Intermediate Food Levels Research Articles

Effects of scarcity and excess of larval food on life history traits of Aedes aegypti (Diptera: Culicidae).

Few studies have assessed the effects of food scarcity or excess on the life history traits of Aedes aegypti (L.) (Diptera: Culicidae) independently from larval density. We assessed immature survival, development time, and adult size in relation to food availability. We reared cohorts of 30 Ae. aegypti larvae from newly hatched to adult emergence with different food availability. Food conditions were kept constant by transferring larvae each day to a new food solution. Immature development was completed by some individuals in all treatments. The shortest development time, the largest adults, and the highest survival were observed at intermediate food levels. The most important effects of food scarcity were an extension in development time, a decrease in the size of adults, and a slight decrease in survival, while the most important effects of food excess were an important decrease in survival and a slight decrease in the size of adults. The variability in development time and adult size within sex and treatment increased at decreasing food availability. The results suggest that although the studied population has adapted to a wide range of food availabilities, both scarcity and excess of food have important negative impacts on fitness.

Larval Survivorship and Settlement of Crown-of-Thorns Starfish (Acanthaster cf. solaris) at Varying Algal Cell Densities

The dispersal potential of crown-of-thorns starfish (CoTS) larvae is important in understanding both the initiation and spread of population outbreaks, and is fundamentally dependent upon how long larvae can persist while still retaining the capacity to settle. This study quantified variation in larval survivorship and settlement rates for CoTS maintained at three different densities of a single-celled flagellate phytoplankton, Proteomonas sulcata (1 × 103, 1 × 104, and 1 × 105 cells/mL). Based on the larval starvation hypothesis, we expected that low to moderate levels of phytoplankton prey would significantly constrain both survival and settlement. CoTS larvae were successfully maintained for up to 50 days post-fertilization, but larval survival differed significantly between treatments. Survival was greatest at intermediate food levels (1 × 104 cells/mL), and lowest at high (1 × 105 cells/mL) food levels. Rates of settlement were also highest at intermediate food levels and peaked at 22 days post-fertilization. Peak settlement was delayed at low food levels, probably reflective of delayed development, but there was no evidence of accelerated development at high chlorophyll concentrations. CoTS larvae were recorded to settle 17–43 days post-fertilization, but under optimum conditions with intermediate algal cell densities, peak settlement occurred at 22 days post-fertilization. Natural fluctuations in nutrient concentrations and food availability may affect the number of CoTS that effectively settle, but seem unlikely to influence dispersal dynamics.

The optimal combination of standard metabolic rate and aerobic scope for somatic growth depends on food availability

Summary Metabolic rates can vary as much as threefold among individuals of the same size and age in a population, but why such variation persists is unclear given that they determine the energetic cost of living. Relationships between standard metabolic rate (SMR), growth and survival can vary with environmental conditions, suggesting that the fitness consequences of a given metabolic phenotype may be context‐dependent. Less attention has focused on the link between absolute aerobic scope (AS, the difference between standard and maximum metabolic rate) and fitness under different environmental conditions, despite the importance of aerobic scope to an organism's total energetic capacity. We examined the links between individual variation in both SMR and AS and somatic growth rates of brown trout (Salmo trutta) under different levels of food availability. Standard metabolic rate and AS were uncorrelated across individuals. However, SMR and AS not only had interactive effects on growth, but these interactions depended on food level: at ad libitum food levels, AS had a positive effect on growth whose magnitude depended on SMR; at intermediate food levels, AS and SMR had interactive effects on growth, but at the low food level, there was no effect of either AS or SMR on growth. As a result, there was no metabolic phenotype that performed best or worst across all food levels. These results demonstrate the importance of aerobic scope in explaining somatic growth rates and support the hypothesis that links between individual variation in metabolism and fitness are context‐dependent. The larger metabolic phenotype and the environmental context in which performance is evaluated both need to be considered in order to better understand the link between metabolic rates and fitness and thereby the persistence of individual variation in metabolic rates.

Growth performance of the rotifers Brachionus plicatilis, B. ‘Nevada’ and B. ‘Cayman’ under different food concentrations

Numerical response experiments were conducted using three strains of the Brachionus plicatilis species complex. They are commonly used in aquaculture, and could also coexist in nature although this has not been studied yet. Brachionus plicatilis sensu stricto (s.s.), B. ‘Cayman’ and B. ‘Nevada’ were cultured at different concentrations of Nannochloropsis oculata (0–40 mg C l−1). Growth rate, egg female ratio (EF), fecundity and percentage of ovigerous females were determined at each food concentration. From the growth rate and EF, the egg development time and mortality rate were estimated using the Paloheimo equation. Monod curves with a threshold for zero growth described the relationship between growth rate and food concentration. The three strains had different growth rates at low (<0.04 mg C l−1) and high (>5.0 mg C l−1) food concentrations, but were similar at intermediate food levels. B. ‘Cayman’ had the highest maximum growth rate (1.57±0.07 d−1), whereas B. plicatilis s.s. had the less negative minimum growth rate and the highest maximum EF (0.96±0.02 eggs ind−1). These differences were attributed to the lowest egg development time of B. ‘Cayman’ (5.95±0.24 h) and the higher starvation resistance of B. plicatilis s.s. (84±12 h), respectively. B. ‘Nevada’ was the inferior competitor at high and low food concentrations due to the higher investment in body growth rather than reproduction. The outcome of competition in hatcheries is predicted to favor B. ‘Cayman’ under high food concentrations, whereas the three species may coexist under moderate food limitation.

AN EXPERIMENTAL TEST OF FRAGMENTATION AND LOSS OF HABITAT WITH ORYZAEPHILUS SURINAMENSIS

This research used a model experimental system to explore the joint effects of resource fragmentation and resource deterioration on population dynamics. The system provides a rapid test of conservation theory and can aid in planning for large and expensive studies. The investigation subjected the sawtooth grain beetle, Oryzaephilus surinamensis (L.), to five experiments in which population abundance was monitored over time. Metapopulations were created by connecting adjacent vials of flour with rubber tubes. The metapopulations were arranged in rings, and food was replenished at two-week intervals. The experiments examined populations in (1) habitats becoming increasingly fragmented, (2) dynamically fragmenting structure with concurrent loss of resources over time, (3) static structure, dynamic resource loss and differing numbers of patches, (4) static structural arrangements with dynamic resource loss that mimic resource deterioration and destruction, and (5) static arrangements of connected fragments with an equal total amount of food in a few large patches or several small patches. Time series of stage-specific abundances were compared in repeated-measures ANOVA. The results show that populations respond in nonlinear ways to both the amount of food and area provided. Within a confined area, when food is plentiful, individual interaction limits population abundance. When the amount of food is reduced, population abundance becomes food dependent. Immediate increase in immature abundance was a characteristic of fragmentation while a delayed reduction of adults was characteristic of food reduction. The stage-specific interaction at intermediate food levels shows birth and death processes that control this system, and elucidates the relationship between area and food that is integral to the dynamics of patchy populations.

Influence of food abundance on individual behaviour strategy and growth rate in juvenile brown trout (Salmo trutta)

We studied the benefit of being territorial as an effect of food abundance by measuring the proportions of individuals that displayed a territorial, floating (individuals occasionally displayed territorial behaviour), or nonterritorial (shoaling) behaviour strategy, and individual growth rates. Also, swimming activity was monitored as an indicator of emigration. Replicate groups of 12 juvenile brown trout (Salmo trutta) were released into an artificial stream channel, fed according to one of four food regimes for 10 days, and allocated to one of three behaviour categories. There was no significant relationship between food abundance and the number of territorial individuals. Instead, the proportions of individuals that displayed the alternative behaviour strategies, i.e., floating and nonterritorial, changed with food abundance. At the lowest food level, more individuals displayed nonterritorial than floating behaviour, but these two strategies were equally represented at the highest food level. The difference in growth rates with respect to behaviour category was highest at intermediate food levels. At the highest and lowest food levels, nonterritorial fish and floaters grew nearly as fast or as poorly, respectively, as the territorial individuals. Swimming activity between the two sections of the stream tank was greatest at the lowest food level, and there was a general tendency (not significant) for the floaters to exhibit the greatest activity.

Intraspecific competition in a social spider

An individual-based model was developed to investigate the population dynamics of a permanently-social spider, Stegodyphus dumicola (Eresidae), in relation to intraspecific competition for food. This further develops our initial model that demonstrated that individual variability in body mass causes contest competition and is based on it. This confirmed field observations. In the present study, we examined whether intraspecific contest competition for food could be a mechanism for density regulation in S. dumicola. We investigated the consequences of contest competition on the individual variability in body mass as well as on the individual fitness of female spiders. Results demonstrate that colony size stabilises above a threshold of food level. At low food levels the risk of colony extinction is high because only few individuals survive and reproduce. Contest competition for food leads to high variance in body mass under conditions of food scarcity. Those variances tend to decrease with increasing food resources and a threshold of food level separates ranges of high and low variances. Model results indicate that individual females differ significantly by their fitness. An optimum of fitness was found at intermediate food levels. Results suggest that contest competition is a mechanism of density regulation in spider societies which stabilises colony size as well as variability in body mass and fitness.

Egg size, offspring quality and food level in planktonic rotifers

1. For two species of planktonic rotifers, Brachionus calyciflorus and Synchaeta pectinata, neonates from larger eggs were more resistant to starvation than neonates from smaller eggs, so egg size was positively correlated with offspring quality.2. Theories of optimal offspring size in zooplankton predict either a decrease in offspring size with increasing food level, or maximum offspring size at intermediate food levels. In experiments using acclimated, even‐aged individuals, neither rotifer species changed its egg size as predicted by theory. B. calyciflorus egg size increased with increasing food concentration, while S. pectinata egg size did not change.3. The lack of fit to theory may have occurred because of constraints on minimum or maximum egg size, constraints on total reproductive effort, or because offspring fitness is influenced by traits in addition to neonate starvation time.

The Allocation of Resources to Reproduction in Daphnia Galeata: Against the Odds?

Variation in offspring is a size common phenomenon in many organism. In cladoceran zooplankton large offspring are known to have a high starvation resistance. One could, therefore, expect offspring to be large at low food levels, whereas at higher food levels the production of more, but smaller, offspring would yield the highest parental fitness. However, in Daphnia galeata I found that individual offspring were smallest at a low food level and largest at intermediate food levels. Moreover, in contrast to the predictions made by several theoretical models, I found that larger mothers produce larger offspring. The first discrepancy between data and theory could be explained by the existence of a maximum offspring size, combined with the difficulty of Daphnia to produce one egg less than was done in reality. The dependence of offspring size on maternal size could be explained by the higher likelihood of intraspecific competition when large (=old) females are present, and hence higher starvation risks for the offspring. It was found that embryonic respiration was lowest at the lower food levels and hence the smaller individual egg size seemed to be compensated by lower carbon losses during development.

Behavioural Aggregation of Daphnia pulex in Response to Food Gradients

1. Natural populations are heterogeneously distributed in space and often form dense local aggregations. 2. Here, we examine the ability of the freshwater zooplankter Daphnia pulex to modify its spatial position in response to food gradients that span the natural range encountered in lakes and ponds. 3. When presented with a food gradient, individuals chose to feed at the high-food end of gradients whose upper range is typical of field conditions, and they avoided the high-food end of very «high-food» gradients that are atypical of the field. 4. In addition, the degree to which individuals aggregated along the gradient varied in a non-linear manner with overall food concentration. The strongest aggregative response was obtained at intermediate food levels typical of the field

Studies on the functional role of tintinnids in the Southern California Bight. I. Grazing and growth rates in laboratory cultures

Five species of tintinnids, Amphorella quadrilineata (Claparede and Lachmann, 1858), Tintinnopsis cf. beroidea Stein (1867), T. cf. acuminata Daday (1887), Eutintinnus pectinis (Kofoid and Campbell, 1929), and Helicostomella subulata (Ehrenberg, 1833), were isolated from Southern California coastal waters and maintained in laboratory cultures which were used to investigate, several aspects of the feeding dynamics and population growth rates of this group of planktonic ciliates. Data are most complete for the latter three species. Both E. pectinis and H. subulata displayed ingestion and growth rates which increased with increasing food concentration until a maximum rate was obtained which then remained essentially constant as food levels increased further. Maximum hourly ingestion was equivalent to approximately 10 to 20% of the body weight of the tintinnids. T. cf. acuminata showed no such maximum ingestion rate, as ingestion rates increased throughout the entire range of food concentrations used. The dependence of growth rate of T. cf. acuminata on food concentration also differed from the other species, being characterized by a broad region of maximum growth at intermediate food levels with reduced growth at both lower and higher food concentrations. Maximum observed growth rates represented doubling times of approximately 12 h for both E. pectinis and T. cf. acuminata and 24 h for H. subulata. The data suggested gross growth efficiencies exceeding 50% over much of the range of food concentrations used. No strong evidence supporting the existence of feeding thresholds or “switching” behavior was collected during these experiments, although apparently selective feeding was observed in one experiment with E. pectinis.

FOOD LEVEL IN RELATION TO RATE OF DEVELOPMENT AND EYE PIGMENTATION IN DROSOPHILA MELANOGASTER

In the development of larvae of Drosophila melanogaster an organizational change takes place at about 70 hours after egg-laying (25° C.). This change is referred to as the "70-hour change."Larvae deprived of all food for periods at any time up to the 70-hour change are retarded in development, the retardation being a function of the length of the starvation period.Larvae deprived of food after the 70-hour change are not retarded, but are probably somewhat accelerated in development.If deprived of food during a portion of the larval developmental period after the 70-hour change, the final size attained by the fly appears to depend largely on the proportion of this period spent on full food.Periods of complete removal of food from vermilion brown larvae are without apparent effect on eye pigment production. This is true whether the starvation period be short or long and whether it comes before or after the 70-hour change.Partial starvation (at intermediate food levels) of larvae, under certain conditions, greatly increases the amount of eye pigment formed by vermilion brown flies.The period of development most sensitive to this "starvation effect" on pigment production by the eye, lies shortly before the 70-hour change.The production of a supposedly specific eye color hormone, v+ hormone, can be influenced greatly by the amount of food given to vermilion brown larvae during a limited period of development, the period of maximum sensitivity referred to above. This increase is not less than a hundredfold.