A physiological perspective on the response of body size and development time to simultaneous directional selection

Natural selection typically acts on multiple traits simultaneously.Quantitative genetics provides the theory for predicting theresponse to selection of multiple traits and predicts symmetricalresponses to selection (the response to upward selection onboth traits is equal to their response to downward selection).In reality, however, the response to simultaneous selectionon two traits is often asymmetrical. We provide a physiology-basedframework to explain the asymmetrical response to simultaneousselection on two important life history traits: body size anddevelopment time. The tobacco hornworm, Manduca sexta, is particularlywell suited for such a study, as the physiological control ofbody size and development time is well known in this species.Three physiological factors control both life history traitsin M. sexta: growth rate, the critical weight that measuresthe timing of the onset of the cessation of juvenile hormonesecretion (which initiates the processes leading to pupation)and the time interval between the critical weight and secretionof the molting hormone 20-hydroxyecdysteroid (the interval tocessation of growth, ICG). Asymmetry in the response to simultaneousselection on the two life history traits is due to the differenttypes of selection acting on the three physiological factors.The critical weight and ICG are always under synergistic selectionwhen both focal traits are selected in the same direction andunder antagonistic selection when the focal traits are selectedin opposite directions. Growth rate follows the opposite pattern.We propose a general model to explain the asymmetric responseto simultaneous selection. This model emphasizes the importanceof physiological processes in understanding evolutionary responsesto selection and the control of complex traits.     

Effects of food quality on individual growth and development in the freshwater copepod Boeckella triarticulata

To quantify the demographic effects of food quality, and specificallyof the ‘poorquality’ cyanobecterium Anabaena flos-aquae,we reared individual Boeckella triarticulata (Copepoda, Calanoida)on two diets (monospecific Cryptomom sp. versus mixed Cryptomonas-Anabanmadiets) and quantified individual growth and developmental trajectoriesby examining exuviae produced at each molt, from hatching tomaturity. Size at molting was less variable among individuals,within and between diets, than age. Food quality had significanteffects on male sizes at molting and on stage-specific dailygrowth rates of both sexes; these effects were strongest duringlate naupliar and all copepodite stages Tke med Cryptomonas-Anabaenadiet significantly slowed development, particularly of copepoditestages. As a consequence of these effects, individuals raisedon the mixed diet were smaller and older at maturity. Withina given diet, individual differences explained much, if notmost, of the variation exhibited in growth and development.By following growth and development of a large number of individualsthroughout their Life cycles, we show that individual femalesproduce variable offspring, indicative of a bet-hedging life-historystrategy, and that B.triarticulata (like other calanoids) cangrow, develop and survive on diets that include ‘poorquality’ cyanobacteria. ¹Present address: Department of Biological Sciences, Universityof Rhode Island, Kingston, RI 02881, USA     

Distribution and abundance of hyperiid amphipods in relation to summer mesoscale features in the southern Gulf of Mexico

Seventy-one hyperiid species were identified from 97 zooplanktonsamples collected in the southern Gulf of Mexico during July1988. About 91% of the adult individuals belonged to five species:Lestrigonus bengalensis (86.56% of total hyperiid numbers),Anchylomera blossevillei (1.20%), Phronimopsis spinifera (1.05%),Hyperioides longipes (1.00%) and Hyperietta vosseleri (0.99%).Overall, up to 74% of the hyperiids were collected at night,although a reverse migration was observed in the anticyclones.The mean abundance was 5-fold higher at the neritic stationsthan in the oceanic areas. This tendency was even more markedat night. Night samples yielded about the same number of speciesas during the daytime (56 taxa versus 61). Four mesoscale features(two anticyclones, one cyclone and one upwelling) characterizedthe oceanic mesoscale circulation in the surveyed area. Theabundance of the oceanic hyperiid community showed significantdifferences related to some of the mesoscale features activein the area, i.e. the abundance in the Lazy Eddy anticyclonewas lower than that in the cyclone (day and night). Overall,the upwelling areas showed a tendency to have higher abundancesthan the downwelling features (anticyclones). Cluster analysisindicated neritic–oceanic differences rather than mesoscalefeature-related differences in the local hyperiid community.The neritic community showed differences that were attributedto the effect of upwelling. The summer and spring hyperiid communitieshad important differences in the same area, thus suggestinga seasonal succession of the gulf hyperiid community.     

Laboratory studies on the larval growth and development of Nyctiphanes capensis (Euphausiacea)

Larvae of Nyctiphanes capensis Hansen were reared in the laboratoryunder different trophic conditions (i.e., algal, animal andmixed diets) from the second calyptopis stage to the first juvenilestage. It is demonstrated that diet is important in determininggrowth rates and development of the larvae. The highest growthrates were achieved on diets of Artemia nauplii mixed with eitherthe flagellate Tetraselmis or Pseudodiaptomus nauplii. Poorerdietary conditions such as with the diatom Phaeodactylum yieldedslower growth rates where additional moults were needed to completefurcilia development. Under favourable trophic conditions theintermoult period was 4–6 days and was not age dependent.Three dominant pathways of pleopod development were found inhealthy individuals.     

Energetic constraints, size gradients, and size limits in benthic marine invertebrates

Populations of marine benthic organisms occupy habitats witha range of physical and biological characteristics. In the intertidalzone, energetic costs increase with temperature and aerial exposure,and prey intake increases with immersion time, generating sizegradients with small individuals often found at upper limitsof distribution. Wave action can have similar effects, limitingfeeding time or success, although certain species benefit fromwave dislodgment of their prey; this also results in gradientsof size and morphology. The difference between energy intakeand metabolic (and/or behavioral) costs can be used to determinean energetic optimal size for individuals in such populations.Comparisons of the energetic optimal size to the maximum predictedsize based on mechanical constraints, and the ensuing mortalityschedule, provides a mechanism to study and explain organismsize gradients in intertidal and subtidal habitats. For specieswhere the energetic optimal size is well below the maximum sizethat could persist under a certain set of wave/flow conditions,it is probable that energetic constraints dominate. When theopposite is true, populations of small individuals can dominatehabitats with strong dislodgment or damage probability. Whenthe maximum size of individuals is far below either energeticoptima or mechanical limits, other sources of mortality (e.g.,predation) may favor energy allocation to early reproductionrather than to continued growth. Predictions based on optimalsize models have been tested for a variety of intertidal andsubtidal invertebrates including sea anemones, corals, and octocorals.This paper provides a review of the optimal size concept, andemploys a combination of the optimal energetic size model andlife history modeling approach to explore energy allocationto growth or reproduction as the optimal size is approached.     

Growth and development rates in a riparian spider are altered by asynchrony between the timing and amount of a resource subsidy

Rapid growth in response to increased prey abundance may be induced by environmental variability associated with resource subsidies. Spiders living in riparian areas are subject to frequent, episodic bursts of aquatic prey (subsidies). These periods of high resource abundance may occur at different points in recipient consumers’ development through variation in emergence patterns of prey between years or across a landscape. We examine how variable timing of subsidy abundance intersects with life history scheduling to produce different growth and development outcomes for individuals within a population. Through a series of controlled feeding experiments, we tested the hypotheses that the spider Tetragnatha versicolor: (1) exhibits compensatory growth in response to subsidy variability, (2) that rapid increases in mass may result in a greater risk of mortality, and (3) that the timing of subsidy resources relative to the development schedule of this spider may produce different outcomes for individual growth patterns and adult condition. Spiders fed at very high rates grew fastest but also showed evidence of increased mortality risk during moulting. T. versicolor is capable of exhibiting strong growth compensation—individuals suffering initial growth restriction were able to catch up completely with animals on a constant diet utilising the same amount of food. Spiders that received an early pulse of resources (simulating an early arrival of an aquatic insect subsidy to riparian forests) did worse on all measures of development and fitness than spiders that received either a constant supply of food or a late pulse of resources. Importantly, receiving large amounts of food early in life appears to actually confer relative disadvantages in terms of later performance compared with receiving subsidies later in development. Subsidies may provide greater benefits to individuals or age cohorts encountering this resource abundance closer to the onset of reproductive efforts than subsidies arriving early in development.     

Establishment of in vitro Cultures of Tapinanthus bangwensis (Engler and K. Krause) Danser and the Effects of Sugars on Growth

Culturing of organs in vitro has been successfully employedin studies on morphogenesis and nutritional requirements ofparasitic and semi-paraaitic angiosperms. Tapinanthus bangwensis,a semi-parasite, has been successfully cultured on chemicallydefined media. By and large the parasite will thrive well ina medium of mineral salts and sucrose at its optimal concentration(4 per cent). However, the parasite is able to metabolize awide range of sugars most of which show similar concentrationoptima Although the growth in vivo was simulated in vitro inthe early stages, it was found that in the later stages growthin vitro was much slower than growth in vivo. The growth differencesobserved in the different media may reflect some of the physiologicaldifferences that are responsible for the selective nature ofthe parasite's development and establishment on different hosts     

A Canopy Photosynthesis Model for the Dynamics of Size Structure and Self-thinning in Plant Populations

A dynamic model for growth and mortality of individual plantsin a stand was developed, based on the process of canopy photosynthesis,and assuming an allometric relationship between plant heightand weight, i.e. allocation growth pattern of plant height andstem diameter. Functions G(t, x), for the mean growth rate ofindividuals of size x at time t, and M(t,x), for the mortalityrate of individuals of size x at time t, were developed fromthis model and used in simulations. The dynamics of size structurewere simulated, combining the continuity equation model, a simpleversion of the diffusion model, with these functions. Simulationsreproduced several well-documented phenomena: (1) size variabilityin terms of coefficient of variation and skewness of plant weightincreases at first with stand development and then stabilisesor decreases with an onset of intensive self-thinning; (2) duringthe course of self-thinning, there is a power relationship betweendensity and biomass per unit ground area, irrespective of theinitial density and of the allocation-growth pattern in termsof the allometric parameter relating plant height and weight.The following were further shown by simulation: (a) competitionbetween individuals in a crowded stand is never completely one-sidedbut always asymmetrically two-sided, even though competitionis only for light; (b) plants of ‘height-growth’type exhibit a greater asymmetry in competition than plantsof ‘diameter-growth’ type, (c) the effect of competitionon the growth of individuals in a crowded stand converges toa stationary state, even when the stand structure still changesgreatly. All of these theoretical results can explain recentempirical results obtained from several natural plant communities.Finally, a new, general functional form for G(t, x) in a crowdedstand is proposed based on these theoretical results, insteadof a priori or empirical growth and competition functions. Canopy photosynthesis, competition mode, continuity equation, self-thinning, simulation, size distribution     

Growth Analysis of Soybean Seedlings During the Lifespan of the Cotyledons

Patterns of seedling growth of Glycine max in light and darknesswere compared during the period from germination to cotyledonabscission. Fitted growth curves and the derived functions,relative growth rate, unit leaf rate, leaf area ratio and specificleaf area, were used to assess the relative importance in seedlinggrowth of cotyledon storage reserves, cotyledon photosynthesisand leaf photosynthesis. The cotyledons are of an intermediatetype with a predominant storage and a minimal photosyntheticfunction. Cotyledon reserves support seedling growth until theprimary leaves expand, after which growth depends on leaf photosynthesis. Glycine max (L.) Merr., soybean, cotyledons, growth analysis, seedling development     

Control and Consequences of Alternative Developmental Modes in a Poecilogonous Polychaete

SYNOPSIS. The poecilogonous polychaete Streblospio benedicti(Webster) exhibits both planktotrophic and lecithotrophic modesof larval development. The alternative trophic modes are associatedwith differences in age and size at maturation, offspring number,size and energetic investment, larval planktonic period, morphologyand survivorship. This paper reviews a decade of research intothe control and consequences of the traits associated with planktotrophyand lecithotrophy in S. benedicti. The dominant control on reproductiveand developmental characters is genetic. Significant additivegenetic variance has been detected for egg diameter, fecundity,larval planktonic period and aspects of larval morphology. However,environmental factors such as temperature, food quality andphotoperiod, and intrinsic factors such as maternal age, exertconsiderable influence on non-trophic developmental traits (e.g.,offspring number, size and energy content). Demographic consequencesof development mode are reviewed for field and laboratory demesof S. benedicti dominated by individuals exhibiting either planktotrophyor lecithotrophy. Similar population size structure, fluctuationsin abundance, P: B ratios, and estimated population growth ratesare achieved through trade-offs between survivorship and fecundity. Development mode may best be viewed as a complex set of traitsthat are intimately linked developmentally and evolutionarilyto other aspects of an organism's life history. Greater insightinto the control and consequences of development mode shouldresult from further investigation of these linkages     

The Interactions of Auxins and Anti-auxins in the Stimulation of Root Growth

Experiments on the interactions of auxins and anti-auxins inroots have been extended to studies with concentrations givinggrowth stimulations using 2-mm. sections excised from the extensionzone of roots of Pisum sativum. The curves relating growth responsesto log₁₀ concentration for ß-indolylacetic acid (IAA)and three anti-auxins, -(I-naphthylmethylsulphide) propionicacid (NMSP), I-naphthylmethylsulphide acetic acid (NMSA), and4-chloro-3-nitrobenzoic acid (CNB), are of very similar shape.A fourth anti-auxin, p-chlorophenoxy-iso butyric acid (PCIB),shows negligible stimulation of growth in any concentration.In multifactorial experiments involving stimulatory concentrationsof IAA (10^–4 and 10^–5 p.p.m.) with several stimulatoryconcentrations of the first three anti-auxins (NMSP, NMSA, andCNB), consistent mutual antagonisms were clearly demonstrated.PCIB in non-inhibitory concentrations markedly antagonized stimulationby IAA Similar mutual antagonisms were shown in various mixturesof two anti-auxins. Both the similarities of the concentration-response curves andthe consistent mutual antagonisms suggest that both auxins andanti-auxins in stimulating root growth are exerting identicalphysiological actions in the same growth system. On the assumptionthat these substances are active when adsorbed at some enzymeor other protoplasmic surface, it has been demonstrated thatthese experimental results are more easily explained by a directaction at those growth centres than by a competitive antagonismof a natural endogenous growth inhibitor, as suggested in aprevious paper. The bearing of these results on current theories relating auxinand anti-auxin activities to molecular structure is discussed.     

Daylength Effects on Growth, Tracheid Development, and Photosynthesis in Seedlings of Picea sitchensis and Pinus sylvestris

Effects of change in daylength on tracheid expansion and tracheidwall thickening are considered in relation to shoot and rootgrowth, assimilation rate, and starch content, in seedlingsof Picea sitchensis and Pinus sylvestris grown in controlledenviroments. Rates of wall thickening decreased in the first tracheids tobegin wall thickening after decrease in daylength. A sharp decreasein starch-grain size at the same time supports previous suggestionsthat wall thickness is related to available substrate; however,rates of assimilation were found to be far greater in shortdays than in long, suggesting that a surplus of carbohydratemay be produced in long days. It is pointed out that if availablesubstrate does limit wall thickening it is likely to determineto al accumulation of wall material; wall material per tracheidwould follow logistically from this depending on the numberof tracheids around the xylem, and wall thickness would dependalso on the radial tracheid diameter. Unexpectedly, rate of shoot growth accelerated after transferto short days before its final cessation with terminal-bud formation.Root growth declined in short days, but later increased againafter terminal bud formation. Rate of root growth did not appearto be associated with tracheid development in the shoot.     

Spatio-temporal patterns in the copepod community in Malangen, Northern Norway

The objective of this study was to identify the key copepodspecies and their life cycles, and provide evidence for anyseasonal and spatial changes in the copepod community in Malangen,a fjord located 30 km to the south of Tromsø in NorthernNorway (69°30'N, 18°21'E). As a result of high levelsof freshwater run-off in May, the fjord became highly stratifiedwith a sharp pycnocline at 10–30 m depth from May to August.The generation patterns of six copepod species are described.Calanusfinmarchicus produced one generation during the spring thatyear, whereas two generations appeared to be produced by bothPseudocalanus acuspes and P.minutus: one in spring (March-June)and the other in autumn (August-December). However, it is uncertainto what extent P.minutus regularly produces a second generation.Two peaks of CI-CIII Metridia spp. were found; there were differencesalong the length of the fjord in the timing of these, but therelative contributions of M.longa and M.lucens are uncertain.Chiridius armatus CI-CIII peaked in abundance in the spring,which indicates that one main generation was produced at theouter station of the fjord. The copepod community in Malangencould be grouped into three entities according to their numericalabundance during the year one group of highly abundant forms,generally with maxima >50 000 individuals m^–3 (C.finmarchicus,Microcalanus sp., Oithona similis , Oithona spinirostris, Acartiasp. and Pseudocalanus spp.), a second group of less abundantspecies with a clear seasonality in abundance, varying from500 to 50 000 individuals m^–3 (M.longa, M.lucens, Calanushyperboreus, Carmatus, Tenwra longicornis, Oncaea sp., Euchaetanorvegica and Scolecithrwella minor), and a third group of 14holoplanktonic species, sporadically occurring in the fjord.The study demonstrates clear gradients in the abundance of fivespecies along the length of the fjord: the recruiting generationof C.finmarchicus occurred in higher abundances at the outerstation in May and June compared to the other inner sites. Laterin the season, the reverse situation appeared, in which thepopulation was more abundant in the inner part of the fjorcCalanw hyperboreus increased abruptly in abundance from lowwinter levels to a maximum in April-May, and declined steadilyduring the season (except at the innermost station). Metridialucens, M.longa and C.armatus demonstrated different distributionpatterns in Malangen that matched their preferred areas of distribution.Both M.lucens and C.armatus are known as oceanic and deep-waterspecies, respectively, and these were prevalent at the two outersites in Malangen. Metridia longa is a more nentic species andwas found in highest numbers at the two innermost sites. Themechanisms for the differences in abundance among these specieswithin the fjord are discussed.     

Behavioral and Morphological Adaptations by Galapagos Land Iguanas (Conolophus subcristatus) to Water and Energy Requirements of Eggs and Neonates

We examine the importance of both the hydric environment ofnaturally incubating reptilian eggs and the energetic needsof hatchlings via an investigation of reproduction in Galapagosland iguanas (Conolophus subcristatus). Hatching success ofegg clutches and the size of subsequent hatchlings are bothpositively correlated with the water potential of natural nests,as predicted from previous laboratory experiments. Water potentialsrepresenting optimal incubation environments are available foronly a brief period in nature, and depend upon the same seasonalrainfall as does food abundance for the emerging hatchlings.The temporal placement of the reproductive season of Conolophussubcristatus balances these conflicting needs for water by eggsand energy by hatchlings. Oviposition occurs slightly beforesuitable water potentials are reached and hatchling emergenceoccurs after the peak in food abundance. Morphological adaptations by Conolophus subcristatus to theirprecarious reproductive phenology include greater amounts ofalbumen in their eggs, and greater energy reserves in emergenthatchlings than most other lizard species. These adaptationslessen the severity of an arid environment where water becomesavailable for periods too short to allow both oviposition andhatching to be temporally placed in an optimal manner.     

The Chloroplast and Leaf Developmental Mutant, pale cress, Exhibits Light-conditional Severity and Symptoms Characteristic of its ABA Deficiency

The PALE CRESS gene (PAC) is essential for proper chloroplastand leaf development in Arabidopsis thaliana. The ability ofpac mutants to accumulate significantly more chlorophyll whengrown in low light conditions than in high light conditionssuggests that carotenoid deficiency is at least partly responsiblefor premature cessation of chloroplast development. In additionto accumulation of low levels of chlorophyll and carotenoidpigments,pac mutants are abscisic acid (ABA) deficient and havecharacteristics which may be explained by this deficiency. Theseinclude reduced seed viability and, in enclosed growth conditions,increased leaf growth. Plants transformed with an antisensePAC construct often bear viviparous embryos which may be symptomaticof a deficiency in ABA. Since carotenoids are precursors ofABA, a role for PAC in carotenoid biosynthesis is further supported.The nuclear-encoded, chloroplast-localized PAC protein has beenimplicated in the maturation of plastid-encoded mRNAs. Thus,PAC may affect the abundance of one or more chloroplast proteinswhich function in the synthesis or stability of carotenoids.Using thePROLIFERA gene as a marker for cell division, it isshown that cell division profiles in the pac shoot apex aredisrupted. pac leaves are relatively normal in size and shapedespite the light intensity-induced variability of leaf celldefects. Copyright 2000 Annals of Botany Company Abscisic acid, carotenoid, chloroplast development, leaf development, organismal theory, PALE CRESS,PROLIFERA , vivipary     

Food regulation of growth and maturation in a natural population of Aurelia aurita (L.)

Growth and maturity development of the moon jellyfish. Aureliaaurita, were recorded in Vgsbpollen, a small and semi-enclosedbay on the Norwegian west coast, and compared to those of medusaetransferred to excess food and starving conditions, respectively.Mesozooplankton were extremely scarce in Vgsbpollen. The abundanceand biomass of the medusae in the poll were higher than thosetypicallyfound in open waters, reaching a maximum of 22 ind.m^–3 and 710 mg C m^–3 in June. The average diameterof medusae in the p increased to 8 cm until the last part ofJune, with an instantaneous growth rate between 1.5 and 20%day^–1, thereafter retarding somewhat, giving a negativegrowth rate of up to 2.6% day^–1. Starving medusae showeda negative growth rate ofup to 13.4% day^–1, and all thernedusae were dead after 49 days. Well-fed medusae showed avery stable growth over a 56 day period, diverging from thepollpopulation from early June, and with a growth rate between3.8 and 9.8% day^–1. Medusae from the pollpopulation begancarrying planulae on their oral arms when at least 5 cm in diameter,whereas not even the largest medusa of 15.6 cm diameter amongthose in the well-fed group produced any planulae. For the firsttime, it is thus explicitly shown that thesize and maturityof A.aurita are externally controlled through food availability.Scarcity of food reduces the growth rate, but also changes theenergy allocation towards reproduction, which thus occurs ata smaller size than for well-fed rnedusae. Its plasticity makesit possible for this species to exploit environments with lowadvection of food and develop high abundance in such environments,without losing fecundity.     

Infection of the diatom Asterionella by a chytrid. II. Effects of light on survival and epidemic development of the parasite

A model is formulated to investigate the ability of chytridparasites to survive or become epidemic within populations oftheir algal hosts The model is used for an analysis of the effectsof light on the occurrence of Rhizophydium planktonicum Canteremend., a chytrid parasite of the freshwater diatom Asterionellaformosa Hass., using the information on the growth parametersof host and parasite presented in the first part of this article(J. Plankton Res ., 13, 103–117). According to the model,conditions for survival of the parasite are optimal when thehost grows at saturating light conditions. Under limiting lightconditions, Rhizophydium needs higher host densities in orderto maintain itself. The parasite is not able to survive prolongedperiods of severe light limitation of the host Epidemic development,however, turned out to be facilitated by a moderate light limitationof the host. Both light saturation and severe light limitationhamper epidemic development, but in the first case, epidemicdevelopment is still possible at sufficiently high host densities.     

Nutrient levels and the development of diatom and blue-green algal blooms in a shallow Australian estuary

In the Peel-Harvey estuary system, Western Australia, some 90%of riverflow and nutrient loading occurs in three winter months.Diatom blooms follow riverflow, but are replaced by blooms ofthe blue-green Nodularia spumigena Mert., especially in HarveyEstuary. By analysis of time series data from 1977–1983,it is shown that the magnitude of the Nodularia bloom in summeris related to the minimum salinity of the estuary (and hencetotal river flow), maximum phosphate concentration and totalriverine phosphorus loading, in the previous winter. The relationshipshave a predictive capacity. It is argued that diatom bloomstrap phosphorus, which is sedimented largely as faecal pellets;the phosphorus is recycled and supports Nodularia growth underwarmer conditions, and the amount available determines Nodulariabiomass. Nodularia blooms collapse when summer salinities reach30     

Growth Dynamics of Shoot Height and Foliage Structure of a Rhizomatous Perennial Herb, Polygonum cuspidatum

The growth dynamics of shoot populations of Polygonum cuspidatumwere investigated at the Houei crater (approx. 2380 m abovesea level) on the south-eastern slope of Mount Fuji. At thisstudy site, a genetic individual of this species produces apopulation of shoots in the form of a patch occupying a certainground area. Generally, genetic individuals are located awayfrom each other and hence there is little interaction betweenindividuals. A large-sized individual occupying 31·2m² ground area with shoots, a medium-sized individual (5·6m²) and a small-sized individual (1·4 m²) were selectedfor this study. In each individual, growth was investigatedat the shoot level. The results were analysed based on the diffusionmodel. Early in the growing season in 1990, there was littledifference in LAI (leaf area index) and shoot density betweenthe individuals. Shoots of the small- and medium-sized individualsshowed size-independent height growth, whilst those of the larger-sizedindividual showed size-dependent height growth. Consequently,small-sized shoots of the small- and the medium-sized individualshad greater RGRs of shoot height growth than those of the largeindividual at the early stage. As a result, in the small- andmedium-sized individuals, cv (coefficient of variance) and skewnessof shoot height decreased with time. Increases in cv and skewnessof shoot height were found in the large-sized individual. Thesize-independent growth pattern of shoot height in the small-and medium-sized individuals during early growing stages isdifferent from the growth pattern of non-clonal plant species,in which plant height growth is positively size-dependent. Theexistence of a regulatory mechanism of shoot height growth issuggested for the small- and medium-sized individuals. The foliagestructure of the large-sized individual was different from thatof the medium- and small-sized individuals. The foliage structureof small- and medium-sized individuals was similar to the theoretical"optimal foliage structure" of plants. In clonal plant species,a genetic individual occupies a certain ground area with itsshoots. Therefore, "optimal foliage structure" per unit groundarea brings about maximization of photosynthetic rate for agenetic individual, which is consistent with the maximizationof fitness at the level of the individual plant.Copyright 1994,1999 Academic Press Clonal plant species, diffusion model, growth regulatory mechanism, genetic individual, maximization of fitness     

Differentiation and frequency distributions of body weights in plants and animals

The basic and simplest system that one can consider in ecology is a group of individuals of equal age and representing one species, that is, a cohort. This paper is an attempt to show that analysis of such a system may be of great importance to understanding basic ecological problems, such as, intraspecific competition and the dynamics of a single population. It is easy to observe that in even-aged populations individuals differ in weights. A close look can show that weight distributions in even-aged populations may have different skewness. Most common are distributions with coefficients of skewness greater than zero. Sometimes weight distributions are symmetrical or with skewness coefficients less than zero. In a cohort of growing individuals the coefficient of skewness changes with time: most often starting from zero (symmetrical distribution), it increases in time; sometimes after an initial increase it can decrease in the final stage of growth, which is related to an increased mortality of individuals. The rate of change in skewness, and the skewness itself depend on the density of individuals in a cohort and on food conditions. They are greater at higher densities and increase with deteriorating food conditions. Weight distributions are symmetrical at low densities and optimal food conditions. The differences in individual weights measured by variance of weight distributions or coefficient of variation follow the same pattern, but observed changes with time, density and food conditions are not so clear. These conclusions rest upon the review of numerous papers concerning both plants and animals, which is presented in this paper. In the past, the properties of weight distributions in even-aged populations were explained not by interactions between individuals, but rather as a natural outcome of the growth process of non-interacting individuals. The exponential equation of growth, with relative growth rate having a normal distribution in populations, was used to support this hypothesis. Obtained weight distributions were of positive skewness; however, this model, which in fact is able to describe the growth process only in its initial stage, cannot explain the changes of skewness of weight distributions with density and food conditions. A model has been developed which includes competitive interactions among members of even-aged populations to explain observed properties of weight distributions in them. The basic assumption is that intraspecific competition leads to uneven partitioning of resources, which are the object of competition. Functions describing resource partitioning among individuals are included into the model.(ABSTRACT TRUNCATED AT 400 WORDS)