Theoretical studies on the role of food exploitation for the competition of scaamble type

Summary A model was made to clarify the basic processes of competition to occur among larvae by the exploitation as defined byBakker (1969). It was found that this model is applicable to the experimental results on the food exploitation amongDroshophila larvae obtained byBakker (1961). In the model the preimaginal stage is divided into two periods;T _f which is the time that a group of larvae spends in exhausting the food after hatching, andT _s which is the duration of the starvation period afterT _f .T _f and thenW _l (larval body weight) just after the end ofT _f are decided byF _s (amount of food supplied per larva at larval hatching) andF _c (amount of food consumed per larva).T _f affects on the onset ofT _s as well asR _l (rate of decrease in the individual body weight duringT _s ).W _a (weight of emerging adults) is gotten by a subtraction ofR _l fromW _l just after the end ofT _f ,R _e is affected directly by these components ofW _l andR _l . As a result,W _a andR _e are expressed by functions ofF _s . This model confirmed that the food exploitation lead to the competition of scramble type. Finally it was suggested that there exist some strategies which prevent ill-effects owing to the food exploitation.     

Relationship between female body size and demographic parameters inBactrocera Malaysian A (Diptera: Tephritidae)

Summary The effect of body size, as measured by the head width, of the femaleBactrocera sp. Malaysian A (kept separately in sexual pairs) on the demographic parameters was investigated in the laboratory under ambient conditions of 28–30°C, 78–85% RH and natural photoperiod. Body size was shown to influence significantly all the demographic parameters. The expectation of life of females at eclosion from pupae was respectively for head widths of 1.6, 1.8, 1.9, 2.0 and 2.1 mm: 76.2, 73.4, 73.8, 102.4 and 115.2 days. The mean number of eggs laid per female in its life time was respectively: 86.4±48.7, 181.8±56.1, 229.7±72.6, 364.3±69.4 and 477.5±109.3 which was significantly different from one another (F=3.73,P<0.05) especially the two smaller sizes from the two larger sizes. The regression line for total eggs laid (Y) against head width (X) wasY=785.2X−1208.7 (R ²=0.35,P<0.001). The net reproductive rate (R ₀) was respectively 15.8, 34.0, 43.5, 66.9 and 88.8 eggs, while the intrinsic rate of increase (r) was respectivley 0.0435, 0.0538, 0.0670, 0.0665 and 0.0711. The results confirm that for mass rearing purposes, larger females which produce more offspring are to be preferred.     

Yearly variation in recruitment and its effect on population dynamics inYoldia notabilis (Mollusca: Bivalvia), analyzed using projection matrix model

Summary Long-term variation in recruitment was estimated by constructing projection matrices for a marine bivalve,Yoldia notabilis, at two stations in Otsuchi Bay, northeastern Japan, and the effects of its variation on population dynamics were examined using a simple matrix model. The matrix model was developed from the Leslie matrix, in which the population growth rate λ was expressed as a function of recruitment rater ₀. The equilibrium recruitment rater _s, or the recruitment rate required to maintain population at constant size (λ=1), was expressed by the reciprocal of the reproductive value of a newly recruited individual. The estimates ofr _s for the field population were lower at the shallower station than at the deeper station, reflecting higher survivorship and fecundity. Past recruitment rate estimated both by the field samplings for 3 years and by the back-calculation from the current age structure for over 10 years showed large yearly variation, ranging between 0 and 58.6×10⁻⁴. The estimates were larger thanr _s, and hence, large enough to increase population size (λ>1) only in approximately one-third of the estimated years. This suggests that the population has been maintained by occasional successful recruitment occurring once every few years.     

Dynamics of buried seed population and seedling cohorts of two dominant weeds in a hill agroecosystem of the humid subtropics of India

Dynamics of the buried seeds and plant population of two dominant weeds, viz.,Emilia sonchifolia (Linn.) DC. andRichardsonia pilosa HBK were studied in the crop fields of Meghalaya, north-east India during radish and maize cropping and intervening fallow periods. The total buried seed population ofR. pilosa was always larger than that ofE. sonchifolia, but the germinable fraction was invariably greater in the latter. A major portion (39–41%) of the viable (germinable+dormant) seed population in both weeds was confined to the surface soil layer (0–5 cm). The viable seed population ofE. sonchifolia peaked during April, while that ofR. pilosa showed two peaks (during August and December). The survival pattern and half-lives of seedling cohorts showed, some differences in the two weed species, but both being summer annuals, their populations behaved in a similar manner by showing higher seedling recruitment (K) and survivorship (p) rates in the summer crop (maize) than in the winter crop (radish). However, the density of plants that could attain adulthood was significantly higher inE. sonchifolia thanR. pilosa which might have resulted in greater seed input of the former to the soil leading to its greater abundance in the crop fields. Supported by the University Grants Commission, New Delhi (Grants No. F. 3-37/87 SR II)     

Bayesian Estimation for Quantification by Real-time Polymerase Chain Reaction Under a Branching Process Model of the DNA Molecules Amplification Process

The aim of Quantitative Polymerase Chain Reaction is to determine the initial amount X ₀ of specific nucleic acids from an observed trajectory of the amplification process, the amplification being achieved through successive replication cycles. This process depends on the efficiency {p _n } _n of replication of the DNA molecules, p _n being the probability that a molecule will duplicate at replication cycle n. Assuming p _n  = p for all n, Bayesian estimators of the unknown parameter θ = (p, X ₀) are constructed by Markov Chain Monte Carlo methods under a Bienaymé-Galton-Watson branching model of the amplification process. The Bayesian approach takes into account some prior information on the parameter. Relying on simulated data, the proposed Bayesian estimators and their credibility sets are shown to be quite accurate.     

The outbreak mechanisms of the green rice leafhopper,Nephotettix cincticeps Uhler, in northern Japan

Intraspecific regulatory processes keep the population ofNephotettix cincticeps stable at a low density in southern Japan. In northern Japan, however, the yearly population density of the insect fluctuates violently, and large outbreaks occasionally take place. To clarify the difference in the population dynamics between the two regions, we analyzed light-trap and sweep-net sampling records from prefectural and national agricultural experimental stations. The survival rate of the overwintering population decreased with increases in the period of continuous snow cover (PCSC) in the north, and initial population densities in the years of long PCSC were too low for populations to reach equilibrium density by the end of the active breeding season. This made yearly population fluctuations in the north much larger than in the south. The equilibrium density in the north was higher than in the south. The higher equilibrium density presumably permits the higher population density and larger yearly population fluctuations in the north. A major factor responsible for the difference in equilibrium densities between the two regions is the difference in heading dates of the host plant (rice). Qualitative differences among rice plant varieties, and among biotypes ofN. cincticeps, may also be important.     

Stabilization of the population of a parasite on fruit by the population of a cleptoparasite

Summary When there exist two species such that one is a parasite on fruit and the other exploits the parasitized fruits, they must compete for a limited resource with each other. The relation betweenDacus cucurbitae andAtherigona orientalis is an example of such a situation. We raise a question whether the population of a parasite on fruit can be stabilized by the existence of the cleptoparasite of the parasite on fruit. The changes in their population densities are represented as a differential equation with time delayed parameters, which is deduced from the context of life histories of the two species. An index representing degree of overlapping of generations (g) is defined as an average oviposition period devided by the average preoviposition period, and the value is assumed to be the same in the two species. The stability of the system is classified by three parameters: the reproductive rate of the parasite on fruits (R), the survival probability of it through competition against the cleptoparasite (p), and the generation overlapping index (g). For small values ofg, e.g. less than some 0.5, the stability is determined mainly by a product ofRp: the values larger than 1 result in no equilibrium and infinite increase of both species, the values near 0 lead to large amplitude oscillations, and the intermediate values make stable equilibria or regular small oscillations. Asg takes the larger values, the stability region in the space (p, R) occupies the larger area. The model presented here is well adjusted to the fluctuating pattern of the population ofD. cucurbitae on Okinawa Is., and would also be applied to analysis of both hyperparasitisms and inquilin. This study was supported partly by Science Research Fund (No. 574092) from the Ministry of Education, Japan.     

Influence of instantaneous fertility decline to replacement level on population growth: An alternative model

If age-specific birth rates m, of a stable population drop abruptly tom _x/R ₀, whereR ₀ is the net reproduction rate, then, according to Keyfitz, the size of the ultimate stationary population relative to.that at the beginning of the process is given byI =be ⁰ ₀ R ₀ ? 1)/(rμR ₀, whereb andr are the birth rate and the rate of growth, respectively, of the stable population,e ₀ ⁰ the life expectancy at birth, andμ the average age at childbirth in the resulting stationary population. Noting that the decline inm _x need not necessarily be uniform, investigation has been carried out to examine the effect on Iwhen fertility decline is more rapid at higher ages. In particular, the effect of the reduced age-specific rates such asm _xe^? rx (which also produces a stationary population) has been analyzed, and simplifications of the results carried out separately for three different models of the net maternity function. It has also been shown that when m, drops abruptly to somem _x ^*, where the form ofm _x ^* need not be specified except for the restriction that the resulting population will be stationary, the value of the index can be approximately obtained fromI ^* =be ₀ ⁰ (1 -rμ/2), whereμ is the average age at childbearing of the initial stable population.     

Niche modification and stability of competitive systems. II. Persistence of interspecific competitive systems with parasitoid wasps

Summary Effects of niche shift in ecological time scale on the population dynamics of competing species were studied in the experimental populations of two parasitoid wasp species,Anisopteromalus calandrae andHeterospilus prosopidis (both are solitary parasites), on a host, the azuki bean weevil,Callosobruchus chinensis. Four resource conditions were set up with combination of kind of bean (azuki or black eye), and host distribution (uniform or clumped). In each resource condition, four developmental stages of hosts were provided as a resource spectrum for parasitoid wasps. Population dynamics of the two wasp populations were investigated in each resource condition in Multi-Generation Competitive Systems (MGCS), in which fresh hosts of four developmental stages were periodically introduced and were parasitized competitively by the two wasp species. Competitive coexistence of both wasps occurred in the azuki-clumped condition, where the peaks of the resource utilization curves separated in the two species; pupae inA. calandrae and the early or late fourth instar inH. prosopidis, A. calandrae was eliminated in the azuki-uniform condition andH. prosopidis went extinct in two black eye conditions irrespective of host distributions. The degrees of overlap of the resource utilization patterns of the two wasp species during population dynamics were not significantly different among resource conditions irrespective of the results of coexistence or extinction. Even in the azuki-clumped condition, however, extinction ofA. calandrae was observed when resource partitioning could not be realized with only the late fourth instar larvae available to wasps. Further analytical experiments showed that parasitizing ability ofA. calandrae increased with host density per bean with azuki beans, butA. calandrae could express higher parasitizing ability with black eye beans thanH. prosopidis irrespective of host density per bean. The flexibility in parasitizing ability byA. calandrae for various host stages under different resource conditions was thought to be the major factor in determining the competitive coexistence or the extinction of either species under different resource conditions. The present experiments also suggested that different second-best host stages between competitors could be a major contributing factor to competitive coexistence.     

Resource exploitation of larvae ofGastrophysa atrocyanea Motschulsky andgalerucella vittaticollis Baly (Coleoptera: Chrisomelidae) under a limited resource condition

Summary Resource exploitation by and intraspecific competition in larvae ofGastrophysa atrocyanea andGalerucella vittaticollis were investigated in field and laboratory experiments. Larvae of both species frequently suffered from food shortages in the field. WhenG. atrocyanea larvae suffered from a food shortage, severe intraspecific competition occurred because of lack of predation and parasitism. This exploitive competition was caused by a local food shortage of the host plant. Individuals survived by fast exploitation when food became abundant (contest type competition). TheG. atrocyanea larvae were wasteful of the food resource, and no mechanism by which to economize on the utilization of the resource was acquired because of their exploitation of the abundant resource. In contrast, theG. vittaticollis population probably is regulated by extrinsic factors such as predation and parasitism. Those larvae grew into smaller adults than those ofG. atrocyanea under a food shortage, so that their wasted food consumption was lower than that ofG. atrocyanea. Although intraspecific competition was similar to that forG. atrocyanea, it was not as severe. The food forG. vittaticollis was apt to be appropriated by other wasteful exploitators such asG. atrocyanea, which was superior in resource exploitation; thereforeG. vittaticollis frequently suffered a food shortage. Consequently selection in relation to tolerance to starvation became more acute forG. vittaticollis than forG. atrocyanea, and individuals ofG. vittaticollis that could endure starvation better may have been selected.     

A model for analyzing insect stage-frequency data when mortality varies with time

Summary A model is developed for the analysis of insect stage-frequency data which may be applied to populations with age-dependent mortality. The analysis of stage-frequency data is divided into two steps. In the first step, the number of different mortality rates and their values are estimated. The second step provides estimates of developmental rates and variances for each developmental stage and in addition provides estimates of the number of recruits to each stage. The model may be used both in analysis and prediction of insect stage frequencies. Hence, in addition to estimating developmental and mortality rates from stage-frequency data, it may also be used as a simulation model for an insect population. The model is applied to two populations ofHemileuca oliviae Cockerell, a lepidopterous pest of New Mexico grasslands. The model identifies, in the two populations, different mortality rates that are related to plant productivity.     

Some extensions of the keyfitz momentum relationship

A stable population, such that the total birthrateB(t) =B _o e ^ro^t, is abruptly altered by modifying the age-specific birth rate,m(x). The survivor function remains unaltered. The modified population ultimately settles down to a stable behavior, such thatB(t) =B ₁ e ^r ₁ ^t . It is shown thatB ₁/B ₀ = (R ₀ ?R ₁)/[(r ₀ ?r ₁)R ₀ Z ₁], whereR ₀,R ₁ are the net reproduction rates before and after the change, and \(\bar Z_1 \) expected age giving birth for the stable population after the change. The age structure and transients resulting from the change are also described. The effect of an abrupt change in the survivor functionl(x) is also investigated for the simple case where the change is caused by alteringl(x) toe ^?λx l(x). It is shown that the above ratio becomes \(B_1 /B_0 = N_1 /N_0 = [1 - \smallint _0^\infty e^{ - kx} g(x)dx]/\bar Z_1 \lambda \) , whereN refers to the numbers in the population,k =r ₀ + λ, andg(x) =m(x)l(x), the value before the change. A measure for the reproductive worth of the population is also established.     

Natural control of populations of the southern green stink bug,Nezara viridula

Summary Based on the results of five-year study on theNezara population, it was concluded that the level of abundance of the insect was determined partly by the combination and relative adundance of the host plants and partly by the climatic factors. The endless increase of the population was checked by the deleterious effects associated with the overpopulation which occurred locally. The degree of such overpopulation was determined by the relative abundance of host plants to the current level of the population. Therefore, even when the general level ofNezara population seemed to be low, there still exists the possibility of overpopulation in some areas. Winter was the critical period which threatened the persistence ofNezara population. The survival of the hibernating adults was partly dependent on the physiological state of adults which was determined in the period of pre-hibernation. But when the severity of a winter exceeded the tolerance limit of the insect, the only safety-valve which prevented the elimination ofNezara was the heterogeneity of hibernacula. Therefore,Milne's theory (1957) was closest to the known facts.     

Competitive exclusion through reproductive interference

Summary A simple differential equation model was developed to describe the competitive interaction that may occur between species through reproductive interference. The model has the form comparable to Volterra's competition equations, and the graphical analysis of the outcome of the two-species interaction based on its zero-growth isoclines proved that: (1) The possible outcome in this model, as in usual models of resource competition, is either stable coexistence of both species or gradual exclusion of one species by the other, depending critically upon the values of the activity overlapping coefficientc _ij ; (2) but, for the samec _ij -values, competitive exclusion is much more ready to occur here than in resource competition; (3) and moreover, the final result of the competition is always dependent on the initial-condition due to its non-linear isoclines, i.e., even under the parameter condition that generally allows both species to coexist, an extreme bias in intial density to one species can readily cause subsequent complete exclusion of its counterparts. Thus, it may follow that the reproductive interference is likely to be working in nature as an efficient mechanism to bring about habitat partitioning in either time or space between some closely related species in insect communities, even though they inhabit heterogeneous habitats where resource competition rarely occurs so that they could otherwise attain steady coexistence.     

Time-specific life tables for the pea aphid,Acyrthosiphon pisum (Harris ), on alfalfa

Summary Time-specific life tables were constructed for three pea aphid,Acyrthosiphon pisum (Harris) (Homoptera: Aphididae), populations using a modification ofHughes' analytical procedure. All populations were studied on second-growth alfalfa (mid-June to mid-July) in south central Wisconsin; data for two populations were collected during 1980, and data for the third population were collected during 1982. The intrinsic rate of increase (r _m) estimated on a physiological time (day-degree) scale under field conditions but in the absence of natural enemies, provided a reliable estimate of potential population growth rate and was used in preference toHughes' approach of estimating potential population growth rates directly from stage structure data. Emigration by adult alatae and fungal disease were the major sources ofA. pisum mortality in each of the three populations studied. These factors were most important because of their impact on reducing birth rates within the local population. Parasitism was never greater than 9 percent. Mortality attributable to predation ranged from 0.0 to about 30.0%; however, even at the highest predator densitiesA. pisum populations increased exponentially.     

The precision of population projections studied by multiple prediction methods

A population projection is a prediction of a random vector variable X _T . which represents the size and age/sex distribution of the population in year T. The population is assumed to be closed and to develop according to fixed and known schedules of birth and death probabilities as a multitype branching process. The precision of the usual projection e _T (= EX _T) is studied by a family of prediction intervals of linear functions of the vector of deviations X _T — e _T , which has a preassigned probability level. This family is obtained by a multi-normal approximation and an argument similar to the one leading to Scheffé's method of multiple comparison. From the family of prediction intervals, an upper limit of the total absolute deviation Σ |X _iT ? e _it | is obtained, and the ratio of this limit to the projected total population is proposed as a measure of the relative precision of the projection. For a numerical study, Norwegian population data is used.     

Preference in flower visits and partitioning in pollen diets of stingless bees in an Asian tropical rain forest

Floral resource partitioning among stingless bees (Trigona, Meliponini, Apidae) in a lowland rain forest in Sarawak, Malaysia, was investigated using tree towers and walkways in a 4-year study that included a general flowering period. We obtained 100 collections of insect visitors to flowers of varying floral location and shape representing 81 plant species. The tendency of 11 species of stingless bees to visit specific flowers with a particular floral location and shape was analyzed by logistic regression analysis. This analysis showed that the proportion of flower visitor collections containing Trigona fuscobalteata and T. melanocephala differed according to floral location. The former was frequently collected at canopy and gap flowers, whereas the latter was most often collected at understory flowers. The analysis also suggested that T. erythrogastra was more rarely collected at shallow flowers than at deep flowers. Analysis of the pollen diets of T. collina, T. fuscobalteata, T. melanocephala, and T. melina revealed that similarity of pollen sources differed among the six permutated pairs of the four species. The lowest mean rank of similarity found was between T. fuscobalteata and T. melanocephala. This result supports the hypothesis that preference in visiting flowers in different locations leads to pollen resource partitioning. Received: May 14, 1997 / Accepted: April 23, 1999     

The estimation of the fitness function from two samples taken from a population

Summary The fitness of animals subjected to natural selection can be defined as the probability of surviving selection for a given interval of time, or some convenient multiple of this probability. If the fitness of animals is related to some quantitative variableX (such as size) then this relationship is expressed mathematically in the fitness functionw(x) and this function can be estimated by comparing the distribution ofX in samples taken before and after selection. In this note five methods for estimating the fitness function on the basis of samples from a large population are discussed. They are compared on three previously published sets of data and as a result estimation according to weighted multiple regression is recommended.     

Natural mortality at different population densities of a pine shoot moth,Evetria cristata

Summary A field population ofEvetria cristata was studied in 10 plots in 1962 and in 6 plots in 1963. These plots were divided into 2 or 3 groups of different population levels of the shoot moth in respective years. The survival of the insect was then analysed in these different groups of plots. The survival rate ofE. cristata from eggs to adults in the first generation was found always higher in the group with low population density, which indicates the existence of some factors that affect the population more severely when the insect is more abundant.Lissonota evetriae andPediobius sp. seemed to have killed more proportion of the hosts where the shoot moth density was high. However, the total effect of the all natural enemies was not always great in the plots with high density of the moth. The survival of the second generation of the moth in 1963 was observed to be much higher at any population level than in the other generations.     

The feeding from edge towards inner part in soybean plot in rufous turtle dove,Streptopellia orientalis (Latham) and the estimation of passing rate of the flock

Summary Rufous turtle dove,Streptopelia orientalis, coming to the soybean field entered it from the outer part to eat soybean cotyledons. As a result, the injured plants extended from the outer to inner parts in the field. A model expressing these behaviours was constructed here, by assuming that the amount of food birds can eat in one block determines whether they stay there or move into neighbour block. As the food decrease due to exploitation of them by birds, birds enter into farther parts with the passage of time. The rate of feeding in all visiting birds (an ₀ wherea is the rate of feeding per individual andn ₀ the number of birds visiting) and the rate of staying at a block,b, was estimated from the field experimental results, using the above model. The value ofan ₀ fluctuated greatly, depending upon the season in which soybean seeds sowed. The value ofb also fluctuated inversely with that ofan ₀, suggesting the the staying rate decreases with an increase in the number of doves coming, probably because of interference among individuals.