Aziz Subach

Interference competition and temporal and habitat partitioning in two gerbil species

We have tested two hypotheses which may explain two different patterns which underlie coexistence in two species of desert gerbils (Gerbillus allenbyi and G. pyramidum). The two patterns are temporal partitioning of foraging activity and shared preference habitat selection. When sympatric, G. pyramidum uses the early part of the night most heavily while G. allenbyi is most active in the late part of the night. Although both species prefer the same habitat type (semistabilized sand dune), in the presence of G, pyramidum, G. allenbyi utilizes mainly the secondary habitat type (stabilized sand dune)

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.

Mechanisms of species coexistence of optimal foragers: temporal partitioning by two species of sand dune gerbils

Mechanistic approaches to understanding species coexistence and community structure have recently proved highly successful. We apply this approach to a sand dune community containing two common species of gerbils, Gerbillus allenbyi and G. pyramidum. Previous work suggested that coexistence is based on temporal variability in seed resource density on a daily time scale and a tradeoff of foraging efficiency at high versus low resource abundances. We tested the predictions of this mechanism by quantifying rodent foraging activity and its timing during the night over 6 nights using sand tracking plots, sand tracking strips, and seed trays. We demonstrated temporal partitioning with all three methods

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.

THE COSTS OF APPREHENSIVE FORAGING

We used a behavioral bioassay, in the form of foraging behavior of Gerbillus allenbyi, and the ideal free distribution to estimate the costs associated with risk of predation. Experiments were conducted in two pairs of 2-ha field enclosures. Risk of predation was introduced to one 1-ha subplot of each pair of enclosures either by simulating the light of a full moon or by causing trained owls to fly over our experimental subplots. We used a titration method to estimate the energetic costs associated with risk of predation. We added seeds to the 1-ha subplot of each enclosure that experienced the risk of predation. The adjacent subplot served as a control. Without seed addition the subplot without risk of predation had more foraging activity: the foragers were avoiding the risk of predation. As the amount of seeds in the treatment increased, the gerbils responded smoothly and quantitatively and shifted their foraging activity back to the subplot with the seeds (and risk). At addition rates of 4.24–8.47 g se...

Do gerbils care more about competition or predation

We used trained barn owls to introduce a controlled predation threat to two species of gerbil, Gerbillus allenbyi and G. pyramidum in a system of 2-ha, sandy-substrate field enclosures in the Negev Desert, Israel. Using the principles of optimal density-dependent habitat selection, we estimated several coefficients of population interaction focusing on G. allenbyi. G. allenbyi exhibits strong intraspecific competition. In the absence of owls, G. pyramidum competes with it (α = - 0.35). We estimated the slope of the G. allenbyi victim isocline to be - 0.60. The competitive effect of G. pyramidum disappeared in the presence of owl, although the intraspecific competition remained. Our results indicated that in the presence of owls, the threat of predation overwhelms the cost of interspecific competition.

Foraging behaviour and habitat selection in pit-building antlion larvae in constant light or dark conditions

Pit-building antlion larvae are small sit-and-wait arthropod predators that dig conical pits in sandy soils. We investigated the effect of exposure to constant light versus constant dark conditions on antlion behaviour. Antlions tended to relocate less often, construct pits more frequently and construct larger pits in constant light. We interpret this as an indication of dichotomous behaviour under light versus dark conditions such that antlion larvae hunt during the day and modify their positions at night. Antlion larvae immediately responded to the complete switch in this illumination treatment and adapted their behaviour to fit the new conditions. Prey capture success did not differ between the constant light and constant dark conditions. When provided with a choice between light and dark conditions, the vast majority of larvae preferred the former. Those that preferred constant dark, however, were on average larger than those that preferred constant light. We thus suggest that since larger antlion larvae possess larger fat reserves, they can afford reductions in foraging activity (possibly to increase safety). An examination of the antlion trade-off between their preference for light and their preference for sand depth (which may affect their ability to evade predators) showed that their response to illumination was much more prominent, irrespective of the sand depth. Finally, combining light with other factors such as temperature and density may further elucidate microhabitat selection in pit-building antlion larvae.

Effect of spatial pattern and microhabitat on pit construction and relocation in Myrmeleon hyalinus (Neuroptera: Myrmeleontidae) larvae

Abstract 1. In two sets of enclosure experiments, we studied the spatial pattern, relocation rates, pit construction rates, and microhabitat preference of Myrmeleon hyalinus larvae.

A comparison between desert and Mediterranean antlion populations: differences in life history and morphology

We performed a transplant experiment to compare the life histories and morphologies of five geographically representative antlion Myrmeleon hyalinus populations along a sharp climatic gradient, from a Mediterranean climate in Israel’s north to a desert climate in the south. Larvae were raised in two environmental chambers simulating Mediterranean and desert climates to investigate the extent to which the different populations exhibit phenotypic plasticity. Along the north‐to‐south climatic gradient, we observed a gradient in body mass prior to pupation and in pupation rate. Mediterranean populations suffered higher mortality rate when exposed to desert conditions, whereas the mortality rate of desert populations was consistent between Mediterranean and desert conditions. Our results regarding body mass, pupation rate and mortality rate suggest that Mediterranean populations had a more flexible response compared with desert populations. An analysis of digital photographs was used to measure population morphological differences, which were usually indicative of a decrease in trait size along the north‐to‐south gradient. We show how climatic gradients translate into phenotypic differences in an antlion population and provide a morphometric tool to distinguish between instar stages.

The interplay between foraging mode, habitat structure, and predator presence in antlions

Antlion larvae are sand-dwelling insect predators, which ambush small arthropod prey while buried in the sand. In some species, the larvae construct conical pits and are considered as sit-and-wait predators which seldom relocate while in other species, they ambush prey without a pit but change their ambush site much more frequently (i.e., sit-and-pursue predators). The ability of antlion larvae to evade some of their predators which hunt them on the sand surface is strongly constrained by the degree of sand stabilization or by sand depth. We studied the effect of predator presence, predator type (active predatory beetle vs. sit-and-pursue wolf spider), and sand depth (shallow vs. deep sand) on the behavioral response of the pit building Myrmeleon hyalinus larvae and the sit-and-pursue Lopezus fedtschenkoi larvae. Predator presence had a negative effect on both antlion species activity. The sit-and-wait M. hyalinus larvae showed reduced pit-building activity, whereas the sit-and-pursue L. fedtschenkoi larvae decreased relocation activity. The proportion of relocating M. hyalinus was negatively affected by sand depth, whereas L. fedtschenkoi was negatively affected also by the predator type. Specifically, the proportion of individual L. fedtschenkoi that relocated in deeper sand was lower when facing the active predator rather than the sit-and-pursue predator. The proportion of M. hyalinus which constructed pits decreased in the presence of a predator, but this pattern was stronger when exposed to the active predator. We suggest that these differences between the two antlion species are strongly linked to their distinct foraging modes and to the foraging mode of their predators.