Efficient Forager Research Articles

Scale‐dependent foraging tradeoff allows competitive coexistence

In spatially heterogeneous environments, coexistence between competing species can be facilitated by spatially mediated tradeoffs. In this paper we develop a mechanistic model to investigate under which circumstances interspecific differences in the tradeoff between foraging efficiency and travel costs can allow two central place foraging species to coexist in spite of considerable overlap in resource use. One species (Flyer) has a high basal metabolic rate, but a low relative cost of travelling such that it can use patches at a greater distance from its central place while the minimum patch quality it can economically use is high. The other species (Forager), by contrast, has a lower basal metabolic rate, but higher relative cost of travelling, and can therefore be a more efficient forager and able to use foraging patches of low quality, as long as they are not too far from the nest. We demonstrate that the coexistence of these two species critically depends on landscape composition and structure, with the Flyer outcompeting the Forager in structurally simple, coarse‐grained, landscapes with abundant high‐quality forage and the Forager outcompeting the Flyer in fine‐grained highly diverse landscapes. Coexistence between the two species is possible when the landscape is structurally and compositionally complex, fine‐grained, and has both high and low quality forage. Our results demonstrate that exploitative competition between two contrasting life histories can produce very different community dynamics depending on landscape composition and structure.

Temperature‐ and light‐dependent ratio of energy gain to metabolic costs explains spatial and temporal habitat use of zooplanktivorous fish

AbstractUnderstanding the forces that drive habitat selection of species in communities is important in both ecology and evolution. In nature, species face variation in competition, predation and physical characters among habitats. Vendace (Coregonus albula (L.)) is a specialised zooplanktivorous fish predominantly using deeper water in lakes during summer, while roach (Rutilus rutilus (L.)) uses mainly the shallow littoral zone as well as the upper layer of the pelagic zone. To understand mechanisms behind habitat use of these species, I first conducted a predation experiment to investigate their sensitivity to predation by perch (Perca fluviatilis L.). Second, I performed a foraging experiment using different temperature and light treatments. I then used metabolic calculations to estimate energetic costs when foraging. I found no difference between species regarding sensitivity to predation. Vendace was the most efficient forager on zooplankton but also swam faster spending more energy compared to roach. Roach had a comparatively high metabolic rate in the lowest temperature, where their foraging efficiency was lowest. The energy gain ratio at 6°C was highest for vendace, while it was lowest for roach. In the highest temperature (18°C) and the lowest light level (1 lux), both species were similar in their energy gain ratio. The relative energy gain ratio provides a mechanism to explain habitat distribution for the two species. An increased understanding of the role of metabolism in combination with biotic interactions and habitat use may help to foresee effects of environmental change for different species.

Environmental filtering of foraging strategies mediates patterns of coexistence in the fire antsSolenopsis geminataandSolenopsis xyloni, and their interspecific hybrids

Tradeoffs in functional traits facilitate species coexistence within communities and are important contributors to community assembly. Two aspects of a life‐history tradeoff known to structure ant communities include foraging efficiency (i.e. resource discovery and retrieval) and competitive dominance (territoriality and survivorship following competition).This work examined these aspects in workers of two fire ant species (Solenopsis xyloniandSolenopsis geminata) and theirF1hybrids using laboratory experiments.Parental foraging strategies were consistent with a tradeoff between discovery efficiency and competitive territorial dominance. Desert‐adaptedS. xyloniwas the most efficient forager but the least effective territory defender; mesic‐adaptedS. geminatawas a less efficient forager, but fared best in competition trials. Hybrid workers were generally intermediate, leading to increased survival by hybrids in competitive encounters withS. geminatacompared with the performance ofS. xyloni.These findings suggest that traits associated with foraging and competitive ability in the parental species are subject to environmental filtering; across habitats, species with a similar dominance rank in their respective communities differ in their placement along the discovery–dominance axis.Interspecific hybridisation may play a role in extending eastern range limits ofS. xylonivia shifting colony allocation towards the strategy of its competitor. In the overlap zone,S. xyloniwith hybrid workers may be sufficiently competitive to persist withS. geminata, while colonies withS. xyloniworkers cannot, providing a substantial selective advantage to hybridisation forS. xyloniqueens that may explain the unusual hybridogenetic social structure of this population.

Adaptive plasticity and genetic divergence in feeding efficiency during parallel adaptive radiation of whitefish (Coregonusspp.)

Parallel phenotypic divergence in replicated adaptive radiations could either result from parallel genetic divergence in response to similar divergent selection regimes or from equivalent phenotypically plastic response to the repeated occurrence of contrasting environments. In post-glacial fish, replicated divergence in phenotypes along the benthic-limnetic habitat axis is commonly observed. Here, we use two benthic-limnetic species pairs of whitefish from two Swiss lakes, raised in a common garden design, with reciprocal food treatments in one species pair, to experimentally measure whether feeding efficiency on benthic prey has a genetic basis or whether it underlies phenotypic plasticity (or both). To do so, we offered experimental fish mosquito larvae, partially burried in sand, and measured multiple feeding efficiency variables. Our results reveal both, genetic divergence as well as phenotypically plastic divergence in feeding efficiency, with the phenotypically benthic species raised on benthic food being the most efficient forager on benthic prey. This indicates that both, divergent natural selection on genetically heritable traits and adaptive phenotypic plasticity, are likely important mechanisms driving phenotypic divergence in adaptive radiation.

Cognitive Processes inVespula germanicaWasps (Hymenoptera: Vespidae) When Relocating a Food Source

AbstractThe German yellowjacket Vespula germanica (F.) (Hymenoptera: Vespidae), an invasive wasp, is a highly efficient forager. We studied wasp cognitive ability while varying landmark disposition and cue conspicuity. Learning flights were used as an indicator of forager cognition while relocating a food source. We recorded the number of learning flights (circling above the food location) performed after each visit by each wasp. We studied the effect of modifying a learned location and analyzed how the addition of conspicuous cues affected wasp learning. Cognitive ability also was studied in relation to food manipulation efficiency, i.e., the time taken to extract a piece of meat and fly away. We found that one feeding visit was sufficient for wasps to learn relevant cues associated with a rewarded location, as shown by the reduction of learning flights after just one experience. Moreover, wasps reached asymptotic levels in food manipulation after only one feeding visit. The introduction of a contextual change, such as moving the food location a few centimeters from the original feeding site, affected returning foragers, who increased the number of learning flights when leaving the array. Interestingly, enriched contexts with conspicuous cues seemed to facilitate wasp cognition as they performed fewer learning flights than in nonenriched ones. Learning flights seem to be a good cognitive indicator, reflecting wasp experience with a certain context, thus revealing their sensitivity to landmark cue conspicuity. This study highlights the cognitive capacities of V. germanica foragers.

Ecological Factors Affecting the Foraging Behaviour ofXerus rutilus

The African unstriped ground squirrel (Xerus rutilus) is widely dispersed across various habitats in East Africa and hence encounters a diverse suite of predators and plant communities. It is not known how different habitats and plant characteristics affect the foraging behaviour of X. rutilus. We used giving-up densities (GUDs) as a measure of foraging efficiency to explore the foraging costs of environmental heterogeneity. To determine foraging efficiency across spatial scales, we established food patches in two microhabitats (open and cover), which were nested within three habitats (koppie, edge and bushland). When foraging in a cover microhabitat, foraging efficiency decreased away from the koppie, but when in the open microhabitat, foraging efficiency was lowest near the koppie edge. Second, to determine foraging efficiency with common plant toxins, we presented the squirrels with seeds soaked in either tannic acid, oxalic acid or distilled water (control). Foraging efficiency did not differ between tannic-treated and control seeds, but oxalic-treated seeds had higher GUDs. Overall, our results suggest that X. rutilus is a remarkably efficient forager across multiple axes of environmental heterogeneity, which may have intriguing consequences for the ecological community.

Ecological factors affecting the foraging behaviour of Xerus rutilus

The African unstriped ground squirrel (Xerus rutilus) is widely dispersed across various habitats in East Africa and hence encounters a diverse suite of predators and plant communities. It is not known how different habitats and plant characteristics affect the foraging behaviour of X. rutilus. We used giving-up densities (GUDs) as a measure of foraging efficiency to explore the foraging costs of environmental heterogeneity. To determine foraging efficiency across spatial scales, we established food patches in two microhabitats (open and cover), which were nested within three habitats (koppie, edge and bushland). When foraging in a cover microhabitat, foraging efficiency decreased away from the koppie, but when in the open microhabitat, foraging efficiency was lowest near the koppie edge. Second, to determine foraging efficiency with common plant toxins, we presented the squirrels with seeds soaked in either tannic acid, oxalic acid or distilled water (control). Foraging efficiency did not differ between tannic-treated and control seeds, but oxalic-treated seeds had higher GUDs. Overall, our results suggest that X. rutilus is a remarkably efficient forager across multiple axes of environmental heterogeneity, which may have intriguing consequences for the ecological community.

A comparison of giving-up densities of five species of granivorous birds

The foraging efficiencies of four sympatric southern African seed-eating birds, namely Bronze Mannikin Spermestes cucullatus, Cape Sparrow Passer melanurus, Southern Red Bishop Euplectes orix and Thick-billed Weaver Amblyospiza albifrons, and a domesticated species, the Bengalese Finch Lonchura domestica, were measured and compared using giving-up densities (the amount of food remaining following patch exploitation) in experimental food patches. Foraging efficiency was quantified using giving-up densities by offering individual birds equal foraging opportunities. A low giving-up density displays the ability of a forager to profitably harvest food at low resource densities and to gainfully exploit the foraging opportunities overlooked by a less efficient forager. Ten individuals of each of the five species were allowed to forage on six different seed types. Thick-billed Weavers had significantly lower giving-up densities for all seed types except the smallest, namely red manna. Bronze Mannikins showed the converse trend, foraging most efficiently on the smallest seeds. The results of the present study revealed that Thick-billed Weavers were the most efficient foragers (i.e. had the lowest giving-up densities on seeds in feeding trays).

Diet segregation between two sympatric ‘small’Barbus spp: an experimental study of mechanisms

Gut contents of two co‐occurring species of ‘small’ diploid barbs (<10 LF cm) in Lake Tana revealed that zooplankton is the major diet component for B. tanapelagius(75% based on volume), but less prominent in B. humilis(40%). Functional response experiments in the laboratory were conducted to elucidate the mechanisms causing this difference. The type of functional response by the two ‘small’ barbs under different microcrustacean zooplankton densities (10, 20, 40, 60 and 80 ind.l−1) was examined. The functional response of B. tanapelagius to increasing prey densities corroborates with Holling Type II model, whereas B. humilis exhibits a Type III functional response. Predation rate is higher for B. tanapelagius at low zooplankton density (<40 ind.l−1) and equals the level of B. humilis at higher densities (>40 ind.l−1). This suggests that at lower zooplankton densities B. humilis is a less efficient forager on zooplankton prey items than B. tanapelagius. In Lake Tana average zooplankton density is relatively low (<35 ind.l−1). Under these food conditions, B. humilis is forced to feed on other food items (e.g. benthic invertebrates), whereas B. tanapelagius primarily feeds on zooplankton. The feeding potentials of the two ‘small’ barbs, as deduced from their morphology explain their different performances and their segregation in space and food resources.

Mechanisms of Coexistence of Optimal Foragers as Determinants of Local Abundances and Distributions of Desert Granivores

Foraging theory can be applied to the study of ecological communities. We developed and tested several local mechanisms of species coexistence based on foraging theory for a desert granivore community in a heterogeneous landscape in the Negev Desert, Israel. The most common species were Allenby's gerbil ( Gerbillus allenbyi ), Wagner's gerbil ( Gerbillus dasyurus ), and crested lark ( Galerida cristata ). Allenby's gerbils were most abundant on sandy substrates; Wagner's gerbils were most abundant on cobble and loess substrates; crested larks occurred across all three. To test among predictions of the different mechanisms of coexistence, we used live-trapping and counts to census rodents and birds, recorded spoor in sand-tracking plots to determine habitat-specific activity patterns, and measured giving-up densities in depletable food patches to estimate relative foraging efficiencies on different substrates and within different habitats. Allenby's gerbil had an advantage in habitat selection by being the most efficient forager (lowest giving-up densities on seeds in feeding trays) in the sandy habitat. The crested lark was the tolerant forager. Its foraging abilities were affected little by escape substrate, foraging substrate, or food type. As a result, the crested lark had three advantages: it was the most efficient forager in the loess habitat, it was a better insectivore than the gerbils, and its diurnal habit allowed it first access to seed patches that renew in the afternoon in the sandy habitat when winds typically arise. In response to our measures of foraging aptitude, Wagner's gerbil revealed no advantage over the other species, and factors promoting its success eluded our proposed mechanisms of species coexistence. Foraging ecology and community ecology can be integrated to understand local species distributions, abundances, and community structure and organization. Uniform conceptual and methodological techniques can be applied across taxa and communities. We were able to relatively quickly test which mechanisms of coexistence apply. In the community we studied, there were two species whose presence we feel we understood and one species whose presence remained an enigma.

How Do Red Squirrels, Tamiasciurus hudsonicus, and Eastern Chipmunks, Tamias striatus, Coexist?

This study tests five mechanisms for the coexistence of red squirrels (Tamiasciurus hudsonicus) and eastern chipmunks (Tamias striatus). These mechanisms, based on optimal foraging theory, depend on differences between the two species in habitat over space and time. The mechanisms are related to: 1) habitat; 2) microhabitat; 3) foraging and travel costs; 4); seasonal variation in predation risk; and 5) seasonal variation in thermoregulatory costs. Each species' foraging efficiency was estimated, using GUDs (Giving-up-densities) from February to July 1995 at the Morgan Arboretum. Sainte-Anne-de-Bellevue, Quebec, Canada. GUDs were measured in 36 aluminum seed trays, each containing 20 g of sunflower seeds, mixed with 2.25 1 of kiln dried sand. Trays were set in three habitats (coniferous, deciduous and mixed forest), separated by at least 300 m with 6 pairs of trays per habitat. In general, the chipmunk was more efficient (left lower GUDs) than the red squirrel. The red squirrel was more efficient than the chipmunk in the coniferous forest during the spring and had exclusive access to food during the winter when the chipmunk was in hibernation. The chipmunk was the more efficient forager in the mixed forest in both spring and summer. This spatio-temporal trade-off in foraging efficiency constitutes a mechanism which can promote coexistence between these two diurnal rodents.

Competition between birds and mammals: A comparison of giving-up densities between crested larks and gerbils

We combined the concept of mechanisms of co-existence with the approach of giving-up densities to study inter-taxon competition between seed-eating birds and mammals. We measured feeding behaviour in food patches to define and study the guild of seed-eating vertebrates occupying sandy habitats at Bir Asluj, Negev Desert, Israel. Despite a large number of putatively granivorous rodents and birds at the site, two gerbil species (Allenby's gerbil, Gerbillus allenbyi, and the greater Egyptian gerbil, G. pyramidum) dominated nocturnal foraging, and a single bird species (crested lark, Galerida cristata) contributed all of the daytime foraging. We used giving-up densities to quantify foraging behaviour and foraging efficiencies. A low giving-up density demonstrates the ability of a forager to profitably harvest food at low abundances and to profitably utilize the foraging opportunities left behind by the less efficient forager. Gerbils had lower giving-up densities in the bush than open microhabitat, and lower giving-up densities in the semi-stabilized than stabilized sand habitats. Crested larks showed the opposite: lower giving-up densities in the open than bush, and on the stabilized than semi-stabilized sand habitats. Despite these patterns, gerbils had substantially lower giving-up densities than crested larks in both microhabitats, all sand habitats, and during each month. Several mechanisms may permit the crested lark to co-exist with the gerbils. Larks may be cream skimmers on the high spatial and temporal variability in seed abundances. Larks may rely on insects, fruit or smaller seeds. Or, larks may rely on adjacent rocky habitats.

Foraging Theory, Patch Use, and the Structure of a Negev Desert Granivore Community

We tested five mechanisms of coexistence in a community of three common rodent species (two gerbils, Gerbillus allenbyi and G. pyramidum, and one jerboa, Jaculus jaculus) inhabiting sand dunes in the Negev Desert, Israel. The five mechanisms, based on foraging theory, considered various forms of habitat selection in time and space. From November 1986 until January 1988, we live—trapped to census rodent populations, counted rodent spoor in tracking plots to quantify activity, and measured the rodents' giving—up densities (GUDs: the amount of food remaining within a resource patch following exploitation by a forager) in seed trays to determine relative foraging efficiencies. The population sizes of the two gerbil species tended to fluctuate synchronously (unfortunately, we could not live—trap any jerboas). G. allenbyi biased its activity towards the stabilized sand habitat and towards the bush microhabitat. In contrast, G. pyramidum and J. jaculus biased their activity towards semistabilized sand habitats, and J. jaculus also biased its activity towards the open microhabitat. Despite divergent patterns of habitat use among species, G. allenbyi tended to be the most efficient forager and J. jaculus the least efficient forager regardless of microhabitat, sand habitat, or month. G. allenbyi's presence in the community may be assured by its higher foraging efficiency. J. jaculus's presence in the community may either result from its ability to travel greater distances and utilize rare, rich resource patches, or result from an herbivorous diet. G. pyramidum's presence with G. allenbyi in the community appears to require the less stabilized sand habitats and the ability of G. pyramidum to dominate rich patches by interference or nightly temporal partitioning.

Design and Application of a Hive-Mounted Device That Uses Honey Bees (Hymenoptera: Apidae) To Disseminate Heliothis Nuclear Polyhedrosis Virus

The honey bee, Apis mellifera L., is a highly efficient forager on numerous flowering plants that host phytophagous insect pests. Because of these associations, honey bees theoretically provide an economical and nonintrusive means of conveying biorational mortality agents against pest species. This hypothesis was tested during April 1989 by employing a specially designed plexiglass and sheet metal applicator that causes honey bees to disseminate a talc formulation of Heliothis nuclear polyhedrosis virus (HNPV) into fields of crimson clover, Trifolium incarnatum L. The device when integrated into a specifically designed substructure of a conventional beehive provided separate entry and departure pathways for honey bees, which allowed bees to be surface-contaminated with the HNPV formulation as they exited the hive. The mean percentage of HNPV-induced mortality was significantly higher among Helicoverpa zea (Boddie) larvae that fed on clover heads from fields foraged by HNPV-contaminated A. mellifera and among Heliothis or Helicoverpa spp. larvae collected from those fields than among similarly exposed control larvae. HNPV in honey collected from treatment hives and stored at 26 ± 1°C in total darkness remained active for at least 170 d. The extent to which the technology for using A. mellifera to disseminate insect pathogens is applicable for pest control will depend on an improved understanding of the interrelationships among A. mellifera , flowering plants on which they forage, targeted phytophages which reside on the plants, and the availability of pathogens which attack the phytophages.

Efficiency of area-concentrated searching behaviour in a continuous patchy environment

In many natural situations involving terrestrial predators, prey items are clustered in patches that have no perceptible boundaries and can only be defined as areas where the local resource density is higher than the mean overall resource density. In this type of continuous patchy environment, an efficient forager has to regulate the sinuosity (klinokinesis) and the speed (orthokinesis) of its random search movements so as to concentrate its search effort in the high resource density areas. This study determined the theoretical efficiency of this area-concentrated behaviour using computer simulations. Two search modes were assumed to be used by the model predator: an intensive search mode involving high sinuosity and low speed within high resource density areas, which is triggered by the detection of a prey item, and an extensive search mode involving low sinuosity and high speed between these areas, which is resumed after an unsuccessful search. Among the four habitat types included in this study, the efficiency of this movement control was higher in coarse-grained than in fine-grained habitats and increased also with the level of heterogeneity. The optimal values of the movement control parameters were similar whatever the habitat type and a large range of values led to nearly maximal efficiency. This suggests that even if they can only poorly estimate the resource distribution, terrestrial predators may ultimately search efficiently for clustered prey items by using elementary proximate mechanisms roughly controlling their random search movements.

Desert Rodent Community Structure: A Test of Four Mechanisms of Coexistence

Four mechanisms of coexistence are considered that may contribute to the diversity of desert granivorous rodent communities. In the first, bush/open microhabitat selection, coexistence is possible if there is a trade—off between foraging efficiency in the bush and open microhabitats. In the second, temporal variation in resource abundances, coexistence is possible if there is a trade—off between foraging efficiency and maintenance efficiency. The first species can forage profitably on low resource abundances while the second uses dormancy to travel inexpensively in time between periods of high resource abundances. In the third, spatial variation in resource abundance, coexistence is possible if there is a trade—off between foraging efficiency and the cost of travel. The first species forages patches to a lower giving—up density, (the density of resource at which a forager ceases foraging), while the second can inexpensively travel between patches with high resource abundances. In the fourth, seasonal rotation in foraging efficiencies, coexistence is possible if there is a trade—off between the costs of foraging during different seasons. The species that is the more efficient forager changes seasonally. The first mechanism of coexistence has received much empirical attention and support. The other three have not previously been considered with desert rodents. In a community of four granivorous rodent species, I used artificial seed patches to measure species— and habitat—specific foraging efficiencies and live—trapping to measure population sizes and mean distances between recaptures. Of the four, the fourth mechanism of coexistence best explained the presence of Perognathus amplus, Dipodomys merriami, and Spermophilus tereticaudus in the community. Each species enjoyed a period of the year during which it was the most efficient forager. Furthermore, the annual population densities of these three species fluctuated out of phase. Seasonal changes in species—specific predation risks and body size—dependent metabolic costs may have accounted for these results. The third mechanism of coexistence best explained the presence of Ammospermophilus harrisii in the community. This species preferred to forage a large number of widely spaced patches to a high giving—up density rather than forage a few patches to a low giving—up density.

The Economics of Seed Handling

The effects of food handling costs on the feeding behavior of domestic rats was explored in three experiments. Sunflower seeds with or without hulls were presented to the rats, either sequentially or simultaneously, along with a purified carbohydrate. Rats do not eat the hull of these seeds and, thus, must add the cost of removing the hulls from the seeds with hulls (handling) to any other costs associated with eating these seeds. The lower cost seeds without hulls were strongly preferred to the seeds with hulls in all conditions, but were not eaten exclusively. When the preferred seeds without hulls were made increasingly difficult to obtain, the rats eat an increasingly greater proportion of the seeds with hulls. The results indicated the importance of handling costs in determining prey selection and demonstrated that the omnivorous rat is relatively efficient forager.