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Plastic Responses (plastic + response)
Selected AbstractsRELATIVE ROLE OF GENETIC DETERMINATION AND PLASTIC RESPONSE DURING ONTOGENY FOR SHELL-SHAPE TRAITS SUBJECTED TO DIVERSIFYING SELECTIONEVOLUTION, Issue 5 2009Paula Conde-Padín We studied the relative role of genetic determination versus plastic response for traits involved in ecological adaptation of two ecotypes of Littorina saxatilis living at different shore levels. To investigate the magnitude of the plastic response across ontogeny, we compared morphological data from individuals grown in the laboratory and taken from the wild at three developmental stages: shelled embryos, juveniles, and adults. The results indicate that most shell shape variation (72,99%) in adaptive traits (globosity and aperture of the shell) is explained by the ecotype irrespective of the growth environment, suggesting that direct genetic determination is the main factor responsible for the process of adaptation in the wild. There was a tendency for the contribution of plasticity to increase over ontogeny but, in general, the direction of the plastic response did not suggest that this was adaptive. [source] REPLICATED EVOLUTION OF INTEGRATED PLASTIC RESPONSES DURING EARLY ADAPTIVE DIVERGENCEEVOLUTION, Issue 4 2006Kevin J. Parsons Abstract Colonization of a novel environment is expected to result in adaptive divergence from the ancestral population when selection favors a new phenotypic optimum. Local adaptation in the new environment occurs through the accumulation and integration of character states that positively affect fitness. The role played by plastic traits in adaptation to a novel environment has generally been ignored, except for variable environments. We propose that if conditions in a relatively stable but novel environment induce phenotypically plastic responses in many traits, and if genetic variation exists in the form of those responses, then selection may initially favor the accumulation and integration of functionally useful plastic responses. Early divergence between ancestral and colonist forms will then occur with respect to their plastic responses across the gradient bounded by ancestral and novel environmental conditions. To test this, we compared the magnitude, integration, and pattern of plastic character responses in external body form induced by shallow versus open water conditions between two sunfish ecomorphs that coexist in four postglacial lakes. The novel sunfish ecomorph is present in the deeper open water habitat, whereas the ancestral ecomorph inhabits the shallow waters along the lake margin. Plastic responses by open water ecomorphs were more correlated than those of their local shallow water ecomorph in two of the populations, whereas equal levels of correlated plastic character responses occurred between ecomorphs in the other two populations. Small but persistent differences occurred between ecomorph pairs in the pattern of their character responses, suggesting a recent divergence. Open water ecomorphs shared some similarities in the covariance among plastic responses to rearing environment. Replication in the form of correlated plastic responses among populations of open water ecomorphs suggests that plastic character states may evolve under selection. Variation between ecomorphs and among lake populations in the covariance of plastic responses suggests the presence of genetic variation in plastic character responses. In three populations, open water ecomorphs also exhibited larger plastic responses to the environmental gradient than the local shallow water ecomorph. This could account for the greater integration of plastic responses in open water ecomorphs in two of the populations. This suggests that the plastic responses of local sunfish ecomorphs can diverge through changes in the magnitude and coordination of plastic responses. Although these results require further investigation, they suggest that early adaptive evolution in a novel environment can include changes to plastic character states. The genetic assimilation of coordinated plastic responses could result in the further, and possibly rapid, divergence of such populations and could also account for the evolution of genes of major effect that contribute to suites of phenotypic differences between divergent populations. [source] No Plastic Responses to Experimental Manipulation of Sperm Competition per se in a Free-Living FlatwormETHOLOGY, Issue 4 2010Peter Sandner In the absence of sperm competition evolutionary theory predicts low mating rates and low ejaculate expenditure per mating, and sex allocation theory for simultaneous hermaphrodites predicts a strongly female-biased sex allocation. In the presence of sperm competition a shift towards a more male-biased sex allocation and a higher ejaculate expenditure are predicted. The free-living flatworm Macrostomum lignano has been shown to respond plastically in mating rate, testis size, and sperm transfer to manipulation of the social group size, a proxy of the strength of sperm competition. However, manipulation of social group size may manipulate not only sperm competition, but also other factors, such as food supply and metabolite concentration. In this study we therefore manipulated sperm competition per se by repeatedly exposing individuals to partners that have either mated with rivals or not, while keeping the social group size constant. Our results suggest that M. lignano does not have the ability to detect sperm competition per se, as worms experimentally exposed to the presence or absence of sperm competition did not differ in sex allocation, sperm transfer or mating behavior. A response to our manipulation would have required individual recognition, the ability to detect self-referencing tags, or tags or traces left by rivals on or in the mating partners. We first discuss the possibility that highly efficient sperm displacement may have decreased the difference between the treatment groups and then propose three alternative cues that may allow M. lignano to respond plastically to the social group size manipulation used in earlier studies: assessment of the mating rate, chemical cues, or tactile cues. [source] REPLICATED EVOLUTION OF INTEGRATED PLASTIC RESPONSES DURING EARLY ADAPTIVE DIVERGENCEEVOLUTION, Issue 4 2006Kevin J. Parsons Abstract Colonization of a novel environment is expected to result in adaptive divergence from the ancestral population when selection favors a new phenotypic optimum. Local adaptation in the new environment occurs through the accumulation and integration of character states that positively affect fitness. The role played by plastic traits in adaptation to a novel environment has generally been ignored, except for variable environments. We propose that if conditions in a relatively stable but novel environment induce phenotypically plastic responses in many traits, and if genetic variation exists in the form of those responses, then selection may initially favor the accumulation and integration of functionally useful plastic responses. Early divergence between ancestral and colonist forms will then occur with respect to their plastic responses across the gradient bounded by ancestral and novel environmental conditions. To test this, we compared the magnitude, integration, and pattern of plastic character responses in external body form induced by shallow versus open water conditions between two sunfish ecomorphs that coexist in four postglacial lakes. The novel sunfish ecomorph is present in the deeper open water habitat, whereas the ancestral ecomorph inhabits the shallow waters along the lake margin. Plastic responses by open water ecomorphs were more correlated than those of their local shallow water ecomorph in two of the populations, whereas equal levels of correlated plastic character responses occurred between ecomorphs in the other two populations. Small but persistent differences occurred between ecomorph pairs in the pattern of their character responses, suggesting a recent divergence. Open water ecomorphs shared some similarities in the covariance among plastic responses to rearing environment. Replication in the form of correlated plastic responses among populations of open water ecomorphs suggests that plastic character states may evolve under selection. Variation between ecomorphs and among lake populations in the covariance of plastic responses suggests the presence of genetic variation in plastic character responses. In three populations, open water ecomorphs also exhibited larger plastic responses to the environmental gradient than the local shallow water ecomorph. This could account for the greater integration of plastic responses in open water ecomorphs in two of the populations. This suggests that the plastic responses of local sunfish ecomorphs can diverge through changes in the magnitude and coordination of plastic responses. Although these results require further investigation, they suggest that early adaptive evolution in a novel environment can include changes to plastic character states. The genetic assimilation of coordinated plastic responses could result in the further, and possibly rapid, divergence of such populations and could also account for the evolution of genes of major effect that contribute to suites of phenotypic differences between divergent populations. [source] Effects of four generations of density-dependent selection on life history traits and their plasticity in a clonally propagated plantJOURNAL OF EVOLUTIONARY BIOLOGY, Issue 3 2003M. Van Kleunen Abstract Life history evolution of many clonal plants takes place with long periods of exclusively clonal reproduction and under largely varying ramet densities resulting from clonal reproduction. We asked whether life history traits of the clonal herb Ranunculus reptans respond to density-dependent selection, and whether plasticity in these traits is adaptive. After four generations of exclusively clonal propagation of 16 low and 16 high ramet-density lines, we studied life history traits and their plasticities at two test ramet-densities. Plastic responses to higher test-density consisted of a shift from sexual to vegetative reproduction, and reduced flower production, plant size, branching frequency, and lengths of leaves and internodes. Plants of high-density lines tended to have longer leaves, and under high test-density branched less frequently than those of low-density lines. Directions of these selection responses indicate that the observed plastic branching response is adaptive, whereas the plastic leaf length response is not. The reverse branching frequency pattern at low test-density, where plants of high-density lines branched more frequently than those of low-density lines, indicates evolution of plasticity in branching. Moreover, when grown under less stressful low test-density, plants of high-density lines tended to grow larger than the ones of low-density lines. We conclude that ramet density affects clonal life-history evolution and that under exclusively clonal propagation clonal life-history traits and their plasticities evolve differently at different ramet densities. [source] Density-dependent polyphenism and geographic variation in size among two populations of lubber grasshoppers (Romalea microptera)ECOLOGICAL ENTOMOLOGY, Issue 5 2009JASON E. JANNOT Abstract. 1. Density-dependent phase polyphenism occurs when changes in density during the juvenile stages result in a developmental shift from one phenotype to another. Density-dependent phase polyphenism is common among locusts (Orthoptera: Acrididae). 2. Previously, we demonstrated a longitudinal geographic cline in adult body size (western populations = small adults; eastern populations = large adults) in the eastern lubber grasshopper (Romalea microptera) in south Florida. As lubbers are confamilial with locusts, we hypothesised that the longitudinal size cline was partly due to density-dependent phase polyphenism. 3. We tested the effect of density, population, and density×population interaction on life-history traits (pronotum length, mass, cumulative development time, growth rate) of, and proportion surviving to, each of the five instars and the adult stage in a 2 × 3 factorial laboratory experiment with two lubber populations, each reared from hatchling to adult at three different densities. 4. The effect of density on life history and survival was independent of the effects of population on life history and survival. Higher densities led to larger adult sizes (pronotum, mass) and lower survivorship. The western population had smaller adult masses, fewer cumulative days to the adult stage, and higher survivorship than the eastern population. 5. Our data suggest that lubber grasshoppers exhibit density-dependent phase polyphenism initiated by the physical presence of conspecifics. However, the plastic response of adult size to density observed in the laboratory is not consistent with the relationship between phenotypes and adult density in the field. Genetic differences between populations observed in the laboratory could contribute to size and life-history differences among lubber populations in the field. [source] Environmentally induced migration: the importance of foodECOLOGY LETTERS, Issue 6 2006Ivan C. Olsson Abstract The decision to migrate or not is regarded as genetically controlled for many invertebrate and vertebrate taxa. Here, we show that the environment influences this decision. By reciprocally transplanting brown trout (Salmo trutta L.) between two sections in a river, we show that both migratory and non-migratory behaviour can be environmentally induced; migratory behaviour developed in a river section with high brown trout densities and low specific growth rates, whereas non-migratory behaviour developed in a section with low brown trout densities and high specific growth rates. In a laboratory experiment, we tested the effect of food availability on the development of migratory and non-migratory body morphologies and found that most brown trout became migrants when food levels were low but fewer did so at high food levels. Thus, the decision to migrate seems to be a plastic response, influenced by growth opportunities. [source] Does morphological variation between young-of-the-year perch from two Swedish lakes depend on genetic differences?ECOLOGY OF FRESHWATER FISH, Issue 2 2010M. Heynen Heynen M, Hellström G, Magnhagen C, Borcherding J. Does morphological variation between young-of-the-year perch from two Swedish lakes depend on genetic differences? Ecology of Freshwater Fish 2010: 19: 163,169. © 2009 John Wiley & Sons A/S Abstract,,, Different local environmental conditions have often been found to generate phenotypic diversity. In the present study we examined morphological differences between young-of-the-year perch from two lake populations with differences in size-specific predation risk. A common garden setup was used to examine the genetic and environmental components of the morphological variation. We found differences in head and jaw length and slight differences in body depth between the wild young-of-the-year perch from Lake Ängersjön and Lake Fisksjön. The differences found between the wild fish from the two lakes were not maintained under common garden rearing. The observed morphological divergence between the wild young-of-the-year perch from Lake Ängersjön and Lake Fisksjön seems to stem mainly from a plastic response to different environmental conditions in the two lakes. It is clear that the morphological traits are not influenced by direct reaction to the size-specific risk of cannibalism, but probably stem from a combination of different environment characteristics, including resource and habitat use, and the density of other piscivores, such as pike. [source] RELATIVE ROLE OF GENETIC DETERMINATION AND PLASTIC RESPONSE DURING ONTOGENY FOR SHELL-SHAPE TRAITS SUBJECTED TO DIVERSIFYING SELECTIONEVOLUTION, Issue 5 2009Paula Conde-Padín We studied the relative role of genetic determination versus plastic response for traits involved in ecological adaptation of two ecotypes of Littorina saxatilis living at different shore levels. To investigate the magnitude of the plastic response across ontogeny, we compared morphological data from individuals grown in the laboratory and taken from the wild at three developmental stages: shelled embryos, juveniles, and adults. The results indicate that most shell shape variation (72,99%) in adaptive traits (globosity and aperture of the shell) is explained by the ecotype irrespective of the growth environment, suggesting that direct genetic determination is the main factor responsible for the process of adaptation in the wild. There was a tendency for the contribution of plasticity to increase over ontogeny but, in general, the direction of the plastic response did not suggest that this was adaptive. [source] CROSS-GENERATIONAL ENVIRONMENTAL EFFECTS AND THE EVOLUTION OF OFFSPRING SIZE IN THE TRINIDADIAN GUPPY POECILIA RETICULATAEVOLUTION, Issue 2 2006Farrah Bashey Abstract The existence of adaptive phenotypic plasticity demands that we study the evolution of reaction norms, rather than just the evolution of fixed traits. This approach requires the examination of functional relationships among traits not only in a single environment but across environments and between traits and plasticity itself. In this study, I examined the interplay of plasticity and local adaptation of offspring size in the Trinidadian guppy, Poecilia reticulata. Guppies respond to food restriction by growing and reproducing less but also by producing larger offspring. This plastic difference in offspring size is of the same order of magnitude as evolved genetic differences among populations. Larger offspring sizes are thought to have evolved as an adaptation to the competitive environment faced by newborn guppies in some environments. If plastic responses to maternal food limitation can achieve the same fitness benefit, then why has guppy offspring size evolved at all? To explore this question, I examined the plastic response to food level of females from two natural populations that experience different selective environments. My goals were to examine whether the plastic responses to food level varied between populations, test the consequences of maternal manipulation of offspring size for offspring fitness, and assess whether costs of plasticity exist that could account for the evolution of mean offspring size across populations. In each population, full-sib sisters were exposed to either a low- or high-food treatment. Females from both populations produced larger, leaner offspring in response to food limitation. However, the population that was thought to have a history of selection for larger offspring was less plastic in its investment per offspring in response to maternal mass, maternal food level, and fecundity than the population under selection for small offspring size. To test the consequences of maternal manipulation of offspring size for offspring fitness, I raised the offspring of low- and high-food mothers in either low- or high-food environments. No maternal effects were detected at high food levels, supporting the prediction that mothers should increase fecundity rather than offspring size in noncompetitive environments. For offspring raised under low food levels, maternal effects on juvenile size and male size at maturity varied significantly between populations, reflecting their initial differences in maternal manipulation of offspring size; nevertheless, in both populations, increased investment per offspring increased offspring fitness. Several correlates of plasticity in investment per offspring that could affect the evolution of offspring size in guppies were identified. Under low-food conditions, mothers from more plastic families invested more in future reproduction and less in their own soma. Similarly, offspring from more plastic families were smaller as juveniles and female offspring reproduced earlier. These correlations suggest that a fixed, high level of investment per offspring might be favored over a plastic response in a chronically low-resource environment or in an environment that selects for lower reproductive effort [source] Development and evolution of adaptive polyphenismsEVOLUTION AND DEVELOPMENT, Issue 1 2003H. Frederik Nijhout SUMMARY Phenotypic plasticity is the primitive character state for most if not all traits. Insofar as developmental and physiological processes obey the laws of chemistry and physics, they will be sensitive to such environmental variables as temperature, nutrient supply, ionic environment, and the availability of various macro- and micronutrients. Depending on the effect this phenotypic plasticity has on fitness, evolution may proceed to select either for mechanisms that buffer or canalize the phenotype against relevant environmental variation or for a modified plastic response in which some ranges of the phenotypic variation are adaptive to particular environments. Phenotypic plasticity can be continuous, in which case it is called a reaction norm, or discontinuous, in which case it is called a polyphenism. Although the morphological discontinuity of some polyphenisms is produced by discrete developmental switches, most polyphenisms are due to discontinuities in the environment that induce only portions of what is in reality a continuous reaction norm. In insect polyphenisms, the environmental variable that induces the alternative phenotype is a token stimulus that serves as a predictor of, but is not itself, the environment to which the polyphenism is an adaptation. In all cases studied so far, the environmental stimulus alters the endocrine mechanism of metamorphosis by altering either the pattern of hormone secretion or the pattern of hormone sensitivity in different tissues. Such changes in the patterns of endocrine interactions result in the execution of alternative developmental pathways. The spatial and temporal compartmentalization of endocrine interactions has produced a developmental mechanism that enables substantial localized changes in morphology that remain well integrated into the structure and function of the organism. [source] Inaccurate or disparate temperature cues?FUNCTIONAL ECOLOGY, Issue 5 2010Seasonal acclimation of terrestrial, aquatic locomotor capacity in newts Summary 1.,Many organisms respond to seasonal temperature fluctuations by the reversible modification of whole-animal performance. Semiaquatic ectotherms, which possess this acclimatory capacity in swimming speed, lack the plastic response in terrestrial locomotor performance and vice versa. Theory predicts that the presence of reversible (seasonal) thermal acclimation or fixed phenotypes depends on the predictability of future thermal conditions (i.e. accuracy of temperature cues) in a given environment. Alternatively, comparative data suggest that thermal acclimation is induced by disparate temperature cues in water and on land. 2.,We tested both predictions by examining the seasonal acclimation response in thermal sensitivity of maximal swimming and running speed in adult alpine newts, Ichthyosaura (formerly Triturus) alpestris. 3.,Following the seasonal variation in environmental temperatures, we exposed newts to 5 °C from November to March and, after a gradual temperature increase, to either a constant (15 °C) or fluctuating (10,20 °C) thermal regime from May to June. At the end of each treatment, we measured newt swimming and running capacity at five temperatures (range 5,25 °C). In the field, hourly temperatures were recorded in various aquatic and terrestrial microhabitats to obtain information about the predictability of thermal conditions in both environments. 4.,Seasonal acclimation shaped the thermal sensitivity of swimming speed under both constant and fluctuating temperature treatments. Thermal sensitivity of running speed was markedly modified by a fluctuating thermal regime so that newts ran at the highest test temperature faster than cold-acclimated individuals. Natural thermal environment contained a similar proportion of predictable variation in water and on land. 5.,Complex seasonal acclimation of locomotor capacity in newts was influenced by the disparate thermal cues, i.e. mean acclimation temperature or diel temperature fluctuations, rather than by the different accuracy of these cues in water and on land. Future confrontations of theory with empirical data will require more attention not only on the assumptions of adaptive thermal acclimation but also on the ecologically relevant thermal conditions during acclimation experiments. [source] Adaptive versus non-adaptive phenotypic plasticity and the potential for contemporary adaptation in new environmentsFUNCTIONAL ECOLOGY, Issue 3 2007C. K. GHALAMBOR Summary 1The role of phenotypic plasticity in evolution has historically been a contentious issue because of debate over whether plasticity shields genotypes from selection or generates novel opportunities for selection to act. Because plasticity encompasses diverse adaptive and non-adaptive responses to environmental variation, no single conceptual framework adequately predicts the diverse roles of plasticity in evolutionary change. 2Different types of phenotypic plasticity can uniquely contribute to adaptive evolution when populations are faced with new or altered environments. Adaptive plasticity should promote establishment and persistence in a new environment, but depending on how close the plastic response is to the new favoured phenotypic optimum dictates whether directional selection will cause adaptive divergence between populations. Further, non-adaptive plasticity in response to stressful environments can result in a mean phenotypic response being further away from the favoured optimum or alternatively increase the variance around the mean due to the expression of cryptic genetic variation. The expression of cryptic genetic variation can facilitate adaptive evolution if by chance it results in a fitter phenotype. 3We conclude that adaptive plasticity that places populations close enough to a new phenotypic optimum for directional selection to act is the only plasticity that predictably enhances fitness and is most likely to facilitate adaptive evolution on ecological time-scales in new environments. However, this type of plasticity is likely to be the product of past selection on variation that may have been initially non-adaptive. 4We end with suggestions on how future empirical studies can be designed to better test the importance of different kinds of plasticity to adaptive evolution. [source] Fitness consequences of temperature-mediated egg size plasticity in a butterflyFUNCTIONAL ECOLOGY, Issue 6 2003K. Fischer Summary 1By randomly dividing adult females of the butterfly Bicyclus anynana, reared in a common environment, among high and low temperatures, it is demonstrated that oviposition temperature induces a plastic response in egg size. Females kept at a lower temperature laid significantly larger eggs than those ovipositing at a higher temperature. 2Cross-transferring the experimentally manipulated eggs between temperatures and investigating hatching success showed that a lower rearing temperature is more detrimental for the smaller eggs produced at a higher temperature than for the larger eggs produced at a lower temperature, supporting an adaptive explanation. 3However, when examining two potential mechanisms for an increased fitness of larger offspring (higher desiccation resistance of larger eggs and higher starvation resistance of larger hatchlings), no direct link between egg size and offspring fitness was found. Throughout, i.e. even under benign conditions, larger offspring had a higher fitness. 4Therefore, egg size should be viewed as a conveniently measurable proxy for the plastic responses induced by temperature, but caution is needed before implying that egg size per se is causal in influencing offspring traits. [source] Contrasting responses of migration strategies in two European thrushes to climate changeGLOBAL CHANGE BIOLOGY, Issue 1 2007PHILIPPE RIVALAN Abstract Migration is a widespread strategy that enables animals to escape harsh winter conditions. It has been well documented that migration phenology in birds is changing in response to recent climate warming in the northern hemisphere. Despite the existence of large temporal and geographical scale ringing data on birds in Europe, changes in migration strategies in relation to climate warming have not been well studied, mainly because of a lack of appropriate statistical methods. In this paper, we develop a method that enables us to investigate temporal changes in migration strategies from recoveries of dead ringed birds. We estimated migration probability as the ratio between recovery probabilities of conspecific birds originating from different countries but potentially wintering in the same country. We applied this method to two European thrushes: the entirely migrant redwing Turdus iliacus, and the partially migrant blackbird T. merula. We tested for an immediate and a 1-year lagged relationship between our migration probability and climatic covariates (i.e. mean winter temperature in France and the North Atlantic Oscillation). Using ringing-recovery data collected in Finland, Germany, Switzerland and France from 1970 to 1999, we detected contrasting responses in these two species, likely related to their different migratory behaviours. Both species showed a decline in the probability for northern and eastern birds to winter in France. The entirely migratory redwing exhibited a year-to-year plastic response to climate, whereas the decline in the partially migrant blackbird was smooth, suggesting underlying genetic processes. The proposed method, thus, allows us to identify useful indicators of climatic impacts on migration strategies, as well as highlighting differences between closely related species. [source] Plant,soil feedback induces shifts in biomass allocation in the invasive plant Chromolaena odorataJOURNAL OF ECOLOGY, Issue 6 2009Mariska Te Beest Summary 1. ,Soil communities and their interactions with plants may play a major role in determining the success of invasive species. However, rigorous investigations of this idea using cross-continental comparisons, including native and invasive plant populations, are still scarce. 2. ,We investigated if interactions with the soil community affect the growth and biomass allocation of the (sub)tropical invasive shrub Chromolaena odorata. We performed a cross-continental comparison with both native and non-native-range soil and native and non-native-range plant populations in two glasshouse experiments. 3. ,Results are interpreted in the light of three prominent hypotheses that explain the dominance of invasive plants in the non-native range: the enemy release hypothesis, the evolution of increased competitive ability hypothesis and the accumulation of local pathogens hypothesis. 4. ,Our results show that C. odorata performed significantly better when grown in soil pre-cultured by a plant species other than C. odorata. Soil communities from the native and non-native ranges did not differ in their effect on C. odorata performance. However, soil origin had a significant effect on plant allocation responses. 5. ,Non-native C. odorata plants increased relative allocation to stem biomass and height growth when confronted with soil communities from the non-native range. This is a plastic response that may allow species to be more successful when competing for light. This response differed between native and non-native-range populations, suggesting that selection may have taken place during the process of invasion. Whether this plastic response to soil organisms will indeed select for increased competitive ability needs further study. 6. ,The native grass Panicum maximum did not perform worse when grown in soil pre-cultured by C. odorata. Therefore, our results did not support the accumulation of local pathogens hypothesis. 7. ,Synthesis. Non-native C. odorata did not show release from soil-borne enemies compared to its native range. However, non-native plants responded to soil biota from the non-native range by enhanced allocation in stem biomass and height growth. This response can affect the competitive balance between native and invasive species. The evolutionary potential of this soil biota-induced change in plant biomass allocation needs further study. [source] Greater capacity for division of labour in clones of Fragaria chiloensis from patchier habitatsJOURNAL OF ECOLOGY, Issue 3 2007SERGIO R. ROILOA Summary 1Unlike non-clonal plants, clonal plants can develop a division of labour in which connected ramets specialize to acquire different, locally abundant resources. This occurs as a plastic response to a patchy environment where two resources tend not to occur together and different ramets experience high availabilities of different resources. We hypothesized that if division of labour is an important advantage of clonal growth in such environments in nature, then clones from habitats where resource availabilities are negatively associated should show a greater capacity for division of labour than clones from habitats where resource availabilities are more uniform. 2To test this, we collected clones of Fragaria chiloensis from sand dune and grassland sites in each of three regions of the central coast of California, grew pairs of connected or severed ramets under low light and high N or under high light and low N, and measured leaf area, chlorophyll content and final dry mass. Given that previous work has indicated that high availabilities of light and N show a stronger tendency not to occur together in the dune than in the grassland sites, we expected that clones from dunes would show greater capacity for division of labour than clones from grasslands. 3Clones from dunes showed a greater capacity than clones from grasslands to specialize for acquisition of abundant N via high proportional mass of roots. Clones from the two types of habitats showed similar capacity to specialize for acquisition of abundant light via high leaf area and chlorophyll content of leaves. Specialization via leaf area and chlorophyll content took place mainly within the first half of the 60-day experiment. 4These results provide evidence that division of labour in a clonal plant has been selected for in natural habitats where high levels of different resources tend to be spatially separated. Results also show that division of labour can occur, not just via allocation of mass, but also via physiological traits, and that both morphological and physiological specialization can take place within a few weeks. 5Clonal plants dominate many habitats and include many highly invasive species. Division of labour is one of the most striking potential advantages of clonal growth, and is a remarkable instance of phenotypic plasticity in plants. This study further suggests that division of labour in clonal plants is an instance of adaptive plasticity and could therefore play a part in their widespread ecological success. [source] Within and between generation phenotypic plasticity in trichome density of Mimulus guttatusJOURNAL OF EVOLUTIONARY BIOLOGY, Issue 6 2007L. M. HOLESKI Abstract Mimulus guttatus (yellow monkeyflower) frequently produce glandular trichomes, a trait that may resist herbivory. Constitutive production of trichomes is variable both within and among populations of M. guttatus and most of this variation is genetic. This study demonstrates that damage on early leaves can induce increased trichome production on later leaves, a plastic response that is likely adaptive. Moreover, this study shows that this induction can be maternally transmitted, increasing trichome density in progeny before they experience herbivory. This transgenerational response must involve a yet undescribed epigenetic mechanism. These experiments also show genetic variation among plants in the capacity for both within and between plant generation induction. Despite the clear evolutionary importance of variation in constitutive and induced herbivory-resistance traits, few other studies have noted genetic variation in both within a plant species. [source] Application of Hertzian Tests to Measure Stress,Strain Characteristics of Ceramics at Elevated TemperaturesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2007Estíbaliz Sánchez-González A new method for evaluating the elastic,plastic properties of ceramics from room temperature up to the onset of creep based on Hertzian indentation testing is proposed. Indentation stress,strain curves are compiled for representative alumina and zirconia ceramics at prescribed temperatures. Deconvolution of the indentation stress,strain curves for each material provides a measure of Young's modulus, yield stress, and work-hardening coefficient as a function of temperature, enabling construction of true stress,strain curves. The stress,strain curves flatten out with increasing temperature, in accordance with an expected increased plastic response at elevated temperatures. [source] Theoretical and experimental analysis of plastic response of isotropic circular plates subjected to underwater explosion loadingMATERIALWISSENSCHAFT UND WERKSTOFFTECHNIK, Issue 2 2008J. Z. Ashani Abstract Dynamic response analysis of structures subjected to underwater explosion (UndEx) loading has been always an interesting field of study for ship designers and metal forming specialists. Understanding the deformation and rupture mechanism of simple structures plays an important role in successful designing of a reliable structure under this kind of loading. In this paper, first the major parameters of the UndEx phenomenon (peak overpressure, impulse per unit area, dimensionless damage parameter ,, etc.) are discussed and determined by means of available experimental relations mostly offered by R. Cole. After that, the maximum deflection of a fully clamped circular plate has been calculated using a theoretical procedure assuming two different conditions: (1) neglecting the effect of strain rate, (2) considering the strain rate sensitivity of the material. Some experiments have been conducted on 5010 aluminum circular plates, using C4 as explosive. In order to simulate reality, a fixture was designed so that the plates are loaded in air-backed condition. Plates were fixed on top of the fixture, so the fully clamped condition which was assumed as the boundary condition was achieved. The test specimens were measured, not only their maximum deflection but also their thickness at different radii were determined. The results are compared to experimental-based predictions offered by Nurick and Rajendran who has conducted similar experiments. The results show reasonable agreement with theoretical predictions, especially when strain rate effects are considered. In addition, two new material constants (D, q) for this special aluminum alloy are introduced. Theoretische und experimentelle Analyse der plastischen Umformung isotroper kreisförmiger Platten bei Unterwasser-Explosionsbelastung Die Analyse der dynamischen Reaktion einer Struktur bei einer Explosionsbelastung unter dem Wasser ist für die Konstrukteure von Schiffen und Umformspezialisten eine interessante Forschungsaufgabe. Das Verständnis der Umformung und der Bruchmechanismen einfacher Strukturen spielt eine wichtige Rolle im erfolgreichen Konstruieren einer festen Struktur gegen diese Belastungsart. In diesem Beitrag werden erstens die Hauptparameter der Explosionserscheinung unter Wasser (maximaler Druck, Impulse per Flächeneinheit, dimensionsloser zerstörender Parameter usw.) dargestellt und zweitens durch die vorhandenen Methoden, die überwiegend von R. Cole vorliegen, berechnet. Danach ist die maximale Durchbiegung einer ganz gespannten kreisförmigen Platte durch eine analytische Methode in zwei Formen bestimmt worden: (1) Vernachlässigung der Dehnungsgeschwindigkeit, (2) Berücksichtigung der Dehnungsgeschwindigkeitsempfindlichkeit des Werkstoffes. Einige Untersuchungen wurden an kreisförmigen Platten aus Aluminium 5010 mittels Sprengstoff C4 durchgeführt. Zur Praxissimulation wurde die Spannvorrichtung so konstruiert, so dass die Platten Luft-gepuffert belastet wurden. Die Platten wurden oben auf der Spannvorrichtung vollständig eingespannt, um Grenzbedingungen zu erhalten. Bei den Proben wurde nicht nur die maximale Durchbiegung sondern auch die Dicke in verschiedenen Radien ermittelt. Die Ergebnisse wurden mit den experimentellen Abschätzungen von Nurick und Rajendran, die ähnliche Versuche durchgeführt haben, verglichen. Die Ergebnisse zeigen eine gute Übereinstimmung mit den theoretischen Abschätzungen, insbesondere bei Betrachtung der Dehngeschwindigkeit. Zusätzlich wurden für diese spezielle Aluminiumlegierung zwei neue Materialkonstanten (D, q) eingeführt. [source] Endogenous sink,source interactions and soil nitrogen regulate leaf life-span in an evergreen shrubNEW PHYTOLOGIST, Issue 4 2009C. Marty Summary ,,How the balance between exogenous and endogenous nitrogen for shoot growth varies with soil nitrogen availability, and its consequences on leaf life-span, have rarely been studied within a single species in the field. ,,In this study, we investigated two Rhododendron ferrugineum populations with contrasting leaf life-span. Soil nitrogen availability and nitrogen resorption of different leaf age classes were assessed, as were the interactions between plant compartments, using 15N labelling and sink organ suppression. ,,The population growing on poorer soil had a shorter leaf life-span (17.9 vs 21.5 months) and a higher net contribution of leaf reserves to shoot growth (32% vs 15%), achieved by faster nitrogen resorption and greater shedding of young nitrogen-rich leaves. For both populations, wood contributed to over 40% of shoot nitrogen demand. Both the negative relationship between current-year shoot mass and the percentage of 1-yr-old attached leaves and the delay of leaf shedding after bud removal suggest that shoot development has a strong effect on leaf life-span. ,,Our results suggest that, contrary to the evolutionary response, plastic response to low soil nitrogen could reduce leaf life-span in evergreen plants. In addition, leaf life-span seems to be strongly influenced by the discrepancy between shoot nitrogen demand and soil nitrogen uptake rather than nitrogen demand alone. [source] Size-dependent sex allocation in Aconitum gymnandrum (Ranunculaceae): physiological basis and effects of maternal family and environmentPLANT BIOLOGY, Issue 6 2008Z.-G. Zhao Abstract Theory predicts size-dependent sex allocation (SDS): flowers on plants with a high-resource status should have larger investment in females than plants with a low-resource status. Through a pot experiment with Aconitum gymnandrum (Ranunculaceae) in the field, we examined the relationship between sex allocation of individual flowers and plant size for different maternal families under different environmental conditions. We also determined the physiological base of variations in plant size. Our results support the prediction of SDS, and show that female-biased allocation with plant size is consistent under different environmental conditions. Negative correlations within families showed a plastic response of sex allocation to plant size. Negative genetic correlations between sex allocation and plant size at the family level indicate a genetic cause of the SDS pattern, although genetic correlation was influenced by environmental factors. Hence, the size-dependency of sex allocation in this species had both plastic and genetic causes. Furthermore, genotypes that grew large also had higher assimilation ability, thus showing a physiological basis for SDS. [source] Habitual throwing and swimming correspond with upper limb diaphyseal strength and shape in modern human athletesAMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY, Issue 1 2009Colin N. Shaw Abstract Variation in upper limb long bone cross-sectional properties may reflect a phenotypically plastic response to habitual loading patterns. Structural differences between limb bones have often been used to infer past behavior from hominin remains; however, few studies have examined direct relationships between behavioral differences and bone structure in humans. To help address this, cross-sectional images (50% length) of the humeri and ulnae of university varsity-level swimmers, cricketers, and controls were captured using peripheral quantitative computed tomography. High levels of humeral robusticity were found in the dominant arms of cricketers, and bilaterally among swimmers, whereas the most gracile humeri were found in both arms of controls, and the nondominant arms of cricketers. In addition, the dominant humeri of cricketers were more circular than controls. The highest levels of ulnar robusticity were also found in the dominant arm of cricketers, and bilaterally amongst swimmers. Bilateral asymmetry in humeral rigidity among cricketers was greater than swimmers and controls, while asymmetry for ulnar rigidity was greater in cricketers than controls. The results suggest that more mechanically loaded upper limb elements,,unilaterally or bilaterally,,are strengthened relative to less mechanically loaded elements, and that differences in mechanical loading may have a more significant effect on proximal compared to distal limb segments. The more circular humerus in the dominant arm in cricketers may be an adaptation to torsional strain associated with throwing activities. The reported correspondence between habitual activity patterns and upper limb diaphyseal properties may inform future behavioral interpretations involving hominin skeletal remains. Am J Phys Anthropol 2009. © 2009 Wiley-Liss, Inc. [source] Latitudinal clines in body size, but not in thermal tolerance or heat-shock cognate 70 (HSC70), in the highly-dispersing intertidal gastropod Littorina keenae (Gastropoda: Littorinidae)BIOLOGICAL JOURNAL OF THE LINNEAN SOCIETY, Issue 3 2010HYUK JE LEE Natural populations of widely-distributed animals often exhibit clinal variation in phenotypic traits or in allele frequencies of a particular gene over their geographical range. A planktotrophic intertidal snail, Littorina keenae is broadly distributed along the north-eastern Pacific coast through a large latitudinal range (24°50,N,43°18,N). We tested for latitudinal clines in two complex phenotypic traits , thermal tolerance and body size , and one single locus trait , heat shock cognate 70 (HSC70) , in L. keenae along almost its entire geographical range. We found only weak evidence for a latitudinal cline in the thermal tolerance and no evidence for a cline in allele frequencies at HSC70. However, as predicted by Bergmann's rule, we detected a strong latitudinal cline that accounted for 60% of the variance in body size (R2 = 0.598; P < 0.001). In contrast, body size did not significantly affect thermal tolerance. HSC70 showed no genetic differentiation among the populations, supporting our previous mitochondrial gene-based estimate of high gene flow during this snail's free-swimming larval stage. Given that L. keenae experiences panmixia along its species range, the observed size cline may be partially or entirely caused by a phenotypically plastic response to local thermal environments rather than by genetic divergence in body size among populations in response to locally optimizing natural selection. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 494,505. [source] Root/Shoot Allocation and Root Architecture in Seedlings: Variation among Forest Sites, Microhabitats, and Ecological Groups1BIOTROPICA, Issue 3 2003Horacio Paz ABSTRACT I analyzed patterns of variation in root mass allocation and root morphology among seedlings of woody species in relation to environmental factors in four Neotropical forests. Among forests, I explored the response of root traits to sites varying in water or nutrient availability. Within each forest, I explored the plastic response of species to different microhabitats: gaps and understory. Additionally, I explored evidence for life history correlation of root and shoot traits by comparing species differing in their successional group (light-demanding [22 spp.] or shade tolerant [27 spp.]) and germination type (species with photosynthetic cotyledons or species with reserve cotyledons). At each forest site, young seedlings from 10 to 20 species were excavated. A total of 55 species was collected in understory conditions and 31 of them were also collected in gaps. From each seedling, six morphological ratios were determined. Allocation to roots was higher in forest sites with the lowest soil resources. Roots were finer and longer in the most infertile site, while roots were deeper in the site with the longest dry season. Seedling traits did not differ between germination types. Shade tolerant species allocated more to roots and developed thicker roots than light-demanding species. Light-demanding species showed stronger plastic responses to habitat than shade tolerant species, and species with photo-synthetic cotyledons showed lower plasticity than species with reserve cotyledons. Overall, these results suggest that among Neotropical species, root allocation and root morphology of seedlings reflect plant adjustments to water or nutrient availability at geographic and microhabitat scales. In addition, life history specialization to light environments is suggested by differences among groups of species in their allocation to roots and in their root morphology. RESUMEN Analicé los patrones de variación en la asignación de biomasa y en la morfología de raíces de plántulas de especies leñosas en relación a factores ambientales en cuatro bosques Neotropicales. Entre los bosques, exploré la respuesta promedia de características de las plántulas en sitios que varían en disponibilidad de agua o nutrientes en el suelo. Dentro de cada bosque, exploré la respuesta fenotípica de las especies entre claros y sotobosque. Además, explore correlaciones de la historia de vida de las especies con características de raíces y parte aérea, comparando especies con diferente hábito sucesional (demandante de luz [22 spp.] o tolerante a la sombra [27 spp.]) y tipo de germinación (especies con cotiledones fotosintéticos o cotiledones de reserva). En cada bosque, se excavaron plántulas de 10 a 20 especies manteniendo la integridad de sus raíces. Se colectaron 55 especies en el sotobosque y 31 de ellas fueron colectadas también en claros. De cada plántula obtuve seis parámetros morfológicos de raíces y parte aérea. La asignación de biomasa a raíces fue mayor en los bosques con menos recursos. Las plántulas desarrollaron raíces más finas, y con mayor longitud por unidad de superficie fotosintética, en el sitio menos fértil. En cambio, las raíces tendieron a ser más profundas en relación al área fotosintética en el sitio más estacional. Las especies tolerantes a la sombra asignaron más biomasa a raíces y desarrollaron raíces más gruesas que las especies demandantes de luz. Las características morfológicas de las plántulas no difirieron entre tipos de germinación. Las especies demandantes de luz mostraron mayor plasticidad al hábitat en la asignación de biomasa a raíces, que las especies tolerantes a la sombra. Las especies con cotiledones fotosintéticos mostraron menor plasticidad al hábitat en la asignación de biomasa a raíces que las especies con cotiledones de reserva. En conjunto, mis resultados sugieren que para especies Neotropicales la asignación de biomasa y la morfología de raíces en plántulas reflejan ajustes de las plantas a la disponibilidad de agua o nutrientes a escala geográfica y de micro-hábitat. Además, se sugiere que la asignación de biomasa y la morfología de raíces son componentes del síndrome de especialización a ambientes lumínicos en el bosque. [source] Rearing Environment Affects the Brain Size of Guppies: Lab-Reared Guppies have Smaller Brains than Wild-Caught GuppiesETHOLOGY, Issue 2 2009James G. Burns Animals bred for captivity often have smaller brains and behave differently than their wild counterparts. These differences in brain size have been attributed to genetic changes resulting from, for example, inbreeding depression and pleiotropic effects of artificial selection for traits such as docility. A critical question, though, is whether these differences in brain size are due to plastic responses to the environment, not just genetic changes. We observed a large reduction in brain size in first generation, lab-reared female guppies compared with wild-caught ones (19% smaller telencephalon, 17% smaller optic tectum). We then reared first-generation, lab-born guppies in environments varying in spatial complexity and size in an attempt to isolate factors that might increase brain size and change temperament, but no significant differences in phenotype were observed. The results of these experiments show that, although the environmental factors responsible for the effect have not been found, even first generation lab-reared individuals can have smaller brains than wild individuals. [source] THE EVOLVABILITY OF GROWTH FORM IN A CLONAL SEAWEEDEVOLUTION, Issue 12 2009Keyne Monro Although modular construction is considered the key to adaptive growth or growth-form plasticity in sessile taxa (e.g., plants, seaweeds and colonial invertebrates), the serial expression of genes in morphogenesis may compromise its evolutionary potential if growth forms emerge as integrated wholes from module iteration. To explore the evolvability of growth form in the red seaweed, Asparagopsis armata, we estimated genetic variances, covariances, and cross-environment correlations for principal components of growth-form variation in contrasting light environments. We compared variance,covariance matrices across environments to test environmental effects on heritable variation and examined the potential for evolutionary change in the direction of plastic responses to light. Our results suggest that growth form in Asparagopsis may constitute only a single genetic entity whose plasticity affords only limited evolutionary potential. We argue that morphological integration arising from modular construction may constrain the evolvability of growth form in Asparagopsis, emphasizing the critical distinction between genetic and morphological modularity in this and other modular taxa. [source] REPLICATED EVOLUTION OF INTEGRATED PLASTIC RESPONSES DURING EARLY ADAPTIVE DIVERGENCEEVOLUTION, Issue 4 2006Kevin J. Parsons Abstract Colonization of a novel environment is expected to result in adaptive divergence from the ancestral population when selection favors a new phenotypic optimum. Local adaptation in the new environment occurs through the accumulation and integration of character states that positively affect fitness. The role played by plastic traits in adaptation to a novel environment has generally been ignored, except for variable environments. We propose that if conditions in a relatively stable but novel environment induce phenotypically plastic responses in many traits, and if genetic variation exists in the form of those responses, then selection may initially favor the accumulation and integration of functionally useful plastic responses. Early divergence between ancestral and colonist forms will then occur with respect to their plastic responses across the gradient bounded by ancestral and novel environmental conditions. To test this, we compared the magnitude, integration, and pattern of plastic character responses in external body form induced by shallow versus open water conditions between two sunfish ecomorphs that coexist in four postglacial lakes. The novel sunfish ecomorph is present in the deeper open water habitat, whereas the ancestral ecomorph inhabits the shallow waters along the lake margin. Plastic responses by open water ecomorphs were more correlated than those of their local shallow water ecomorph in two of the populations, whereas equal levels of correlated plastic character responses occurred between ecomorphs in the other two populations. Small but persistent differences occurred between ecomorph pairs in the pattern of their character responses, suggesting a recent divergence. Open water ecomorphs shared some similarities in the covariance among plastic responses to rearing environment. Replication in the form of correlated plastic responses among populations of open water ecomorphs suggests that plastic character states may evolve under selection. Variation between ecomorphs and among lake populations in the covariance of plastic responses suggests the presence of genetic variation in plastic character responses. In three populations, open water ecomorphs also exhibited larger plastic responses to the environmental gradient than the local shallow water ecomorph. This could account for the greater integration of plastic responses in open water ecomorphs in two of the populations. This suggests that the plastic responses of local sunfish ecomorphs can diverge through changes in the magnitude and coordination of plastic responses. Although these results require further investigation, they suggest that early adaptive evolution in a novel environment can include changes to plastic character states. The genetic assimilation of coordinated plastic responses could result in the further, and possibly rapid, divergence of such populations and could also account for the evolution of genes of major effect that contribute to suites of phenotypic differences between divergent populations. [source] CROSS-GENERATIONAL ENVIRONMENTAL EFFECTS AND THE EVOLUTION OF OFFSPRING SIZE IN THE TRINIDADIAN GUPPY POECILIA RETICULATAEVOLUTION, Issue 2 2006Farrah Bashey Abstract The existence of adaptive phenotypic plasticity demands that we study the evolution of reaction norms, rather than just the evolution of fixed traits. This approach requires the examination of functional relationships among traits not only in a single environment but across environments and between traits and plasticity itself. In this study, I examined the interplay of plasticity and local adaptation of offspring size in the Trinidadian guppy, Poecilia reticulata. Guppies respond to food restriction by growing and reproducing less but also by producing larger offspring. This plastic difference in offspring size is of the same order of magnitude as evolved genetic differences among populations. Larger offspring sizes are thought to have evolved as an adaptation to the competitive environment faced by newborn guppies in some environments. If plastic responses to maternal food limitation can achieve the same fitness benefit, then why has guppy offspring size evolved at all? To explore this question, I examined the plastic response to food level of females from two natural populations that experience different selective environments. My goals were to examine whether the plastic responses to food level varied between populations, test the consequences of maternal manipulation of offspring size for offspring fitness, and assess whether costs of plasticity exist that could account for the evolution of mean offspring size across populations. In each population, full-sib sisters were exposed to either a low- or high-food treatment. Females from both populations produced larger, leaner offspring in response to food limitation. However, the population that was thought to have a history of selection for larger offspring was less plastic in its investment per offspring in response to maternal mass, maternal food level, and fecundity than the population under selection for small offspring size. To test the consequences of maternal manipulation of offspring size for offspring fitness, I raised the offspring of low- and high-food mothers in either low- or high-food environments. No maternal effects were detected at high food levels, supporting the prediction that mothers should increase fecundity rather than offspring size in noncompetitive environments. For offspring raised under low food levels, maternal effects on juvenile size and male size at maturity varied significantly between populations, reflecting their initial differences in maternal manipulation of offspring size; nevertheless, in both populations, increased investment per offspring increased offspring fitness. Several correlates of plasticity in investment per offspring that could affect the evolution of offspring size in guppies were identified. Under low-food conditions, mothers from more plastic families invested more in future reproduction and less in their own soma. Similarly, offspring from more plastic families were smaller as juveniles and female offspring reproduced earlier. These correlations suggest that a fixed, high level of investment per offspring might be favored over a plastic response in a chronically low-resource environment or in an environment that selects for lower reproductive effort [source] Variation in leaf functional trait values within and across individuals and species: an example from a Costa Rican dry forestFUNCTIONAL ECOLOGY, Issue 1 2010Catherine M. Hulshof Summary 1.,Patterns of species co-existence and species diversity in plant communities remain an important research area despite over a century of intensive scrutiny. To provide mechanistic insight into the rules governing plant species co-existence and diversity, plant community ecologists are increasingly quantifying functional trait values for the species found in a wide range of communities. 2.,Despite the promise of a quantitative functional trait approach to plant community ecology, we suggest that, along with examining trait variation across species, an assessment of trait variation within species should also be a key component of a trait-based approach to community ecology. Variability within and between individuals and populations is likely widespread due to plastic responses to highly localized abiotic and biotic interactions. 3.,In this study, we quantify leaf trait variation within and across ten co-existing tree species in a dry tropical forest in Costa Rica to ask: (i) whether the majority of trait variation is located between species, within species, within individuals or within the leaves themselves; (ii) whether trait values collected using standardized methods correlate with those collected using unstandardized methods; and (iii) to what extent can we differentiate plant species on the basis of their traits? 4.,We find that the majority of variation in traits was often explained by between species differences; however, between leaflet trait variation was very high for compound-leaved species. We also show that many species are difficult to reliably differentiate on the basis of functional traits even when sampling many individuals. 5.,We suggest an ideal sample size of at least 10, and ideally 20, individuals be used when calculating mean trait values for individual species for entire communities, though even at large sample sizes, it remains unclear if community level trait values will allow comparisons on a larger geographic scale or if species traits are generally similar across scales. It will thus be critical to account for intraspecific variation by comparing species mean trait values across space in multiple microclimatic environments within local communities and along environmental gradients. Further, quantifying trait variability due to plasticity and inheritance will provide a better understanding of the underlying patterns and drivers of trait variation as well as the application of functional traits in outlining mechanisms of species co-existence. 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