The utilization of nitrogen from insect capture by different growth forms of Drosera from Southwest Australia

Summary Plants of Drosera species, neighbouring noncarnivorous plants, and arthropods on or near each Drosera sp. were collected at 11 contrasting habitat locations in SW Australia. At three of the sites clones of the rare glandless mutant form of D. erythrorhiza were collected alongside fully glandular counterparts. The ¹⁵N value (¹⁵N/¹⁴N natural isotope composition) of insect-free leaf and stem fractions was measured, and the data then used to estimate proportional dependence on insect N (%N_dI) for the respective species and growth forms of Drosera. The data indicated lower %N_dI values for rosette than for self-supporting erect or for climbing vine species. The latter two groups showed an average %N_dI value close to 50%. The %N_dI increased with length and biomass of climbing but not erect forms of Drosera. ¹⁵N values of stems were positively correlated with corresponding values for leaves of Drosera. Leaf material was on average significantly more ¹⁵N enriched than stems, possibly due to delayed transport of recent insect-derived N, or to discrimination against ¹⁵N in transfer from leaf to the rest of the plant. The comparison of ¹⁵N values of insects and arthropod prey, glandless and glandular plants of D. erythrorhiza indicated %N_dI values of 14.3, 12.2 and 32.2 at the respective sites, while matching comparisons based on ¹⁵N of insect, reference plants and glandular plants proved less definitive, with only one site recording a positive %N_dI (value of 10.4%) despite evidence at all sites of feeding on insects by the glandular plants. The use of the ¹⁵N technique for studying nutrition of carnivorous species and the ecological significance of insect feeding of different growth forms of Drosera growing in a large range of habitats is discussed.     

Fluorescent prey traps in carnivorous plants

Carnivorous plants acquire most of their nutrients by capturing ants, insects and other arthropods through their leaf‐evolved biological traps. So far, the best‐known attractants in carnivorous prey traps are nectar, colour and olfactory cues. Here, fresh prey traps of 14 Nepenthes, five Sarracenia, five Drosera, two Pinguicula species/hybrids, Dionaea muscipula and Utricularia stellaris were scanned at UV 366 nm. Fluorescence emissions of major isolates of fresh Nepenthes khasiana pitcher peristomes were recorded at an excitation wavelength of 366 nm. N. khasiana field pitcher peristomes were masked by its slippery zone extract, and prey capture rates were compared with control pitchers. We found the existence of distinct blue fluorescence emissions at the capture spots of Nepenthes, Sarracenia and Dionaea prey traps at UV 366 nm. These alluring blue emissions gradually developed with the growth of the prey traps and diminished towards their death. On excitation at 366 nm, N. khasiana peristome 3:1 CHCl₃–MeOH extract and its two major blue bands showed strong fluorescence emissions at 430–480 nm. Masking of blue emissions on peristomes drastically reduced prey capture in N. khasiana pitchers. We propose these molecular emissions as a critical factor attracting arthropods and other visitors to these carnivorous traps. Drosera, Pinguicula and Utricularia prey traps showed only red chlorophyll emissions at 366 nm.     

Prey-induced changes in the accumulation of amino acids and phenolic metabolites in the leaves of <Emphasis Type="Italic">Drosera capensis</Emphasis> L

Effect of prey feeding (ants Formica fusca) on the quantitative changes in the accumulation of free amino acids, soluble proteins, phenolic metabolites and mineral nutrients in the leaves of carnivorous plant Drosera capensis was studied. Arginine was the most abundant compound in Drosera leaves, while proline was abundant in ants. The amount of the majority of amino acids and their sum were elevated in the fed leaves after 3 and 21 days, and the same, but with further enhancement after 21 days, was observed in ants. Accumulation of amino acids also increased in young non-fed leaves of fed plants. Soluble proteins decreased in ants, but were not enhanced in fed leaves. This confirms the effectiveness of sundew’s enzymatic machinery in digestion of prey and suggests that amino acids are not in situ deposited, but rather are allocated within the plant. The content of total soluble phenols, flavonoids and two selected flavonols (quercetin and kaempferol) was not affected by feeding in Drosera leaves, indicating that their high basal level was sufficient for the plant’s metabolism and prey-induced changes were mainly N based. The prey also showed to be an important source of other nutrients besides N, and a stimulation of root uptake of some mineral nutrients is assumed (Mg, Cu, Zn). Accumulation of Ca and Na was not affected by feeding.     

Three carnivorous plant species (Drosera spp.) in European Russia: peaceful coexistence?

In spite of the recent improvements in the understanding of carnivorous plants’ biology, some questions have remained unanswered. In this study, the segregation of food niches (i.e. specialization on different categories of prey) for three sympatric carnivorous temperate Drosera species with different shapes of trapping leaves is tested. Potentially available prey was also taken into account, by using artificial traps. Almost all the prey trapped by the three Drosera species and by passive traps belonged to four insect orders: Diptera, Hymenoptera, Coleoptera, Homoptera, as well as Araneae. Diptera specimens were the main prey for all the species. This study demonstrates that arthropods caught by the three temperate sympatric sundew species (D. rotundifolia, D. obovata and D. anglica), belong to the same orders. The proportions of prey from different orders, caught by different sundew species did not differ significantly. The result does not necessarily imply the absence of interspecific competition for prey: arthropods were identified only to order, and competition may have resulted in specialization on prey taxa of lower rank.     

Mixotrophy in gyrodinium galatheanum (DINOPHYCEAE): grazing responses to light intensity and inorganic nutrients*

This paper presents results of field and laboratory studies on mixotrophy in the estuarine dinoflagellate Gyrodinium galatheanum (Braarud) Taylor. We tested the hypotheses that this primarily photosynthetic organism becomes phagotrophic when faced with suboptimal light and/or nutrient environments. In Chesapeake Bay, incidence of feeding of this species on cryptophytes is positively correlated with prey density and concentrations of nitrate and nitrite, but negatively correlated with depth, salinity, and phosphate concentration. Feeding in natural assemblages and cultures increased hyperbolically with light intensity. The stoichiometric proportions of dissolved inorganic P and N (DIP:DIN) at the stations where G. galatheanum was present were far below the optimal growth P:N (1:10). Incidence of feeding was negatively related to the ratio of DIP to DIN, suggesting that P limitation may have induced feeding. Addition of nitrate, or addition of both nitrate and phosphate, inhibited feeding in a natural population, indicating that N limitation may also induce feeding. Ingestion of the cryptophyte, Storeatula major, by cultured G. galatheanum was higher in media low in nitrate or phosphate or both, but moderate rates of feeding occurred in nutrient‐replete cultures. When cells were grown in media with varying concentrations of nitrate and phosphate, N deficiency resulted in greater cellular N and Chl a losses than did P deficiency, but P deficiency stimulated feeding more than N deficiency. Both N and P deficiency, or P:N ratios that deviated greatly from 1:10, result in an increase of cellular carbon content and an increase in propensity to feed. Our results suggest that feeding in G. galatheanum is partly a strategy for supplementing major nutrients (N and P) that are needed for photosynthetic carbon assimilation. Feeding in G. galatheanum may also be a strategy for supplementing C metabolism or acquiring trace organic growth factors, since feeding occurs, although at a reduced rate, in nutrient‐replete cultures.     

The significance of opportunistic predators for the sympatric carnivorous plant species Drosera intermedia and Drosera rotundifolia

Summary Earlier feeding experiments with Drosera in the field using adult Drosophila melanogaster as prey had shown that D. intermedia reacts three times as strong with respect to biomass production as the sympatric species D. rotundifolia. The present study shows that in D. rotundifolia only 29% of added flies remain on the leaves for more than 24 h, but 95% in D. intermedia. Opportunistic predators, mostly ants, are likely to be responsible for this difference. Ants were often observed robbing food from the leaves of D. rotundifolia, and showed a much higher activity in the microhabitat of this species. In both species of Drosera larger individuals were better than smaller ones in retaining added flies. The activity of ants significantly increased with air temperature and the duration of sunshine. The advantage of plundering seems to be more important for the ants than the danger of being caught. The prey collected from Drosera may be an important source of food for bog-dwelling ants.     

Responses of a carnivorous plant to prey and inorganic nutrients in a Mediterranean environment

We have analysed the effect of prey and fertilization by inorganic nutrients on the survival, growth, reproduction (sexual and vegetative) and mucilage secretion of Pinguicula vallisneriifolia (Lentibulariaceae), a carnivorous plant inhabiting rocky substrates of southern Spain. We tested the hypothesis that carnivorous plants are more prey dependent when root access to nutrients is strongly limited by (1) analysing the importance of the carnivorous habit to the fitness of P. vallisneriifolia in its natural rocky habitat, and (2) determining whether the effect of trapped prey varies with soil nutrient levels. Our 2-year experimental results indicated prey to be limiting to P. vallisneriifolia growth on its natural rocky substrate. Animal food supply substantially increased the chance of survival, growth, vegetative propagation, sexual reproductive success and mucilage secretion. The differences between prey levels were more evident at the end of the experiment when all the surviving Prey-exclusion plants had lost weight, and the probability of sexual reproduction and of vegetative propagation by axillary buds had accordingly diminished. Furthermore, there were clear benefits from carnivory at the population level, since both the expected individual life span and the lifetime vegetative and sexual output correlated positively with the quantity of prey trapped. Application of insects to non-fertilized plants stimulated growth, but similar application to fertilized plants grown on a complete nutrient solution failed to enhance growth. There was no obvious benefit from the provision of a balanced mineral nutrient solution (alone or with prey). The greatest absolute growth and sexual and vegetative output resulted from providing a surplus of insects to plants on their natural rocky substrate. The strong dependence of P. vallisneriifolia on prey can therefore be considered a useful preadaptation enabling colonization of rocky substrates. Received: 11 November 1996 / Accepted: 31 March 1997     

Stoichiometry of actual vs. potential predator–prey interactions: insights into nitrogen limitation for arthropod predators

Literature‐compiled data sets demonstrate wide interspecific variation in nitrogen content among terrestrial arthropods and raise the possibility of nitrogen (N) limitation for predatory species. It remains unclear, however, whether the same disparities between N supply and demand that appear in literature compilations also exist in particular ecological communities. To address this uncertainty, we compared arthropod predator–prey stoichiometries derived from a compiled database with those from a natural Spartina saltmarsh community. Separate assessments of potential N‐limitation were made for arthropod predators feeding on herbivores and for intraguild predators feeding on intraguild prey. Relative to the compiled database, saltmarsh consumer–resource interactions exhibited increased disparity between N‐content of herbivores and N‐demand by predators. The high N content of saltmarsh arachnids relative to predatory insects at large may contribute to the supply‐demand disparity. Whether N‐limitation of terrestrial arthropod predators is widespread in the marsh, and in nature in general, depends sensitively on the predatory species’ gross growth efficiencies for N and carbon. Obtaining hard empirical data for these efficiency parameters should be a research goal.     

Mineral allocation to reproduction in Sorhum bicolor and Sorghum halepense in relation to parental nutrient supply

Summary This experiment investigated the effect of parental nutrient shortage on the allocation of five nutrients to seeds and rhizomes in Sorghum halepense, a perennial, noxious weed, and to seeds in Sorghum bicolor, an annual, cultivated species. Plants from both species were grown from seeds and supplied with fertilizer at three concentrations. The allocation of biomass and nutrients (N, P, K, Ca and Mg) to reproductive and vegetative parts was determined. Relative biomass allocation to reproduction (either sexual or vegetative) remained constant in S. halepense in spite of large differences in total plant weight. In S. bicolor, however, biomass allocation to sexual reproductive structures decreased significantly with decreasing nutrient supply. Individual seed weight was not modified by parental nutrient supply in S. halepense, but it increased with decreasing nutrient availability in S. bicolor. Important differences in mineral allocation to seeds were found between the two species. While S. bicolor seeds were largely buffered from the differences in parental nutrient status, concentration of nutrients in S. halepense seeds decreased significantly with decreasing supply for all the nutrients analyzed except Ca. However, mineral nutrient concentration in S. halepense rhizomes remained remarkably constant despite differences in the external supply, evincing the priority given to vegetative reproduction at the expense of sexual reproduction. Overall, the pattern of nutrient allocation in S. bicolor seeds under different nutrient supply resembled the pattern observed in S. halepense rhizomes, but it had little resemblance to the pattern of nutrient allocation in S. halepense seeds. The results are discussed in terms of differences and similarities in the reproductive strategy of these two species.     

Assimilation of carbon and nitrogen from pollen and nectar by a predaceous larva and its effects on growth and development

Abstract. 1. Predaceous insects may benefit from feeding on non‐prey foods, such as pollen, nectar, and honeydew, because they can provide nutrients that help maintain metabolism and enhance overall nutrient intake. Yet, the extent to which predaceous insects can assimilate non‐prey food and the importance of diet mixing during particular life history stages is poorly understood. In this study the relative contribution of an omnivorous diet to the growth and survivorship of a predaceous larva was tested in a hypothetical situation in which nutritionally optimal prey was not available. The study system comprised a predaceous larva (second‐ and third‐instar larvae of the green lacewing Chrysoperla carnea), nutritionally poor prey (larvae of Drosophila melanogaster), and non‐prey food (pollen suspension, a mixture of bee pollen and artificial nectar (1 M sucrose solution)). Chrysoperla carnea larvae in the mixed diet treatment were provided with both Drosophila larvae and pollen suspension, while those reared on the prey and non‐prey diet treatments received only Drosophila larvae or pollen suspension respectively. 2. The inclusion of pollen and sucrose in their diet enhanced the growth of C. carnea larvae. Second instars reared on the mixed diet developed significantly faster than their cohorts reared on the prey diet, however third instars reared on the mixed diet did not develop faster than their cohorts reared on the prey diet. Larvae reared on the mixed diet became larger adults than did those reared on either the prey or non‐prey diets. Third instars reared on the non‐prey diet completed their development while second instars in the non‐prey diet treatment failed to pupate. 3. Stable isotope analysis indicated that the larvae obtained most of their carbon (55–73%) and nitrogen (71–73%) from Drosophila but acquired only a minor amount of carbon (2–5%) and nitrogen (3–11%) from pollen. Larvae reared on the mixed and non‐prey diets acquired a relatively significant amount of carbon (23–51%) from sucrose. 4. A model, which included a novel fractionation factor to account for the isotopic effect of metamorphosis, was developed to explain the proportion of larval growth attributable to each diet item. It explained the adult δ¹³C values to within 0.2‰ and adult δ¹⁵N values to within 0.7‰ in all treatments. 5. Adults fed ¹⁵N‐labelled pollen as larvae retained the ¹⁵N signal of the pollen as adults. 6. The collective results of this study support the view that, despite their dependence on prey arthropods to obtain most of their dietary nitrogen, omnivorous lacewing larvae can enhance their growth and development by supplementing their diets with alternative non‐prey food resources. This finding is consistent with the notion that omnivory has evolved as a feeding strategy to acquire both additional nitrogen as well as trace nutrients.     

Trap diversity and evolution in the family Droseraceae

We review trapping mechanisms in the carnivorous flowering plant family Droseraceae (order Caryophyllales). Its members are generally known to attract, capture, retain and digest prey animals (mainly arthropods) with active snap-traps (Aldrovanda, Dionaea) or with active sticky flypaper traps (Drosera) and to absorb the resulting nutrients. Recent investigations revealed how the snap-traps of Aldrovanda vesiculosa (waterwheel plant) and Dionaea muscipula (Venus’ flytrap) work mechanically and how these apparently similar devices differ as to their functional morphology and shutting mechanics. The Sundews (Drosera spp.) are generally known to possess leaves covered with glue-tentacles that both can bend toward and around stuck prey. Recently, it was shown that there exists in this genus a higher diversity of different tentacle types and trap configurations than previously known which presumably reflect adaptations to different prey spectra. Based on these recent findings, we finally comment on possible ways for intrafamiliar trap evolution.     

Bacterial communities in predatory mites are associated with species and diet types

The symbiotic bacterial communities of phytophagous arthropods are affected by host species and feeding habits, but such effects have been poorly studied in natural enemies. Here, we investigated the entire bacterial microbiome of two species of predatory mites, Neoseiulus californicus and Neoseiulus barkeri, feeding on three types of diets (artificial diet, pollen and their natural prey, the spider mite Tetranychus urticae) by high-throughput sequencing of the 16S rRNA gene. We found that the bacterial diversity of predatory mites feeding on artificial diet was significantly different from pollen and spider mite feeding groups in both N. californicus and N. barkeri, while bacterial diversity also differed strikingly between the two species even when feeding on the same artificial diet. This finding suggests that the bacterial community of predatory mites is determined by both species and diet. Alphaproteobacteria and Gammaproteobacteria were the two dominant bacterial classes in both predatory mite species, except for N. californicus feeding on artificial diet. The bacterium Bosea sp. was detected in all samples as the core microbial species in predatory mites. Additionally, we discuss whether Bradyrhizobiaceae and Rhodobacteraceae bacteria could be used as probiotics in the artificial diet of N. californicus for better mass rearing.

     

Importance of Terrestrial Arthropods as Subsidies in Lowland Neotropical Rain Forest Stream Ecosystems

The importance of terrestrial arthropods has been documented in temperate stream ecosystems, but little is known about the magnitude of these inputs in tropical streams. Terrestrial arthropods falling from the canopy of tropical forests may be an important subsidy to tropical stream food webs and could also represent an important flux of nitrogen (N) and phosphorus (P) in nutrient‐poor headwater streams. We quantified input rates of terrestrial insects in eight streams draining lowland tropical wet forest in Costa Rica. In two focal headwater streams, we also measured capture efficiency by the fish assemblage and quantified terrestrially derived N‐ and P‐excretion relative to stream nutrient uptake rates. Average input rates of terrestrial insects ranged from 5 to 41 mg dry mass/m²/d, exceeding previous measurements of aquatic invertebrate secondary production in these study streams, and were relatively consistent year‐round, in contrast to values reported in temperate streams. Terrestrial insects accounted for half of the diet of the dominant fish species, Priapicthys annectens. Although terrestrially derived fish excretion was found to be a small flux relative to measured nutrient uptake rates in the focal streams, the efficient capture and processing of terrestrial arthropods by fish made these nutrients available to the local stream ecosystem. This aquatic‐terrestrial linkage is likely being decoupled by deforestation in many tropical regions, with largely unknown but potentially important ecological consequences.     

Foraging ecology and niche overlap in pygmy (Kogia breviceps) and dwarf (Kogia sima) sperm whales from waters of the U.S. mid‐Atlantic coast

A complementary approach of stomach content and stable isotope analyses was used to characterize the foraging ecology and evaluate niche overlap between pygmy (Kogia breviceps) and dwarf (K. sima) sperm whales stranded on the U.S. mid‐Atlantic coast between 1998 and 2011. Food habits analysis demonstrated both species were primarily teuthophagous, with 35 species of cephalopods, and 2 species of mesopelagic fishes represented in their overall diets. Pianka's Index of niche overlap suggested high overlap between whale diets (O_n = 0.92), with squids from the families Histioteuthidae, Cranchidae, and Ommastrephidae serving as primary prey. Pygmy sperm whales consumed slightly larger prey sizes (mean mantle length [ML] = 10.8 cm) than dwarf sperm whales (mean ML = 7.8 cm). Mean prey sizes consumed by pygmy sperm whales increased with growth, but showed no trend in dwarf sperm whales. Significant differences were not detected in δ¹⁵N and δ¹³C values of muscle tissues from pygmy (10.8‰ ± 0.5‰, ?17.1‰ ± 0.6‰), and dwarf sperm whales (10.7‰ ± 0.5‰, ?17.0‰ ± 0.4‰), respectively. Isotopic niche widths also did not differ significantly and dietary overlap was high between the two species. Results suggest the feeding ecologies of the pygmy and dwarf sperm whales are similar and both species occupy equivalent trophic niches in the region.     

Nutrient requirements of exotic tree species in Zimbabwe

Pot and field experiments were conducted in the greenhouse and at three field sites (Marondera, Domboshawa and Makoholi) in Zimbabwe to examine the effects of soils and fertilizers on nutrient uptake and growth of 6 exotic tree species (Eucalyptus camaldulensis, E. grandis, E. tereticornis, Leucaena leucocephala, Casuarina cunninghamiana, and Acacia holosericea). Plant growth, N and P contents of all species were increased by the application of N, P, K and micronutrient fertilizers. The effect of individual nutrients (N, P, K and micronutrients) and their combination on Eucalyptus species was further investigated in a pot experiment using soil from Domboshawa. Eucalyptus species responded only to N application and no significant interactions were found between N and the other elements. Nutrient uptake results showed that E. camaldulensis and E. tereticornis removed more cations than the N-fixing trees but only in the fertilized treatments. L. leucocephala and C. cunninghamiana were higher in P, but no clear trends were observed for N. Plant growth and nutrient uptake by E. camaldulensis. C. cunninghamiana and A. holosericea were assessed in the field at the three sites. Plant species grown in the Marondera site had greater height and diameter at breast height (DBH) than those in the two other sites. These results followed trends in soil nutrient contents. The analysis of foliage revealed differences in the nutrient concentration of leaves from different trees, but no effect of site was found. The interrelationships between plant characteristics, soil and foliage nutrients were examined. In a pot experiment, mineral N was the only variable correlated with growth response and nutrient uptake, while in the field the only significant correlation was obtained with soil pH.     

Birds exploit herbivore‐induced plant volatiles to locate herbivorous prey

Arthropod herbivory induces plant volatiles that can be used by natural enemies of the herbivores to find their prey. This has been studied mainly for arthropods that prey upon or parasitise herbivorous arthropods but rarely for insectivorous birds, one of the main groups of predators of herbivorous insects such as lepidopteran larvae. Here, we show that great tits (Parus major) discriminate between caterpillar‐infested and uninfested trees. Birds were attracted to infested trees, even when they could not see the larvae or their feeding damage. We furthermore show that infested and uninfested trees differ in volatile emissions and visual characteristics. Finally, we show, for the first time, that birds smell which tree is infested with their prey based on differences in volatile profiles emitted by infested and uninfested trees. Volatiles emitted by plants in response to herbivory by lepidopteran larvae thus not only attract predatory insects but also vertebrate predators.     

Growth of dinoflagellates, Ceratium furca and Ceratium fusus in Sagami Bay, Japan: The role of nutrients

In order to study the influence of nutrients on the growth characteristics of the dominant dinoflagellates, Ceratium furca and Ceratium fusus, in the temperate coastal area of Sagami Bay, Japan, we conducted field monitoring from January 2000 to December 2005 and performed laboratory culture experiments. In the field study, population densities of C. furca and C. fusus were high, even in low nutrient concentrations (N: 1.58 μM, P: 0.17 μM). Both species were more abundant in the surface and sub-surface layers than in the bottom layers during the stratification periods. In the laboratory study, the specific growth rates of C. furca and C. fusus increased gradually along with increasing nutrients up to the T₅ (N: 5 μM, P: 0.5 μM) and T₁₀ (N: 10 μM, P: 1 μM) concentration levels, after which the growth rate plateaued at the T₅₀ (N: 50 μM, P: 5 μM) concentration level. In contrast, the nutrient uptake rates of both species continuously increased, indicating “luxury consumption”, i.e., excessive cellular storage not related to growth rate. The half-saturation constants (K_s) of C. furca for nitrate (0.49 μM) and phosphate (0.05 μM) were slightly higher than C. fusus (0.32 and 0.03 μM, respectively). We offer two reasons why the two Ceratium population densities were maintained at high levels in low nutrient conditions. First, these two species have a competitive advantage over other algal species because of low K_s values and specific characteristics for nutrient uptake such as luxury consumption. Their ability to obtain nutrients through alternative methods, such as phagotrophy, might contribute to bloom formation and population persistence. Second, the cell densities of both Ceratium species increased along with nitrate concentrations in the media even when phosphorus was held constant. In particular, the growth of C. furca was directly supported by various nitrogen sources such as nitrate, ammonium, and urea, although the highest growth rates were observed only in the nitrate-enriched cultures. Our field and laboratory results revealed that the growth rates of the two Ceratium species increased readily in high N:P nutrient conditions (i.e., conditions of P limitation) indicating an advantage over other algal species in phosphorus-limited environments such as Sagami Bay.     

A specialized araneophagic predator's short‐term nutrient utilization depends on the macronutrient content of prey rather than on prey taxonomic affiliation

A specialist predator that has a specialized diet, prey‐specific prey‐capture behaviour and a preference for a particular type of prey may or may not be specialized metabolically. Previous studies have shown that jumping spiders of the genus Portia prey on other spiders using prey‐specific prey‐capture behaviour, prefer spiders as prey to insects and gain long‐term benefits in terms of higher survival and growth rates on spider diets than on insect diets. However, it is unclear whether there are substances uniquely present in spiders on which Portia depends, or, alternatively, spiders and insects all contain more or less the same nutrients but the relative amounts of these substances are such that Portia perform better on a spider diet. These questions are addressed by testing the hypothesis that prey specialization includes metabolic adaptations that allow Portia an enhanced nutrient extraction or nutrient utilization efficiency when feeding on spider prey compared with insect prey. Three groups of Portia quei Zabka are fed either their preferred spider prey or one of two types of flies (Drosophila melanogaster Meigen) that differ in nitrogen and lipid content. Portia quei shows a higher feeding rate of high‐protein flies than of high‐lipid flies and spiders but, after 5 days of feeding, there is no significant difference in growth between treatments, and the diets lead to significant changes in the macronutrient composition of P. quei as a result of variable extraction and utilization of the prey. The short‐term utilization of spider prey is similar to that of high‐lipid flies and both differ in several respects from the utilization of high‐protein flies. Thus, the short‐term nutrient utilization is better explained by prey macronutrient content than by whether the prey is a spider or not. The results suggest that spider prey may have a more optimal macronutrient composition for P. quei and that P. quei does not depend on spider‐specific substances.     

Zoophytophagy in the plantbug Nesidiocoris tenuis

1 The zoophytophagy of Nesidiocoris tenuis (Reuter) (Heteroptera: Miridae) was characterized in relation to prey availability and environmental factors by: (i) monitoring its population dynamics in tomato greenhouses; (ii) analysis of the influence of N. tenuis and whitefly density, temperature and humidity on the intensity of N. tenuis plant feeding; and (iii) laboratory assays under controlled conditions to determine the intensity of plant feeding in relation to prey availability, temperature and humidity. 2 A negative relationship was found between plant feeding and predated whiteflies in tomato greenhouses. Plant feeding was directly related to N. tenuis density and temperature and inversely related to whitefly density. The significance of prey availability and temperature was corroborated in laboratory assays. The intensification of plant feeding at low prey density indicates switching from zoophagy to phytophagy as prey become scarce. 3 Nesidiocoris tenuis showed a typical predator dynamic in relation to variance in prey density. Populations increased after whitefly outbreaks and decreased after whitefly had been depleted. The rapid decrease of N. tenuis populations after whitefly decreased, however, suggests that plants are a poorer nutrient source than whitefly for this species.     

Gastropod grazers and nutrients,but not light,interact in determining periphytic algal diversity

The potential interactions of grazing, nutrients and light in influencing autotroph species diversity have not previously been considered. Earlier studies have shown that grazing and nutrients interact in determining autotroph species diversity, since grazing decreases species diversity when nutrients (i.e. N or P) limit autotroph growth, but increases it when nutrients are replete. We hypothesized that increased light intensities would intensify the interactions between grazing and nutrients on algal species diversity, resulting in even stronger reductions in algal species diversity through grazing under nutrient–poor conditions, and to even stronger increases of algal species diversity through grazing under nutrient-rich conditions. We studied the effects of grazing (absent, present), nutrients (ambient, N + P enriched) and light (low light, high light) on benthic algal diversity and periphyton C:nutrient ratios (which can indicate algal nutrient limitation) in a factorial laboratory experiment, using the gastropod grazer Viviparus viviparus. Grazing decreased algal biomass and algal diversity, but increased C:P and N:P ratios of periphyton. Grazing also affected periphyton species composition, by decreasing the proportion of Spirogyra sp. and increasing the proportion of species in the Chaetophorales. Grazing effects on diversity as well as on periphyton N:P ratios were weakened when nutrients were added (interaction between grazing and nutrients). Chlorophyll a (Chl a) per area increased with nutrient addition and decreased with high light intensities. Light did not increase the strength of the interaction between grazing and nutrients on periphytic algal diversity. This study shows that nutrient addition substantially reduced the negative effects of grazing on periphytic algal diversity, whereas light did not interact with grazing or nutrient enrichment in determining periphytic algal diversity.