Physiological responses to stoichiometric constraints: nutrient limitation and compensatory feeding in a freshwater snail

Nitrogen (N) and phosphorus (P) are considered to be essential nutrients that control secondary production in various ecosystems; insufficient availability of N and P can limit herbivore growth. Here, data are presented from field samplings and from a laboratory experiment on the potential of primary producers low in P, N, or P and N to constrain growth of the freshwater gastropod Radix ovata . The filamentous green alga Ulothrix fimbriata was cultured under different nutrient regimes, resulting in algae with different C:N:P ratios. The pure algae were fed in high and low quantities to juvenile R. ovata . Low availability of N and especially P in the algae strongly constrained the biomass accrual of the herbivore. In accordance with theoretical predictions, these food quality differences were highly dependent on the food quantity. The snails' growth rate was significantly related to their body C:P ratio, thereby supporting the growth rate hypothesis. R. ovata displayed a pronounced compensatory feeding response to low-nutrient food that could partly dampen but not fully compensate the food quality effects on snail growth. Increased feeding of gastropods at low P and/or N availability leads to depletion of periphyton biomass; hence compensatory feeding would shift the benthic herbivore community from a P or N limitation to a C limitation and thus have whole-ecosystem effects.     

Extrinsic vs. intrinsic food shortage and the strength of feeding links: effects of density and food availability on feeding rate of Hyphydrus ovatus

Summary We use field and laboratory experiments to determine whether Hyphydrus ovatus, a predatory aquatic beetle, is food limited, and whether any food shortage results from depletion of prey by these predators (intrinsic food shortage) or is independent of predation by these beetles (extrinsic food shortage). In the laboratory, differences in feeding rate influence body fat content, thus making fat content a useful index of recent feeding history. H. ovatus collected during the breeding season have fat contents significantly greater than those of H. ovatus starved for 25 days, but not significantly different from those of H. ovatus fed ad libitum for 25 days, indicating that natural feeding rates are near the maximum possible. H. ovatus confined at a density 60 times greater than natural show reduced fat content and feeding rate relative to natural, indicating that at very high densities H. ovatus is capable of depleting its prey. Addition of supplemental natural prey (primarily Cladocera) to experimental enclosures resulted in an order of magnitude increase in prey availability, and a significant increase in fat content and feeding rate of confined H. ovatus. Adults of this species do not appear to be food limited during the breeding season, and extraordinarily high densities of adults seem to be necessary to produce intrinsic food shortage. These results suggest that feeding links between H. ovatus an its principal prey do not have major effects on population dynamics under typical field conditions, and call into question the assumption that closely coupled predator-prey interactions are the sole explanation for observed food-web patterns.     

Grasshoppers cope with low host plant quality by compensatory feeding and food selection: N limitation challenged

The effect of low host plant nitrogen (N) content on herbivore performance has rarely been studied together with the herbivore's feeding behaviour. We explored this relationship with juvenile Omocestus viridulus (Orthoptera: Acrididae) grasshoppers using fertilized and unfertilized host grasses. Due to lower growth rates, grasshoppers reared on N-poor grasses exhibited slightly prolonged development and smaller adult size, while mortality was similar among the fertilizer treatments. This was found both in the laboratory and in outdoor cages under natural climatic conditions. A parallel analysis of feeding behaviour revealed that the grasshoppers counterbalance N shortage by compensatory feeding, and are capable of selectively feeding among grasses of contrasting nutritional quality when given a choice. This indicates a striking ability of O. viridulus to regulate nutrient intake in the face of imbalanced food sources. Although the species exploits a relatively very poor autotroph nutrient base in the wild, as underpinned by N analysis of natural host grasses and grasshopper tissue, our data suggest that natural food quality imposes no relevant constraint on the herbivore's performance. Our study thus challenges the importance of simple plant-mediated control of herbivore populations, such as N limitation, but supports the view that herbivores balance their intake of N and energy.     

Wild Bornean orangutan (Pongo pygmaeus wurmbii) feeding rates and the Marginal Value Theorem

The Marginal Value Theorem (MVT) is an integral supplement to Optimal Foraging Theory (OFT) as it seeks to explain an animal's decision of when to leave a patch when food is still available. MVT predicts that a forager capable of depleting a patch, in a habitat where food is patchily distributed, will leave the patch when the intake rate within it decreases to the average intake rate for the habitat. MVT relies on the critical assumption that the feeding rate in the patch will decrease over time. We tested this assumption using feeding data from a population of wild Bornean orangutans (Pongo pygmaeus wurmbii) from Gunung Palung National Park. We hypothesized that the feeding rate within orangutan food patches would decrease over time. Data included feeding bouts from continuous focal follows between 2014 and 2016. We recorded the average feeding rate over each tertile of the bout, as well as the first, midpoint, and last feeding rates collected. We did not find evidence of a decrease between first and last feeding rates (Linear Mixed Effects Model, n = 63), between a mid-point and last rate (Linear Mixed Effects Model, n = 63), between the tertiles (Linear Mixed Effects Model, n = 63), nor a decrease in feeding rate overall (Linear Mixed Effects Model, n = 146). These findings, thus, do not support the MVT assumption of decreased patch feeding rates over time in this large generalist frugivore.     

Compensatory growth in juvenile roach Rutilus caspicus: effect of starvation and re‐feeding on growth and digestive surface area

The aim of this study was to investigate compensatory growth in juvenile Rutilus caspicus during starvation and re‐feeding periods. The results confirmed the existence of compensatory growth in R. caspicus which depended on the duration of food deprivation. Complete compensatory growth occurred in the fish that were food deprived for at least 3 weeks. Starvation and re‐feeding had no significant effect on the digestive somatic index and intestinal surface areas in the fish that were food deprived for 1 week, while they showed a significant decrease and increase, during starvation and re‐feeding in the fish that were food deprived for 2 and 3 weeks. This knowledge may have application in aquaculture, as appropriate exploitation of compensatory growth can give increased growth rate and feeding efficiency.     

Compensation and resistance to herbivory in seagrasses: induced responses to simulated consumption by fish

Herbivory can induce changes in plant traits that may involve both tolerance mechanisms that compensate for biomass loss and resistance traits that reduce herbivore preference. Seagrasses are marine vascular plants that possess many attributes that may favour tolerance and compensatory growth, and they are also defended with mechanisms of resistance such as toughness and secondary metabolites. We quantified phenotypic changes induced by herbivore damage on the temperate seagrass Posidonia oceanica in order to identify specific compensatory and resistance mechanisms in this plant, and to assess any potential trade-offs between these two strategies of defence. We simulated three natural levels of fish herbivory by repeatedly clipping seagrass leaves during the summer period of maximum herbivory. Compensatory responses were determined by measuring shoot-specific growth, photosynthetic rate, and the concentration of nitrogen and carbon resources in leaves and rhizomes. Induced resistance was determined by measuring the concentration of phenolic secondary metabolites and by assessing the long-term effects of continued clipping on herbivore feeding preferences using bioassays. Plants showed a significant ability to compensate for low and moderate losses of leaf biomass by increasing aboveground growth of damaged shoots, but this was not supported by an increase in photosynthetic capacity. Low levels of herbivory induced compensatory growth without any measurable effects on stored resources. In contrast, nitrogen reserves in the rhizomes played a crucial role in the plant’s ability to compensate and survive herbivore damage under moderate and high levels of herbivory, respectively. We found no evidence of inducibility of long-term resistance traits in response to herbivory. The concentration of phenolics decreased with increasing compensatory growth despite all treatments having similar carbon leaf content, suggesting reallocation of these compounds towards primary functions such as cell-wall construction.     

Simple models of feeding with time and enery contstraints

We analyze how the foraging currencies "rate" (net energy gainper unit time) and "efficiency" (net energy gain per unit energyexpenditure) relate to the workload adopted by a forager. Weconsider feeding (gathering food for immediate consumption)as opposed to provisioning and investigate the influence oftime and energy constraints. In our model the forager may varythe level of energy expenditure while foraging; increased expenditureincreases the rate of gain, but with diminishing returns. Weshow that rate maximizing requires a higher rate of energy expenditurethan efficiency-maximizing, and we compare the performance ofrate- and efficiency-maximizing tactics when the feeding strategyis (1) to maximize the total net gain while foraging; (2) tomaximize the total net daily gain; or (3) to meet a requirement.Generally, the rate-maximizing tactic only performs best whentime is limiting; otherwise, a lighter workload and slower feedingrate perform better. Under the restricted conditions analyzedhere, no general statement can be made about the best tacticwhen the strategy is to meet a requirement. These results mayhelp explain several instances of "submaximal" foraging describedin the literature.     

Mechanical determinants of nectar-feeding energetics in butterflies: muscle mechanics,feeding geometry,and functional equivalence

Summary We develop a mechanistic model for nectar feeding in butterflies that integrates the two basic components of the feeding process: the fluid dynamics of nectar flow through the food canal and the contractile mechanics of the muscular, cibarial pump. We use the model to predict the relation between rate of energy intake during feeding and nectar concentration. We then identify nectar concentations that maximize energy intake rates (the optimal concentrations). We illustrate the model using measurements of the food canal and cibarium of Pieris butterflies. The model predicts an overall optimal range of nectar concentration of 31–39% sucrose for butterflies, which is in agreement with previously reported laboratory values. The model also predicts an interaction among the geometries of the food canal, the cibarial cavity, and the cibarial muscles, that allows us to identify the combinations of food canal, cibarium, and muscle dimensions that yield the highest rates of energy intake. Nectar-feeding is functionally equivalent in butterflies and hummingbirds: two physically different feeding mechanisms can yield identical energy intake rates. This equivalence results from a mathematical and physical similarity between quasi-steady-state fluid flow in hummingbrid tongues and the force-velocity characteristics of contracting cibarial muscle in butterflies.     

Plant compensatory growth: a conquering strategy in plant–herbivore interactions?

We present a theoretical analysis that considers the phenotypic trait of compensatory growth ability in a context of population dynamics. Our model depicts a system of three interactors: herbivores and two different plant types referred to as ordinary and compensating. The compensating plant type has the ability to increase its intrinsic rate of biomass increase as a response to damage. This compensatory growth ability is maintained at the expense of a reduced growth rate in the absence of damage, where the ordinary plant type has the higher growth rate. Analysis of this system suggests that, even though a compensatory capacity of this kind will not imply an increase in equilibrium plant density, it will give a competitive advantage in relation to other plants, in the presence of a sufficiently efficient herbivore. Invasion of compensating plants into a population of non-compensating plants is facilitated by a high compensatory growth ability and a high intrinsic rate of plant biomass increase. Conversely, an ordinary plant can invade and outcompete a compensating plant when the herbivore is characterised by a relatively low attack rate, and/or when plant intrinsic growth rate is decreased.     

Individual foraging in the antPachycondyla apicalis

Summary A model of individual foraging in social insects as presented that formalises the dynamics of foraging and concentrates on the collective rather than the individual benefit, quantifying the relationships between a colony's foraging area, number of foragers and foraging energy budget and the food sources' rate of arrival, disappearance and capture. A series of experiments, in which a number of prey were offered to colonies of the individually foraging antPachycondyla (ex-Neoponera) apicalis confirm the hypotheses implicit in the model and measured the rates of capture and competition. 60 days observation of 3P. apicalis colonies' foraging activity are summarised and used in conjunction with the model to obtain estimations of the density and rate of arrival of available prey in the foraging area. We examine how a colony's foraging benefit may be influenced by its foraging area, the number of foragers, and the forager/non-forager ratio and show that a colony's jocial structure strongly limits its potential foraging benefit. Within these limits,P. apicalis does not appear to be an optimal forager.     

Understanding the stoichiometric limitation of herbivore growth: the importance of feeding and assimilation flexibilities

Ecological stoichiometry suggests that herbivore growth is limited by phosphorus when this element in the diet is < 8.6 μg P mg C^?1 (C : P atomic ratio > 300). However, in nature, it is not necessarily related to the relative phosphorus content in diets. This may be the result of complex feeding and assimilation responses to diets. We examined these possibilities using herbivorous plankton fed mono‐specific and mixed algae varying in phosphorus content of 1.6 to 8.1 μg P mg C^?1. The herbivores showed a 10‐fold growth rate difference among the diets. Growth rates related poorly with phosphorus content in the diets (r² = 0.07), better with P ingestion rate (r² = 0.41) and best with phosphorus assimilation rate (r² = 0.69). Inclusion of assimilation rates for carbon and fatty acids increased 7% of the explained growth variance. These results indicate that the feeding and assimilation flexibilities play pivotal roles in acquiring a deficient element and in regulating growth rate.     

Stoichiometric tracking of soil nutrients by a desert insect herbivore

Abstract Biogeochemistry and population biology have developed independently, with few attempts at linkage, almost none of which were mechanistically based. We hypothesize that biogeochemical cycling is linked to herbivore population dynamics through the influence of soil nutrient availability on foliar nutrient content, which constrains herbivore investment in phosphorus (P)‐rich molecules necessary for growth. We show that variation in desert soil P availability is linked to abundance of an insect herbivore (Sabinia setosa) through the influence of soil P on the C:P ratio of the host plant (Prosopis velutina). Low P availability increases C:P ratio of Prosopis leaves, leading to a decline in body %P, %RNA and abundance of Sabinia. Tight association between soil, plant and herbivore P provides the first evidence of a mechanistic pathway linking P biogeochemistry to terrestrial food webs by altering the supply of dietary P to herbivores, thus limiting their capacity for growth by constraining the production of P‐rich cellular ribosomal RNA (rRNA).     

Filtration and digestion responses of an elementally homeostatic consumer to changes in food quality: a predictive model

In the study of the stoichiometric relationship between autotrophs and herbivores, attention has been largely focused on effects of the encountered mismatch between needs and supplies of an element on herbivore growth and ecosystem processes. Herbivore adaptation to poor food quality has rarely been investigated. This study presents a predictive model of feeding, assimilation, digestion and excretion of Daphnia facing a dietary deficiency in phosphorus. Biochemical compounds in the food were divided into phosphorous and non-phosphorus compounds. It was assumed that Daphnia is able to differently assimilate both types of compounds by regulation of target specific digestive enzymes. Feeding rate was regulated by optimal gut residence time of food particles, and assimilation efficiency by gut residence time and optimal secretion of both classes of gut enzymes. The model predicted the optimal strategy for a consumer facing an elementally imbalanced diet: (1) increase the ingestion rate, and (2) increase the secretion rate of both classes of gut enzymes. It resulted in decreased C and nutrient assimilation efficiencies, increased C feeding costs, and reduced growth rate. Sensitivity analysis showed that these predictions were qualitatively not influenced by parameter values. An alternative model was tested that includes an additive term allowing the direct excretion of C assimilated in excess. Results showed that this strategy is not optimal for the consumer growth rate. In conclusion, the model supports the hypothesis that carbon ingested in excess may generate energy that can be used to obtain more nutrients by increased feeding rate.     

Importance of primary metabolites in canola in mediating interactions between a specialist leaf-feeding insect and its specialist solitary endoparasitoid

The role of primary plant chemistry on trophic interactions is not well studied. We examined the effect of primary plant metabolites, focusing on nitrogen, on several biological indices of second and third trophic level insects in a model tritrophic system, consisting of two strains of the crucifer, Brassica napus (canola) (SLM₀₄₆ and RGS₀₀₃), the specialist insect herbivore Plutella xylostella (L.) (Lepidoptera: Plutellidae), and its specialist koinobiont larval-pupal parasitoid Diadegma semiclausum (Hellén) (Hymenoptera: Ichneumonidae). In particular, we measured relative growth rate of the herbivore in relation to an index for plant quality (nitrogen content of leaf tissues), developmental time of the herbivore (sum of second, third, and fourth larval instars durations), and intrinsic rate of increase (r _m ) of the herbivore and the parasitoid. Tritrophic studies were conducted on development, survivorship curve analysis, reproductive potential, life history, parasitism, and several other fitness correlates of the parasitoid. The life table parameters of D. semiclausum were determined under laboratory conditions. The intrinsic rate of increase (r _m ) of the parasitoid was significantly higher on RGS₀₀₃ than SLM₀₄₆. In this tritrophic model, the results indicated that the bottom-up direct effect on the herbivore population growth rate was marginally as strong as the direct effect of top-down force due to the parasitoid population growth rate; but it was higher than its indirect counterpoint mediated with the parasitoid population growth rate. Consequently, D. semiclausum performed better on RGS₀₀₃, which was the most inferior host to P. xylostella in comparison with another plant cultivar and had the lowest content of nitrogen in its leaves.     

Increased temperature alters feeding behavior of a generalist herbivore

Temperature can regulate a number of important biological processes and species interactions. For example, environmental temperature can alter insect herbivore consumption, growth and survivorship, suggesting that temperature‐driven impacts on herbivory could influence plant community composition or nutrient cycling. However, few studies to date have examined whether rising temperature influences herbivore preference and performance among multiple plant species, which often dictates their impact at the community level. Here, we assessed the effects of temperature on the performance and preference of the generalist herbivore Popillia japonica among nine plant species. We show that, on average, consumption rates and herbivore performance increased at higher temperatures. However, there was considerable variation among plant species with consumption and performance increasing on some plant species at higher temperatures but decreasing on others. Plant nutritional quality appeared to influence these patterns as beetles increased feeding on high‐nitrogen plants with increasing temperature, suggesting stronger nitrogen limitation. In addition to changes in feeding rates, feeding preferences of P. japonica shifted among temperatures, a pattern that was largely explained by differential deterrence of plant chemical extracts at different temperatures. In fact, temperature‐induced changes in the efficacy of plant chemical extracts led P. japonica to reduce its diet breadth at higher temperatures. Our results indicate that rising temperatures will influence herbivore feeding behavior by altering the importance of plant nutritional and chemical traits, suggesting that climate change will alter the strength and sign of plant–insect interactions.     

Thermoregulatory control of feeding and sleep in premature infants

Objective: The aim of the present study was to test the thermoregulatory feeding control hypothesis in sleeping, premature infants. Research Methods and Procedures: In premature infants, the energy supply from food intake is crucial for (in order of importance): organ operation, body homeothermia, and optimal growth. The Himms‐Hagen model of thermoregulatory feeding control involving activation of heat production by brown adipose tissue (BAT) was formulated on the basis of work in (awake) rats. This hypothesis has also been put forward for the human neonate, which can also use BAT to produce metabolic heat. According to the model, feeding episodes occur during a transient increase in body temperature. Feeding is initiated by a dip in blood glucose concentration after sugar uptake by activated BAT. Results: In 14 neonates (bottle‐fed on demand), food intake always took place during an increase in skin temperature (+0.19 ± 0.21 °C). Awakening occurred 18 ± 17 minutes after the minimum skin temperature level had been reached. When feeding time was imposed, feeding was not necessarily situated during an increase in skin temperature, and the sleep duration after food intake increased significantly (+43%). This could be considered as an adaptive response to the short‐term sleep deprivation and/or stress elicited by an imposed feeding rhythm. Discussion: The validity of the model supports the use of on‐demand feeding in neonatal care units, in accordance with the infant's physiological body temperature oscillations.     

Linking herbivore-induced defences to population dynamics

1. Theoretical studies have shown that inducible defences have the potential to affect population stability and persistence in bi‐ and tritrophic food chains. Experimental studies on such effects of prey defence strategies on the dynamics of predator–prey systems are still rare. We performed replicated population dynamics experiments using the herbivorous rotifer Brachionus calyciflorus and four strains of closely related algae that show different defence responses to this herbivore. 2. We observed herbivore populations to fluctuate at a higher frequency when feeding on small undefended algae. During these fluctuations minimum rotifer densities remained sufficiently high to ensure population persistence in all the replicates. The initial growth of rotifer populations in this treatment coincided with a sharp drop in algal density. Such a suppression of algae by herbivores was not observed in the other treatments, where algae were larger due to induced or permanent defences. In these treatments we observed rotifer population densities to first rise and then decline. The herbivore went extinct in all replicates with large permanently defended algae. The frequency of herbivore extinctions was intermediate when algae had inducible defences. 3. A variety of alternative mechanisms could explain differential herbivore persistence in the different defence treatments. Our analysis showed the density and fraction of highly edible algal particles to better explain herbivore persistence and extinctions than total algal density, the fraction of highly inedible food particles or the accumulation of herbivore waste products or autotoxins. 4. We argue that the rotifers require a minimum fraction and density of edible food particles for maintenance and reproduction. We conjecture that induced defences in algae may thus favour larger zooplankton species such as Daphnia spp. that are less sensitive to shifts in their food size spectrum, relative to smaller zooplankton species, such as rotifers and in this way contributes to the structuring of planktonic communities.     

Qualitative and Quantitative Interactions between Microphytobenthos and Herbivorous Meiofauna on a Brackish Intertidal Mudflat

Micro- and meiofauna are the predominant consumers of diatoms on a brackish intertidal mudflat. The impact of grazing on the benthic diatom populations was studied by field observations and feeding experiments on a few abundant members of the community. Only small fractions of the microphytobenthic biomass and production are converted by herbivores. A hypothesis is presented explaining the growth kinetics and productivity of diatom populations and the inefficient transfer of carbon into herbivore foodchains. Data on feeding rate and population dynamics of the nematode species, Eudiplogaster pararmatus, are discussed in view of the seasonal succession of edible diatom species.     

Dynamics of herbivores and resources on a landscape with interspersed resources and refuges

A tradeoff between energy gain from foraging and safety from predation in refuges is a common situation for many herbivores that are vulnerable to predation while foraging. This tradeoff affects the population dynamics of the plant–herbivore–predator interaction. A new functional response is derived based on the Holling type 2 functional response and the assumption that the herbivore can forage at a rate that maximizes its fitness. The predation rate on the herbivore is assumed to be proportional to the product of the time that the herbivore spends foraging and a risk factor that reflects the habitat complexity; where greater complexity means greater interspersion of high food quality habitat and refuge habitat, which increases the amount of the edge zone between refuge and foraging areas, making foraging safer. The snowshoe hare is chosen as an example to demonstrate the resulting dynamics of an herbivore that has been intensely studied and that undergoes well-known cycling. Two models are studied in which the optimal foraging by hares is assumed, a vegetation–hare–generalist predator model and a vegetation–hare–specialist predator model. In both cases, the results suggest that the cycling of the snowshoe hare population will be greatly moderated by optimal foraging in a habitat consisting of interspersed high quality foraging habitat and refuge habitat. However, there are also large differences in the dynamics produced by the two models as a function of predation pressure.     

Competitor avoidance drives within‐host feeding site selection in a passively dispersed herbivore

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