Rate Of Net Gain Research Articles

Modes of Berm and Beachface Recovery Following Storm Reset: Observations Using a Continuously Scanning Lidar

AbstractFollowing the rapid and destructive impacts of storm erosion, beach recovery is a key natural process of restoration, returning eroded sediment to the subaerial beach and rebuilding coastal morphology. While the effects of storm erosion have commonly been investigated, detailed studies into poststorm recovery are currently lacking. This study investigates wave‐driven recovery processes of the berm and beachface on a microtidal, swash‐aligned sandy beach. Following complete removal of the berm by a significant storm event, the entire 76‐day rebuilding of a swash berm is analyzed at the timescale of every semidiurnal tidal cycle, utilizing high‐resolution (5 Hz) swash and subaerial beach profile measurements from a continuously scanning fixed lidar. Tide‐by‐tide rates of subaerial volume change during berm recovery were most frequently observed between 1 and 2 m3/m/day, including losses and gains an order of magnitude larger than the more gradual rate of net gain (0.7 m3/m/day) observed for the entire recovery period. Patterns of berm crest formation and vertical growth were found to be primarily governed by the neap‐spring tide variations in total water levels. Tide‐by‐tide beachface and berm volume changes were used to classify four principal behavioral modes of subaerial profile variability during recovery. Using decision tree classification, modes were differentiated according to nearshore dimensionless fall velocity, swash exceedance of the berm crest, and ocean water levels. The findings provide novel behavioral and parametric insight into the tide‐by‐tide rebuilding of the beachface and berm by swash throughout a complete poststorm recovery period.

The cost of disturbance: a waste of time and energy?

Quantifying the effect of disturbance is a central issue in conservation. Using time and energy budgets, we obtain a range of ways to assess the importance of disturbance. One measure is the time that must be spent foraging in order to balance the energy budget. From this we derive critical levels of wastage (rate of disturbance multiplied by duration of disturbance) at which the animal runs out of time or reaches a limit on energy expenditure. In the case of the time constraint, the critical wastage is the net rate of energetic gain while foraging divided by the rate of energetic expenditure during a disturbance. The associated critical rate of disturbance is the net rate of energetic gain while foraging divided by the energy spent during a disturbance. The model is illustrated using data from the African wild dog, which suffers disturbance from lions and kleptoparasitism from hyenas. Findings suggest that disturbance imposes significant costs on wild dog time and energy budgets. We show how alternative environments can be evaluated in terms of their effective rate of gain, which is the net rate of gain from foraging minus the rate of energy expenditure as a result of disturbance.

Evolutionary models of metabolism, behaviour and personality.

I explore the relationship between metabolism and personality by establishing how selection acts on metabolic rate and risk-taking in the context of a trade-off between energy and predation. Using a simple time budget model, I show that a high resting metabolic rate is not necessarily associated with a high daily energy expenditure. The metabolic rate that minimizes the time spent foraging does not maximize the net gain rate while foraging, and it is not always advantageous for animals to have a higher metabolic rate when food availability is high. A model based on minimizing the ratio of mortality rate to net gain rate is used to determine how a willingness to take risks should be correlated with metabolic rate. My results establish that it is not always advantageous for animals to take greater risks when metabolic rate is high. When foraging intensity and metabolic rate coevolve, I show that in a particular case different combinations of foraging intensity and metabolic rate can have equal fitness.

Floral Temperature and Optimal Foraging: Is Heat a Feasible Floral Reward for Pollinators?

As well as nutritional rewards, some plants also reward ectothermic pollinators with warmth. Bumble bees have some control over their temperature, but have been shown to forage at warmer flowers when given a choice, suggesting that there is some advantage to them of foraging at warm flowers (such as reducing the energy required to raise their body to flight temperature before leaving the flower). We describe a model that considers how a heat reward affects the foraging behaviour in a thermogenic central-place forager (such as a bumble bee). We show that although the pollinator should spend a longer time on individual flowers if they are warm, the increase in total visit time is likely to be small. The pollinator's net rate of energy gain will be increased by landing on warmer flowers. Therefore, if a plant provides a heat reward, it could reduce the amount of nectar it produces, whilst still providing its pollinator with the same net rate of gain. We suggest how heat rewards may link with plant life history strategies.

The Costs of Carnivory

Mammalian carnivores fall into two broad dietary groups: smaller carnivores (<20 kg) that feed on very small prey (invertebrates and small vertebrates) and larger carnivores (>20 kg) that specialize in feeding on large vertebrates. We develop a model that predicts the mass-related energy budgets and limits of carnivore size within these groups. We show that the transition from small to large prey can be predicted by the maximization of net energy gain; larger carnivores achieve a higher net gain rate by concentrating on large prey. However, because it requires more energy to pursue and subdue large prey, this leads to a 2-fold step increase in energy expenditure, as well as increased intake. Across all species, energy expenditure and intake both follow a three-fourths scaling with body mass. However, when each dietary group is considered individually they both display a shallower scaling. This suggests that carnivores at the upper limits of each group are constrained by intake and adopt energy conserving strategies to counter this. Given predictions of expenditure and estimates of intake, we predict a maximum carnivore mass of approximately a ton, consistent with the largest extinct species. Our approach provides a framework for understanding carnivore energetics, size, and extinction dynamics.

Self-organization in collective honeybee foraging: emergence of symmetry breaking, cross inhibition and equal harvest-rate distribution

De Vries and Biesmeijer described in 1998 an individual-oriented model that simulates the collective foraging behaviour of a colony of honeybees. Here we report how this model has been expanded and show how, through self-organization, three colony-level phenomena can emerge: symmetry breaking, cross inhibition and the equal harvest-rate distribution. Symmetry breaking is the phenomenon that the numbers of foragers visiting two equally profitable food sources will diverge after some time. Cross inhibition is the phenomenon that, by increasing the profitability of one of two equal food sources, the number of foragers visiting the other source will decrease. In some circumstances, the bees foraging on two sources of different profitabilities will be distributed between these sources such that the two average energy harvest rates are equal. We will refer to this phenomenon as the equal harvest-rate distribution. For each of these three phenomena, we show what the necessary behavioural rules to be followed by the individual forager bees are, and what the necessary circumstances are (that is, what values the model parameters should take) in order for these phenomena to arise. It seems that patch size and forager group size largely determine when each of these phenomena will arise. Experimenting with two types of currency, net gain rate and net gain efficiency, revealed that only gain rate may result in an equal harvest-rate distribution of foragers visiting different food sources.

To walk or to fly? How birds choose among foraging modes.

We test the predictive value of the main energetic currencies used in foraging theory using starlings that choose between two foraging modes (walking versus flying). Walking is low-cost, low-yield, whereas flying is the opposite. We fixed experimentally, at 11 different values, the amount of flight required to get one food reward, and for each flight cost value, we titrated the amount of walking until the birds showed indifference between foraging modes. We then compared the indifference points to those predicted by gross rate of gain over time, net rate of gain over time, and the ratio of gain to expenditure (efficiency). The results for the choice between modes show strong qualitative and quantitative support for net rate of gain over time over the alternatives. However, the birds foraged for only a fraction of the available time, indicating that the choice between foraging and resting could not be explained by any of these currencies. We suggest that this discrepancy could be accounted for functionally because nonenergetic factors such as predation risk may differ between resting and foraging in any mode but may not differ much between foraging modes, hence releasing the choice between foraging modes from the influence of such factors. Alternatively, the discrepancy may be attributable to the use of predictable (rather than stochastic) ratios of effort per prey in our experiment, and it may thus be better understood with mechanistic rather than functional arguments.

Maternal nutrition and spontaneous preterm birth.

Previous studies suggesting that maternal undernutrition increases the risk of preterm birth have suffered from several methodological shortcomings, including use of total gestational weight gain rather than net rate of gain in maternal tissue, inclusion of induced preterm deliveries, and error-prone gestational age measurements based solely on menstrual dates. The authors have attempted to overcome these shortcomings by investigating the potential etiologic roles of prepregnancy body mass index, net rate of maternal weight gain, height, and a number of other potential biological and sociodemographic determinants of spontaneous (i.e., noninduced) preterm birth in a cohort of 13,102 women with early ultrasound-confirmed gestational age who delivered at the Royal Victoria Hospital in Montreal, Quebec, Canada, between January 1, 1980 and March 31, 1989. Total weight gain, but not body mass index, was highly significantly associated with spontaneous preterm birth, averaging 14.6, 12.5, 9.9, and 9.1 kg, in women delivering at 37 or more, less than 37, less than 34, and less than 32 completed weeks, respectively. Although the relation persisted when weight gain was expressed as an overall rate, it disappeared when the analysis was based on net rate; mean net rates of gain were 0.28, 0.29, 0.27, and 0.27 kg/week, respectively. On the basis of multiple logistic regression analyses, significant determinants of birth at less than 37 weeks included maternal short stature; noncompletion of high school; unmarried status; smoking; diabetes; urinary tract infection within 2 weeks of delivery; prepregnancy hypertension; severe pregnancy-induced hypertension; and previous history of preterm delivery, low birth weight, or neonatal death. Most of these factors retained their significance for birth at less than 34 and less than 32 weeks. In fact, the effect of low maternal education was even stronger at these more severe "levels" of preterm birth. The authors conclude that prepregnancy weight-for-height and gestational weight gain are not important determinants of spontaneous preterm birth and that some previous studies have mistaken an effect of shortened gestation for its cause. Other biologic and social determinants, however, indicate priorities for future research and intervention.

Stimulation of nitrogen and whole-body protein metabolism in growth hormone-deficient children by recombinant human growth hormone: relationship to growth.

The effect of a mammalian-cell-derived recombinant human growth hormone (rhGH) on nitrogen and whole-body protein metabolism was assessed in 12 children with complete growth hormone (GH) deficiency. All the patients received single oral doses of 15N-glycine (95 atom % 15N), 20 mg/kg body weight, prior to and following 7 days of treatment with rhGH, 1.7 IU/m2 body surface area (BSA) per day, administered subcutaneously. Prior to rhGH, mean urinary 15N-nitrogen excretion was 42.8 +/- 8% of the administered dose, which fell significantly to 22.8 +/- 7% during rhGH administration (p < 0.0001). Stimulation of protein metabolism by rhGH resulted in a protein net gain rate of 1.1 +/- 0.4 g/kg/day, which was significantly higher than the 0.6 +/- 0.5 g/kg/day rate seen prior to rhGH (p < 0.001). In patients subsequently placed on daily subcutaneous injections of rhGH 1.7 IU/m2 BSA, mean height velocity standard deviation score (HV SDS) for chronological age significantly increased from -3.8 +/- 2.6 to +8.5 +/- 3.1 and +3.3 +/- 2.2, during the 1st and 2nd years of treatment, respectively. However, there was no correlation between the long-term response to rhGH treatment and the short-term changes in nitrogen or protein metabolism in GH-deficient children.

Foraging profits and thoracic temperature of honey bees (Apis mellifera)

Body temperature and duration of foraging activities were affected by the concentration of sucrose solution imbibed. When experienced foragers of Apis mellifera arrived at a gravity feeder from the hive, thoracic temperature (TTH) was independent of sucrose concentration (X = 36.3 °C). While imbibing 40% and 60% (g solute per g of solution) solutions bees maintained TTH at approximately the same high level as upon arrival, but those imbibing 10%, 20%, and 30% solutions regulated TTH lower (X = 33.5 °C). All bees departed the feeder for the hive at the same TTH (X = 36.1 °C). Bees that imbibed 40% and 60% solutions sometimes immediately took flight after imbibition and averaged less than 15 s to takeoff. Time to takeoff was 2–3 times longer for bees that had imbibed 10% and 20% solutions because warmup preceded takeoff. The rate of energy expenditure at TTH=36.3°C (at 40% and 60% solutions) was 20% greater than that at 33.3°C (at 10%, 20%, and 30% solution). Bees that fed on the highly concentrated solutions regulated TTH so that rate of net energy gain was enhanced, but bees that fed on less concentrated solutions could have increased rate of net gain by maintaining a higher TTH which would have reduced time required for takeoff. The latter bees lowered rate of expenditure of their limited energetic costs and thereby lowered short-term net profits in favor of improved long-term contribution to the colony.

Foraging Strategy, Worker Mortality, and the Growth of the Colony in Social Insects

Models of optimal foraging often assume that maximizing the long-term net rate of gain is equivalent to maximizing fitness. We present a model that challenges this view on theoretical grounds for foragers from colonies of social insects. To maximize the number of reproductive forms produced by a colony, the production of new workers should be at a maximum during the ergonomic stage of a colony's development. However, when the mortality rate of workers depends on foraging performance, then maximization of the net rate no longer maximizes fitness. Under such conditions, the model predicts deviations from net-rate maximization (gain per unit of time) that may resemble the maximization of efficiency (gain per metabolic expenditure), as found in our earlier studies.

Population structure of tree species in forests as an indicator of regeneration and future stability

The size class distribution in tree populations will continue to be used for analysing the regeneration of tree species and successional patterns in forest communities at extensive levels. However, information regarding the approximate estimates of lifetime expectancy (residence time) for each of the size classes, viz seedlings, saplings and trees, and the estimates of rate of net gain for them (population divided by residence time) can considerably enhance our understanding of the regeneration trends of species. It is shown that several population structures, based on size class distribution may indicate declining population trend, although the population, as indicated by rate of net gain parameter, in fact may foretell future expansion or stability. It appears that high species richness and high percentages of species with poor regeneration are interrelated with each other.

Load size selection by foraging leaf‐cutter ants (Atta cephalotes)

ABSTRACT. Velocity of load‐carrying Atta cephalotes (L.) foragers increases with increasing ant size and decreasing load size. Foragers are selective in the sizes of loads they carry, but heavier loads would apparently increase their rate of leaf transport to the nest (mg of leaf m s−1). Even for very thin leaves, leaf diameter is not correlated with ant body size despite the method of cutting (rotating around a fixed point on the leaf edge). When cutting leaves of different densities, load mass is more closely matched to ant size than is load surface area. This implies that ants choose loads based on mass rather than surface area, and thus the several possible disadvantages associated with carrying loads of large surface area (e.g. increased disturbance by wind or rain) are unlikely explanations of why ants do not select larger loads. The relationship beween forager size and load size is made more complex by further selectivity at the level of colony recruitment: larger ants recruit to higher‐density (thicker) leaf types. Gross leaf transport rate is not maximized by foraging A.cephalotes, but net rate of energy intake cannot be assumed to follow the same pattern. If costs/time (not measured) are constant with changing load size, then the net rate of energy intake is not maximized. An alternative hypothesis is that costs/time increase with larger loads, thereby decreasing net rate of gain for larger loads.

Central-place foraging in honey bees: the effect of travel time and nectar flow on crop filling

Crop-filling by honeybees foraging at sources of variable nectar flow at a fixed distance from the hive has been shown to maximize energetic efficiency, defined as ratio of energy gained to energy spent. Predictions based on maximisation of rate of energy gain, defined as net energy gained per unit time foraging, are significantly different from observed behaviour (Schmid-Hempel et al. 1985). In this paper we consider the effect of varying travel times in addition to flow rate. The predictions of an extended version of our theoretical model are confronted with experimental results obtained by Nunez (1982). Nunez found that bees filled their crops more fully for higher flows and longer travel times. We show that when the cost of carrying a load is considered, this trend can be predicted by maximising either energetic efficiency or net rate of gain. Figure 1 shows, however, that maximisation of net rate of gain can only produce an acceptable quantitative fit if unreasonably high costs are assumed to result from carrying the load. Energetic efficiency instead generates a good quantitative fit for acceptable assumptions about this cost (Fig. 2).

Chemical Reactor Analysis and Optimal Digestion

of energy and nutrient gain. According to the principles of dynamic programming, a method for formulating and solving optimization problems (Bellman 1957), the entire process is optimized only if digestion follows an optimal path constrained by the food items actually ingested. An animal feeding on a variety of foods should thus exhibit the flexibility to adjust digestion with respect to that range of food types. Conversely, the net rate of gain to an animal lacking such flexibility will be maximized over a restricted set of food types, making it a dietary specialist. To test these general predictions and to formulate more specific ones, we must identify the operating variables for each stage. Many foraging theories exist (e.g., Charnov 1976, Orians and Pearson 1979, Pyke 1984, Schoener 1971), but there are only rudiments of an optimal digestion theory (Milton 1981, Sibly 1981, Taghon 1981, Troyer 1984). Without such a theory, studies of digestive systems (e.g. gut anatomy, histology, or enzymology) tend to be static, piecemeal, and most often nonpredic-

Components of Manufacturing Employment Change in Ireland 1973-1981

Employment change in Irish manufacturing industry is disaggregated into its basic components of openings, expansions, contractions and closures over the period 1973 to 1981. The components of change are cross-classified at plant level by ownership, grant type, size and sector and this shows that rates of indigenous new firm formation are higher in Ireland than in either of the UK regions. Comparisons of employment change by plant size with the US and the UK reveals that although the direction of change is the same for Irish manufacturing with only plants of below 100 employed recording net increases, the rate of net gain achieved by small plants in Ireland is greater than in either the US or the UK.

A PHYSIOLOGICAL INTERPRETATION OF THE DYNAMIC RESPONSES OF POPULATIONS OF A PLANKTONIC DIATOM TO PHYSICAL VARIABILITY OF THE ENVIRONMENT

SummaryThe dynamic responses of Fragilaria crotonensis populations to artificial variations in the mixed depth of the water column of a Lund Tube (Enclosure C, Blelham Tarn) are shown to conform to a descriptive model involving only two environmental variables: mixed depth and optical depth. The physiological interpretation of these responses invokes a second model, relating the rates of net population change to photosynthetic productivity, corrected for respiration, and to in situ sinking loss rates. Population increases coincide with mixing episodes, when it is shown that the maximum photosynthetically‐derived growth rate exceeds the sinking low rate by an amount similar to the observed rate of net gain. Abrupt declines occur when the column rapidly restratifies, at rates explicable in terms of accelerated sinking and sinking‐loss rates, i.e. that there is also an abrupt cessation in growth. Evidence that this effect is due to extreme near‐surface photoinhibition is presented. The ecology of Fragilaria populations is briefly discussed in relation to these findings.

Why Black Goats in Hot Deserts? Effects of Coat Color on Heat Exchanges of Wild and Domestic Goats

Black goats are the most abundant domestic animal in the Negev and Sinai deserts. These goats possess a number of complex physiological adaptations for dealing with a hot desert environment. It seemed possible that their black coat might also be an adaptation for this environment, reducing gain and the use of water for evaporative cooling. To test this possibility, we compared the rates at which black goats, white goats, and ibex (a wild goat native to the desert) gained and evaporated water in a hot desert at midday. Two methods were used simultaneously to calculate the net rate of gain by radiation and convection (Ḣgain): a heat balance and a heat flux method. The balance method involved measuring rates of production, storage, and evaporative loss. Ḣgain is the sum of rates of evaporative loss plus storage minus metabolic production. The flux method involved calculating avenues of non-evaporative exchange (radiation, conduction, and ...

Radiant energy transfer in waters

A model is presented for predicting the local radiant flux and the net rate of gain or loss of radiant energy per unit volume on spectral and total bases for a layer of natural water by using exact radiative transfer theory. The model considers attenuation (absorption and scattering) and multiple scattering by water and accounts not only for the transmission and internal reflection of radiation at the air‐water interface but also for reflection at the bottom. Some numerical results are reported for pure water, multiple scattering being neglected. The results show that analyses that neglect the spectral nature of the incident flux and assume water to be gray lead to appreciable errors in total radiant flux computations and in the amount of internal absorption of solar radiation. For accurate predictions, the reflection from the bottom and the internal reflection from the air‐water interface must be properly accounted for in shallow waters.