Predator-prey Ratios Research Articles

Mass entrapment and lysis of Mesodinium rubrum cells in mucus threads observed in cultures with Dinophysis

The entrapment and death of the ciliate Mesodinium rubrum in the mucus threads in cultures with Dinophysis is described and quantified. Feeding experiments with different concentrations and predator–prey ratios of Dinophysis acuta, Dinophysis acuminata and M. rubrum to study the motility loss and aggregate formation of the ciliates and the feeding behaviour of Dinophysis were carried out. In cultures of either Dinophysis species, the ciliates became entrapped in the mucus, which led to the formation of immobile aggregates of M. rubrum and subsequent cell lysis. The proportion of entrapped ciliates was influenced by the concentration of Dinophysis and the ratio of predator and prey in the cultures. At high cell concentrations of prey (136 cells mL−1) and predator (100 cells mL−1), a maximum of 17% of M. rubrum cells became immobile and went through cell lysis. Ciliates were observed trapped in the mucus even when a single D. acuminata cell was present in a 3.4mL growth medium. Both Dinophysis species were able to detect immobile or partly immobile ciliates at a distance and circled around the prey prior to the capture with a stretched out peduncle. Relatively high entrapment and lysis of M. rubrum cells in the mucus threads indicates that under certain conditions Dinophysis might have a considerable impact on the population of M. rubrum.

Plant spatial distribution and predator–prey ratio affect biological control of the twospotted spider mite Tetranychus urticae (Acari: Tetranychidae) by the predatory mite Phytoseiulus persimilis (Acari: Phytoseiidae)

ABSTRACTA greenhouse experiment was conducted to determine the effect of plant spacing and predator–prey ratio on dispersal and foraging efficiency of the predatory mite, Phytoseiulus persimilis, on the twospotted spider mite, Tetranychus urticae. When predators were released at the end of spider mite-infested arrays of lima bean plants that had either no spacing or two different patterns of spacing among plant rows, plant damage was uniformly low throughout the experiment at both predator–prey ratios (1:10 and 3:10) in the treatment with no spacing. In contrast, damage was higher in both treatments where plant rows were interrupted by spacing. At the 1:10 ratio, more plants closer to the predator release point experienced moderate damage than at the 3:10 ratio where only the plant rows farthest from the release point had unacceptable damage. Our findings suggest that point releases of P. persimilis at the standard 1:10 predator–prey ratio should be effective within a diameter of at least 65 cm on mite-infested patches of plants where pots are touching. However, if gaps in plant rows exist, even large numbers of predators may not be sufficient to protect parts of the crop unless predators are released at shorter fixed points in the greenhouse crop.

Interactions of gelatinous zooplankton within marine food webs

Gelatinous zooplankton (GZ) comprise a taxonomically and functionally diverse group of marine organisms which includes ctenophores, cnidarians and pelagic tunicates, sharing a soft, mostly transparent body texture, a high body water content and a lack of exoskeleton. They range in size from less than a millimetre to nearly 2 m for the cnidarian jellyfish Nemopilema nomurai, and comprise some of the fastest growing metazoans on Earth (Hopcroft et al., 1998), sometimes surpassing crustacean zooplankton in their contribution to secondary production (i.e. in subtropical waters; Jaspers et al., 2009). They feed on a wide range of prey sizes, with predator–prey ratios comparable in some cases to those of baleen whales and krill (Deibel and Lee, 1992), and with prey removal rates which are similar to those of their non-gelatinous competitors (Acuna et al., 2011). In spite of early work pointing to gelatinous zooplankton as a trophic dead end (Verity and Smetacek, 1996), evidence is rapidly accumulating which shows that they may potentially channel energy from the picoplankton-sized, microbial loop organisms up to the higher trophic levels, including fish (Llopiz et al., 2010). However, this pathway is still largely neglected in most food web investigations even though it is now becoming clear that GZ represent a major fraction of the diet of several commercially important fish species such as bluefin tuna (Thunnus thynnus) (Cardona et al., 2012).

Predicting Consumer Biomass, Size-Structure, Production, Catch Potential, Responses to Fishing and Associated Uncertainties in the World's Marine Ecosystems.

Existing estimates of fish and consumer biomass in the world’s oceans are disparate. This creates uncertainty about the roles of fish and other consumers in biogeochemical cycles and ecosystem processes, the extent of human and environmental impacts and fishery potential. We develop and use a size-based macroecological model to assess the effects of parameter uncertainty on predicted consumer biomass, production and distribution. Resulting uncertainty is large (e.g. median global biomass 4.9 billion tonnes for consumers weighing 1 g to 1000 kg; 50% uncertainty intervals of 2 to 10.4 billion tonnes; 90% uncertainty intervals of 0.3 to 26.1 billion tonnes) and driven primarily by uncertainty in trophic transfer efficiency and its relationship with predator-prey body mass ratios. Even the upper uncertainty intervals for global predictions of consumer biomass demonstrate the remarkable scarcity of marine consumers, with less than one part in 30 million by volume of the global oceans comprising tissue of macroscopic animals. Thus the apparently high densities of marine life seen in surface and coastal waters and frequently visited abundance hotspots will likely give many in society a false impression of the abundance of marine animals. Unexploited baseline biomass predictions from the simple macroecological model were used to calibrate a more complex size- and trait-based model to estimate fisheries yield and impacts. Yields are highly dependent on baseline biomass and fisheries selectivity. Predicted global sustainable fisheries yield increases ≈4 fold when smaller individuals (< 20 cm from species of maximum mass < 1kg) are targeted in all oceans, but the predicted yields would rarely be accessible in practice and this fishing strategy leads to the collapse of larger species if fishing mortality rates on different size classes cannot be decoupled. Our analyses show that models with minimal parameter demands that are based on a few established ecological principles can support equitable analysis and comparison of diverse ecosystems. The analyses provide insights into the effects of parameter uncertainty on global biomass and production estimates, which have yet to be achieved with complex models, and will therefore help to highlight priorities for future research and data collection. However, the focus on simple model structures and global processes means that non-phytoplankton primary production and several groups, structures and processes of ecological and conservation interest are not represented. Consequently, our simple models become increasingly less useful than more complex alternatives when addressing questions about food web structure and function, biodiversity, resilience and human impacts at smaller scales and for areas closer to coasts.

Implications of scaled δ15N fractionation for community predator-prey body mass ratio estimates in size-structured food webs.

Nitrogen stable isotope ratios (δ(15) N) may be used to estimate community-level relationships between trophic level (TL) and body size in size-structured food webs and hence the mean predator to prey body mass ratio (PPMR). In turn, PPMR is used to estimate mean food chain length, trophic transfer efficiency and rates of change in abundance with body mass (usually reported as slopes of size spectra) and to calibrate and validate food web models. When estimating TL, researchers had assumed that fractionation of δ(15) N (Δδ(15) N) did not change with TL. However, a recent meta-analysis indicated that this assumption was not as well supported by data as the assumption that Δδ(15) N scales negatively with the δ(15) N of prey. We collated existing fish community δ(15) N-body size data for the Northeast Atlantic and tropical Western Arabian Sea with new data from the Northeast Pacific. These data were used to estimate TL-body mass relationships and PPMR under constant and scaled Δδ(15) N assumptions, and to assess how the scaled Δδ(15) N assumption affects our understanding of the structure of these food webs. Adoption of the scaled Δδ(15) N approach markedly reduces the previously reported differences in TL at body mass among fish communities from different regions. With scaled Δδ(15) N, TL-body mass relationships became more positive and PPMR fell. Results implied that realized prey size in these size-structured fish communities are less variable than previously assumed and food chains potentially longer. The adoption of generic PPMR estimates for calibration and validation of size-based fish community models is better supported than hitherto assumed, but predicted slopes of community size spectra are more sensitive to a given change or error in realized PPMR when PPMR is small.

Predator and detritivore niche width helps to explain biocomplexity of experimental detritus-based food webs in four aquatic and terrestrial ecosystems

In the study of food webs, the existence and explanation of recurring patterns, such as the scale invariance of linkage density, predator–prey ratios and mean chain length, constitute long-standing issues. Our study focused on litter-associated food webs and explored the influence of detritivore and predator niche width (as δ13C range) on web topological structure. To compare patterns within and between aquatic and terrestrial ecosystems and take account of intra-habitat variability, we constructed 42 macroinvertebrate patch-scale webs in four different habitats (lake, lagoon, beech forest and cornfield), using an experimental approach with litterbags. The results suggest that although web differences exist between ecosystems, patterns are more similar within than between aquatic and terrestrial web types. In accordance with optimal foraging theory, we found that the niche width of predators and prey increased with the number of predators and prey taxa as a proportion of total taxa in the community. The tendency was more marked in terrestrial ecosystems and can be explained by a lower per capita food level than in aquatic ecosystems, particularly evident for predators. In accordance with these results, the number of links increased with the number of species but with a significantly sharper regression slope for terrestrial ecosystems. As a consequence, linkage density, which was found to be directly correlated to niche width, increased with the total number of species in terrestrial webs, whereas it did not change significantly in aquatic ones, where connectance scaled negatively with the total number of species. In both types of ecosystem, web robustness to rare species removal increased with connectance and the niche width of predators. In conclusion, although limited to litter-associated macroinvertebrate assemblages, this study highlights structural differences and similarities between aquatic and terrestrial detrital webs, providing field evidence of the central role of niche width in determining the structure of detritus-based food webs and posing foraging optimisation constraints as a general mechanistic explanation of food web complexity differences within and between ecosystem types.

Hit or miss: Functional mechanisms underlying prey capture efficiency in three cordylid lizards

Prey capture is one of the most important behaviours in organisms as it directly determines energy acquisition, which in turn is vital for fitness and survival. The contribution of specific prey capture mechanisms to prey capture efficiency, however, remains largely unresolved, especially in groups where a variety of mechanisms is present, such as lizards. Using three sympatric cordylid lizards as model organisms, we investigated (1) the effect of lingual prehension on prey capture efficiency when feeding on small prey (i.e. termites) and (2) whether in species that use jaw prehension to capture prey, prey capture efficiency decreases with increasing predator-prey size ratio. Prey capture efficiency, defined as the proportion of termites that was captured at first attempt, was higher in the species using lingual prehension than in the similarly-sized species using jaw prehension. In contrast, the two species using jaw prehension had a similar prey capture efficiency, despite differences in body size. The observed variation in prey capture efficiency between lingual and jaw prehension is discussed in the light of foraging mode in lizards.

Shift in a large river fish assemblage: body-size and trophic structure dynamics.

As the intensity and speed of environmental change increase at both local and global scales it is imperative that we gain a better understanding of the ecological implications of community shifts. While there has been substantial progress toward understanding the drivers and subsequent responses of community change (e.g. lake trophic state), the ecological impacts of food web changes are far less understood. We analyzed Wabash River fish assemblage data collected from 1974-2008, to evaluate temporal variation in body-size structure and functional group composition. Two parameters derived from annual community size-spectra were our major response variables: (1) the regression slope is an index of ecological efficiency and predator-prey biomass ratios, and (2) spectral elevation (regression midpoint height) is a proxy for food web capacity. We detected a large assemblage shift, over at least a seven year period, defined by dramatic changes in abundance (measured as catch-per-unit-effort) of the dominant functional feeding groups among two time periods; from an assemblage dominated by planktivore-omnivores to benthic invertivores. There was a concurrent increase in ecological efficiency (slopes increased over time) following the shift associated with an increase in large-bodied low trophic level fish. Food web capacity remained relatively stable with no clear temporal trends. Thus, increased ecological efficiency occurred simultaneous to a compensatory response that shifted biomass among functional feeding groups.

Demography of plains zebras (Equus quagga) under heavy predation

AbstractIn natural ecosystems, ungulate densities show strong temporal variations. The ecological processes driving these fluctuations are complex: food limitation and predation are both important and can interact. Survival rates are central to this debate, but data are sparse for tropical ecosystems. Here, we estimate age‐ and sex‐specific survival rates for plains zebra in Hwange National Park, a nutrient‐poor savanna with a high predator–prey ratio. We estimated survival from a detailed Capture‐Mark‐Recapture (CMR) monitoring based on 248 individual life histories, for the first time in an African grazer. We controlled for variations in detection probabilities among adult females, which resulted from their social structure. As expected, annual survival was low during the first year (0.441); increased in yearlings (0.560) and peaked at 0.795 and 0.847 in adult males and females respectively. The survival of adult females was lower during the dry season, which probably resulted from higher predation due to predictable movements of zebras to waterholes. Survival at all ages was low compared to ungulates without predators. The demographic model we constructed showed a declining trend (λ = 0.94), which was consistent with the data from road counts ( = 0.92). Life Table Response Experiment (LTRE) analyses using the Serengeti and Kruger populations as references showed that the main cause of this declining trend in the Hwange population was low survival in yearling and adult females; low foal survival also contributed. In this ecosystem, predation is likely to be the main ecological process causing low survival, and therefore a decline in the zebra population.

Examining predator-prey body size, trophic level and body mass across marine and terrestrial mammals.

Predator-prey relationships and trophic levels are indicators of community structure, and are important for monitoring ecosystem changes. Mammals colonized the marine environment on seven separate occasions, which resulted in differences in species' physiology, morphology and behaviour. It is likely that these changes have had a major effect upon predator-prey relationships and trophic position; however, the effect of environment is yet to be clarified. We compiled a dataset, based on the literature, to explore the relationship between body mass, trophic level and predator-prey ratio across terrestrial (n = 51) and marine (n = 56) mammals. We did not find the expected positive relationship between trophic level and body mass, but we did find that marine carnivores sit 1.3 trophic levels higher than terrestrial carnivores. Also, marine mammals are largely carnivorous and have significantly larger predator-prey ratios compared with their terrestrial counterparts. We propose that primary productivity, and its availability, is important for mammalian trophic structure and body size. Also, energy flow and community structure in the marine environment are influenced by differences in energy efficiency and increased food web stability. Enhancing our knowledge of feeding ecology in mammals has the potential to provide insights into the structure and functioning of marine and terrestrial communities.

Model-to-data comparisons reveal influence of jellyfish interactions on plankton community dynamics

Taxonomic shifts can alter predator feeding preference and modify ecosystem dynamics through top-down control. In Barnegat Bay-Little Egg Harbor Estuary (New Jersey, USA), sea nettle Chrysaora quinquecirrha abundances have increased in the northern portions of the estuary. We evaluated the geographical variation in top-down influence of C. quinquecirrha on plankton community dynamics. We simulated a range of jellyfish- to copepod-dominated eco- systems using a size-resolved nutrientphytoplanktonzooplankton (NPZ) model. Zooplankton feeding was parameterized as a community average based on predatorprey size ratios and breadth of prey sizes of dominant species. We compared model outputs to data collected in the estuary during 2 summer months of high C. quinquecirrha abundance. We predicted that data from the northern region would be more similar to the jellyfish-dominated model outputs, because C. quinquecirrha abundance is higher in the north. Contrary to expectations, all northern sites had observational data more similar to the copepod-dominated model outputs, and the site that was most similar to the jellyfish-dominated model outputs was in the C. quinquecirrha-free southern region. These results may indicate complex interactions between C. quinquecirrha and the ctenophore Mnemiopsis leidyi, a voracious copepod predator that is nearly absent in the northern region despite having wide environmental tolerances. Predation by C. quinquecirrha may limit the distribution of M. leidyi and indirectly strengthen copepod dominance in the northern region of the estuary. These results suggest that top-down control by jellyfish can be strongly influenced by competition among gelatinous taxa.

Supplemental food that supports both predator and pest: a risk for biological control?

Supplemental food sources to support natural enemies in crops are increasingly being tested and used. This is particularly interesting for generalist predators that can reproduce on these food sources. However, a potential risk for pest control could occur when herbivores also benefit from supplemental food sources. In order to optimize biological control, it may be important to select food sources that support predator populations more than herbivore populations. In this study we evaluated the nutritional quality of four types of supplemental food for the generalist predatory mites Amblyseius swirskii Athias-Henriot and Amblydromalus (Typhlodromalus) limonicus (Garman and McGregor), both important thrips predators, and for the herbivore western flower thrips Frankliniella occidentalis Pergande, by assessing oviposition rates. These tests showed that application of corn pollen, cattail pollen or sterilized eggs of Ephestia kuehniella Zeller to chrysanthemum leaves resulted in three times higher oviposition rates of thrips compared to leaves without additional food. None of the tested food sources promoted predatory mites or western flower thrips exclusively. Decapsulated cysts of Artemia franciscana Kellogg were not suitable, whereas cattail pollen was very suitable for both predatory mites and western flower thrips. In addition, we found that the rate of thrips predation by A. swirskii can be reduced by 50 %, when pollen is present. Nevertheless, application of pollen or Ephestia eggs to a chrysanthemum crop still strongly enhanced the biological control of thrips with A. swirskii, both at low and high release densities of predatory mites through the strong numerical response of the predators. Despite these positive results, application in a crop should be approached with caution, as the results may strongly depend on the initial predator-prey ratio, the nutritional quality of the supplemental food source, the species of predatory mites, the distribution of the food in the crop and the type of crop.

Modelling size structured food webs using a modified niche model with two predator traits.

The structure of food webs is frequently described using phenomenological stochastic models. A prominent example, the niche model, was found to produce artificial food webs resembling real food webs according to a range of summary statistics. However, the size structure of food webs generated by the niche model and real food webs has not yet been rigorously compared. To fill this void, I use a body mass based version of the niche model and compare prey-predator body mass allometry and predator-prey body mass ratios predicted by the model to empirical data. The results show that the model predicts weaker size structure than observed in many real food webs. I introduce a modified version of the niche model which allows to control the strength of size-dependence of predator-prey links. In this model, optimal prey body mass depends allometrically on predator body mass and on a second trait, such as foraging mode. These empirically motivated extensions of the model allow to represent size structure of real food webs realistically and can be used to generate artificial food webs varying in several aspects of size structure in a controlled way. Hence, by explicitly including the role of species traits, this model provides new opportunities for simulating the consequences of size structure for food web dynamics and stability.

Temperature alters food web body-size structure.

The increased temperature associated with climate change may have important effects on body size and predator-prey interactions. The consequences of these effects for food web structure are unclear because the relationships between temperature and aspects of food web structure such as predator-prey body-size relationships are unknown. Here, we use the largest reported dataset for marine predator-prey interactions to assess how temperature affects predator-prey body-size relationships among different habitats ranging from the tropics to the poles. We found that prey size selection depends on predator body size, temperature and the interaction between the two. Our results indicate that (i) predator-prey body-size ratios decrease with predator size at below-average temperatures and increase with predator size at above-average temperatures, and (ii) that the effect of temperature on predator-prey body-size structure will be stronger at small and large body sizes and relatively weak at intermediate sizes. This systematic interaction may help to simplify forecasting the potentially complex consequences of warming on interaction strengths and food web stability.

Trophic links and the relationship between predator and prey body sizes in food webs

The relationship between predator and prey body sizes is an important property of food webs with potential implications for community dynamics and ecosystem functioning. To shed more light on this issue I here analyze the relationships between prey size, predator size and trophic position of consumers, using body size estimates of 697 species in 52 food webs. First I show that the relationship between predator and prey body sizes across many systems can be different from, and potentially obscure the true relationship within systems. More specifically, when data from all webs are aggregated average prey size is positively correlated to predator size with a regression slope less than unity, suggesting that predators become less similar in size to their average prey the larger the predator is, and consequently that the relative size difference between a predator and its prey should increase with the trophic position of the consumer. However, despite this I find the predator-prey body mass ratio to be negatively correlated to the trophic position of the consumer within many webs. The reason for this is that the across-webs pattern is not representative for the within-web relationship. Second, I show that the pattern observed is not compatible with a simple null-model for the distribution of trophic links between predators and prey. The observed relationship between predator size and mean prey size is for most webs significantly steeper than that predicted by the cascade model. Furthermore, the observed relationship also deviates significantly (but less so) from an ecologically more realistic model for the distribution of trophic links (the niche model). The results contradict the traditional Eltonian paradigm that predator-prey body mass ratios do not vary consistently across trophic levels. It is concluded that more studies are needed to establish the generality of the results and explore its dynamical implications.

Benchmarking successional progress in a quantitative food web.

Central to ecology and ecosystem management, succession theory aims to mechanistically explain and predict the assembly and development of ecological communities. Yet processes at lower hierarchical levels, e.g. at the species and functional group level, are rarely mechanistically linked to the under-investigated system-level processes which drive changes in ecosystem properties and functioning and are comparable across ecosystems. As a model system for secondary succession, seasonal plankton succession during the growing season is readily observable and largely driven autogenically. We used a long-term dataset from large, deep Lake Constance comprising biomasses, auto- and heterotrophic production, food quality, functional diversity, and mass-balanced food webs of the energy and nutrient flows between functional guilds of plankton and partly fish. Extracting population- and system-level indices from this dataset, we tested current hypotheses about the directionality of successional progress which are rooted in ecosystem theory, the metabolic theory of ecology, quantitative food web theory, thermodynamics, and information theory. Our results indicate that successional progress in Lake Constance is quantifiable, passing through predictable stages. Mean body mass, functional diversity, predator-prey weight ratios, trophic positions, system residence times of carbon and nutrients, and the complexity of the energy flow patterns increased during succession. In contrast, both the mass-specific metabolic activity and the system export decreased, while the succession rate exhibited a bimodal pattern. The weighted connectance introduced here represents a suitable index for assessing the evenness and interconnectedness of energy flows during succession. Diverging from earlier predictions, ascendency and eco-exergy did not increase during succession. Linking aspects of functional diversity to metabolic theory and food web complexity, we reconcile previously disjoint bodies of ecological theory to form a complete picture of successional progress within a pelagic food web. This comprehensive synthesis may be used as a benchmark for quantifying successional progress in other ecosystems.

The Coevolution of Tyrannosaurus &amp; Its Prey

Students will analyze the coevolution of the predator–prey relationships between Tyrannosaurus rex and its prey species using analyses of animal speeds from fossilized trackways, prey-animal armaments, adaptive behaviors, bite marks on prey-animal fossils, predator–prey ratios, and scavenger competition. The students will be asked to decide whether T. rex was a predator, an opportunistic scavenger, or an obligate scavenger.

Effects of inter-annual landscape change on interactions between cereal aphids and their natural enemies

The agricultural intensification and the subsequent habitat changes in agroecosystem can strongly affect biological control services. We here examine the influence of inter-annual landscape change in wheat field area on interactions of cereal aphids and their natural enemies, as well as the efficacy of biological control using data collected from a 4-year experiment in Northwest China. Two hypotheses were tested. (i) Population densities decline following an inter-annual expansion of wheat crop proportion cover due to dilution and crowding effects. (ii) Species that are specialists or at higher trophic levels are more sensitive to bottom-up disturbance by inter-annual change in percent cover of wheat crop. Results showed the population densities of one cereal aphid (Macrosiphum avenae), one parasitic wasp (Aphidius avenae), two specialist predators (ladybirds: Hippodamia variegata and H. tredecimpunctata) and one hyperparasitic wasp (Pachyneuron aphidis) declined following the expansion of wheat crop areas, supporting the predictions of inter-annual dilution and crowding effects. In contrast, the populations of one cereal aphid (Schizaphis graminum), one parasitic wasp (A. gifuensis), two generalist predators (spiders: Pardosa astrigera; carabid beetles: Chlaenius pallipes), and two hyperparasitic wasps (Asaphes suspensus, and Alloxysta sp.) did not respond to inter-annual landscape change. The two hypotheses were partially supported but with noticeable exceptions, and the bio-control efficiency declined with the increase of the proportion cover of wheat field in agricultural landscape. Overall, different responses of cereal aphids and their natural enemies make it difficult but still possible to optimize inter-annual landscape change for enhancing the parasitism rate and predator-prey ratio.

Increasing zooplankton size diversity enhances the strength of top‐down control on phytoplankton through diet niche partitioning

1. The biodiversity-ecosystem functioning debate is a central topic in ecology. Recently, there has been a growing interest in size diversity because body size is sensitive to environmental changes and is one of the fundamental characteristics of organisms linking many ecosystem properties. However, how size diversity affects ecosystem functioning is an important yet unclear issue. 2. To fill the gap, with large-scale field data from the East China Sea, we tested the novel hypothesis that increasing zooplankton size diversity enhances top-down control on phytoplankton (H1) and compared it with five conventional hypotheses explaining the top-down control: flatter zooplankton size spectrum enhances the strength of top-down control (H2); nutrient enrichment lessens the strength of top-down control (H3); increasing zooplankton taxonomic diversity enhances the strength of top-down control (H4); increasing fish predation decreases the strength of top-down control of zooplankton on phytoplankton through trophic cascade (H5); increasing temperature intensifies the strength of top-down control (H6). 3. The results of univariate analyses support the hypotheses based on zooplankton size diversity (H1), zooplankton size spectrum (H2), nutrient (H3) and zooplankton taxonomic diversity (H4), but not the hypotheses based on fish predation (H5) and temperature (H6). More in-depth analyses indicate that zooplankton size diversity is the most important factor in determining the strength of top-down control on phytoplankton in the East China Sea. 4. Our results suggest a new potential mechanism that increasing predator size diversity enhances the strength of top-down control on prey through diet niche partitioning. This mechanism can be explained by the optimal predator-prey body-mass ratio concept. Suppose each size group of zooplankton predators has its own optimal phytoplankton prey size, increasing size diversity of zooplankton would promote diet niche partitioning of predators and thus elevates the strength of top-down control.

ECOLOGICAL STUDIES ON SOME MEALYBUG SPECIES ATTACKING MANDARIN TREES AND THEIR PREDATORY INSECTS AT MANSOURA DISTRICT

The present investigation was carried out to study the population density of the common mealybug species attacking mandarin trees and their predatory insects, also to evaluate the effect of certain weather factors on these insects. The obtained resultus revealed that there were four mealybug species infesting mandarin trees namely Planococcus citri Risso, Icerya seychellarum (Westwood), Icerya aegyptiaca Douglas and Icerya purchase Mask. The highest peak for P. citri in 15th of October 2010 and 15th of September 2012, for I. seychellarum, I. aegyptiaca and I. purchase recorded in the 15th of September 2011 and 2012during the two successive years of study, respectively. The maximum activity of Rodolia cardinalis (Mulsant) recorded in the beginning of October 2010 (31 indiv.) and in 15th of June 2012 (55 indiv.), Chrysoperla carnea (Steph.) recorded in 15th of June 2011 (21 indiv.) and in 15th of September 2012 (25 indiv.) and Nephus includens Kirsch in 15th of September 2011 (14 indiv.) and in 15th of June and 15th of September 2012 (16 indiv.) during the two successive years, respectively. The highest average number for I. purchasi, I. aegyptiaca, I. seychellarum and P. citri were recorded in summer during the two years. Statistical analysis showed a highly significant differences for each insect pest between the four seasons during the two successive years of study. The highest average number of R. cardinalis recorded in summer during the two years. Also, N. includens recorded in summer during the two seasons. Meanwhile, C. carnea recorded in spring during 2010/11 and in summer during 2011/12. Statistical analysis showed a highly significant differences for each insect predator between the four seasons during the two successive years. The predator-prey ratios ranged between 40.0 in the third week of January 2011 and 9.1 in the first week of August 2011. The ratios was narrowed during the period from May to August 2011 during the second year. This ratio ranged between 29.3 in 15th of January and 8.4 in 15th of May 2012. The ratio was narrowed during the period from April to August 2012 during the second year. Results of statistical analysis of simple correlation coefficient indicated that, the relationship between the mealybug species and their associated predator on mandarin trees showed a highly positive significant effect for R. cardinalis, N. includens and C. carnea during the two years of study. The maximum and minimum as well as average temperature showed a highly positive significant effects on the population density of I. purchasi, I. aegyptiaca, I. seychellarum, P. citri, R. cardinalis, N. includens and C. carnea. On the other hand, the maximum, minimum and average relative humidity showed a highly negative significant effects or negatively significant effects of the population density for the previously insects during the first year. Meanwhile, I. purchasi and I. seychellarum showed insignificantly effects with minimum relative humidity during the second year.