Foraging and community consequences of seed size for coexisting Negev Desert granivores

We examined the effects of seed size on patch use and diet selection for three co-existing Negev Desert granivores: Allenby's gerbil ( Gerbillus allenbyi ), greater Egyptian sand gerbil ( Gerbillus pyramidum ), and crested lark ( Galerida cristata ). We manipulated size and spatial distribution of seeds in experimental food patches and quantified foraging behavior by measuring giving-up densities (GUDs: the amount of food remaining in a resource patch following exploitation by a forager). In one experiment, we presented small (<1.4 mm in diameter cracked wheat), medium (2.0–3.3 mm), and large (>3.4 mm) seeds in separate trays; in a second, we presented small and medium seeds separately and mixed together. Gerbils had a higher handling time efficiency on smaller seeds, but a much higher encounter probability on larger seeds (20 times higher on large than medium seeds, and 2–5 times higher on medium than small seeds). This led gerbils to have significantly lower GUDs on larger seeds than smaller seeds and to harvest a higher proportion of the larger seeds. When presented with rich and poor patches, G. allenbyi tended to equalize GUDs in both patches, indicating a quitting harvest rate rule for patch exploitation. In contrast, larks appeared to use a fixed time rule for patch exploitation. For larks, seed size did not influence encounter probabilities, and they showed no seed-size selectivity. Still, larks had higher handling efficiencies on smaller than larger seeds, and consequently had a significantly lower GUD on small than medium seeds. Despite large differences between the gerbils and larks in their foraging, our results do not support species coexistence via seed-size partitioning: the larks had much higher GUDs than the gerbils on all seed sizes. Nonetheless, seed size, seed abundance, seed distribution and the animal's patch use behavior all played major roles in determining gerbils' and larks' diet selectivities and GUDs.     

Taxonomic status of the Forest Buzzard Buteo oreophilus trizonatus

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).     

Foraging behavior of the midday gerbil (Meriones meridianus): combined effects of distance and microhabitat

We used the giving-up density (GUD) method and direct observation to study the combined effects of travel distance and microhabitat on foraging behavior of the midday gerbil (Meriones meridianus), which often acts as a central place forager. We provided animals with artificial seed trays in which dry and unhusked pumpkin seeds were mixed with fine sand. Gerbils practiced an eat-and-carry strategy in patches of bush microhabitat that were far from central places (BF patches), and tended to carry all seeds back in the other three treatments. Resource protection, predation risk avoidance and the balance between future and present value of food items may contribute to the eat-and-carry strategy. When distance was held constant, GUDs in open patches were higher than in bush patches, which was consistent with most studies. When microhabitat was held constant, GUDs in nearer patches were normally lower than in farther patches. In most cases, gerbils preferred to carry more seeds back rather than consume them immediately. We concluded that this tendency was due to the gerbils balancing the factors of future value and present value, and individual fitness and inclusive fitness.     

The use of time and space by male and female gerbils exploiting a pulsed resource

Foraging theory postulates that interference is a foraging cost and affects patch exploitation and activity times. One such system contains two species of seed-eating gerbils inhabiting sandy habitats in the Negev Desert of Israel. Low population densities of the dominant species allowed us to examine the interaction between males and females of the subordinate species, Gerbillus andersoni allenbyi , as a function of interference and resource renewal. We used giving-up densities (GUDs; the amount of food left in a resource patch when a forager abandons the patch) in seed trays to quantify patch use by gerbils. By placing 6 trays at each foraging station and either presenting all 6 trays at the start of the night (pulse treatment) or presenting one tray at a station 6 times per night (renewal treatment), we were able to manipulate characteristics of resource renewal. We used radio telemetry to obtain an independent assessment of activity. Male and female G. a. allenbyi differed in their timing of activity, with males beginning earlier than females and remaining active later. This was most pronounced for the pulse treatment. For the renewal treatment, female activity in trays was more intense early in the night, but thereafter male activity was more intense. At the same time, telemetry showed that males and females did not differ in their total activity in or out of trays. This suggests that males begin their activity on the renewal treatment by exploiting the richest natural patches of seeds. Only later when these are depleted do they move to dominate the renewing seed trays. Finally, females exploited stabilized sand habitats more than did males, especially during the renewal treatment. Taken together, these findings suggest that male G. a. allenbyi interfere with foraging in females, causing temporal shifts in their use of space and resources.     

Effects of predatory risk and resource renewal on the timing of foraging activity in a gerbil community

The foraging decisions of animals are often influenced by risk of predation and by the renewal of resources. For example, seed-eating gerbils on sand dunes in the Negev Desert of Israel prefer to forage in the bush microhabitat and during darker hours due to risk of predation. Also, daily renewal of seed resource patches and timing of nightly foraging activity in a depleting environment play important roles in species coexistence. We examined how these factors influence the timing of gerbil foraging by quantifying foraging activity in seed resource patches that we experimentally renewed hourly during the night. As in previous work, gerbils showed strong preference for the safe bush microhabitat and foraged less in response to high levels of illumination from natural moon light and from artificial sources. We demonstrate here for the first time that gerbils also responded to temporal and spatial heterogeneity in predatory risk through their timing of activity over the course of each night. Typically, gerbils concentrated their activity early in the night, but this changed with moon phase and in response to added illumination. These results can be understood in terms of the nature of patch exploitation by gerbils and the role played by the marginal value of energy in determining the cost of predation. They further show the dynamic nature of gerbil foraging decisions, with animals altering foraging efforts in response to time, microhabital, moon phase, illumination, and resource availability.     

Mismatches between habitat preferences and risk avoidance for birds in intensive Mediterranean farmland

Land use intensification may create habitats that organisms perceive as suitable, but where reproduction or survival is insufficient to maintain self-sustaining populations. Such conditions may qualify as ecological traps, but their existence is often hard to prove. Here, we provide a practical framework to evaluate a potential ecological trap resulting from mismatch between habitat preferences and predation risk, focusing on ground-nesting farmland birds of conservation concern. The framework is based on species-specific associations with safe or unsafe habitat types (i.e. field and landscape types with high or low nest survival), and the occurrence of risk avoidance (i.e. negative responses to predator abundances or to nest failure rates after controlling for habitat effects). Bird densities were far more influenced by field characteristics than landscape context. Corn bunting and fan-tailed warbler were associated with tall swards (safe habitats), and did not show risk avoidance. Tawny pipit and and Galerida larks were associated with short swards (unsafe habitats), with the former avoiding fields with high nest predation rates, and the later avoiding high mongoose abundances. Short-toed lark was associated with fields with short swards and low nest trampling rates. Results suggest that short-toed lark may be the most vulnerable to ecological trapping, because it nests on unsafe habitats and did not show predation risk avoidance. Our approach provides a practical first step to infer vulnerability to a potential ecological trap, though further research is needed to confirm this effect. Management actions increasing nest survival in short sward fields will likely favour grassland bird conservation in intensive Mediterranean farmland.     

The role of the nasal passages in the water economy of crested larks and desert larks

Condensation of water vapor in the exhaled air stream as it passes over previously cooled membranes of the nasopharynx is thought to be a mechanism that reduces respiratory water loss in mammals and birds. Such a mechanism could be important in the overall water economy of these vertebrates, especially those species occupying desert habitats. However, this hypothesis was originally based on measurements of the temperature of exhaled air (Tex), which provides an estimate of water recovered from exhaled air as a proportion of water added on inhalation but does not yield a quantitative measure of the reduction in total evaporative water loss (TEWL). In this study, we experimentally occluded the nares of crested larks (Galerida cristata), a cosmopolitan species, and desert larks (Ammomanes deserti), a species restricted to arid habitats, to test the hypothesis that countercurrent heat exchange in the nasal passages reduces TEWL. Tex of crested larks increased linearly with air temperature, (Ta): Tex=8.93+0.793xTa. Following Schmidt-Nielsen and based on measurements of Tex, we predicted that crested larks would recover 69%, 49%, 23%, and -5% of the water added to the inhaled air at Ta's of 15 degrees, 25 degrees, 35 degrees, and 45 degrees C, respectively. However, with the nares occluded, crested larks increased TEWL by only 27%, 10%, and 6% at Ta's of 15 degrees, 25 degrees, and 35 degrees C, respectively. At Ta=45 degrees C, TEWL of the crested lark was not affected by blocking the nares. In contrast to our expectation, occluding the nares of desert larks did not affect their TEWL at any Ta.     

Agricultural farming alters predator–prey interactions in nearby natural habitats

Agricultural farming is a major consumer of global arable lands and has a direct effect on species decline through habitat destruction. However, agricultural endeavours can also evoke indirect threats that will result in behavioural modifications of indigenous species. In a desert ecosystem, where a political border led to a farming dichotomy between intensive cultivates in Israel and intact lands in Jordan, we compared the foraging behaviours and abundances of the red fox and two species of gerbils, close to and distant from farms, and during two moon phases. We estimated fox and gerbil foraging levels by track counts, and measured gerbil time allocation, vigilance and apprehension by the giving-up density method. While foxes were significantly more abundant and active at locations close to farms, gerbils were significantly more abundant and active at locations distant from farms. Moreover, the typical reduction in food consumption during full-moon nights was exhibited only at locations close to farms. These results could suggest that indicators of predation risk, such as illumination intensity or distance to cover, are not universal, and their effectiveness may depend indirectly on anthropogenic activities, such as agricultural farming. The results could also suggest that although intensive agricultural endeavours benefit foxes, they might increase the predatory pressure on gerbils in addition to the already known effects of habitat loss. Therefore, agriculture acts as a double-edged sword by reducing natural habitats, while at the same time changing the predator–prey natural balance.     

Driven to distraction: detecting the hidden costs of flea parasitism through foraging behaviour in gerbils

Gerbilline rodents such as Allenby's gerbils (Gerbillus andersoni allenbyi), when parasitized by fleas such as Synosternus cleopatrae pyramidis, devote long hours of grooming to remove the ectoparasites. Yet no detrimental energetic or immunological effects of the ectoparasites have been found in adult Allenby's gerbil. Why should gerbils go to such trouble? We tested for the various ways that fleas can negatively affect gerbils by manipulating flea infestation on gerbils and the presence of a fox. We demonstrate that gerbils responded to fleas by leaving resource patches at higher giving-up densities. Furthermore, they stayed in those resource patches less time and left them at higher quitting harvest rates so long as a fox was also present. When flea-ridden, gerbils also abandoned using vigilance to manage risk and relied mainly on time allocation. Thus, having fleas imposed a foraging cost similar in nature to that arising from the risk of predation from foxes and may be even larger in magnitude. More than that, the presence of fleas acted as a magnifier of foraging costs, especially those arising from the risk of predation. The fleas reduced the gerbils' foraging aptitude and altered how they went about managing risk of predation. We hypothesize that fleas reduce the attention that gerbils otherwise have for foraging and predator detection. We suggest that this is the major cost of ectoparasitism.     

Immune indexes of larks from desert and temperate regions show weak associations with life history but stronger links to environmental variation in microbial abundance

Abstract Immune defense may vary as a result of trade-offs with other life-history traits or in parallel with variation in antigen levels in the environment. We studied lark species (Alaudidae) in the Arabian Desert and temperate Netherlands to test opposing predictions from these two hypotheses. Based on their slower pace of life, the trade-off hypothesis predicts relatively stronger immune defenses in desert larks compared with temperate larks. However, as predicted by the antigen exposure hypothesis, reduced microbial abundances in deserts should result in desert-living larks having relatively weaker immune defenses. We quantified host-independent and host-dependent microbial abundances of culturable microbes in ambient air and from the surfaces of birds. We measured components of immunity by quantifying concentrations of the acute-phase protein haptoglobin, natural antibody-mediated agglutination titers, complement-mediated lysis titers, and the microbicidal ability of whole blood. Desert-living larks were exposed to significantly lower concentrations of airborne microbes than temperate larks, and densities of some bird-associated microbes were also lower in desert species. Haptoglobin concentrations and lysis titers were also significantly lower in desert-living larks, but other immune indexes did not differ. Thus, contrary to the trade-off hypothesis, we found little evidence that a slow pace of life predicted increased immunological investment. In contrast, and in support of the antigen exposure hypothesis, associations between microbial exposure and some immune indexes were apparent. Measures of antigen exposure, including assessment of host-independent and host-dependent microbial assemblages, can provide novel insights into the mechanisms underlying immunological variation.     

From inter-specific behavioural interactions to species distribution patterns along gradients of habitat heterogeneity

The strength of the behavioural processes associated with competitor coexistence may vary when different physical environments, and their biotic communities, come into contact, although empirical evidence of how interference varies across gradients of environmental complexity is still scarce in vertebrates. Here, I analyse how behavioural interactions and habitat selection regulate the local distribution of steppeland larks (Alaudidae) in a gradient from simple to heterogeneous agricultural landscapes in Spain, using crested lark Galerida cristata and Thekla lark G. theklae as study models. Galerida larks significantly partitioned by habitat but frequently co-occurred in heterogeneous environments. Irrespective of habitat divergence, however, the local densities of the two larks were negatively correlated, and the mechanisms beyond this pattern were investigated by means of playback experiments. When simulating the intrusion of the congener by broadcasting the species territorial calls, both larks responded with an aggressive response as intense with respect to warning and approach behaviour as when responding to the intrusion of a conspecific. However, birds promptly responded to playbacks only when congener territories were nearby, a phenomenon that points to learning as the mechanisms through which individuals finely tune their aggressive responses to the local competition levels. Heterospecifics occurred in closer proximity in diverse agro-ecosystems, possibly because of more abundant or diverse resources, and here engage in antagonistic interactions. The drop of species diversity associated with agricultural homogenisation is therefore likely to also bring about the disappearance of the behavioural repertoires associated with species interactions.     

The role of site, habitat, seasonality and competition in determining the nightly activity patterns of psammophilic gerbils in a centrifugally organized community

Major ecological forces affecting diel activity patterns include predation, competition, resource dynamics, and ambient conditions. In this work we studied if, how, and why the nocturnal activity patterns of two gerbil species differed with respect to site, habitat, season, and inter-specific competition. The study system included two gerbil species- Gerbillus pyramidum and G. andersoni allenbyi in a sandy landscape composed of three habitats-shifting, semi-stabilized, and stabilized sand habitats. Conforming to previous studies, in the semi-stabilized sand habitat clear temporal partitioning in activity times occurred, with G. pyramidum being active earlier in the night and G. a. allenbyi being active in the later part of the night. As shown in previous studies in a different, more xeric site, removal of G. pyramidum demonstrated that this activity pattern is the result of interference competition by the latter on the former. Nocturnal activity pattern differed between habitats and seasons. Temporal partitioning occurred in the semi-stabilized sand habitat where both gerbil species were common. In the shifting sand habitat, G. pyramidum activity pattern was similar to that in the semi-stabilized sand habitat, but at a lower level. In the stabilized sand habitat where G. a. allenbyi occurred alone, its activity was the highest among the three habitats and was fairly constant throughout the night. Temporal partitioning occurred in the fall and early summer, but not in the early spring. The roles of predation risk, competition and resource dynamics in determining the gerbils' temporal activity together with their implications for community structure, are discussed.     

Inter-specific association and habitat use in a farmland passerine assemblage

We studied the pattern of inter-specific association of breeding territories in a passerine assemblage of dry cereal farmland in central Spain and evaluated the role of the presence of heterospecifics in the habitat use patterns exhibited by different species. Bird territories showed a non-random inter-specific spatial aggregation pattern. We studied territory abundance variation in the three more abundant species: the corn bunting, the crested lark, and the fan-tailed warbler. Crested lark and fan-tailed warbler territories were more abundant in plots where corn bunting territories were present and vice versa, while their respective abundances did not vary with the breeding presence of the other species. We used landscape and agricultural management variables to analyze the relationships between habitat and each species?? breeding territories by means of classification trees. While the corn bunting showed a marked pattern of nesting habitat use, the crested lark and the fan-tailed warbler exhibited a much more generalist one. Corn Bunting presence was affected negatively by intensification-related variables, such as field size and percent cover of cereal crops. Similarly, the presence of crested larks was negatively related to high yielding areas. However, when the presence of hetero-specific territories was considered, the presence of corn bunting territories was the most important variable explaining the occurrence of breeding fan-tailed warblers, and the second most important in the case of the crested lark. These results suggest that inter-specific attraction could play a role in the formation of farmland bird assemblages, while adding further evidence for the detrimental effect of agricultural intensification at the community level.     

Measuring the benefit of habitat selection

We used a behavioral bioassay to estimate the advantages thattwo species of gerbils (Gerbillus allenbyi and G. pyramidum)experienced by preferring a semistabilized dune habitat overa stabilized sand habitat. We used the magnitude of foragingeffort by the gerbils to signal the difference between thetwo habitats. When they were foraging as much in stabilizedsand as in semistabilized dune, we inferred that these habitatswere providing equivalent rewards. We performed a series ofexperiments in two 1-ha field enclosures, each containing similarproportions of stabilized sand and semistabilized dune. Eachenclosure contained a population of only one of the species.By varying the amount of seeds added (either 0.5, 1, 2, or 3g of seeds in 18 seed trays) to each habitat and monitoringthe behavior of the gerbils, we were able to fit a curve thatreflected the change in habitat preference as a function ofseed addition rate. We were also able to show how much seedaddition had to be added to bring the two habitats into equaluse. Each species required only 13 g/ha/night to entirely offsetthe advantage of the semistabilized dune.     

Two gerbils of the Negev: A long-term investigation of optimal habitat selection and its consequences

Optimal foraging theory has entered a new phase. It is not so much tested as used. It helps behavioural ecologists discover the nature of the information in an animals brain. It helps population ecologists reveal coefficients of interaction and their patterns of density-dependent variation. And it helps community ecologists examine niche relationships. In our studies on two species of Negev desert gerbil, we have taken advantage of the second and third of these functions. Both these gerbils prefer semi-stabilized dune habitat, and both altered their selective use of this habitat and stabilized sand according to experimental changes we made in their populations. Their changes in selectivity agree with a type of optimal foraging theory called isoleg theory. Isoleg theories provide examples of dipswitch theories – bundles of articulated qualitative predictions – that are easier to falsify than single qualitative predictions. By linking behaviour to population dynamics through isoleg theory, we were able to use the behaviour of the gerbils to reveal the shapes of their competitive isoclines. These have the peculiar non-linear shapes predicted by optimal foraging theory. Finally, when owl predation threatens, the behaviour of Gerbillus allenbyi reveals the shape of their victim isocline. As has long been predicted by predation theory and laboratory experiments, it is unimodal.     

The effects of gerbil foraging on the natural seedbank and consequences on the annual plant community

We explicitly tested the following predictions using Gerbillus allenbyi and G. pyramidum foraging on artificial arrays of natural seedbank on a semi-stabilized sand dune: 1. that the gerbils will forage using a quitting harvest rate rule, 2. that larger seeds are preferred due to higher encounter rates, and 3. that there are community level consequences for the annual plants as a direct result of foraging by the desert rodents. Natural seedbank, separated into two size classes, was placed in seed trays in the field at four different densities (1/16×, 1/4×, 1×, and 2×normal). Following exposure to granivory, the remaining seeds were weighed and germinated to test for community level effects. Only the 1× and 2×normal density plots were heavily foraged, and at both seed sizes, which suggests that the gerbils employed a quitting harvest rate rule. In support of our second prediction, the two species of gerbils tended to consume more of the larger seeds, particularly at higher densities. The mean number and total number of plant species that germinated in plots exposed to granivores was not significantly different from unexposed samples. At the community level however, there was no net association of germinated plant species in the four treatment groups exposed to granivores, but a significant net positive association in the unexposed control seedbank. Gerbil foraging on annual plant seedbank may thus subtly change the entire community structure (from positive to neutral), although not necessarily shift the species distributions significantly. Our results corroborate other studies involving artificial food types such as millet and suggest that foraging decisions may affect the plant community.     

Linking optimal foraging behavior to bird community structure in an urban-desert landscape: field experiments with artificial food patches

Urban bird communities exhibit high population densities and low species diversity, yet mechanisms behind these patterns remain largely untested. We present results from experimental studies of behavioral mechanisms underlying these patterns and provide a test of foraging theory applied to urban bird communities. We measured foraging decisions at artificial food patches to assess how urban habitats differ from wildlands in predation risk, missed-opportunity cost, competition, and metabolic cost. By manipulating seed trays, we compared leftover seed (giving-up density) in urban and desert habitats in Arizona. Deserts exhibited higher predation risk than urban habitats. Only desert birds quit patches earlier when increasing the missed-opportunity cost. House finches and house sparrows coexist by trading off travel cost against foraging efficiency. In exclusion experiments, urban doves were more efficient foragers than passerines. Providing water decreased digestive costs only in the desert. At the population level, reduced predation and higher resource abundance drive the increased densities in cities. At the community level, the decline in diversity may involve exclusion of native species by highly efficient urban specialists. Competitive interactions play significant roles in structuring urban bird communities. Our results indicate the importance and potential of mechanistic approaches for future urban bird community studies.     

The survival-rate-maximizing policy for Bayesian foragers: wait for good news

We present a model of the survival-maximizing foraging behaviorof an animal searching in patches for hidden prey with a clumpeddistribution. We assume the forager to be Bayesian: it updatesits statistical estimate of prey number in the current patchwhile foraging. When it arrives at the parch, it has an expectationof the patch's quality, which equals the average patch qualityin the environment While foraging, the forager uses its informationabout the time spent searching in the patch and how many preyhas been caught during this time. It can estimate both the instantaneousintake rate and the potential intake rate during the rest ofthe parch visit. When prey distribution is clumped, potentialintake rate may increase with time spent in the parch if preyis caught in the near future. Being optimal, a Bayesian foragershould therefore base its patch-leaving decision on the estimatedpotential patch value, not on the instantaneous parch value.When patch value is measured in survival rate and mortalitymay occur either as starvation or predation, the patch shouldbe abandoned when the forager estimates that its potential survivalrate dining the rest of the patch visit equals the long termsurvival rate in the environment This means that the instantaneousintake rate, when the patch is left, is nor constant but isan increasing function of searching time in the patch. Therefore,the giving-up densities of prey in the patches will also behigher the longer the search times. The giving-up densitiesare therefore expected to be an increasing, but humped, functionof initial prey densities. These are properties of Bayesianforaging behavior not included in previous empirical studiesand model tests.     

Effects of season, water and predation risk on patch use by birds on the African savannah

Birds from semi-arid regions may suffer dehydration during hot, dry seasons with low food availability. During this period, both energetic costs and water requirements for thermoregulation increase, limiting the scope of activity. For granivorous birds feeding on dry seeds, this is a major challenge and availability of water may affect the value of food. Water availability could (1) increase the value of a food patch when the surrounding environment is poor, due to an increase in the marginal value of energy, and (2) increase the value of the entire environment to the forager when environmental quality increases, due to an increase in the marginal value of time. We aimed to test this by measuring giving-up densities (GUDs, remaining food densities after foraging) of granivorous birds in the presence or absence of filled water pots, at different seasons differing in background food and water availability. We predicted that GUDs will increase with water provision during the dry season with moderate food, but in the early wet season with low food and water availability, GUDs will decrease with water provision. Later in the wet season, our experimental addition of water should have no effect. During seasons with low water availability but differing in food availability, results confirmed our predictions. However, when water became more abundant as the wet season progressed, birds still foraged more intensely during days with added water. In all seasons, birds fed more intensely in cover than in exposed areas, suggesting that predation risk rather than heat influenced microhabitat selection.     

Seeds redistribution in sand dunes: a basis for coexistence of two rodent species

Spatial and temporal heterogeneity is a major factor structuring communities and contributing to coexistence of the species they contain. In this study we examine a critical aspect of environmental heterogeneity that is assumed to promote coexistence in two gerbil species of the Western Negev Desert. Previous studies assumed that temporal partitioning, in activity time, is the result of daily redistribution of seeds that the dominant species is the first to utilize while the sub-ordinate and efficient species is being pushed to use the later and poorer part of the night. We tested the assumption that daily afternoon winds generating spatial and temporal heterogeneity in seed availability by the redistribution of sand and seeds. This was done by comparing plots experiencing normal wind condition with manipulated plots where wind action was diminished by a shade-cloth fence. Our results show that considerable amount of sand and seeds are redistributing regularly on a time scale of a single day. Our results also show that gerbil foraging behavior is strongly related to the pattern of the redistribution dynamics of the seeds. When we prevented redistribution of seeds, gerbil foraging activity was reduced considerably. However, both seed redistribution and gerbil activity did not change much on control plots. Furthermore, the two gerbil species responded differently to the reduction in seed redistribution. The larger Gerbillus pyramidum was shown to be more sensitive to the reduction than the smaller G. a. allenbyi . Daily variability in the availability of seed resources is probably the niche axis which, together with the trade-off in foraging efficiency of the species, forms the mechanism for the coexistence of the two gerbil species in the semi-stabilized sands.