Geographical variation in food web structure in Nepenthes pitcher plants

Abstract. 1. Relative to Nepenthes species in West Malaysia near the evolutionary centre of the genus, outlying species of Nepenthes in the Seychelles, Sri Lanka and Madagascar have fewer species of both prey and predator living in them, fewer and smaller guilds of species, much apparently empty niche space, less complex food webs, and a greater connectance. The ratios of prey to predators, and of connectance (C₁) to the total number of trophic types present remain approximately constant.
2. Differences between the food webs appear to be related in a complex way to the size of the country and its degree of spatial and temporal isolation, the size of the local species pool capable of colonizing the pitchers, and the number of Nepenthes species present. However, the maximal length of food chains in the richest and most complex food webs is probably limited by energetic constraints or environmental predictability.
3. The data may illustrate how food webs change to become more complex, both by the addition of new guilds of species and the addition of species to existing guilds, while at the same time certain properties of the food web are kept approximately constant.     

An experimental study of the effects of predatory fish on macroinvertebrates in a Hong Kong stream

SUMMARY. 1. Predation upon macroinvertebrates by the loach Oreonectes platycephalus Günther (Cobitidae) was studied using predator inclusion/exclusion cages in a series of pools along a Hong Kong stream. Treatments employed were predator exclusion, medium (approximately natural) predator densities (1 fish cage⁻¹) and high predator densities (2 fish cage⁻¹). Macroinvertebrate abundance in cages was monitored after 2 and 4-weeks exposure to predators.
2. The presence of fish was associated with significant declines in the total numbers of macroinvertebrates colonizing cages. However, taxa were influenced differently, with mayflies decreasing by a factor of two while the more mobile shrimps (Atyidae) were unaffected. Chironomid abundance (largely Chironominae) was unaffected by predator density and increased in week 4. Detritus acted as a confounding variable at this time because chironomid abundance was significantly correlated with the weight of accumulated detritus in cages.
3. While invertebrates were more abundant in cages lacking fish, there were no fewer invertebrates in cages with 2 fish than with 1 fish. This may indicate the presence of secure refuges among substrates in the cages, preventing the additional fish from depleting prey further, or a lack of precision of methods due to natural variations in prey densities and spatial patchiness.
4. No significant effects of predators on relative prey abundance or species richness were detected.
5. The impact of predation on prey abundance weakened on week 4, perhaps due to extra refuges among the accumulated detritus. However, drying of the stream increased fish densities in pools so that cages may have become zones of relative safety that were colonized readily by macroinvertebrates. This result highlights the need for year-round investigations to quantify predation effects in Hong Kong's seasonal tropical climate.     

The feeding ecology of Morone americana larvae in the Chesapeake Bay estuarine turbidity maximum: the influence of physical conditions and prey concentrations

Feeding ecology of white perch Morone americana larvae, a major component of the ichthyoplankton community in the estuarine turbidity maximum (ETM) region of upper Chesapeake Bay, was evaluated in years of contrasting physical conditions. In 1998, high river flow led to an ETM with high turbidity and pronounced salinity stratification. In 1999, under low river flow conditions, turbidities in the ETM region were lower and the salt front was located 15 km up‐stream of its location in 1998. Copepodites and adults of Eurytemora affinis were the predominant prey in guts of all length classes of larvae (3·2–9·8 mm standard length, L _S) that were examined. Repeated‐measures, multiple regression analyses were conducted to evaluate the influence of several factors on feeding success by determining if physical conditions (temperature, current velocity, salinity, turbidity and light) and prey concentrations explained variability in mean gut fullness (gut contents mass per body mass) of small (<5 mm L _S) and large (>5 mm L _S) white perch larvae. Eurytemora affinis concentrations were significant in the statistical models for both small and large larvae. High concentrations of E. affinis , which enhanced encounter rates of white perch larvae with prey, may have been the most important factor determining feeding success in the ETM region. Larval feeding incidence (percentage of guts with food) was higher in 1998, suggesting that annual variability in river flow influenced larval feeding success by controlling physical and biological conditions in the ETM region.     

Predation on meiobenthic assemblages: resource use of a tanypod guild (Chironomidae, Diptera) in a gravel stream

1. Three predatory chironomid species constituted numerically 8.8% (± 95% CL 2.2) of the macro- and meiobenthic community at the sediment surface and in the hyporheic zone of Oberer Seebach, a gravel stream in Lower Austria. Larvae of Thienemannimyia geijskesi (Goetghebuer) and Nilotanypus dubius (Meigen) occurred in higher densities in sediment depths between 10 and 40 cm, whereas Conchapelopia pallidula (Meigen) achieved higher densities at the sediment surface. The three species completed one generation in a year. 2. A total of ninety-seven prey species and instars were identified by gut analyses, of which forty-one benthic rotifer species constituted 69.5% of individuals and twenty-three chironomid species and their instars, 22.9%. The three tanypod species showed shifts from mainly rotifer species in early instars to chironomids and diverse other meio- and macrofaunal taxa in later instars. Rather than shifting towards larger prey sizes, growing predators expanded their upper size thresholds and continued to include smaller prey species in their diet. The extent to which tanypod instars fed on similar prey size classes declined with increasing larval size. Predation by tanypods amounted to 2.2% (± 95% CL 0.1) of the combined prey densities and prey consumption averaged 1.32 (bootstrap 95% CL 1.26–1.39) individuals per predator individual. 3. Preferences for microhabitat flow differed between predator species and in the prey assemblage. Prey densities and densities of T. geijskesi and C. pallidula were highest in pool areas, whereas N. dubius achieved high densities in riffle sites. 4. Tanypod larvae fed non-selectively among prey types. To test the significance of observed size(instar)-specific spatial and dietary overlap values amongst tanypod species, simulations were generated from random models for pairs of intra- and interspecific associations of individuals and groups of prey and predator species. Groups and individuals of tanypod instars fed near randomly on groups of prey types and a high proportion (P > 0.60) of prey individuals are quasi-randomly chosen by tanypods in those patches. Tanypod instar-pairs did not show a sustained trophic resource partitioning in time, thus reducing the degree of competitive interactions for food in this predator guild. Spatially segregated and non-segregated tanypod instars formed random aggregations independent of each other at different flow microhabitats. 5. Species-rich prey assemblages such as benthic rotifers and larval chironomids increased the probability of non-selective feeding upon a wide spectrum of prey species by tanypods. Prey choice was governed by prey availability and tanypod individuals fed on many species at rather even proportions independent of each other.     

The life histories, trophic relations and production of Stenoperla prasina (Plecoptera) and Deleatidium sp. (Ephemeroptera) in a New Zealand river

The life history, feeding relations and production of the stonefly Stenoperla prasina were studied for 16 months in the Selwyn River, South Island, New Zealand. Larval life is about 1 year with growth occurring in all months. Although emergence of adults was observed only from November to February, egg hatching probably occurred over an extended period as small larvae were present in most months. Larvae were mainly carnivorous except in early instars when most guts contained detritus. Larvae of a mayfly, Deleatidium sp. and a chironomid, Orthocladiinae sp. were the most important prey items. In April of two successive years the guts of most stoneflies examined were filled with diatoms (Gomphonema sp.) and filamentous algae which were abundant in the river only at these times. Algal feeding was not found in other months. Larvae of the main prey species, Deleatidium, were present in all months, being most abundant in summer and declining in numbers during winter. Maximum emergence occurred in March and April. The annual cycle of Deleatidium was diflicuit to interpret as larvae of all sizes were present in all months. Two generations probably occurred in a year, a fast-growing summer generation and a slower-growing and less synchronized winter generation. Mean annual standing biomass, annual production and turnover ratios (P/B) were calculated for both species. The latter were within the range of values given by Waters (1969) but may be subject to error from several sources. Shortcomings in the method used to estimate production are discussed.     

Spatial and temporal variation of microbial respiration rates in a blackwater stream

1. The extent of spatial and temporal variation of microbial respiration was determined in a first-order, sand-bottomed, blackwater stream on the coastal plain of south-eastern Virginia, U.S.A.
2. Annual mean respiration rates (as g O₂ m^–3 h^–1) differed significantly among substrata: leaf litter, 12.9; woody debris, 2.4; surface sediment, 0.8; hyporheic sediment, 0.4; water column, 0.003. Rates associated with wood were higher than those with leaves when expressed per unit surface area.
3. Highest respiration rates on leaves, wood and in the water column occurred during the summer, whereas rates in the sediments were greatest during the late autumn and winter. Water temperature, as well as particulate organic matter and nitrogen content of the substrata, was correlated positively with respiration rates.
4. A stepwise multiple regression showed that temperature and nitrogen content together explained 88% of the variation in respiration rates of leaves and wood. In contrast, particulate organic matter content and nitrogen content explained 89–90% of the variation in respiration in the sediments. Although water temperature was a significant factor in the sediment multiple regressions, its addition as an independent variable improved the regression models only slightly.
5. Annual mean respiration in the stream channel, based on the proportional amount of respiration occurring associated with each type of substratum during each month, was 1.1 kg O₂ m^–2 yr^–1. Seventy per cent of respiration in the stream occurred in the hyporheic zone, 8–13% occurred in the surface sediment, leaf litter or woody debris, and < 1% occurred in the water column. Approximately 16% of total detritus, or 40% of non-woody detritus, stored in the stream during the year was lost to microbial respiration.     

Spatial and temporal variation of microbial respiration rates in a blackwater stream

1. The extent of spatial and temporal variation of microbial respiration was determined in a first-order, sand-bottomed, blackwater stream on the coastal plain of south-eastern Virginia, U.S.A.
2. Annual mean respiration rates (as g O₂ m^–3 h^–1) differed significantly among substrata: leaf litter, 12.9; woody debris, 2.4; surface sediment, 0.8; hyporheic sediment, 0.4; water column, 0.003. Rates associated with wood were higher than those with leaves when expressed per unit surface area.
3. Highest respiration rates on leaves, wood and in the water column occurred during the summer, whereas rates in the sediments were greatest during the late autumn and winter. Water temperature, as well as particulate organic matter and nitrogen content of the substrata, was correlated positively with respiration rates.
4. A stepwise multiple regression showed that temperature and nitrogen content together explained 88% of the variation in respiration rates of leaves and wood. In contrast, particulate organic matter content and nitrogen content explained 89–90% of the variation in respiration in the sediments. Although water temperature was a significant factor in the sediment multiple regressions, its addition as an independent variable improved the regression models only slightly.
5. Annual mean respiration in the stream channel, based on the proportional amount of respiration occurring associated with each type of substratum during each month, was 1.1 kg O₂ m^–2 yr^–1. Seventy per cent of respiration in the stream occurred in the hyporheic zone, 8–13% occurred in the surface sediment, leaf litter or woody debris, and < 1% occurred in the water column. Approximately 16% of total detritus, or 40% of non-woody detritus, stored in the stream during the year was lost to microbial respiration.     

Growth hormone-induced effects on mortality, energy status and growth: a field study on Brown Trout (Salmo trutta)

1. Growth hormone (GH) treatment increases the growth rate and competitive ability of salmonids under laboratory conditions. Since fast growth should increase fitness, why is endogenous secretion of GH not higher in wild fish? To address this question, three hypotheses were suggested. H₁: high GH levels reduce antipredator responses and may therefore increase mortality from predation. H₂: high GH levels reduce long-term (e.g. over winter) survival by reducing allocation to critical energy reserves. H₃: GH is not beneficial for growth under natural conditions.
2. To test these hypotheses, the performance of GH-treated juvenile Brown Trout ( Salmo trutta ) and control (placebo) trout was compared in an enclosed stream section subjected to natural predation. Four experiments were conducted during winter, spring, summer and autumn, respectively.
3. Mortality rates were not significantly different between GH-treated and control trout in any of the four experiments so H₁ was not supported. Energy reserves were generally lower in GH-treated fish, which is consistent with H₂, whereas growth rates in mass were higher in GH-treated fish than in controls so H₃ was not supported. This suggests that GH promotes growth at the expense of investment in maintenance.
4. Judging from growth and mortality rates, the fitness of GH-treated and control trout appeared similar. Thus, escaped GH-manipulated fish may compete successfully with wild fish.
5. Hatchery-raised trout with higher initial condition index suffered higher mortality rates than more slender fish. This novel finding may be explained by reduced escape ability related to body morphology, reduced behavioural responses towards predators by high-condition trout, or predator preferences for high-condition fish.     

Ocean acidification disrupts the innate ability of fish to detect predator olfactory cues

While ocean acidification is predicted to threaten marine biodiversity, the processes that directly impact species persistence are not well understood. For marine species, early life history stages are inherently vulnerable to predators and an innate ability to detect predators can be critical for survival. However, whether or not acidification inhibits predator detection is unknown. Here, we show that newly hatched larvae of the marine fish Amphiprion percula innately detect predators using olfactory cues and this ability is retained through to settlement. Aquarium-reared larvae, not previously exposed to predators, were able to distinguish between the olfactory cues of predatory and non-predatory species. However, when eggs and larvae were exposed to seawater simulating ocean acidification (pH 7.8 and 1000 p.p.m. CO₂) settlement-stage larvae became strongly attracted to the smell of predators and the ability to discriminate between predators and non-predators was lost. Newly hatched larvae were unaffected by CO₂ exposure and were still able to distinguish between predatory and non-predatory fish. If this impairment of olfactory preferences in settlement-stage larvae translates to higher mortality as a result of increased predation risk, there could be direct consequences for the replenishment and the sustainability of marine populations.
Ecology Letters (2010) 13: 68–75     

The microdistribution of some lotic insect predators in relation to their prey and to abiotic factors

SUMMARY. 1. The microdistribution of three species of insect predators in a Swedish stream was assessed using a multivariate statistical approach. Both abiotic factors and factors pertaining to the prey community were included in the analysis.
2. The factors most strongly correlated to the distribution of large Dinocras cephalotes were the densities of case less caddis larvae ( Wormaldia sp. and Hydropsyche siltalai ) and the median weight of stonefly prey. For small D. cephalotes , the density of H. siltalai and the percentage of moss cover were most important. Isoperla grammatica showed a significant correlation to blackfly density. Rhyacophila nubila occurred predominantly in patches with high prey biomass and with high densities and median weight of case less caddis larvae.
3. It was concluded that the quantity and quality of the prey, such as size, availability and species, influenced the predators'microdistribution. The distributional pattern differed between species of predators and was probably related to their different hunting strategies.     

Diet and predation by three leaf-associated stoneflies (Plecoptera) in an Arkansas mountain stream

SUMMARY. 1. Life cycles, food habits, mouthpart morphologies, prey preferences, and predator-prey size relationships were investigated for the three most common stoneflies in leaf packs in the Little Missouri River, Arkansas: Clioperla clio (Newman), Isoperla namata Prison, and Perlesta spp. Each species was univoltine and had a fast seasonal cycle, with extended egg or nymphal diapause, rapid nymphal growth and emergence in spring to early summer.
2. Foregut analysis showed predictable dietary shifts by C. clio and Perlesta spp., from detritus and diatoms in early instars to invertebrate prey in later instars. Isoperla namata fed facultatively on insects, detritus and diatoms, with no conspicuous ontogenetic shifts. The mouthparts of all three species strongly reflected their diet.
3. Electivity analysis indicated opportunistic feeding by the three species, with feeding rarely differing significantly from random.
4. Sizes of stonefly predators and their ingested prey were highly correlated ( P <0.01). However, rather than shifting toward larger prey sizes, growing predators expanded their size thresholds and continued to include numerous small prey items in their diet. Prey choice was governed most directly by prey availability, because stoneflies selected the most abundant groups, irrespective of size or taxon.
5. The likelihood of competition among the three stoneflies for prey was minimized by timing of their life cycles, differential food use and probably the temporary nature of leaf pack habitats.     

The feeding relationships of two invertebrate predators in a New Zealand river

The size frequency distributions and foods of the larvae of Archichauliodes diversus (Megaloptera: Corydalidae) and Stenoperla prasina (Plecoptera: Eustheniidae) were studied for a year in the Glentui River, South Island, New Zealand. These two species are the largest invertebrate predators inhabiting streams and rivers in New Zealand, where they are the only carnivorous members of their respective orders. Both species occupied the same habitat with A. diversus being slightly more abundant in most months. Many small larvae occurred within the sediments of the stream bed, whereas more larger larvae were found on stones lying on the surface. A wide size range of larvae was present throughout the year and the median size (in terms of head width) of both species was similar in most months. Larval growth could not be determined from field data and the life cycles of both species can be described as non-seasonal. Quantitative sampling in 5 months provided estimates of population densities. These were maximal in December when values of 136/m² for A. diversus and 114/m² for S. prasina were obtained. Larval Chironomidae and mayflies of the genus Deleatidium were the most frequently taken prey of both predators in all months, and no strong relationship between size or species of predator and size of prey was found. Caseless trichopteran larvae formed a less important component of the diets of both species and detritus was ingested by some individuals. No instances of cannibalism and only one example of cross-predation by each predator was seen. Diel sampling in November showed that large Deleatidium larvae were relatively more abundant in the guts of insect predators and in nocturnal drift samples than in the benthos. This suggests that their greater activity at night may increase their susceptibility to predation by nocturnal feeders. No clear ecological segregation of the two species with respect to habitat or prey utilization was found and there was no obvious interspecific competition for food, which appeared to be abundant at all times. Finally, the prevalence of non-seasonal life cycles in New Zealand aquatic insects is discussed in relation to the low degree of speciation in several groups.     

Competition for space by predators in streams: field experiments on a net-spinning caddisfly

SUMMARY. 1. Field experiments in a fishless stream were carried out on an abundant caddisfly with a predatory, net-spinning larva, Plectrocnemia conspersa (Curtis), to assess whether net site availability affects their microdistribution.
2. Net sites were supplemented by adding nought, one or four artificial structures to replicated patches on the stream bed. In each of three experiments at different seasons (summer, autumn and late winter), caddis densities increased significantly in patches with extra net sites.
3. The response of caddis to supplemented net sites could be affected by the subsidiary effects of food and offish. These potential interactions were assessed in each experiment by varying net site density in two additional treatment stretches in which (1) prey abundance was increased by releasing Daphnia , and (2) brown trout ( Salmo trutta L.) were enclosed. The responses of caddis in these two treatments were compared to that in the reference stretch, where only net site density varied.
4. Increased food abundance enhanced the response of caddis to net site supplementation in winter, when natural prey was least abundant, but not in summer or autumn. We suggest that extra food affects the mechanism determining net building only when prey availability is below some threshold.
5. The presence of fish precluded any effect of extra net sites in summer, but had no effect in autumn (the winter fish treatment was lost). We suggest fish predation reduced the densities of caddis in summer, so that net sites no longer limited local densities. In autumn, fallen leaves provided refugia from fish, which consequently were less effective predators of P. conspersa.     

Cellulose digestion and metabolism in the freshwater amphipod Gammarus pseudolimnaeus Bousfield

SUMMARY. 1. Weight-specific respiration and feeding rates of Gammarus pseudolimnaeus were inversely related to body weight at 12°C, the temperature of the stream from which they were collected.
2. At this temperature animals could digest about a third of ingested, purified ¹⁴C-cellulose. In animals greater than 20 mg live weight, about 38% of ingested cellulose was metabolized; animals smaller than 20 mg metabolized about 28%.
3. Respired CO₂ from cellulose metabolism was 33–47% of measured uptake of C₂ in animals greater than 20 mg live weight and approximately 25% in smaller animals.
4. Ability to digest cellulose would be of nutritional benefit to Gammarus feeding on any plant material, but would be of particular advantage in the spring/summer period when fine detritus is the main foodstuff.     

Life cycle, production, microdistribution and diet of the damselfly Euphaea decorata (Odonata: Euphaeidae) in a Hong Kong forest stream

Euphaea decorata in Tai Po Kau Forest Stream (Hong Kong) was univoltine. Most recruitment took place in summer, and larval growth proceeded throughout the year. Life-cycles recorded in 1977–78 and 1978–79 were similar. Annual production estimates, using the removal-summation, instantaneous growth and size-frequency methods, were more similar for the 1978–79 generation (ranging from 158.7–174.7; mean 1671 mg dry wt m^-2) than for the 1977–78 generation (93.9-173-6; mean 131.7 mg dry wt m^-2). Mean biomass was similar for both generations (ranging from 33.5–33.9 mg dry wt m^-2), and mean P/B ratios were 3.9 1977–78) and 5.0 (1978–79). These are the first estimates of annual production by an Oriental stream insect.
Larvae were most abundant at microsites in the middle of the stream. Multiple regression analysis indicated that substratum characteristics were a major determinant of microdistribution. Euphaea decorata apparently favoured poorly-sorted sediments with highly peaked grain size-frequency distributions, containing few fine particles.
The carnivorous larvae showed ontogenetic changes in diet. Small individuals consumed mainly chironomid (Diptera) larvae; the diet expanded to include (successively) larvae of Ephemeroptera and Trichoptera as E. decorata grew. Seasonal changes in diet were also apparent, although larval diets during spring and summer were similar. There was also considerable overlap between autumn and winter diets. Ontogenetic influences upon prey consumed were not sufficient to account for the observed seasonal differences.     

Trophic relationships: integrating meiofauna into a realistic benthic food web

• 1 This paper summarises the most important contributions on trophic relationships of lotic meiofauna. In contrast to marine research, the few quantitative studies of the freshwater meiobenthos have shown that these invertebrates not only take up particulate/fine organic matter, but also dissolved organic substances attached to organic particles. In lotic ecosystems, further estimates of grazing rate and bacterial/algal ingestion rate are needed, particularly in situ measurements.
• 2 The effects of macroinvertebrate predators upon meiofauna are still under debate. Depending on the type of experiments (laboratory vs. field) it seems that macrofauna may or may not affect meiofauna. Field samples and analyses of gut contents of larval tanypod chironomids have shown that the impact upon meiofauna was low and larvae were nonselective predators. Predation amounted to 2.2% of the combined prey density and prey consumption averaged 1.3 individuals per predator individual per year.
• 3 Adding taxonomic resolution by including the meiofaunal component within lotic food webs distinctly increases the number of total species and, as a consequence, changes food web statistics. Webs that included meiofauna revealed that these metazoans contributed substantially to the percentage of intermediate species (species with predators and prey). The resolution of dietary analyses of major consumers of macro‐ and meiobenthos showed that many stream invertebrates feed on meiofauna.

     

Selective predation by larval Agabus (Coleoptera: Dytiscidae) on mosquitoes: support for conservation-based mosquito suppression in constructed wetlands

1. Wetland insect predators can structure aquatic prey communities via selective predation, but receive considerably less attention than vertebrate predators. We conducted laboratory experiments to test selective predation by two species of larval dytiscid beetles ( Agabus ; Coleoptera: Dytiscidae) and the potential contribution of these beetles to suppression of mosquito populations in constructed wetlands.
2.  Agabus consumed copepods, ostracods and mosquito larvae in no-choice tests. When offered a choice, 76% of all prey consumed were mosquito larvae, indicating selective predation. Subsequent experiments revealed this preference was due to ease of capture of mosquito larvae over alternative prey.
3. Cannibalism and intraguild predation were common within and between species of Agabus , which may reduce the overall impact of the observed selective predation.
4.  Agabus larvae selectively preyed on mosquito larvae over alternative prey, which is not characteristic of some fish used as biological control agents for mosquitoes. Predator exclusion or similar experiments in the field could document how these results translate into a natural setting.
5. The findings of this study suggest developing mosquito suppression strategies focused on conservation of native wetland predators. These strategies are preferable to introducing non-native generalist predators, or applying pesticides.     

The effects of temperature and season on the O2 consumption of third-instar larvae of the goldenrod gall fly (Eurosta solidaginis)

Abstract. Third-instar larvae of the goldenrod gall fly ( Eurosta solidaginis Fitch) live inside ball galls on goldenrod plants from summer to the following spring.Because galls are highly exposed to the weather, larvae experience substantial variations in body temperature.This study documents the oxygen consumption of gall fly larvae with regard to the effects of ambient temperature, seasonal conditioning, and prior exposure to subzero temperature.The body mass of larvae doubles between the late summer and the autumn; it subsequently undergoes a modest decline by early winter.The O₂, consumption of field-acclimatized larvae increases with ambient temperature, especially between 0 and 10°C (Q₁₀= 2.6-3.4).The thermal sensitivity of metabolism declines at higher ambient temperatures, most notably during the autumn/early winter.After exposure to 15°C for 1 week, autumn and early winter larvae maintain much lower rates of O₂ consumption than do late summer specimens.Prior exposure to -5°C for 24 h did not influence the O₂ consumption of larvae.Low thermal sensitivity of O₂ consumption, especially at higher ambient temperatures, is an energy-sparing mechanism during seasonal inactivity.Indeed, the persistence of this metabolic pattern in larvae exposed to 15°C suggests that they have entered a state of diapause.     

Growth response of pikeperch larvae in relation to body size and zooplankton abundance

The most critical period at onset of feeding in pikeperch Stizostedion lucioperca is short (<5 days at 20° C). The larvae are sensitive to prey density during the first week of exogenous feeding. First-feeding larvae of 6.5 mm total length ( L_T ) needed prey densities of >585 prey l⁻¹ to maintain mass (C_maint), whereas 5 days older larvae of 7 mm L _T C_maint= 55 prey l⁻¹ and for 11 mm L _T larvae C_maint<10 prey 1⁻¹. Changes in specific growth rates for larvae of 7 and 11 mm were similar to a type-II functional response curve reaching a specific growth rate of 26 and 30% day⁻¹, respectively, at 1000 prey l⁻¹, whereas the 6·5 mm larvae showed a linear growth response reaching a specific growth rate of only 9% day ⁻¹ at 1000 prey l⁻¹. The results suggest that prey density is a limiting factor, which might contribute to the high variation between year-class Strengths.     

Role of benthic insect feces in a Rocky Mountain stream: fecal production and support of consumer growth

The feces of stream insects may be a major component of fine particulate organic matter (FPOM) available to collector organisms. In Mink Creek, Idaho, winter defecation rates for 9 species ranged from 86 mg dry feces (g dry body wt)⁻¹ d⁻¹ (Ephemerella spinifera) to 154 mg g⁻¹ d⁻¹ (Paraleptophlebia heteronea) . Detailed studies of 3 species in summer revealed that rates were much greater than in winter and exhibited high between-individual variation (e.g., 695 ± 184.8 mg (g body wt)⁻¹ d⁻¹ for Baetis tricaudatus ). By combining measured and literature values, it is estimated that the benthic insect community egests approximately 3.4 kg feces m⁻² yr⁻¹. This value lies within the range of FPOM estimated to be derived from degradation of leaf litter input (0.7–9.9 kg m⁻² yr⁻¹, depending upon assimilation efficiency of the organisms).
Short-term winter growth experiments showed that, with two exceptions, collectors fed on fecal detritus will grow as well as on other food resources; relative growth rates depend upon the species. Because fecal detritus is abundant throughout the year, and is treated by many species as a source of food, it could be a major mechanism by which seasonal fluctuation in availability of allochthonous litter input is minimized and homeostasis of structure and function maintained.