Inter-specific competitors reduce inter-gender competition in Negev Desert gerbils

We examined gender-dependent competitive interactions between two nocturnal desert gerbil species, Gerbillus andersoni allenbyi and G. pyramidum, by a field manipulation experiment. The study was done in two 1-ha enclosed plots and included allopatric (only G. a. allenbyi) and sympatric (both species together) treatments. Seed trays and thermal imaging cameras were used to observe the gerbils foraging activities and aggressive interactions. We found that the negative effect of the competitively dominant species, G. pyramidum, on time spent in seed trays, and ability to control these artificial food patches, was stronger on male than on female G. a. allenbyi. Consequently, the aggression of male G. a. allenbyi towards female G. a. allenbyi was markedly reduced, indicating that the dominant species mediated competition between the genders of the subordinate species. Furthermore, this interference-mediated indirect effect was associated with a decrease in the body mass of male G. a. allenbyi and an increase in the survival of female G. a. allenbyi. We suggest that both the reduction in intra-specific aggression and the positive effect on female survival can potentially stabilize competitive interactions and promote coexistence in this small mammal community.     

Harvest rates and foraging strategies in Negev Desert gerbils

We examined the foraging strategy and quantified the foragingtraits of two nocturnal rodent species, Allenby's gerbil (Gerbillusallenbyi) and the greater Egyptian sand gerbil (Gerbillus pyramidum).In the laboratory, both species used two distinct foragingstrategies: either they immediately consumed seeds found ina patch (seed tray); or they collected and delivered the seedsto their nest box for later consumption. Moreover, we founda transition in foraging strategy among individual G. allenbyi under laboratory conditions; they all began by consuming theseeds on the tray and, after 7 days on average, switched tothe collecting strategy. By contrast, in the field both speciesused only one foraging strategy; they collected and deliveredthe seeds to their burrow or to surface caches for later consumption.Furthermore, G. allenbyi and G. pyramidum collected seeds atsignificantly higher rates in the field than in the laboratorybecause the seed encounter rates for both species were higherin the field. This suggests that in natural conditions, probablyinvolving predation risk and competitive pressure, gerbilsmust respond in two ways: (1) they must choose a foraging strategythat reduces predation risk by minimizing time spent feedingoutside their burrows; and (2) they must forage more efficiently.In the field, seed handling time of the larger species, G. pyramidum, was shorter than that of the smaller one, G. allenbyi.This difference may give G. pyramidum an advantage when resourcelevels are high and when most of a forager's time is spent handling seeds rather than searching for more seeds. Additionally,our field study showed that the seed encounter rate of G. allenbyiwas higher than that of G. pyramidum. This difference may giveG. allenbyi an advantage when resource levels are low and whensearching occupies most of the forager's time. The differentadvantages that each species has over the other, under differentconditions, may well be factors promoting their coexistenceover a wide range of resource densities.     

The effect of intra- and interspecific aggression on patch residence time in Negev Desert gerbils: a competing risk analysis

We observed patch-use behavior by two gerbil species in a fieldsetting and investigated how aggression and intrinsic decision-makinginteract to influence patch residence times. Results were interpretedby using a competing risk analysis model, which uniquely enabledus to estimate the intrinsic patch-leaving decisions independentlyof external interruptions of foraging bouts by aggression. Theexperiment was conducted in two 1-ha field enclosures completelysurrounded by rodent-proof fences and included allopatric (onlyGerbillus andersoni allenbyi) and sympatric (G. a. allenbyiand G. pyramidum) treatments. We predicted that increased foodpatch quality (i.e., habitat quality) should decrease intrinsicpatch-leaving rates and increase rates of aggressive interactionsinvolving the forager feeding in the patch (i.e., the occupantindividual). We also anticipated that increasing populationdensity should result in an increase in the rate of aggressiveinteractions involving the occupant individual. Our resultssupported the first two predictions, indicating a trade-offbetween foraging and aggression. However, the third predictionwas realized only for G. a. allenbyi in allopatry. Furthermore,in allopatry, occupant G. a. allenbyi individuals with highcompetitive ranks were involved in aggressive interactions atlower rates than those with low competitive ranks. However,in sympatry, patch-use behavior of occupant G. a. allenbyi individualswas mainly influenced by aggressive behavior of G. pyramidum,which did not respond to their competitive rank. Thus, it shouldpay less for G. a. allenbyi to be aggressive in sympatric populations.The observed reduction in intraspecific aggression among individualG. a. allenbyi in the presence of G. pyramidum supports thisassertion. We suggest that this reduction likely weakens thenegative effect of intra- and interspecific density on the percapita growth rate of G. a. allenbyi. Because this would changethe slope of the isocline of G. a. allenbyi, it could be animportant mechanism promoting coexistence when habitat selectionis constrained.     

Field metabolic rates and water influxes of two sympatric Gerbillidae:Gerbillus allenbyi andG. pyramidum

Summary Two primarily granivorous rodents of Old World deserts,Gerbillus allenbyi (mean adult body mass=26 g) andG. pyramidum (mean adult body mass=40 g), coexist in sandy habitats in the northwestern Negev desert. Both species are burrow dwellers and are nocturnal; however, in their overall distributions,G. pyramidum occurs in more extreme deserts than doesG. allenbyi. In comparing field metabolic rate (FMR) and water influx of the twoGerbillus species, we considered two alternative hypotheses: (1) given the difference in their overall distributions,G. pyramidum has a lower FMR and water influx thanG. allenbyi, and (2) given the similarity in their diets, and that we worked with sympatric populations, FMR and water influx are similar. The latter alternative proved to be correct. Field metabolic rates in summer were 7.29 kJ · g^-0.51 · day^-1 forG. allenbyi and 7.74 kJ · g^-0.51 · day^-1 forG. pyramidum, values that were 69.3% and 74.5%, respectively, of those predicted for rodents of their body masses. Summer water influx ofG. allenbyi was 0.167 ml · g^-0.90 · day^-1 and that ofG. pyramidum was 0.144 ml · g^-0.90 · day^-1; these values were 79.4% and 68.6%, respectively, of water influxes predicted for rodents of their body masses. When compared allometrically, there were no interspecific differences in any of the measurements.     

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.     

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.     

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.     

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.     

Microhabitat use, giving-up densities and temporal activity as short- and long-term anti-predator behaviors in common voles

We used depletable food patches to determine the effect of microhabitat (mowed versus unmowed adjacent grasslands) and time (day versus night) on the foraging behavior of common voles (Microtus arvalis). The food remaining after 12‐h periods (giving‐up density, GUD) measured the vole's habitat selection under predation risk. In accord with several other rodent species and the effects of avian predators, voles had significantly lower GUDs in the unmowed than mowed portion of the grassland. GUDs in patches along the border between adjacent habitats were more similar to the risky mowed grassland than the safe unmowed grass. Time interacted strongly with microhabitat. In the mowed grass, voles had significantly higher GUDs at night than day. Whereas in the unmowed grass, GUDs were significantly higher during the day than night. Vole GUDs did not vary with time along the boundary. This suggests that predators are more abundant or effective in the mowed grass at night (owls?), and in the unmowed grass during the day (weasels?). In terms of predation risk, the voles perceived the mowed grass at night as the riskiest and the unmowed grass at night as the safest. Voles may have difficulties assessing resources under high predation risk: GUDs among patches were well equalized in the unmowed microhabitat whereas in the mowed grass only day GUDs did not vary significantly among patches. We linked these results to the vole's day‐night‐activity and life span. For the 533 voles live‐trapped at the study area, the ratio of day versus night captures for each individual served as an activity index and the span between first and last capture measured minimum life span. In accord with higher GUDs at night, very few individuals behaved selectively towards the night, but individual life expectancy increased with temporal opportunism. Microhabitat differences in GUDs reflect short‐term strategies of predator avoidance and the trapping data reflect long‐term patterns of anti‐predator behavior.     

Scales of aboveground and below-ground competition in a semi-arid woodland detected from spatial pattern

Abstract. Semi-arid woodlands are two-phase mosaics of canopy and inter-canopy patches. We hypothesized that both aboveground competition (within canopy patches), and below-ground competition (between canopy patches), would be important structuring processes in these communities. We investigated the spatial pattern of trees in a Pinus edulis-Juniperus monosperma woodland in New Mexico using Ripley's K-function. We found strong aggregation of trees at scales of 2 to 4 m, which indicates the scale of canopy patches. Canopy patches were composed of individuals of both species. Crown centers of both species were always less aggregated than stem centers at scales less than canopy patch size, indicating morphological plasticity of competing crowns. In the smallest size classes of both species, aggregation was most intense, and occurred over a larger range of scales; aggregation decreased with increasing size as is consistent with density-dependent mortality from intraspecific competition. Within canopy patches, younger trees were associated with older trees of the other species. At scales larger than canopy patches, younger trees showed repulsion from older conspecifics, indicating below-ground competition. Hence, intraspecific competition was stronger than interspecific competition, probably because the species differ in rooting depth. Woodland dynamics depend on the scale and composition of canopy patches, aggregated seed deposition and facilitation, above- and below-ground competition, and temporal changes in the spatial scale of interactions. This woodland is intermediate in a grassland-forest continuum (a gradient of increasing woody canopy cover) and hence we expected, and were able to detect, the effects of both above- and below-ground competition.     

Opportunity costs influence food selection and giving‐up density of dabbling ducks

The interaction of animals with their food can yield insights into habitat characteristics, such as perceived predation risk and relative quality. We deployed experimental foraging patches in wetlands used by migrating dabbling ducks Anas spp. in the central Illinois River Valley to estimate variation in seed removal and giving‐up density (GUD; i.e. density of food remaining in patches following abandonment) with respect to seed density, seed size, seed depth in the substrate, substrate firmness, perceived predation risk, and an energetic profitability threshold (i.e. critical food density). Seed depth and the density of naturally‐occurring seeds outside of experimental plots affected seed removal and GUD in experimental patches more than perceived predation risk, seed density, seed size or substrate firmness. The greatest seed removal and lowest GUDs in experimental patches occurred when food resources in alternative foraging locations outside of plots (i.e. opportunity costs) appeared to be near or below a critical food density (i.e. 119–181 kg ha^–1). Giving‐up densities varied substantially from a critical food density across a range of food densities in alternative foraging locations suggesting that fixed GUDs should not be used as surrogates for critical food densities in energetic carrying capacity models. Foraging and resting rates in and near experimental foraging patches did not reflect patterns of seed removal and were poor predictors of GUD and foraging habitat quality. Our results demonstrated the usefulness of GUDs as indicators of habitat quality for subsurface, benthic foragers relative to other available foraging patches and suggested that food may be limited for dabbling ducks during spring migration in some years in the midwestern USA.     

The effects of habitat manipulation on population distribution and foraging behavior in meadow voles

Individuals, free to choose between different habitat patches, should settle among them such that fitness is equalized. Alternatives to this ideal free distribution result into fitness differences among the patches. The concordance between fitnesses and foraging costs among inhabitants of different quality patches, demonstrated in recent studies, suggests that the mode of habitat selection and the resulting fitness patterns may have important implications to the resource use of a forager and to the survival of its prey. We studied how coarse scale selection between habitat patches of different quality and quitting harvest rate in these patches are related to each other and to fine scale patch use in meadow voles (Microtus pennsylvanicus). To demonstrate these relationships, we manipulated habitat patches within large field enclosures by mowing vegetative cover and adding supplemental food according to a 2×2 factorial design. We tracked vole population densities, collected giving‐up densities (GUDs, a measure of patch quitting harvest rate), and monitored the removal of seeds from lattice grids with 1.5 m intervals (an index of fine‐scale space use) in the manipulated habitat patches. Changes in habitat quality induced changes in habitat use at different spatial scales. In preferred habitats with intact cover, voles were despotic and GUDs were low, but increased with the addition of food. In contrast, voles in less‐preferred mowed habitats settled into an ideal free distribution, GUDs were high and uninfluenced by the addition of food. Seed removal was enhanced by the presence of cover but inhibited by supplemental food. Across all treatments, vole densities and GUDs were strongly correlated making it impossible to separate their effects on seed removal rates. However, this relationship broke down in unmowed habitats, where GUDs rather than vole density primarily influenced seed removal by voles. GUDs and seed removal correlated with predation on tree seedlings formerly planted into the enclosures, demonstrating the mechanisms between coarse‐scale habitat manipulations and community level consequences on a forager's prey.     

Linking foraging decisions to residential yard bird composition

Urban bird communities have higher densities but lower diversity compared with wildlands. However, recent studies show that residential urban yards with native plantings have higher native bird diversity compared with yards with exotic vegetation. Here we tested whether landscape designs also affect bird foraging behavior. We estimated foraging decisions by measuring the giving-up densities (GUD; amount of food resources remaining when the final forager quits foraging on an artificial food patch, i.e seed trays) in residential yards in Phoenix, AZ, USA. We assessed how two yard designs (mesic: lush, exotic vegetation; xeric: drought-tolerant and native vegetation) differed in foraging costs. Further, we developed a statistical model to calculate GUDs for every species visiting the seed tray. Birds foraging in mesic yards depleted seed trays to a lower level (i.e. had lower GUDs) compared to birds foraging in xeric yards. After accounting for bird densities, the lower GUDs in mesic yards appeared largely driven by invasive and synanthropic species. Furthermore, behavioral responses of individual species were affected by yard design. Species visiting trays in both yard designs had lower GUDs in mesic yards. Differences in resource abundance (i.e., alternative resources more abundant and of higher quality in xeric yards) contributed to our results, while predation costs associated with foraging did not. By enhancing the GUD, a common method for assessing the costs associated with foraging, our statistical model provided insights into how individual species and bird densities influenced the GUD. These differences we found in foraging behavior were indicative of differences in habitat quality, and thus our study lends additional support for native landscapes to help reverse the loss of urban bird diversity.     

Competition between perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus): the advantage of turning night into day

1. The outcome of interspecific competition for food resources depends both on the competitors’ sensory abilities and on environmental conditions. In laboratory experiments we tested the influence of daylight and darkness on feeding behaviour and specific growth rate (SGR) of two species with different sensory abilities. 2. We used perch (Perca fluviatilis) as a visually orientated, and ruffe (Gymnocephalus cernuus) as a mechano‐sensory oriented predator and tested their growth rates and behaviour under conditions of interspecific and intraspecific competition. Three different foraging conditions were used: food supplied (i) only during the day, (ii) only during the night or (iii) during both day and night. 3. In perch neither SGR nor feeding behaviour were influenced substantially by interspecific competition during daylight. During darkness their foraging behaviour changed markedly and their access to the food source as well as their SGR were negatively affected by the presence of ruffe. 4. Ruffe's foraging behaviour did not change during either day or night with interspecific competition. During the night ruffe's SGR was higher with interspecific competition, probably because of a release from intraspecific competition and the competitive inferiority of perch during the night. 5. Because of its seonsory abilities ruffe feeds predominantly at night, thereby reducing competitive interference from perch.     

Land-use and edge effects unbalance seed dispersal and predation interactions under habitat fragmentation

The process of fragmentation can greatly influence plant–animal interactions. To assess the degree to which it affects the balance between two interactions of opposite sign, namely seed dispersal and post-dispersal seed predation, we selected 16 patches of chestnut forest in O Courel and El Bierzo, northwestern Spain. We assessed the effect of fragmentation over two different seed dispersal–predation systems using Helleborus foetidus and Ilex aquifolium as model species. In the first case, field experiments consisted of seed-offering trays with selective exclusion of rodents and ants in a two-way orthogonal design. In the second experiment, we placed experimental branches and trays on the floor to assess seed dispersal and predation. The interactions between several fragment traits and the relative contribution of rodents, ants and birds to seed removal were analyzed by means of generalized linear mixed models. Results show that for H. foetidus, differences in seed dispersal–predation were accounted for by patch shape, which affected mainly the dispersal phase. Major seed dispersal took place in patches with a smaller edge to core ratio and high plant cover (abandoned patches), whilst the latter also showed maximum seed predation. For I. aquifolium, fragmentation effects were significant only for seed predation, which was increased in abandoned patches. This shows that the effects of habitat fragmentation can emerge at different phases depending on specific traits of the interacting animals. It also highlights the importance of traditional land-use practices in species interactions.     

Niche partitioning and the effect of interspecific competition on microhabitat use by two sympatric galaxiid stream fishes

1. Numerous interacting abiotic and biotic factors influence niche use and assemblage structure of freshwater fishes, but the strength of each factor changes with spatial scale. Few studies have examined the role of interspecific competition in structuring stream fish assemblages across spatial scales. We used field and laboratory approaches to examine microhabitat partitioning and the effect of interspecific competition on microhabitat use in two sympatric stream fishes (Galaxias‘southern’ and Galaxias gollumoides) at large (among streams and among sites within streams) and small (within artificial stream channels) spatial scales. 2. Diurnal microhabitat partitioning and interspecific competition at large spatial scales were analysed among three sympatry streams (streams with allotopic and syntopic sites; three separate catchments) and four allopatry streams (streams with only allotopic sites; two separate catchments). Electro‐fishing was used to sample habitat use of fishes at 30 random points within each site by quantifying four variables for each individual: water velocity, depth, distance to nearest cover and substratum size. Habitat availability was then quantified for each site by measuring those variables at each of 50 random points. Diet and stable isotope partitioning was analysed from syntopic sites only. Diel cycles of microhabitat use and interspecific competition at small spatial scales were examined by monitoring water velocity use over 48 h in artificial stream channels for three treatments: (i) allopatric G. ‘southern’ (10 G. ‘southern’); (ii) allopatric G. gollumoides (10 G. gollumoides) and (iii) sympatry (five individuals of each species). 3. One hundred and ninety‐four G. ‘southern’ and 239 G. gollumoides were sampled across all seven streams, and habitat availability between the two species was similar among all sites. Galaxias‘southern’ utilised faster water velocities than G. gollumoides in both the field and in channel experiments. Both species utilised faster water velocities in channels at night than during the day. Diet differences were observed and were supported by isotopic differences (two of three sites). No interspecific differences were observed for the other three microhabitat variables in the field, and multivariate habitat selection did not differ between species. Interspecific competition had no effect on microhabitat use of either species against any variable either in the field (large scale) or in channels (small scale). 4. The results suggest that niche partitioning occurs along a subset of microhabitat variables (water velocity use and diet). Interspecific competition does not appear to be a major biotic factor controlling microhabitat use by these sympatric taxa at any spatial scale. The results further suggest that stream fish assemblages are not primarily structured by biotic factors, reinforcing other studies de‐emphasising interspecific competition.     

The effect of barn owls (Tyto alba) on the activity and microhabitat selection of Gerbillus allenbyi and G. pyramidum

Predation plays an important role in ecological communities by affecting prey behavior such as foraging and by physical removal of individual prey. In regard to foraging, animals such as desert rodents often balance conflicting demands for food and safety. This has been studied in the field by indirectly manipulating predatory risk through the alteration of cues associated with increased risk such as cover or illumination. It has also been studied by directly manipulating the presence of predators in aviaries. Here, we report on experiments in which we directly manipulated actual predatory risk to desert rodents in the field. We conducted a series of experiments in the field using a trained barn owl (Tyto alba) to investigate how two species of coexisting gerbils (Gerbillus allenbyi and G. pyramidum) respond to various cues of predatory risk in their natural environment. The gerbils responded to risk of predation, in the form of owl flights and owl hunger calls, by reducing their activity in the risky plot relative to the control plot. The strongest response was to owl flights and the weakest to recorded hunger calls of owls. Furthermore, when risk of predation was relatively high, as in the case with barn owl flights, both gerbil species mostly limited their activity to the safer bush microhabitat. The response of the gerbils to risk of predation disappeared very quickly following removal of the treatment, and the gerbils returned to normal levels of activity within the same night. The gerbils did not respond to experimental cues (alarm clock), the presence of the investigators, the presence of a quiet owl, and recorded white noise. Using trained barn owls, we were able to effectively manipulate actual risk of predation to gerbils in natural habitats and to quantify how gerbils alter their behavior in order to balance conflicting demands of food and safety. The method allows assessment of aspects of behavior, population interactions, and community characteristics involving predation in natural habitats.     

Foraging efficiency of <Emphasis Type="Italic">Akodon azarae</Emphasis> under different plant cover and resource levels

The goal of this work was to determine how the foraging behaviour of Akodon azarae changes with predation risk and food availability in cropfield borders of Buenos Aires, Argentina. Our hypotheses were that A. azarae has a greater foraging efficiency in safe areas than in risky ones and that the foraging behaviour of A. azarae also depends on the level of resources. We measured giving-up densities (GUDs) and food consumption twice a year in artificial foraging patches (bottles with known amounts of millet seed) in covered and open areas and with two different levels of seed abundance. In both periods, GUDs were lower in the covered areas than in the open ones independently of food level. Consumption increased with food level in covered areas but not in open areas. Based on these results, we conclude that A. azarae appears to maximize its consumption depending on predation risk.     

Elevational replacement of two Himalayan titmice: interspecific competition or habitat preference?

Elevational species replacement is a widely documented pattern in montane species. Although interspecific competition has been shown to be important in setting species elevational limits in tropical habitats, its effect in species of temperate regions is poorly studied. We tested the role of interspecific competition for space in the breeding season and for food in the non‐breeding season in mediating the distribution of two resident titmice species in the Himalayas. We show that high elevation green‐backed tits Parus monticolus are behaviourally dominant over low elevation cinereous tits Parus cinereus in both song playback and feeder trials. Despite being subordinate, at their elevational upper limit, cinereous tits occur in sympatry in human modified habitats. Our study suggests that the loss of natural habitats in the sympatric zone, not interspecific competition, might be limiting the distribution of the high‐elevation green‐backed tits and facilitating an upward range shift through human association in cinereous tits.     

Are subordinate ants the best seed dispersers? Linking dominance hierarchies and seed dispersal ability in myrmecochory interactions

True myrmecochory involves the dispersal of elaiosome-bearing seeds by ants. Between the guild of ants that are attracted to these seeds, only a few of them will act as effective dispersers, that is, transporting the seeds to suitable sites (the nests) for germination and plant establishment. Ant communities are known to be highly hierarchical, and subordinate ants quickly deliver resources to their nest rather than consuming it on-site, thereby avoiding encounters with more dominant species. As a result of a series of studies that were carried out during summer in semi-arid Northwest Argentina, we have found that the most important seed disperser of the myrmecochorous plant Jatropha excisa Griseb. (Euphorbiaceae), the ant Pogonomyrmex cunicularius pencosensis Forel, was the most subordinate species during interspecific interactions. The daily timing of release of the J. excisa seeds through ballistic dispersal increased their probability of being removed by the highly thermophilic P. cunicularius pencosensis. Foraging during the warmest hours of the day allowed P. cunicularius pencosensis ants to avoid the risk of interference competition with dominant species, which also behaved as elaiosome predators. As a conclusion, subordinance behaviour appears to be integral to successful myrmecochory, and also the timing of seed release plays a key role in shaping the dynamics of myrmecochorous interactions. Therefore, ant-dispersed plants should not only favour their discovery by subordinate ants, but also should present their seeds at those times of the day when the behaviourally dominant ants are less active.