Effects of food diversity on diatom selection by harpacticoid copepods

The diversity of species assemblages which occupy a basal position in the trophic pyramid (typically unicellular algae in aquatic environments) is known to influence the interaction with organisms of higher trophic levels. A laboratory feeding experiment was conducted with cultures of three benthic diatom species (Navicula phyllepta, Grammatophora marina and Cylindrotheca closterium) as primary producers and three harpacticoid copepod species (Harpacticus obscurus, Paramphiascella fulvofasciata and Tigriopus brevicornis) as grazers to evaluate the effects of food diversity (and concomitant food availability) on grazing selectivity. This kind of selectivity experiments is singular for benthic harpacticoid copepods as so far, information on food selection of harpacticoids is scarce.Uptake of a unispecific food source by a single copepod species decreased as food diversity (and concomitant overall food concentration) increased. All three consumers reacted similarly to changing food diversity, but exhibited strong species-specific responses to food identity i.e. which diatom was added was crucial. Irrespective of level of food diversity, H. obscurus took up high amounts of G. marina, whereas both P. fulvofasciata and T. brevicornis preferred C. closterium when given the choice between different diatoms. As for zooplanktonic taxa, this experiment showed that in lower benthic marine food webs both prey organisms (primary producers) and grazers play a very specific role. Diversity of food and its identity are of critical importance at the base of the trophic pyramid, influencing trophic transfer from primary producers over grazers to higher trophic levels.     

Food selection by calanoid copepods in response to between-lake variation in food abundance

1. Food selection experiments were conducted by acclimating calanoid copepods (Eudiaptomus spp.) in suspensions of natural seston and then adding pairs of dual-labelled (¹⁴C/³²P) algae. Each feeding trial measured selectivity between a small, high-quality reference alga, Chlamydomonas reinhardii, and a test alga that differed in size and/or food quality. The influence of food concentration on food selection was tested by using seston from two lakes with contrasting food abundance and by including treatments with filtered lake water ('starved’) and seston diluted with filtered water or enriched with cultured algae. 2. Copepods that had been starved or acclimated to natural seston with low food abundance preferred the larger of two labelled algae, regardless of the nutritional quality of the algae. In agreement with the predictions of an optimal diet model, however, copepods that were acclimated to high food conditions discriminated against low-quality foods, including digestion-resistant algae and dead algae. 3. Selectivity coefficients showed excellent agreement with a previous study involving the same taxa of copepods and labelled algae but in which the copepods had been acclimated to pairs of cultured algae rather than natural seston. Thus, these comparisons emphasize the importance of food availability in modulating copepod selectivity for foods that differ in nutritional quality and suggest that such behaviour occurs in nature.     

Flower choice in honeybees: Effects of food value and flower density

To test the effects of food value on the flower choice, individual honeybees (Apis mellifera) were offered a choice of 25 % sucrose solution (SS) and 1 of 6 different SSs, ranging from 5 % to 50 % SS, at either a low or a high flower density. Artificial flowers were filled with each SS. The honeybees showed a stronger preference for a concentrated SS to a diluted SS at a high than at a low flower density, and the degree of preference was positively correlated to the difference in the sucrose concentration between paired SSs. These foraging patterns were consistent with qualitative predictions from optimal foraging theory. Furthermore, it was found that experience in feeding on a concentrated SS lowered the foraging motivation for a diluted SS at the high flower density, but not at the low flower density. I discuss the effects of food density, food profitability and experience on the foraging behaviour of honeybees.     

Frequency-dependent food selection by arthropods: a review

We review published studies in which arthropods were presented with a choice of food types at a range of relative densities. Twenty-four of 28 studies showed frequency-independent preference; 11 studies also showed frequency-dependent preference. Both pro- and anti-apostatic selection were found. In the light of these studies we discuss possible behavioural mechanisms which produce such selection; the potential benefits to the predators and the consequences of such selection for diversity both within and between species.     

Food selection and consumption by estuarine nematodes

Summary The feeding strategies and feeding techniques of 12 nematode species, isolated from the Ems-Dollard estuary, were investigated in agar cultures. In the consumption of bacteria, algae, diatoms, protozoa and small metazoa, two main strategies are distinghuished: the non-selective strategy, characteristic of species living on the surface of macrophytes, and the selective strategy, characteristic of sediment-inhabiting species. The selective strategists showed various ingestion techniques, depending on the size and armature of the buccal cavity; food items could be ingested whole, or pierced or cracked and the contents sucked out.Publication No. A 73 of the project Biological Research in the Ems-Dollard estuary.     

Learning by grasshoppers: association of colour/light intensity with food

ABSTRACT. Nymphs of Melanoplus sanguinipes learn to associate food with visual cues such as colour or light intensity. Such an ability in active exploratory insects may improve foraging efficiency by reducing time spent searching in unsuitable areas. Significant effects are demonstrated after a single rewarded trial, and the effects are extinguished after a variable number of unrewarded trials.     

Cannibalistic interactions in two co-occurring decapod species: Effects of density, food, alternative prey and habitat

Cannibalism is a potentially important factor in the regulation of populations in a range of habitats. The intensity of this biotic factor may be determined by both intra and interspecific interactions. Cancer pagurus and Porcellana platycheles are two co-occurring decapods on Atlantic rocky shores. In laboratory mesocosms, we investigated intra and intersize class cannibalistic and interspecific predatory behaviours in those species. We addressed the effects of prey and predator densities, food, starvation, alternative prey and habitat type. No agonistic behaviour was noted in P. platycheles, suggesting a non-aggressive co-existence between gregarious individuals. Predation of C. pagurus on P. platycheles was intense, possibly accounting for the spatial segregation observed in the natural environment. Cannibalism among C. pagurus juveniles was low and only on vulnerable prey (i.e. at moulting), suggesting a non-aggressive co-existence among juveniles. However, intersize class cannibalism in C. pagurus was intense (ontogenetic shift), possibly reflecting the juvenile-adult segregation in the natural environment. Prey and predator densities, food and habitat type strongly influenced this behaviour. Possible interference among cannibals was noted, with lower prey consumption at high predator density. Food supply alone had more effect on cannibalistic rate than did alternative prey (P. platycheles) and predator starvation. Structurally complex habitats (small pebble and Fucus serratus habitats) yielded higher prey survival than the sandy habitat, and the behaviours of both prey and cannibals reflected the small-scale spatial distribution of individuals in the wild. Intersize class cannibalism and interspecific agonistic relationships may account for the intertidal distribution of crab species at low tide.     

The effects of the visual environment on responses to colour by domestic chicks

It is well known that development of vision is affected by experience, but there are few studies of environmental effects on colour vision. Natural scenes contain predominantly a restricted range of reflectance spectra, so such effects might be important, perhaps biasing visual mechanisms towards common colours. We investigated how the visual environment affects colour preferences of domestic chicks ( Gallus gallus), by training week-old birds to select small food containers distinguished from an achromatic alternative either by an orange or by a greenish-blue colour. Chicks that had been raised in control conditions, with long-wavelength-dominated reflectance spectra, responded more readily to orange than to blue. This was not due to avoidance of blue, as increasing saturation enhanced the chicks' preference for the same hue. The advantage of orange was, however, reduced or abolished for chicks raised in an environment dominated by blue objects. This indicates that responses to coloured food are affected by experience of non-food objects. If colours of ordinary objects in the environment do influence responses to specialised visual signals this might help explain why biological signals directed at birds are often coloured yellow, orange or red; long-wavelength-dominated spectra being more prevalent than short-wavelength-dominated spectra.     

Dietary GABA and food selection by rats

To obtain further information pertaining to amino acid-induced alterations in feeding behavior, studies were performed to examine the food choices made by rats fed low protein diets made more or less aversive by the addition of various amino acids. When rats were allowed to choose between two diets, they preferred a low protein control, threonine-imbalanced or nonprotein diet to one containing 2.5% gamma-aminobutyric acid (GABA). Acceptance increased when GABA content was lowered to 1.5%; rats preferred this diet when the alternative diet was made sufficiently aversive. There were large individual differences among rats selecting from pairs of unacceptable diets. Avoidance of, or preference for, a given diet is clearly affected by the relative aversive qualities of the offered pair of diets.     

Effects of food availability on fledging condition and post-fledging survival in king penguin chicks

Effects of summer food shortage on king penguin Aptenodytes patagonicus chicks were studied at South Georgia. Two cohorts were compared, fledging in the austral summers of 1992 (n = 32) and 1994 (n = 33) when availability of food was judged good and poor, respectively. The former cohort had a higher pre-fledging mean mass (12.78 kg vs ≤ 10.03 kg), fledged earlier (median 5 January vs 21 January), and a higher proportion was re-sighted within 2 years of fledging (28% vs 0%). Within 4 years, 47% of the former cohort had been re-sighted (i.e. post-fledge survival); in addition, one was observed at the Falkland Islands, and 22% had bred (i.e. recruitment) in their colony of origin. The re-sighted chicks of the 1992 cohort fledged earlier than those not re-sighted (median 24 December vs 10 January), but it remain unclear if they were heavier at fledging. All chicks in this study (n = 65) were marked with both transponders (subcutaneously implanted) and flipper bands (on one flipper), and no losses of any markings were found (controlled up to 4 years afterwards). Therefore, data on chick post-fledging survival and recruitment were not adjusted for losses of markings, as has been done in other studies. Received: 21 October 1996 / Accepted: 2 February 1997     

Food selection by bacterivorous protists: insight from the analysis of the food vacuole content by means of fluorescence in situ hybridization

A modified fluorescence in situ hybridization (FISH) method was used to analyze bacterial prey composition in protistan food vacuoles in both laboratory and natural populations. Under laboratory conditions, we exposed two bacterial strains (affiliated with beta- and gamma-Proteobacteria -- Aeromonas hydrophila and Pseudomonas fluorescens, respectively) to grazing by three protists: the flagellates Bodo saltans and Goniomonas sp., and the ciliate Cyclidium glaucoma. Both flagellate species preferably ingested A. hydrophila over P. fluorescens, while C. glaucoma showed no clear preferences. Differences were found in the digestion of bacterial prey with B. saltans digesting significantly faster P. fluorescens compared to two other protists. The field study was conducted in a reservoir as part of a larger experiment. We monitored changes in the bacterial prey composition available compared to the bacteria ingested in flagellate food vacuoles. Bacteria detected by probe HGC69a (Actinobacteria) and R-BT065 were negatively selected by flagellates. Bacteria detected by probe CF319a were initially positively selected but along with a temporal shift in bacterial cell size, this trend changed to negative selection during the experiment. Overall, our analysis of protistan food vacuole content indicated marked effects of flagellate prey selectivity on bacterioplankton community composition.     

Size-related selection of food plants by bumblebees

Abstract. 1. A positive correlation between the tongue length of conspecific workers collecting nectar from seven plant species and the corolla length of the flowers probed was found for B.lapidarius and B.pascuomm but not B.terrestris . No simple relationship was found between the volume, sugar weight or concentration of nectar in flowers and the tongue or wing length of probing bees.
2. B.terrestris workers collecting pollen from four plant species producing pollen only, were found to differ in size according to the type of pollen presentation mechanism and the pollen content per flower. Body size variation was also related to the foraging of pollen plus nectar from two other plant species.     

Secondary compounds and food selection by colobus monkeys

The feeding behaviour of black-and-white colobus monkeys, Colobus guereza, has been studied in the rain forest of Uganda. These animals, which are mainly leaf-eaters, are very selective in their choice of food. Only 32 of the over 60 species of tree in the study area were seen to be eaten and over two-thirds of the diet came from only five species, of which Celtis durandii was the most important. Analysis of food and non-food plants showed a strong inverse correlation between food choice by guerezas and the concentration of procyanidin tannins. There was less correlation with alkaloid content.