Is competition important to arctic zooplankton community structure?

1. Daphnia pulex and Daphnia middendorffiana are commonly found in the Toolik Lake region of arctic Alaska. These two species are very similar morphologically, although their natural distributions differ markedly: D. pulex is restricted to shallow ponds, while D. middendorffiana is widely distributed and found in a variety of ponds and lakes. We compared the reproductive capabilities of D. pulex and D. middendorffiana grown under similar resource conditions and in the absence of the invertebrate predator Heterocope septentrionalis. In situ life table and mesocosm experiments were conducted in Toolik Lake and Dam Pond, habitats that have historically contained natural populations of D. middendorffiana, but never D. pulex. 2. Daphnia pulex exhibited a significantly higher net growth rate than D. middendorffiana in both life table and mesocosm experiments although D. pulex has never been found in either Toolik Lake or Dam Pond. Daphnia middendorffiana exhibited a negative net growth rate in Dam Pond, which had lower resource levels then Toolik Lake. Therefore, the smaller D. pulex appears to have a lower food threshold concentration than the larger D. middendorffiana. 3. Our results indicate that D. pulex is a superior resource competitor in the Toolik Lake region. These results combined with distributional patterns suggest that the restricted distribution of D. pulex in these arctic lakes and ponds cannot be explained by resource competition alone. We suggest that in the presence of H. septentrionalis, predation is an important factor structuring arctic zooplankton communities in the Toolik Lake region.     

Changes in vulnerability of Daphnia to an insecticide application depending on the population phase

1. Laboratory populations of cloned Daphnia pulex at three population phases (growing phase, density peak, declining phase) were exposed to the insecticide carbaryl at 15 μg L^?1, which killed juveniles but not adults. 2. The population decline was largest and recovery was slowest when the chemical was applied at the density peak, indicating that the population was most sensitive to the chemical at that stage. Populations in the declining phase were least affected by the treatment. 3. We propose that food condition, which varies with population density, was a major factor affecting vulnerability to carbaryl. In that case, Daphnia populations in lakes would be most vulnerable to toxic chemicals during food‐limited condition such as the spring clear‐water phase.     

On the way to eternity; the success of an aquatic laboratory microecosystem

A microecosystem consisting of three subsystems roughly representing the trophic levels of autotrophs, herbivores and decomposers was developed. A recirculating flow of water connected the three subsystems. Analysis of the data over a period of 7500 days indicates that the system has remained in the same state and has the prospect of being immortal. The Daphnia population showed periods of large oscillations, but also long periods of almost constant numbers. External factors were responsible for the initiation of the oscillations. The threshold food level for population increase was lower than mostly is found for individual growth, indicating adaptation to low food concentrations.     

大型枝角类蚤状溞对小型热带湖泊浮游植物群落影响的研究

2006年4月10日至2006年4月22日,采用原位吊瓶实验的方法,在暨南大学校园明湖中进行了大型枝角类蚤状溞(Daphnia pulex)对浮游植物种群变化和群落结构影响的实验。实验初期,明湖浮游植物群落的组成是以飞燕角甲藻(Ceratium hirundinella)和单角盘星藻(Pediastrum simplex)为优势种。在室内将蚤状溞培养至体长2mm以上,将蚤状溞加入到装满湖水的4.5L的透明瓶中,设置4个梯度:0ind(A,对照),10ind.(B),20ind(C)和30ind(D),每个梯度有3个平行,将实验瓶置于水表层50cm处。12d后实验结束,不同处理之间的浮游植物种群数量和组成有了明显的差异。与对照组相比,飞燕角甲藻和颗粒直链藻(Meclosira granulata)密度在实验组B、C和D中下降明显;小环藻(Cyclotella)和栅藻(Scenedesmus)等小型浮游植物在牧食压力比较大的C和D瓶中密度低于对照组,而在牧食压力相对较低的B瓶中,它们的密度高于对照组,这可能是由于蚤状溞加速了水体的营养盐循环反过来促进这些小型浮游植物的生长。处理组中绿藻门的盘星藻数量没有明显的下降,这与盘星藻不能被浮游动物直接滤食有关。实验结果表明蚤状溞对浮游植物的群落数量和组成的影响程度与其自身的种群密度密切相关,也与浮游植物群落结构有关。由于飞燕角甲藻是一种细胞较大的种类,蚤状溞对它的抑制作用主要是通过机械伤害作用实现的。     

Calcium balance in Daphnia grown on diets differing in food quantity, phosphorus and calcium

1. The influences of dietary phosphorus (P) and food concentration on the calcium (Ca) balance in Daphnia magna were examined in this study at two different ambient Ca concentrations (0.5 and 10 mg Ca L^?1). Daphnia were grown by feeding the young adults differentially under contrasting dietary P conditions [molar C : P ratio = c. 900 and c. 90 as low P (LP) and high P (HP), respectively], ambient Ca concentrations [0.5 mg and 10 mg Ca L^?1 as low Ca (LCa) or high Ca (HCa), respectively] and food levels [0.15 or 1.5 mg C L^?1 as low food (LF) or high food (HF), respectively] for 5 days. 2. The specific Ca contents of daphniids (1.9–6.5% of dry weight^?1) increased with increasing Ca concentration, food level and dietary P content, although the food level did not affect the Ca content in the HPHCa treatment. A radioactive tracer method showed that the food level did not affect the influx of Ca from the water under LP conditions, but the Ca influx under HP conditions doubled with a HF level. A LP condition also led to a decrease in Ca influx with a HF level. 3. During the 3 days of efflux, generally only a small proportion of Ca (2.6–3.3%) was retained by the daphniids, but this retention increased (14–23%) under low ambient Ca concentrations and under P‐limitation. Excretion was the most important pathway for Ca loss (accounting for 50–60% of body Ca), followed by moulting (20–47%), but the relative contribution of these two pathways (excretion and moulting) did not vary among all the different treatments. The absolute loss of Ca through excretion and moulting, on the contrary, differed with different ambient Ca concentrations and dietary P conditions. A HF level led to an increase in the loss rates in most cases. 4. Our study strongly suggested that there is an interaction between an essential metal (Ca) and macronutrients (C and P) in freshwater crustaceans with HCa and P contents. The results imply that variation in environmental nutrient conditions may change the Ca budget in crustaceans and may affect the dynamics of Ca in the epilimnion of freshwaters.     

Ideal free distribution in Daphnia? Are daphnids able to consider both the food patch quality and the position of competitors?

The distribution of the planktonic crustacean Daphnia pulex was tested in a ring-formedflow-through chamber divided into eleven sections. Thedistribution of the animals under both homogeneousfood conditions and in a food gradient were studied.The distribution of repeated registrations of singleanimals was randomly distributed in a homogeneous,low food environment. Single daphnids exposed to afood gradient tended to reside during 88% of the timeat the highest or next highest food concentration,suggesting that daphnids can detect food gradients.When a group of approximately 100 co-occurring animalswere given low homogeneous food conditions, they alsotended to be randomly distributed. In one out of threecases, however, they were slightly less aggregatedthan expected from a random distribution. Exposed toa food gradient (0.5–0.0015 mg C l^-1), thedistribution of the daphnids approximated the idealfree distribution. A very high maximum density in thefood gradient (2 mg C l^-1) resulted in less strongaggregations than expected from the ideal freedistribution.     

Morphological defenses induced in situ by the invertebrate predator Chaoborus : comparison of responses between Daphnia pulex and D. rosea

The presence of plankton predators may induce altered morphology in their potential prey. To date, the mechanism of induction and adaptive value of such defensive responses have been examined in the laboratory. This study investigated the morphological defense structures induced by the invertebrate predator Chaoborus in two coexisting Daphnia species, D. pulex and D. rosea, in the field. In Piscivore Lake (Gr?fenhain, Germany), continuous and intense biomanipulation had led to near elimination of planktivorous fish and greatly increased abundances of Chaoborus (up to >10 larvae l^–1). Here, the density of Chaoborus was manipulated within the lake by an enclosure/exclosure setup and resulting morphological responses of Daphnia spp. were investigated in situ. Three replicate enclosures (4.6 m³) contained no Chaoborus (predator exclusion bags), whereas Chaoborus entered three others at ambient densities (predator enclosures). In both species of Daphnia, formation of neckteeth and elongation of the tail spine were recorded in the predator enclosures, but not in the predator exclusion treatments. Additionally, D. rosea responded to predator inclusion with an increase of the size at first reproduction. Despite the induced defense structures, the presence of Chaoborus caused increased mortality of both Daphnia species. In addition, Chaoborus affected the coexistence of the two populations of Daphnia by causing higher relative mortality in D. rosea. Neckteeth formation was always more pronounced in D. pulex than in D. rosea of the same size. Neckteeth were induced specifically in vulnerably sized juvenile instars of D. pulex, but were not found in all vulnerable instars of D. rosea. In D. rosea, neckteeth were few or absent in the ephippial hatchlings, and neckteeth formation ceased before juveniles reached a body size outside the range that larger larval stages of Chaoborus could ingest. This study provides the first experimental demonstration in the field of the inducibility of morphological defense structures in Daphnia at ambient densities of Chaoborus larvae, and quantifies these in situ responses. This expands on earlier observations of a correlation between predator density in the field and the expression of neckteeth in Daphnia. The term ”maximum size for neckteeth formation” (MSNF) is defined as the limit in body size above which no production of neckteeth was evident. This limit was used to distinguish the size classes of Daphnia that show a sensitive response to Chaoborus kairomone. This new term may be used for further comparisons among species and among different types of predator-induced responses as well as for the evaluation of the adaptive value of defense structures. Received: 10 April 1999 / Accepted: 6 April 2000     

入侵种西花蓟马与其它昆虫的种间竞争

西花蓟马Frankliniella occidentalis作为一种重要的入侵害虫,在全球范围的入侵过程中,为获取更多资源以建立自己的种群,势必会和处于同一营养级的昆虫发生种间竞争。本文对近年来有关西花蓟马与其它种蓟马、粉虱及叶螨等昆虫种间竞争的研究进行了综述,分析了温度、CO 2、寄主植物、农药和天敌等主要因素在竞争取代中的作用,并对今后的研究方向以及种间竞争的研究方法进行了讨论,以期为揭示西花蓟马在我国的种群动态和变化规律奠定基础。     

Speciation with gene flow and the genetics of habitat transitions

Whether speciation can advance to completion in the face of initially high levels of gene flow is a very controversial topic in evolutionary biology. Extensive gene exchange is generally considered to homogenize populations and counteract divergence. Moreover, the role of introgressive hybridization in evolution remains largely unexplored in animals, particularly in freshwater zooplankton in which allopatric speciation is considered to be the norm. Our work investigates the genetic structure of two young ecological species: the pond species, Daphnia pulex and the lake species, Daphnia pulicaria. Phylogenetic and population genetics analyses were conducted on mitochondrial NADH dehydrogenase 5 (ND5) gene, the nuclear Lactate dehydrogenase (Ldh) gene and 21 nuclear microsatellite markers in 416 individuals from habitats with various degrees of permanence. The strong and consistent phylogenetic discordance between nuclear and mitochondrial markers suggests a complex evolutionary history of multiple independent habitat transition events that involved hybridization and introgression between lake and pond Daphnia. On the other hand, the low level of contemporary gene flow between adjacent populations indicates the presence of effective habitat isolating barriers. The Daphnia system provides strong evidence for a divergence-with-gene flow speciation model that involves multiple habitat transition events.     

Costs, benefits and the evolution of inducible defences: a case study with Daphnia pulex

Phenotypic plasticity is one major source of variation in natural populations. Inducible defences, which can be considered threshold traits, are a form of plasticity that generates ecological and evolutionary consequences. A simple cost-benefit model underpins the maintenance and evolution of these threshold, inducible traits. In this model, a rank-order switch in expected fitness, defined by costs and benefits of induction between defended and undefended morphs, predicts the risk level at which individuals should induce defences. Here, taking predator-induced morphological defences in Daphnia pulex as a threshold trait, we provide the first comprehensive investigation into the costs and benefits of a threshold trait, and how they combine to reflect fitness and predict the switchpoint at which induction should occur. We develop reaction norms that show genetic variation in switchpoints. Further experiments show that induction can confer a survival benefit and a cost in terms of lifetime reproductive success. Together, these two traits combine to estimate expected fitness and can predict the switchpoint between an undefended and a defended strategy. The predictions match the reaction norm data for clones that experience these costs and benefits, and correspond well to independent field data on induction. However, predictions do not, and cannot, match for clones that do not gain a benefit from induction. This study confirms that a simple theory, based on life history costs and benefits, is a sufficient framework for understanding the ecology and evolution of inducible, threshold traits.     

The genetic legacy of polyploid Bolivian Daphnia: the tropical Andes as a source for the North and South American D. pulicaria complex

We investigated genetic variation in asexual polyploid members of the water flea Daphnia pulex complex from a set of 12 Bolivian high-altitude lakes. We used nuclear microsatellite markers to study genetic relationships among all encountered multilocus genotypes, and combined this with a phylogenetic approach using DNA sequence data of three mitochondrial genes. Analyses of mitochondrial gene sequence divergence showed the presence of three very distinct clades that likely represent cryptic undescribed species. Our phylogenetic results suggest that the Daphnia pulicaria group, a complex of predominantly North American species that has diversified rapidly since the Pleistocene, has its origin in South America, as specific tests of topology indicated that all three South American lineages are ancestral to the North American members of this species group. A comparison between variation of nuclear and mitochondrial markers revealed that closely related polyploid nuclear genotypes sometimes belonged to very divergent mitochondrial lineages, while distantly related nuclear genotypes often belonged to the same mitochondrial lineage. This discrepancy suggests that these South American water fleas originated through reciprocal hybridization between different endemic, sexually reproducing parental lineages. It is also likely that polyploidy of the investigated lineages resulted from this hybridization. Nevertheless, no putative diploid parental lineages were found in the studied region.     

THE EVOLUTIONARY ECOLOGY OF AN ANTIPREDATOR REACTION NORM: DAPHNIA PULEX AND CHAOBORUS AMERICANUS

Ponds containing the parthenogenetic zooplankter Daphnia pulex with and without chaoborid predators were sampled over the course of a season. A significant (P < 0.05) Spearman rank correlation was found between predator density and the expression of an antipredator defense (neckteeth) by the Daphnia. The reaction norms (percent induction of a single genotype versus predator density) of clones isolated from predator-free and predator-rich habitats were determined in a laboratory setting. There was a statistically significant different response among the six clones tested (P < 0.05). Clones isolated from chaoborid ponds showed significantly greater sensitivity to the presence of predator than clones from predator-free ponds (P < 0.05). In the laboratory, food levels under which prey were cultured affected induction of the antipredator response. Highest induction was found at the lowest food level used.     

Daphnia community analysis in shallow Kenyan lakes and ponds using dormant eggs in surface sediments

1. Water fleas of the genus Daphnia are considered rare in tropical regions, and information on species distribution and community ecology is scarce and anecdotal. This study presents the results of a survey of Daphnia species distribution and community composition in 40 standing waterbodies in southern Kenya. The study sites cover a wide range of tropical standing aquatic habitats, from small ephemeral pools to large permanent lakes between approximately 700 and 2800 m a.s.l. Our analysis combines data on Daphnia distribution and abundance from zooplankton samples and dormant eggs in surface sediments. 2. Nearly 70% (27 of 40) of the sampled waterbodies were inhabited by Daphnia. Although their abundance in the active community was often very low, this high incidence shows that Daphnia can be equally widespread in tropical regions as in temperate regions. 3. Analysis of local species assemblages from dormant eggs in surface sediments was more productive than snapshot sampling of zooplankton communities. Surface‐sediment samples yielded eight Daphnia species in total, and allowed the detection of Daphnia in 25 waterbodies; zooplankton samples revealed the presence of only four Daphnia species in 16 waterbodies. 4. Daphnia barbata, D. laevis, and D. pulex were the most frequently recorded and most abundant Daphnia species. Canonical correspondence analysis of species–environment relationships indicates that variation in the Daphnia community composition of Kenyan waters was best explained by fish presence, temperature, macrophyte cover and altitude. Daphnia barbata and D. pulex tended to co‐occur with each other and with fish. Both species tended to occur in relatively large (>10 ha) and deeper (>2 m) alkaline waters (pH 8.5). Daphnia laevis mainly occurred in cool and clear, macrophyte‐dominated lakes at high altitudes.