Green leaves enhance the growth and development of a stream macroinvertebrate shredder when senescent leaves are available

1. Freshly fallen green leaves and flowers of terrestrial plants enter temperate streams in spring and summer, when senescent leaf litter is often scarce. These resources appear to provide good supplementary food for macroinvertebrate shredders, but have some potential shortcomings as food or case material for caddisflies. 2. To compare suitability of green leaves or flowers and senescent leaves for the growth and development of stream shredders, we reared the caddisfly Lepidostoma complicatum in the laboratory with treatments that provided larvae with senescent (oak) and green (oak or maple) leaves separately, and also together, in case the combined use of both types of leaf may benefit the shredder. 3. Larvae supplied with green leaves alone grew at 65% of the rate of those provided with senescent leaves alone, due to their lower consumption rate. No individuals given green leaves alone developed into adults, whereas 70% of the individuals given senescent leaves alone did. Green leaves may inhibit larval consumption due to their high phenol content, or they may be unsuitable for case material because they are less tough than senescent leaves. 4. Larvae supplied with both senescent and green leaves (or flowers) had a higher growth rate and developed faster, than those given senescent leaves alone, whereas the proportions of successfully emerged individuals did not differ. Lepidostoma probably benefits from the higher nitrogen content of the green leaves when used together with senescent leaves. 5. These results suggest that green leaves (or flowers) cannot serve as an alternative food resource to senescent leaves, but that they can enhance the growth and development of a Lepidostoma stream shredder if senescent leaves are also available.     

Shredder colonization and decomposition of green and senescent leaves during summer in a headwater stream in northern Japan

We conducted a decomposition experiment using green and senescent maple and alder leaves in a coastal headwater stream in Hokkaido, northern Japan, during June and July 2000. We estimated whether shredder colonization on the leaves and leaf breakdown differed between green and senescent leaves during the experimental period. Late-instar Lepidostoma complicatum (Trichoptera) and Sternomoera rhyaca (Amphipoda) were the predominant shredder taxa among the macroinvertebrates that colonized litterbags. There was no significant difference in shredder colonization between green and senescent leaves although we found a significant difference between maple and alder leaves. The colonization patterns of large individuals of L. complicatum and S. rhyaca differed from those of small individuals. All decomposition coefficients of green and senescent leaves were high. During the experiment, decomposition was significantly faster in maple than in alder leaves, although no significant difference was found between green and senescent leaves. However, the fragmented nitrogen portion was higher in green leaves than in senescent leaves during the experiment. Higher nitrogen release (2–2.5 times more) as particulate organic matter in green than in senescent leaves indicates that green leaves may be a potentially valuable food resource for other macroinvertebrate collector–gatherer species.     

Macroinvertebrate community patterns in relation to leaf-associated periphyton under contrasting light and nutrient conditions in headwater streams

1. Temperate headwater streams traditionally have been considered heterotrophic and brown food web dominated with little primary production. Recent work, however, suggests algae on leaves in these streams may play a greater role than previously thought through interactions with microbial decomposers like fungi. Algae also may be important for macroinvertebrates colonizing leaves in streams. Algae are a more nutritious food resource for shredders than fungi and bacteria and provide a food resource for non-shredder macroinvertebrates.
2. In a field experiment, we manipulated light in three low-nutrient and three high-nutrient streams using leaf bags filled with red maple leaves in winter and spring. After four weeks we measured algal and fungal biomass, leaf stoichiometry, and macroinvertebrate abundance and biomass associated with the leaf bags. We also identified the macroinvertebrate community and examined differences in functional feeding guilds and taxa under ambient- and shaded-light treatments and low- and high-nutrient concentrations in relation to measured leaf characteristics.
3. Algal biomass on leaves was greatest in high-nutrient streams and ambient-light treatments in both seasons. Fungal biomass on leaves was greatest in high-nutrient streams and showed a moderate marginally significant positive correlation with algae during the winter. Leaf C:N was negatively correlated to algae in winter and fungi in both seasons, while leaf N:P and C:P were negatively correlated to fungi in winter and algae in spring. Interactions between fungi and algae on leaves and the nutritional importance of each for macroinvertebrates likely change across seasons, potentially impacting macroinvertebrate community composition.
4. Macroinvertebrate diversity did not differ, but biomass was significantly greater in shaded-light treatments during spring. Abundance was highest in the high-nutrient ambient-light conditions in both seasons, corresponding to greatest algal biomass. Functional feeding guild biomass and abundance were related to different leaf characteristics by season and guild. Higher algal biomass was an important factor for colonization of certain macroinvertebrates (e.g., Ephemerella (Ephemeroptera: Ephemerellidae) and Stenonema (Ephemeroptera: Heptageniidae)), while others were more abundant under shaded treatments with lower algal biomass (e.g., Tipula (Diptera: Tipulidae)), indicating taxa-specific responses.
5. Leaf-associated algae may be an important factor mediating macroinvertebrate communities associated with leaves in temperate headwater streams. Our results demonstrate that green and brown food webs intersect within leaf packs, and they cannot be easily disentangled. We therefore should consider both autochthonous and allochthonous resources within headwater streams when examining their communities or developing water management strategies.

     

Nutrients stimulate leaf breakdown rates and detritivore biomass: bottom-up effects via heterotrophic pathways

Most nutrient enrichment studies in aquatic systems have focused on autotrophic food webs in systems where primary producers dominate the resource base. We tested the heterotrophic response to long-term nutrient enrichment in a forested, headwater stream. Our study design consisted of 2 years of pretreatment data in a reference and treatment stream and 2 years of continuous nitrogen (N) + phosphorus addition to the treatment stream. Studies were conducted with two leaf species that differed in initial C:N, Rhododendron maximum (rhododendron) and Acer rubrum (red maple). We determined the effects of nutrient addition on detrital resources (leaf breakdown rates, litter C:N and microbial activity) and tested whether nutrient enrichment affected macroinvertebrate consumers via increased biomass. Leaf breakdown rates were ca. 1.5 and 3× faster during the first and second years of enrichment, respectively, in the treatment stream for both leaf types. Microbial respiration rates of both leaf types were 3× higher with enrichment, and macroinvertebrate biomass associated with leaves increased ca. 2–3× with enrichment. The mass of N in macroinvertebrate biomass relative to leaves tended to increase with enrichment up to 6× for red maple and up to 44× for rhododendron leaves. Lower quality (higher C:N) rhododendron leaves exhibited greater changes in leaf nutrient content and macroinvertebrate response to nutrient enrichment than red maple leaves, suggesting a unique response by different leaf species to nutrient enrichment. Nutrient concentrations used in this study were moderate and equivalent to those in streams draining watersheds with altered land use. Thus, our results suggest that similarly moderate levels of enrichment may affect detrital resource quality and subsequently lead to altered energy and nutrient flow in detrital food webs. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Plant phenology-mediated indirect effects: The gall midge opens the phenological window wider for a leaf beetle

We examined whether larvae of the gall midge Rabdophaga rigidae (Diptera: Cecidomyiidae) can modify the seasonal dynamics of the density of a leaf beetle, Plagiodera versicolora (Coleoptera: Chrysomelidae), by modifying the leaf flushing phenology of its host willow species, Salix serissaefolia and Salix eriocarpa (Salicaceae). To test this, we conducted field observations and a laboratory experiment. The field observations demonstrated that the leaf flushing phenology of the willows and the seasonal dynamics of the beetle density differed between shoots with stem galls and shoots without them. On galled shoots of both willow species, secondary shoot growth and secondary leaf production were promoted; consequently, leaf production showed a bimodal pattern and leaf production periods were 1 to 2 months longer than on non‐galled shoots. The adult beetle density on galled shoots was thus enhanced late in the season, and was found to change seasonally, synchronizing with the production of new leaves on the host willow species. From the results of our laboratory experiment, we attributed this synchrony between adult beetle density and willow leaf flush to beetles’ preference to eat new leaves rather than old. Indeed, beetles consumed five times more of the young leaves when they were fed both young and old leaves. These results indicate that stem galls indirectly enhance the adult beetle density by enhancing food quality and quantity late in the beetle‐feeding season. We therefore conclude that midge galls widen the phenological window for leaf beetles by extending the willows’ leaf flush periods.     

Size class structure and tree dispersion patterns in old-growth cedar-hemlock forests of the northern Rocky Mountains (USA)

Summary The relationship between the food selection of four leaf beetle species (Phratora vitellinae, Plagiodera versicolora, Lochmaea capreae, Galerucella lineola) and the phenolic glycosides of willow (Salix spp.) leaves was tested in laboratory food choice experiments. Four willow species native to the study area (Eastern Finland) and four introduced, cultivated willows were tested.The willow species exhibited profound differences in their phenolic glycoside composition and total concentration. The food selection patterns of the leaf beetles followed closely the phenolic glycoside spectra of the willow species. Both the total amount and the composition of phenolic glycosides affected the feeding by the beetles. Phenolic glycosides apparently have both stimulatory and inhibitory influences on leaf beetle feeding depending on the degree of adaptation of a particular insect. Very rare glycosides or exceptional combination of several glycoside types seem to provide certain willow species with high level of resistance against most herbivorous insects. Analogously the average absolute amount of leaf beetle feeding was lower on the introduced willows than on the native species to which the local herbivores have a good opportunity to become adapted.     

Herbivore functional traits and macroinvertebrate food webs have different responses to leaf chemical compounds of two macrophyte species in a tropical lake’s littoral zone

This research addressed the question of whether invertebrate food web structure varied between a native and an invasive macrophyte leaf species in the littoral zone of a tropical reservoir. We compared macroinvertebrate herbivore functional trait diversity composition with food web structure on the two macrophyte leaves, the invasive white ginger lily (Hedichium coronarium—Zingiberaceae) and the native pickerelweed (Pontederia cordata—Pontederiaceae). We predicted that the herbivore macroinvertebrate trait indices would decrease with macrophyte leaf species due to a lower resource quality with the flow-on effects in the food web structure. We calculated the number of functionally singular species (sing.sp) and herbivore functional trait richness (FRic) indices. For the macroinvertebrate food webs, we calculated the total number of trophic links (L), link density (L/S), connectance (C) and predator–prey ratios using a predator–prey matrix. We analysed the relationship between chemical traits of the macrophyte species’ leaves herbivore traits and food web indices using multivariate regression and Pearson’s correlation. Hedichium coronarium leaves had higher biomass and higher nitrogen content than the native P. cordata, which had higher phosphorus and carbohydrate content. Pontederia cordata leaves were associated with specialist macroinvertebrate species which primarily feed on biofilms (e.g. Ulmeritrus and Scirtidae) and plant leaves (e.g. Beardius). Food webs on P. cordata had lower numbers of trophic links (L), links per species (L/S) and predator–prey ratios. Connectance, which represents food web complexity, was similar between macroinvertebrate assemblages on the two leaf types. Our study suggests that chemical compounds of macrophyte leaves quality may have potential flow-on effects on food web structure.     

Over-winter decomposition and associated macroinvertebrate communities of three deciduous leaf species in forest streams on the Canadian Boreal Shield

The decomposition of deciduous leaf material provides a critical source of energy to aquatic food webs. Changes to riparian forests through harvesting practices may alter the species composition of deciduous leaf material entering streams. We compared over-winter decomposition of three different riparian leaf species (speckled alder (Alnus incana ssp. rugosa (Du Roi) J. Clausen), white birch (Betula papyrifera Marsh.), and trembling aspen (Populus tremuloides Michx.)) to determine their importance as a food resource for macroinvertebrate communities within Boreal Shield streams in northeastern Ontario, Canada. Leaf pack decomposition of the three leaf species formed a processing continuum throughout winter, where alder and birch leaf packs decomposed at a medium rate (k = 0.0065/day and 0.0053/day, respectively) and aspen leaf packs decomposed more slowly (k = 0.0035/day). Macroinvertebrate community colonization on leaf packs changed through time regardless of leaf species. Alder leaf packs supported higher abundances of macroinvertebrates in the fall while aspen leaf packs supported greater shredder abundances in the following spring. The study shows that leaf diversity may be important for providing a sustained food resource for aquatic macroinvertebrates throughout the relatively long over-winter period in Canadian Boreal Shield streams. Riparian forest management strategies should ensure that deciduous plant species richness is sustained in riparian areas.     

The functional role of Gammarus(Crustacea, Amphipoda): shredders, predators, or both?

Gammarus spp. are traditionally viewed under the functional feeding group (FFG) concept as herbivorous `shredders'. Although recent studies suggest that Gammarus should also be viewed as predators, this latter role remains contentious. Here, in a laboratory experiment, we objectively examine the balance between shredder and predator roles in a common freshwater species. Gammarus pulex preyed significantly on mayfly nymph, Baetis rhodani, in both the presence and absence of excess leaf material. There was no significant difference in predation where the alternative food, that is, leaf material, was present as compared to absent. Also, G. pulex shredded leaf material in the presence and absence of B. rhodani. However, shredding was significantly reduced where alternative food, that is, B. rhodani prey, was present as compared to absent. Further, G. pulex had a clear leaf species preference. Our results suggest that Gammarus function as both predators and shredders, with the balance of the two roles perhaps depending on food availability and quality. We discuss implications for the use of the FFG concept in assessing freshwater processes, and the role that Gammarus predation may play in structuring macroinvertebrate communities.     

What physiological processes permit insects to eat Eucalyptus leaves?

Many species of insects eat Eucalyptus foliage despite its relatively low nutritional value and the many plant secondary metabolites (PSMs) present, for example, terpenes, phenols and formylated phloroglucinols (FPGs). Formylated phloroglucinols are a new class of PSMs that act as antifeedants for possums and koalas. What physiological processes are present that permit insects to eat eucalypt foliage and how do PSMs influence insect feeding or digestion? Some trees seem to be repeatedly infested with eucalypt‐feeding insects, possibly as a result of previous chemosensory cues remaining from parental selection of a plant. Avoidance or storage of PSMs permit jarrah leafminers (Perthida glyphopa) and sawflies (Perga sp.) to consume eucalypt foliage without dealing with the majority of these compounds. Some PSMs can be metabolized by polysubstrate membrane oxidases as found in caterpillars or sawflies that feed on eucalypts. High midgut pH may be advantageous for nutrient extraction and PSM metabolism, and midgut pH ranges between 8.5 and 8.9 for caterpillars of Hyalarcta huebneri. Plant secondary metabolites may not be absorbed as a result of the combined presence of the peritrophic matrix and endogenous surfactants. Excretion of PSMs can be as metabolites or intact compounds. Both putative metabolites and sideroxylonal‐A, an FPG, are present in the faeces of larvae of the case moth, H. huebneri. The presence of sideroxylonal‐A in the food had an effect on the presence of 5‐hydroxytryptamine (5HT) in the central nervous system of caterpillars, as larvae fed leaves with a high concentration of sideroxylonal‐A had relatively more 5HT in the brain and central nervous system ganglia than larvae fed leaves containing a low concentration. Further work is necessary to clarify how PSMs are handled by eucalypt‐feeding insects and what effect FPGs have on feeding and digestion.     

Factors affecting litter processing by Anisocentropus kirramus (Trichoptera: Calamoceratidae) from an Australian tropical rainforest stream

• 1 Larvae of the caddisfly Anisocentropus kirramus are common leaf shredders in rainforest streams in tropical Queensland. Laboratory experiments were undertaken to examine the effects of (a) differences in leaf species, age and degree of conditioning, (b) leaf preferences, (c) temperature and (d) larval stage on processing of leaf litter by A. kirramus.
• 2 Leaf species (five tested), age (fresh v senescent) and condition (conditioned in stream v unconditioned) each had a significant effect on the amount of leaf material that was processed. For most species, conditioned leaves were processed faster but for one species unconditioned leaves were processed faster. Senescent leaves were processed more rapidly than green leaves in three species but not in the other two. Given a choice of leaf types A. kirramus actively selected leaves that were processed faster; no preference was shown between two different ‘fast’ leaves.
• 3 Processing occurred at all temperatures tested (10, 18 and 25°C), with the processing rate increasing with temperature. Younger instar larvae processed leaves at a greater rate per unit body weight (up to 343% day^?1) than older instars. Final instar (5) larvae were capable of processing some tough leaves that younger instars did not process.
• 4 In rainforest streams, processing of leaves by A. kirramus takes place throughout the year. Its ability to process green leaves is important because of the high input of fresh green leaves into tropical streams, and because of the severe depletion of the supply of conditioned leaves and fine detritus after floods.

     

Leaf traits induce the feeding preference of a shredder of the genus Triplectides Kolenati, 1859 (Trichoptera) in an Atlantic Forest stream,Brazil: a test with native and exotic leaves

This study assessed the feeding preference of larvae of Triplectides sp. (Trichoptera, Leptoceridae) exposed to leaves of native (Hoffmannia dusenii Standley, 1931) and exotic (Eucalyptus globulus Labillardiere, 1799) trees. We hypothesized that, regardless of the origin of the leaves, larval preference is determined mainly by leaf anatomy and quality. Leaves from both species were conditioned with and without nutrient enrichment (NPK), and the four food items were offered in paired combinations to 162 larvae. Larval preference varied according to leaf combinations. In treatments containing both species, larvae preferred to feed on H. dusenii because of softer tissues and anatomical structure. The only exception was the treatment containing discs of enriched E. globulus and non-enriched H. dusenii where enhanced microbial activity on enriched leaves provided a softer resource to shredders. Our results corroborate the initial hypothesis and suggest that introduction of exotic leaves and changes in nutrient availability may affect shredder activity in streams and, consequently, organic-matter processing and ecosystem functioning.     

Feeding rates,assimilation efficiencies and growth of two amphipod species on biodeposited material from zebra mussels

1. Accumulation of organic material by the zebra mussel Dreissena polymorpha is assumed to be the source of a biodeposition‐based food web. However, only little is known about the importance of the biodeposited material as a food source and its contribution to increased abundances of macroinvertebrates in the presence of D. polymorpha. 2. Feeding, assimilation and growth of the amphipods Gammarus roeselii and Dikerogammarus villosus on food sources directly and indirectly associated with D. polymorpha (biodeposited material and chironomids) and on conditioned alder leaves were measured. The stoichiometry of carbon, nitrogen and phosphorus of the diets was measured as an important determining factor of food quality. 3. Chironomids had the highest nitrogen and phosphorus contents, alder leaves were depleted in nitrogen and phosphorus, and the stoichiometry of biodeposited material was intermediate. 4. Both amphipod species had highest feeding rates and assimilation efficiencies on chironomids. Gammarus roeselii fed more on biodeposited material than on alder leaves, but assimilation efficiencies were similar; D. villosus also had similar feeding rates and assimilation efficiencies on the two diets. 5. Both amphipod species had highest growth rates on chironomids and lowest growth rates on alder leaves. Both grew at intermediate rates on biodeposited material of D. polymorpha. The growth rates of the amphipod species were related to food stoichiometry. Overall, the invasive D. villosus grew faster than the indigenous G. roeselii. 6. Food resources directly and indirectly associated with D. polymorpha are potential diets for amphipods, providing further evidence for a D. polymorpha biodeposition‐based food web.     

Decomposition of litter from three major plant species of jarrah (Eucalyptus marginata Donn ex Sm.) forest in relation to site fire history and soil type

Rates of weight loss and release of N, P, K, S, Ca, Mg, Na, and Cl from litter of several species in jarrah (E. marginata Donn ex Sm.) forest were measured in relation to site fire history and soil type. Weight loss from leaf litter decreased in the order jarrah > marri (E. calophylla R. Br. ex Lindl.) > Banksia grandis Willd. After 18 months on the forest floor senesced leaves of jarrah, marri and Banksia had lost 45%, 42% and 19%, respectively, of their original weight. Although greatest rates of decomposition occurred on a site burnt 3 y previously by an intense autumn fire and slowest rates on a site which had not been burnt for 8 y, the differences between burn sites were small in comparison with the total weight loss from decomposing litter. The order of release of nutrients from decomposing eucalypt litter was P<N<Ca<S<Mg<Cl<K<Na. There appears to be only slow release of N and P from the litter layer of these forests in the period between successive control burns. Fresh jarrah leaves, which are similar in chemical composition to leaf litter falling after crowns have been scorched by intense fire, decompose rapidly in comparison with senescent leaf tissue. Release of nutrients, particularly N and P, is also more rapid from fresh leaves than from leaf litter. Rates of decomposition of green leaves differed between soil types in the order reddish gravels > dark sandy duplex soil > yellow gravels. These differences may be related to the higher nutrient status of the reddish gravel soils.     

Feeding plasticity of two detritivore-shredders

1. The feeding preferences of the trichopteran shredder Sericostoma personatum Spence and the amphipod shredder Gammarus pulex L. were studied using specimens collected in a springbrook where the major food source was beech litter (Fagus sylvatica L.). Six food items were tested: conditioned beech leaves, conditioned alder leaves (Alnus glutinosa L.), conditioned Sitka spruce needles [Picea sitchensis (Bong.) Carr], fresh beech leaves, a fresh macrophyte (Potamogeton perfoliatus L.) and a fresh filamentous green alga (Microspora sp.). 2. The overall preference pattern shown by the two shredders was the same: both preferred conditioned Alnus most, followed by fresh Microspora. The least preferred food items were conditioned Fagus leaves and Picea needles. 3. The feeding preference of the two shredders proved to be unrelated to food source fibre content, toughness, total phosphorus, C:N ratio and total nitrogen (P>0.05). 4. Despite the overall similarity in their feeding preferences, Gammarus was more selective than Sericostoma. The reason for this difference is discussed. 5. We interpret our findings as indicating that detritivore-shredders do not per se prefer leaf litter, but in fact actively select other food items such as filamentous green algae or macrophytes, even when terrestrial leaf litter is abundant. Most shredders in Danish forest streams thus seem to live on a growth-limiting food resource that they do not prefer. This may have important implications for secondary production in such streams.     

Chlorogenic acid as an antiherbivore defence of willows against leaf beetles

In the leaves of 13 Finnish willow species, the content of a phenolic, chlorogenic acid, was found to vary from 0 up to 18 mg g^–1 D.W. Effects of pure chlorogenic acid on insect feeding behaviour were tested using four common leaf beetle species which are in the field mainly found on willows with low-chlorogenic acid leaves. One species, Lochmaea capreae L., was invariably deterred by pure chlorogenic acid applied in naturally occurring concentrations on the willow leaves. Accordingly, in 2-choice laboratory feeding trials L. capreae was found to prefer low-chlorogenic acid leaves of four willow species over high-chlorogenic acid leaves of Salix pentandra L. and S. myrsinifolia Salisb. When presented on the leaves of S. phylicifolia L, pure chlorogenic acid inhibited also the feeding by Phratora polaris Sp.-Schn. Instead, chlorogenic acid had no significant effect on Ph. polaris when it was presented on the leaves of another willow S. cinerea L. In laboratory, Ph. polaris did not show general preference for willow species with low chlorogenic acid content in their leaves. Thus, the response of Ph. polaris to chlorogenic acid seems to depend on the plant species. Apparently variation in other traits such as leaf hairyness may easily override the potential effect of chlorogenic acid content on Ph. polaris. To two other leaf beetle species, Galerucella lineola F. and Plagiodera versicolora Laich., chlorogenic acid is an ineffective deterrent even at unnaturally high concentrations. In laboratory, G. lineola and P. versicolora did not prefer willows with low chlorogenic acid content in their leaves. Thus, among four studied leaf beetle species, only L. capreae seems to be clearly affected by this phenolic. Therefore, overall importance of chlorogenic acid as a defence against willow-feeding leaf beetles appears to be very limited.     

Comparative food web structure of larval macrolepidoptera and their parasitoids on two riparian tree species

Single species or groups of species can be subjected to differing levels of parasitism on different plants. Previous studies have reported that parasitism of larval macrolepidoptera in an assemblage on box elder (Acer negundo L.) was significantly greater than on black willow [Salix nigra (Marsh)]. In this study, quantitative food webs, parasitoid overlap diagrams and other food web attributes were used to identify and describe direct and indirect interactions, and to compare assemblages on each tree species. These comparisons helped identify possible mechanisms explaining the differential parasitism observed. Although links among numerically dominant species in each assemblage were not strong, links between numerically dominant and subdominant species were strong. That is, numerically dominant and subdominant species interacted via shared parasitoids. The degree of parasitoid sharing by numerically dominant and subdominant species differed in each tree. There was less sharing of parasitoids on black willow than on box elder. Further, on box elder, the majority of parasitoids affecting numerically subdominant species originated from numerically dominant species, unlike in willow. These results lead to a working hypothesis—the source/nursery hypothesis—that proposes that community-wide levels of parasitism are highest in circumstances in which numerically subdominant species share parasitoid species in common with numerically dominant species, and most parasitoids attacking subdominant species originate from numerically dominant species. Thus, differences in degree of sharing and the types of herbivores sharing parasitoids may explain differential parasitism. Further, the source/nursery hypothesis may explain why the vast majority of species in most assemblages are numerically subdominant. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Phenotypic plasticity in host plant preference of the willow leaf beetle Phratora vulgatissima: the impact of experience made by adults

相似文献   

Effects of elevated CO2 and temperature on plant growth and herbivore defensive chemistry

Concentration of atmospheric CO₂ and temperature have both been rising for the last three decades. In this century, the temperature has been predicted to rise by 2–5 °C and the CO₂ concentration to double. These changes may affect the primary and secondary metabolism of plants and thus have implications for other trophic levels. However, the biotic interactions in changing climate conditions are poorly known. In this study, two questions were addressed: (i) How will climate change affect growth and the amounts of secondary compounds in flexible plant species? and (ii) How will this affect herbivores living on this species. Four clones of the dark‐leaved willow (Salix myrsinifolia (Salisb.)) seedlings were grown in closed‐top chambers with two controlled factors: concentration of atmospheric CO₂ and temperature (T). There were four combinations of these factors, each combination replicated four times (total of 16 chambers): (i) Control CO₂ (350 ppm) and control T, (ii) Elevated CO₂ (700 ppm) and control T, (iii) Control CO₂ and elevated T (2 °C), and (iv) Elevated CO₂ and elevated T. Stem growth and aerial biomass of the plants were determined; and the leaf phenolics, nitrogen and water concentrations were analysed. In addition the growth rate of larvae and feeding preference of adults of a specialist herbivore, the chrysomelid beetle Phratora vitellinae (L.), on the treated willow leaves were measured. Elevated temperature and CO₂ concentration increased the stem biomass and elevated CO₂ increased leaf biomass and total aerial biomass of the willows. Patterns of biomass allocation were different in different temperature treatments. At elevated temperature there was less branch and leaf material in relation to stems than at the control temperature. Moreover, patterns of biomass allocation differed among clones. CO₂ enhancement increased the specific leaf weight (SLW) and reduced both water and nitrogen content of the leaves, however, leaf area was unaffected by the treatments. Carbon dioxide (CO₂) and T enhancement reduced the concentrations of several phenolic compounds in the leaves. Phenolic compounds, nutrients, and water in the leaves might be diluted partly due to increased carbon allocation to different structures (e.g. thickening of cell wall and increase of trichomes, etc.). In some cases plant clones showed specific responses to treatments. The CO₂ enhancement reduced the relative growth rate (RGR) of the beetle larvae, and in contrast, temperature elevation increased it. Adult beetles did not clearly discriminate between willow leaves grown in different T and CO₂ environments, but tended to eat more leaf material from chambers with doubled CO₂ concentration. At elevated CO₂ adult beetles may need to eat more leaf material in order to reproduce, which may in turn prolong the life cycles, increasing the risk of being eaten and possibly affecting ability to overwinter successfully. Overall, climate change may significantly modify the dynamic interaction between willow and beetle populations.     

Influence of urban tributaries on freshwater mussel populations in a biologically diverse piedmont (USA) stream

We investigated the distribution patterns of senescent and green leaf litter patches on a streambed to evaluate the hypothesis that the different immersion times of senescent leaves in long-term benthic storage and newly retained green leaves affect streambed distribution patterns in summer (June, July, and August). We counted all the leaf litter patches in the streambed of the study reach, comparing the physical condition of patches and classifying the trapping obstacles associated with each patch type. The distribution patterns of senescent and green leaf litter patches differed. Green leaf litter patches were more numerous at every sampling date, with most trapped by cobbles, whereas senescent leaf litter patches were retained by twig obstacles, backwaters, and cobbles. In June and July, senescent leaf litter patches were located in significantly deeper stream areas than were green leaf litter patches. The distribution of senescent leaves would be primarily determined during spring snowmelt-driven floods. We speculate that senescent leaves were originally located at the edges of pools in the main flow pathway of the channel, which overflowed in the floods. We conducted flume experiments to clarify the transport characteristics of senescent and green painted maple and manchurian alder leaves in the water column. Our hypothesis was that the transport characteristics of each leaf type differ when they first enter the water, because of differing leaf properties. The flume experiments showed that duration of surface flotation differed for senescent and green leaves and for the two tree species. These differences in the duration of leaves on the streambed and in the floating time of green leaves of different trees ensure varied food resources for macroinvertebrates in various physical conditions. Handling editor: B. Oertli