result in slightly lower mortality and higher flowering rates for the former species. Simulations where both average competitive ability and relative seedling and adult competitive ability are varied predict that dominant species with high overall competitive ability should experience no advantage or disadvantage from varying competitive ability through development. When overall competitive ability is moderate, the relative costs and benefits of differential competitive abilities among adults and seedlings are variable. High seedling competitive ability relative to adult competitive ability should be favored among species with low overall competitive ability. We predict that communities with high intensity of competition should have a high frequency of species with high seedling competitive ability, while communities with lower intensity of competition should have species with a wide range of relative seedling and adult competitive ability.

Effects of apple orchard management strategies on the great tit (Parus major) in southeastern france

The impact of conventional, organic, and integrated pest management (IPM) strategies of apple orchards on the reproduction of the great tit Parus major was investigated during a three-year period in southeastern France. The colonization process, egg-laying dates, clutch sizes, and fledging success were similar among pairs of P. major nesting in orchards conducted under the three studied management strategies. However, the mean number of young produced per ha (orchard productivity) was significantly higher in organic orchards than in both conventional and IPM orchards. Such divergences between both fledging success and orchard productivity primarily resulted from higher densities of P. major nesting pairs, but also from lower rates of nest abandonment during incubation in organic orchards. We suggest that intensive pesticide use under both IPM and conventional managements may have resulted in a substantial reduction in insect prey availability that enhanced intraspecific competition, which then led to failure in reproduction in pairs with low competitive ability. Our results highlight the relevance of P. major in assessing the environmental impact of apple orchard management strategies.

Effects of Low-growing Perennial Ornamental Groundcovers on the Growth and Fruiting of `Seyval blanc' Grapevines

Greenhouse and field-grown `Seyval blanc' grapevines (Vitis sp.) were grown with low-growing, shallow-rooted, mat-forming, ornamental perennial groundcovers, and the effect of the groundcovers on the vegetative and fruiting growth of the grapevines was evaluated. The groundcovers used in this experiment were `Kentucky-31' tall fescue (Festuca arundinacea); white mazus (Mazus japoonicus albus); english pennyroyal (Mentha pulegium); dwarf creeping thyme (Thymus serpyllum minus); strawberry clover (Trifolium fragiferum); `Heavenly Blue' veronica (Veronica prostrata `Heavenly Blue'); and a companion grass mixture of 75% perennial ryegrass (Lolium perenne) and 25% red fescue (Festuca rubra). A control treatment grown without any groundcover was also used in both the greenhouse and field experiments. All of the groundcovers reduced `Seyval blanc' total shoot length from 22% to 85% in the vineyard. Cluster size was reduced in the field from 7% to 68% by the groundcovers compared to the herbicide control treatment, and from 9% to 66% in the greenhouse experiment, but none of the groundcovers in either the greenhouse or field experiments affected the pH, total acidity, or soluble solids concentrations of the `Seyval blanc' juice. English pennyroyal was the only groundcover that reduced in the leaf area of the grapevine. Single-leaf photosynthesis of the `Seyval blanc' grapevines in the field experiment was reduced by all groundcovers except mazus and creeping thyme. Water infiltration rates were 10 to 50 times higher in the groundcovers compared to the bare soil of the herbicide control treatment. Weed growth in the field caused reduction in shoot length similar to the most competitive groundcovers. Weed growth was reduced in the early season by the english pennyroyal and companion grass, and in the late season by all groundcovers. The reduction in growth of the grapevines caused by groundcovers in the greenhouse was a reasonable screen for the affect of groundcovers in the field. The mazus treatment was the only groundcover in our experiments that coupled fast growth with low competitive ability.

Faster development does not lead to correlated evolution of greater pre-adult competitive ability in Drosophila melanogaster

In comparisons across Drosophila species, faster pre-adult development is phenotypically correlated with increased pre-adult competitive ability, suggesting that these two traits may also be evolutionary correlates of one another. However, correlations between traits within- and among- species can differ, and in most cases it is the within-species genetic correlations that are likely to act as constraints on adaptive evolution. Moreover, laboratory studies on Drosophila melanogaster have shown that the suite of traits that evolves in populations subjected to selection for faster development is the opposite of the traits that evolve in populations selected for increased pre-adult competitive ability. This observation led us to propose that, despite having a higher carrying capacity and a reduced minimum food requirement for completing development than controls, D. melanogaster populations subjected to selection for faster development should have lower competitive ability than controls owing to their reduced larval feeding rates and urea tolerance. Here, we describe results from pre-adult competition experiments that clearly show that the faster developing populations are substantially poorer competitors than controls when reared at high density in competition with a marked mutant strain. We briefly discuss these results in the context of different formulations of density-dependent selection theory.

Faster development does not lead to correlated evolution of greater pre-adult competitive ability in Drosophila melanogaster

In comparisons across Drosophila species, faster pre-adult development is phenotypically correlated with increased pre-adult competitive ability, suggesting that these two traits may also be evolutionary correlates of one another. However, correlations between traits within- and among- species can differ, and in most cases it is the within-species genetic correlations that are likely to act as constraints on adaptive evolution. Moreover, laboratory studies on Drosophila melanogaster have shown that the suite of traits that evolves in populations subjected to selection for faster development is the opposite of the traits that evolve in populations selected for increased pre-adult competitive ability. This observation led us to propose that, despite having a higher carrying capacity and a reduced minimum food requirement for completing development than controls, D. melanogaster populations subjected to selection for faster development should have lower competitive ability than controls owing to their reduced larval feeding rates and urea tolerance. Here, we describe results from pre-adult competition experiments that clearly show that the faster developing populations are substantially poorer competitors than controls when reared at high density in competition with a marked mutant strain. We briefly discuss these results in the context of different formulations of density-dependent selection theory.

Tree seedling growth, survival, and morphology in response to landscape-level variation in soil resource availability in northern Michigan

To investigate causes of tree species distributions across soil resources in northern Michigan, we conducted a seedling transplant experiment with five species showing different site affinities: Acer saccharum Marsh. (sugar maple), Prunus serotina Ehrh. (black cherry), and Fraxinus americana L. (white ash), which are associated with high-fertility mesic moraine; Quercus rubra L. (red oak), associated with intermediate sites; and Quercus velutina Lam. (black oak), associated with low-fertility droughty outwash sites. Seedlings were planted in plots stratified across variation in light and soil nutrient and water availability. After one growing season, under 14%–27% canopy openness, species tended to trade off between high survival on outwash versus high relative growth rate of root + stem mass (RGRrs) on moraine. The high survivorship of black and red oak on outwash was associated with greater root and whole-plant mass in comparison with sugar maple, white ash, and black cherry. High RGRrs on high-fertility moraine for these latter species was associated with high fine root area per unit whole-plant mass and plasticity to increase specific root area in response to increased soil resources. We did not detect a similar survival–growth trade-off for seedlings grown at lower light (3%–10%) on intermediate versus high-fertility sites. Overall, these results suggest that species distributions across soil resource gradients can in part be explained by a trade-off between tolerance of low soil resources versus competitive ability (i.e., growth) under high soil resources.