Density-dependent prophylaxis in the coral-eating crown-of-thorns sea star, Acanthaster planci

The density-dependent prophylaxis hypothesis predicts that individuals at high density will invest more resources into immune defence than individuals at lower densities as a counter-measure to density-dependent pathogen transmission rates. Evidence has been found for this hypothesis in insects, but not in a non-arthropod taxon. To investigate this hypothesis in the coral-eating crown-of-thorns sea star, Acanthaster planci, density treatments were set up over 21 days, and pathogen infection was simulated with bacterial injection. Five immune responses: amoebocyte count, amoebocyte viability, lysosomal membrane integrity, respiratory burst and peroxidase activity were all upregulated at high density. These results demonstrate that immune investment shows phenotypic plasticity with adult population density in agreement with the density-dependent prophylaxis hypothesis. Here I show that the density-dependent prophylaxis hypothesis is neither dependent on larval density nor restricted to insects, and hence may potentially have important consequences on disease dynamics in any species with widely fluctuating population densities. This is the first demonstration of the density-dependent prophylaxis hypothesis outside arthropods.     

Effects of phenotypic plasticity on pathogen transmission in the field in a Lepidoptera-NPV system

In models of insect–pathogen interactions, the transmission parameter (ν) is the term that describes the efficiency with which pathogens are transmitted between hosts. There are two components to the transmission parameter, namely the rate at which the host encounters pathogens (contact rate) and the rate at which contact between host and pathogen results in infection (host susceptibility). Here it is shown that in larvae of Spodoptera exempta (Lepidoptera: Noctuidae), in which rearing density triggers the expression of one of two alternative phenotypes, the high-density morph is associated with an increase in larval activity. This response is likely to result in an increase in the contact rate between hosts and pathogens. Rearing density is also known to affect susceptibility of S. exempta to pathogens, with the high-density morph showing increased resistance to a baculovirus. In order to determine whether density-dependent differences observed in the laboratory might affect transmission in the wild, a field trial was carried out to estimate the transmission parameter for S. exempta and its nuclear polyhedrosis virus (NPV). The transmission parameter was found to be significantly higher among larvae reared in isolation than among those reared in crowds. Models of insect–pathogen interactions, in which the transmission parameter is assumed to be constant, will therefore not fully describe the S. exempta-NPV system. The finding that crowding can influence transmission in this way has major implications for both the long-term population dynamics and the invasion dynamics of insect–pathogen systems. Received: 14 June 1999 / Accepted: 22 March 2000     

Two's a Crowd: Phenotypic Adjustments and Prophylaxis in Anticarsia gemmatalis Larvae Are Triggered by the Presence of Conspecifics

Defence from parasites and pathogens involves a cost. Thus, it is expected that organisms use this only at high population densities, where the risk of pathogen transmission may be high, as proposed by the "density-dependent prophylaxis" (DDP) hypothesis. These predictions have been tested in a wide range of insects, both in comparative and experimental studies. We think it pertinent to consider a continuum between solitarious and gregarious living insects, wherein: (1) solitarious insects are those that are constitutively solitary and do not express any phenotypic plasticity, (2) the middle of the continuum is represented by insects that are subject to fluctuations in local density and show a range of facultative and plastic changes; and (3) constitutively gregarious forms live gregariously and show the gregarious phenotype even in the absence of crowding stimuli. We aimed to chart some of the intermediary continuum with an insect that presents solitarious aspects, but that is subject to fluctuations in density. Thus, Anticarsia gemmatalis (Lepidoptera: Noctuidae) larvae reared at higher densities showed changes in coloration, a greater degree of encapsulation, had higher hemocyte densities and were more resistant to Baculovirus anticarsia, but not to Bacillus thuringiensis. Meanwhile, with increased rearing density there was reduced capsule melanization. Hemocyte density was the only variable that did not vary according to larval phenotype. The observed responses were not a continuous function of larval density, but an all-or-nothing response to the presence of a conspecific. As A. gemmatalis is not known for gregarious living, yet shows these density-dependent changes, it thus seems that this plastic phenotypic adjustment may be a broader phenomenon than previously thought.     

Oviposition in an eruptive moth species, Yponomeuta evonymellus, is insensitive to the population density experienced during the larval period

We studied if the population density experienced during larval development affects the reproductive schedule of Yponomeuta evonymellus L. (Lepidoptera, Yponomeutidae), a moth species characterized by outbreak population dynamics. More specifically, we predicted that reproduction would be delayed to facilitate emigration from sites with suboptimally high densities of conspecifics. We manipulated larval densities in the laboratory, as well as in those pupae collected from the extremes of natural densities. As the response, we also recorded the timing and sizes of egg clutches laid. The results did not support our initial predictions: the timing of oviposition was not dependent on larval growth conditions. This apparent lack of adaptation might be related to the loss of ‘memory’ during metamorphosis in holometabolous insects. However, oviposition schedules were also only minimally sensitive to elevated adult density. An inability to respond to high larval densities may contribute to the outbreak dynamics in this species.     

Baculovirus resistance in the noctuid Spodoptera exempta is phenotypically plastic and responds to population density

Parasite resistance mechanisms can be costly to maintain. We would therefore predict that organisms should invest in resistance only when it is likely to be required. Insects that show density-dependent phase polyphenism, developing different phenotypes at high and low population densities, have the opportunity to match their levels of investment in resistance with the likelihood of exposure to pathogens. As high population densities often precipitate disease epidemics, the high-density form should be selected to invest relatively more in resistance. We tested this prediction in larvae of the noctuid Spodoptera exempta. Larvae reared at a high density were found to be considerably more resistant to a nuclear polyhedrosis virus than those reared in isolation. A conspicuous feature of the high-density phase of S. exempta and other phase-polyphenic Lepidoptera is cuticular melanization. As melanization is controlled by the phenoloxidase enzyme system, which is also involved in the immune response, this suggests a possible mechanism for increased resistance at high population densities. We demonstrated that melanized S. exempta larvae were more resistant than non-melanized forms, independent of rearing density. We also found that haemolymph phenoloxidase activity was correlated with cuticular melanization, providing further evidence for a link between melanization and immunity. These results suggest that pathogen resistance in S. exempta is phenotypically plastic, and that the melanized cuticles characteristic of the high-density form may be indicative of a more active immune system.     

Strong density-dependent survival and recruitment regulate the abundance of a coral reef fish

Debate on the control of population dynamics in reef fishes has centred on whether patterns in abundance are determined by the supply of planktonic recruits, or by post-recruitment processes. Recruitment limitation implies little or no regulation of the reef-associated population, and is supported by several experimental studies that failed to detect density dependence. Previous manipulations of population density have, however, focused on juveniles, and there have been no tests for density-dependent interactions among adult reef fishes. I tested for population regulation in Coryphopterus glaucofraenum, a small, short-lived goby that is common in the Caribbean. Adult density was manipulated on artificial reefs and adults were also monitored on reefs where they varied in density naturally. Survival of adult gobies showed a strong inverse relationship with their initial density across a realistic range of densities. Individually marked gobies, however, grew at similar rates across all densities, suggesting that density-dependent survival was not associated with depressed growth, and so may result from predation or parasitism rather than from food shortage. Like adult survival, the accumulation of new recruits on reefs was also much lower at high adult densities than at low densities. Suppression of recruitment by adults may occur because adults cause either reduced larval settlement or reduced early post-settlement survival. In summary, this study has documented a previously unrecorded regulatory mechanism for reef fish populations (density-dependent adult mortality) and provided a particularly strong example of a well-established mechanism (density-dependent recruitment). In combination, these two compensatory mechanisms have the potential to strongly regulate the abundance of this species, and rule out the control of abundance by the supply of recruits.     

Population dynamics of the beech caterpillar, <Emphasis Type="Italic">Syntypistis punctatella</Emphasis>, and biotic and abiotic factors

The beech caterpillar, Syntypistis punctatella (Motschulsky) (Lepidoptera: Notodontidae), often causes extensive defoliation of beech forests in Japan. Outbreaks have often occurred synchronously among different areas at intervals of 8–11 years. Synchrony of outbreaks was considered to be caused by synchrony of weather. Populations of this insect exhibit periodical dynamics in both outbreak and nonoutbreak areas. Factors that might influence the population dynamics of the beech caterpillar were classified from the point of view of the natural bioregulation com-plex, which includes a coleopteran predator, Calosoma maximowiczi, avian predators, parasitoids, entomopathogenic fungi, and delayed induced defensive response (DIR) of beech trees. Because such periodic population dynamics are believed to be caused by one or more delayed density-dependent factors, delayed density-dependent mortality has been identified as a likely source of population cycles. The DIR and pathogenic diseases showed a high order of density dependence. An infectious pathogen, Cordyceps militaris, was considered to be the most plausible agent responsible for periodic dynamics of the beech caterpillar population because insect diseases were effective in cases in which the S. punctatella population started to decrease without reaching outbreak densities, but DIR was not. Conspicuous defoliation caused by this insect tends to occur at certain elevations, where forests are composed of pure stands of beech trees. I propose three different hypotheses to explain this phenomenon: the diversity–stability hypothesis, the resource concentration hypothesis, and the altitudinal soil nutrient hypothesis. Received: November 20, 1999 / Accepted: August 3, 2000     

Density-dependent effects of larval dispersal mediated by host plant quality on populations of an invasive insect

The success of invasive species is often thought to be due to release from natural enemies. This hypothesis assumes that species are regulated by top-down forces in their native range and are likely to be regulated by bottom-up forces in the invasive range. Neither of these assumptions has been consistently supported with insects, a group which includes many destructive invasive species. Winter moth (Operophtera brumata) is an invasive defoliator in North America that appears to be regulated by larval mortality. To assess whether regulation was caused by top-down or bottom-up forces, we sought to identify the main causes of larval mortality. We used observational and manipulative field and laboratory studies to demonstrate that larval mortality due to predation, parasitism, and disease were minimal. We measured the response of larval dispersal in the field to multiple aspects of foliar quality, including total phenolics, pH 10 oxidized phenolics, trichome density, total nitrogen, total carbon, and carbon–nitrogen ratio. Tree-level declines in density were driven by density-dependent dispersal of early instars. Late instar larvae dispersed at increased rates from previously damaged as compared to undamaged foliage, and in 2015 field larval dispersal rates were related to proportion of oxidative phenolics. We conclude that larval dispersal is the dominant source of density-dependent larval mortality, may be mediated by induced changes in foliar quality, and likely regulates population densities in New England. These findings suggest that winter moth population densities in New England are regulated by bottom-up forces, aligning with the natural enemy release hypothesis.     

A minimalist approach to the effects of density-dependent competition on insect life-history traits

Abstract  1. Due to its effects on the phenotypic and genotypic expression of life-history traits, density-dependent competition is an important factor regulating the growth of populations. Specifically for insects, density-dependent competition among juveniles is often associated with increased juvenile mortality, delayed maturity, and reduced adult size.
2. The aim of the work reported here was to test whether the established phenotypic effects of density-dependent competition on life-history traits could be reproduced in an experimental design requiring a minimal number of individuals. Larvae of the mosquito Aedes aegypti were reared at densities of one, two, or three individuals per standard Drosophila vial and in six different conditions of larval food availability. This design required relatively few individuals per independent replicate and included a control treatment where individuals reared at a density of one larva per vial experienced no density-dependent interactions with other larvae.
3. Increased larval densities or reduced food availability led to increased larval mortality, delayed pupation, and the emergence of smaller adults that starved to death in a shorter time (indicating emergence with fewer nutritional reserves).
4. Female mosquitoes were relatively larger than males (as measured by wing length) but males tended to survive for longer. These differences increased as larval food availability increased, indicating the relative importance of these two traits for the fitness of each sex. The role of nutritional reserves for the reproductive success of males was highlighted in particular.
5. This minimalist approach may provide a useful model for investigating the effects of density-dependent competition on insect life-history traits.     

幼虫密度对甜菜夜蛾生长发育与繁殖的影响

为了研究甜菜夜蛾Spodoptera exigua(Hübner)幼虫的密度对其发育及繁殖的影响,本实验观察了5种幼虫密度下（1,5,10,20,30头/瓶）,幼虫发育和成虫繁殖情况。结果表明：幼虫和蛹历期、存活率和蛹重均差异显著。幼虫和蛹历期均以20头/瓶的最短,1头/瓶的最长,其余随幼虫密度增加而延长;幼虫至蛹存活率以10头/瓶的最高,其余随幼虫密度增加而降低;1头/瓶的蛹最重,显著高于其他密度的,其余随幼虫密度增加而下降。尽管密度间成虫羽化率和产卵前期均无显著差异,但成虫产卵量、寿命和畸形率差异显著。1头/瓶的产卵量最多,其次为10头/瓶的,其余随幼虫密度增加而减少,30头/瓶的产卵量显著少于其他密度的; 密度在1～20头/瓶范围内,雌蛾寿命均较短,显著短于30头/瓶的,而雄蛾寿命以5头/瓶的最短,显著短于其他密度的（10头/瓶除外）,10头/瓶的次之,其余密度间差异不显著;不同幼虫密度下羽化的成虫畸形率差异显著,10头/瓶的最低,其余随幼虫密度增加而升高; 生命表结果表明甜菜夜蛾在10头/瓶下世代存活率和种群增长指数均最高,幼虫密度过低或过高均不利于种群增长;世代存活率（S）和种群增长指数（I）与幼虫密度之间的关系均呈抛物线关系：S =－0.2087x²+2.5694x+211.52 (R²=0.88),I=－0.0552x²+0.9166x+54.168 (R²=0.95)。结果提示幼虫密度影响甜菜夜蛾种群动态的重要生态因子之一。     

Dispersal of sea lamprey larvae during early life: relevance for recruitment dynamics

Dispersal is an important early life history process that influences fish population dynamics and recruitment. We studied larval sea lamprey (Petromyzon marinus) dispersal by combining spatially explicit field sampling, genetic methods, and laboratory experiments to investigate how far sea lamprey larvae can disperse away from nests during their first growing season; subsequent dispersal by age 1 of sea lamprey; and the effect of density on larval dispersal. In two study streams sea lamprey larvae were observed to have moved >150 m downstream from the most likely source nest within 2–3 weeks of hatching. Conversely, randomization trials suggested that for both streams age 0 larvae were found closer to full siblings than would be expected if dispersal was not constrained by distance. Restricted dispersal was also observed for age 1 larvae in five streams, although for this age class full siblings were more commonly found to be separated by >1,000 m. Laboratory experiments indicated a significant effect of density on the movement of larval sea lamprey, with more larval movement at higher densities. Temperature also affected movement significantly, with reduced larval movements at cooler temperatures. Our findings suggest that larval sea lamprey dispersal is sufficient to minimize the likelihood of strong density-dependent effects on recruitment, even with large population sizes.     

Relative importance of density-dependent regulation and environmental stochasticity for butterfly population dynamics

The relative contribution of density-dependent regulation and environmental stochasticity to the temporal dynamics of animal populations is one of the central issues of ecology. In insects, the primary role of the latter factor, typically represented by weather patterns, is widely accepted. We have evaluated the impact of density dependence as well as density-independent factors, including weather and mowing regime, on annual fluctuations of butterfly populations. As model species, we used Maculinea alcon and M. teleius living in sympatry and, consequently, we also analysed the effect of their potential competition. Density dependence alone explained 62 and 42% of the variation in the year-to-year trends of M. alcon and M. teleius, respectively. The cumulative Akaike weight of models with density dependence, which can be interpreted as the probability that this factor should be contained in the most appropriate population dynamics model, exceeded 0.97 for both species. In contrast, the impacts of inter-specific competition, mowing regime and weather were much weaker, with their cumulative weights being in the range of 0.08–0.21; in addition, each of these factors explained only 2–5% of additional variation in Maculinea population trends. Our results provide strong evidence for density-dependent regulation in Maculinea, while the influence of environmental stochasticity is rather minor. In the light of several recent studies on other butterflies that detected significant density-dependent effects, it would appear that density-dependent regulation may be more widespread in this group than previously thought, while the role of environmental stochasticity has probably been overestimated. We suggest that this misconception is the result of deficiencies in the design of most butterfly population studies in the past, including (1) a strong focus on adults and a neglect of the larval stage in which density-dependent effects are most likely to occur; (2) an almost exclusive reliance on transect count results that may confound the impact of environmental stochasticity on butterfly numbers with its impact on adult longevity. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Density-dependent prophylaxis in the mealworm beetle Tenebrio molitor L. (Coleoptera: Tenebrionidae): cuticular melanization is an indicator of investment in immunity

If there are costs involved with the maintenance of pathogen resistance, then higher investment in this trait is expected when the risk of pathogenesis is high. One situation in which the risk of pathogenesis is elevated is at increased conspecific density. This paper reports the results of a study of density-dependent polyphenism in pathogen resistance and immune function in the mealworm beetle Tenebrio molitor. Beetles reared at high larval densities showed lower mortality when exposed to a generalist entomopathogenic fungus and a higher degree of cuticular melanization than those reared solitarily. The degree of cuticular melanization was a strong indicator of resistance, with darker beetles being more resistant than lighter ones regardless of rearing density. No differences were found between rearing densities in the levels of phenoloxidase, an enzyme key to the insect immune response. The results show that pathogen resistance is phenotypically plastic in T. molitor, suggesting that the maintenance of this trait is costly.     

Density-dependent injury in larval salamanders

Summary The effects of initial larval density, food level, and pond drying regime on intraspecific aggression of larval Ambystoma talpoideum were studied in an artificial pond experiment. Aggression was measured by the frequency of injury of feet, limbs, tail, and the extent of tail loss. Initial larval density had a significant effect on the frequency of foot, limb, and tail loss but not on the extent of tail loss. More larvae reared at medium and high densities sustained injuries than larvae reared at low densities but injuries were not more extensive. Food level had no effect on the four measures of injury. Pond drying regime had no effect on foot loss, limb loss, or extent of tail loss but more larvae reared in constant water level ponds had tail loss than in drying ponds. The frequency of limb and tail loss was negatively correlated to density-dependent survival which was the result of intraspecific predation or cannibalism. These results indicate that substantive levels of body injury, particularly tail loss, can occur at high natural larval densities and may result in a subsequent reduction of growth and survival.     

Host–parasitoid interaction as affected by interkingdom competition

Although still underrepresented in ecological research, competitive interactions between distantly related organisms (so-called “interkingdom competition”) are expected to be widespread in various ecosystems, with yet unknown consequences for, e.g. trophic interactions. In the model host–parasitoid system Drosophila melanogaster–Asobara tabida, toxic filamentous fungi have been shown to be serious competitors that critically affect the density-dependent survival of host Drosophila larvae. This study investigates the extent to which the competing mould Aspergillus niger affects key properties of the well-studied Drosophila–parasitoid system and how the host–parasitoid interaction influences the microbial competitor. In contrast to slightly positive density-dependent host mortality under mould-free conditions, competing A. niger mediated a strong Allee effect for parasitised larvae, i.e. mortality decreased with increasing larval density. It was found that the common toxic fungal metabolite kojic acid is not responsible for higher death rates in parasitised larvae. Single parasitised Drosophila larvae were less harmful to fungal reproduction than unparasitised larvae, but this effect vanished with an increase in larval density. As predicted from the negative effect of fungi on host survival and thus on parasitoid fitness at low larval densities, A. tabida females spent less time foraging in fungus-infested patches. Interestingly, even though high host larval densities increased host survival, parasitoids still reduced their search efforts in fungus-infested patches, indicating a benefit for host larvae from feeding in the presence of noxious mould. Thus, this experimental study provides evidence of the potentially important role of interkingdom competition in determining trophic interactions in saprophagous animal communities and the dynamics of both host–parasitoid and microbial populations.     

Incorporating density dependence into the oviposition preference-offspring performance hypothesis

1. Although theory predicts a positive relationship between oviposition preferences and the developmental performance of offspring, the strength of this relationship may depend not only on breeding site quality, but also on the complex interactions between environmental heterogeneity and density-dependent processes. Environmental heterogeneity may not only alter the strength of density dependence, but may also fundamentally alter density-dependent relationships and the preference-performance relationship. 2. Here I present results from a series of field experiments testing the effects of environmental heterogeneity and density-dependent feedback on offspring performance in tree-hole mosquitoes. Specifically, I asked: (i) how do oviposition activity, patterns of colonization and larval density differ among habitats and among oviposition sites with different resources; and (ii) how is performance influenced by the density of conspecifics, the type of resource in the oviposition site, and the type of habitat in which the oviposition site is located? 3. Performance did not differ among habitats at low offspring densities, but was higher in deciduous forest habitats than in evergreen forest habitats at high densities. Oviposition activity and larval densities were also higher in deciduous forests, suggesting a weak preference for these habitats. 4. The observed divergence of fitness among habitats with increasing density may select for consistent but weak preferences for deciduous habitats if regional abundances vary temporally. This would generate a negative preference-performance relationship when population densities are low, but a positive relationship when population densities are high. 5. This study demonstrates that failure to recognize that fitness differences among habitats may themselves be density-dependent may bias our assumptions about the ecological and evolutionary processes determining oviposition preferences in natural systems.     

Importance of endogenous feedback controlling the long-term abundance of tropical mosquito species

Mosquitoes are a major vector for tropical diseases, so understanding aspects that modify their population dynamics is vital for their control and protecting human health. Maximising the efficiency of control strategies for reducing transmission risk requires as a first step the understanding of the intrinsic population dynamics of vectors. We fitted a set of density-dependent and density-independent models to the long-term time series of six tropical mosquito species from northern Australia. The models’ strength of evidence was assessed using Akaike’s Information Criterion (AIC _c ), Bayesian Information Criterion (BIC) and jack-knifed cross-validation (C-V). Density dependence accounted for more than 99% of the model weight in all model-selection methods, with the Gompertz-logistic (Cushing model) being the best-supported model for all mosquito species (negative density feedback expressed even at low densities). The second-most abundant species, Aedes vigilax (a saline breeder), showed no spatial heterogeneity in its density-dependent response, but the remaining five species had different intrinsic growth rates across 11 study sites. Population densities of saline species were high only during the late dry to early wet season following the highest tides of the month or early flood rains when swamps were mostly saline, whereas those of freshwater species were highest during the mid-wet and mid-dry seasons. These findings demonstrate remarkably strong density dependence in mosquito populations, but also suggest that environmental drivers, such as rainfall and tides, are important in modifying seasonal densities. Neglecting to account for strong density feedback in tropical mosquito populations will clearly result in less effective control.     

Asymmetric competition in larval amphibian communities: conservation implications for the northern crawfish frog, Rana areolata circulosa

Asymmetric competition in larval amphibians can influence population dynamics and community structure. This density-dependent regulatory mechanism may be of particular importance for rare or endangered species such as the northern crawfish frog, Rana areolata circulosa. Interspecific competition of R. areolata with two congenerics, R. blairi and R. sphenocephala, was examined in artificial ponds. Analysis of covariance (differential mortality covariate) indicated that interspecific competition increased larval period length and decreased metamorphic body mass of R. areolata. The number of metamorphs produced was lower for R. blairi ponds when reared with R. areolata at high density. Body mass at metamorphosis was larger for R. sphenocephala when reared with R. areolata, suggesting that R. areolata facilitates larval growth in R. sphenocephala. These results indicate that the larval performance of R. areolata was reduced in the presence of interspecific competitors. Although many conservation efforts emphasize the preservation of critical habitat or particular rare species, interactive effects of biotic components in the focal community may also be important demographic regulators. Received: 11 December 1997 / Accepted: 15 April 1998     

Responses of Mamestra brassicae (Lepidoptera: Noctuidae) to crowding: interactions with disease resistance,colour phase and growth

This study examines phenotypic plasticity in relation to rearing density in larvae of the moth, Mamestra brassicae. Larval phase, growth rate, weight at moulting and susceptibility to disease were quantified when reared at five densities. Larvae develop more quickly, but attain a smaller size and are more susceptible to disease, when reared at high than at intermediate densities. They also exhibit a higher degree of melanisation than larvae reared at intermediate densities, or singly. A review of the literature suggests that a switch to a rapidly developing dark phase at high densities is a widespread phenomenon within the Lepidoptera. Rapid development at the expense of attaining a large size, and increased melanisation, are interpreted as adaptive responses to reach pupation before food supplies are depleted, as is likely when larval density is high. High susceptibility to viral infection at high density may be a result of physiological stress associated with rapid development, or due to a shift in allocation of resources from resistance to development: larvae that developed quickly were more susceptible to infection. Larvae reared singly appeared to be less fit than larvae reared at intermediate densities: they exhibited many of the characteristics of larvae reared at high density, particularly low weight, a right-hand skew in their weight frequency distribution, and high susceptibility to disease. I hypothesise that expression of resistance may be phenotypically plastic with regard to environment. Contact with other larvae may, up to a point, stimulate both growth and resistance to infection, for the risk of infection will increase with the density of conspecifics.     

密度对大树蛙蝌蚪生长发育和存活率的影响

种群密度效应主要表现在影响和调节种群的死亡率、发育速率、繁殖率以及扩散、迁移等反应种群数量动态的重要参数。分析密度对大树蛙蝌蚪生长发育和存活率的影响,有助于探究外界环境因子导致大树蛙种群密度迅速下降的作用机制。设置了15、20、25只/L和30只/L 4个密度组,测量大树蛙蝌蚪的尾长、体长、发育时间和存活率。结果表明,在15—30只/L范围内,密度升高显著降低了大树蛙蝌蚪尾长和体长的生长速率,减小其变态期的尾长和体长大小,其中与15只/L相比,20、25只/L和30只/L 3组蝌蚪的尾长分别降低了11.6%、11.8%和13.9%,体长分别降低了11.1%、9.5%和12.9%;随着密度的升高,大树蛙蝌蚪发育至跗蹠部伸长期和前肢伸出期的所需时间显著延长;大树蛙蝌蚪生长后期的存活率随密度升高显著降低,但密度对蝌蚪生长早期的存活率影响不显著。因此,密度升高可显著减小大树蛙蝌蚪的尾长和体长、延长发育时间和降低其生长后期的存活率,可能影响大树蛙蝌蚪变态后的适合度。