In the foraging game between gerbils and their predators, gerbils manage risk of predation using the tools of time allocation (where, when and for how long to forage) and vigilance. Th e optimal level of a forager ' s vigilance should be aff ected by its encounter rate with predators and the eff ectiveness of its vigilance in reducing mortality risk. Th e physical structure of the environment can alter the eff ectiveness of its vigilance and therefore alter its foraging behaviour. We tested this for gerbils at risk of predation from barn owls or foxes in a large vivarium. In particular, we reduced the eff ectiveness of vigilance by placing obstructions around feeding trays that blocked sight lines along either the vertical (vigilance directed against owls) or horizontal axis (vigilance directed against foxes), thereby changing the physical structure of the environment. In addition, we manipulated the presence of foxes and owls. In general, gerbils harvested fewer seeds, allocated less time to foraging in dangerous patches, and used more vigilance while foraging where and when risks were higher (i.e. in the presence of predators and in bright moonlight). Vertical and horizontal sightline treatments interacted synergistically to further raise perceived risk.Th ese results imply that blocking sight lines reduces the eff ectiveness of vigilance, causing gerbils to use it less. Moreover, in the presence of a predator, the gerbils ' response to the blocked sightlines was more severe -harvesting less food and spending less time and vigilance -in the patches with the increased risk. Th is was especially so in the presence of the predator that was expected to most benefi t from blocking that particular type of sight line: cover that blocked vertical sight lines was riskiest in the presence of owls, and cover that blocked horizontal sight lines was riskiest in the presence of foxes. Th ese results strongly indicate the importance of sightlines and landscape features such as bushes in the risk management and forging decisions of gerbils, demonstrating that bush cover provides mixed blessing to gerbils by providing cover, but making vigilance ineff ective.Th e clever forager reacts to the risk of predation by using behavioral tools such as time allocation and vigilance to reduce and manage the risk to which it is exposed (Lima 1987a).Apprehension and vigilance are behavioral tools used by animals to detect risks when foraging, including those from predators (Kotler et al. 2004a) and potential competitors for food and territory (Lima 1987b). Apprehension is defi ned as any reduction in attention to other activities (e.g. foraging) as a result of increasing the allocation of attention given to detecting and/or responding to potential predator activity . Kotler et al. (2004b) showed that gerbils foraged less effi ciently under complex conditions when risk of predation was high, as their attention was not wholly on the task at hand. Vigilance is defi ned as a special case where apprehension is total (i.e. all attention is direct...

Desert rodent assemblages from around the world provide convergent, but independent crucibles for testing theory and deducing general ecological principles. The heteromyid rodents of North America and the gerbils of the Middle East and their predators provide such an example. Both sets of rodents face predation from owls and vipers, but the North American species possess unique traits that may represent macroevolutionary breakthroughs: rattlesnakes have infra-red sensitive sensory pits, and heteromyids have cheek pouches. To test their significance, we brought together two gerbils (Middle East), two heteromyid rodents (a kangaroo rat and a pocket mouse; North America) in a common setting (a vivarium in the Negev Desert), and quantified the “opinions” of the rodents towards the North American sidewinder rattlesnake and the Middle Eastern Saharan horned viper and the foraging behavior of each in the face of these snake predators plus owl predators. Gerbils are fairly evenly matched in their anti-predator abilities, while the heteromyids differ widely, and these seem to match well with and may determine the types of mechanisms of species coexistence that operate in the communities of each continent. Evolutionary history, macroevolutionary traits, and risk management therefore combine to determine the characteristics of the organisms and the organization of their communities.

In predator-prey foraging games, the prey's reaction to one type of predator may either facilitate or hinder the success of another predator. We ask, do different predator species affect each other's patch selection? If the predators facilitate each other, they should prefer to hunt in the same patch; if they interfere, they should prefer to hunt alone. We performed an experiment in a large outdoor vivarium where we presented barn owls (Tyto alba) with a choice of hunting greater Egyptian gerbils (Gerbillus pyramidum) in patches with or without Saharan horned vipers (Cerastes cerastes). Gerbils foraged on feeding trays set under bushes or in the open. We monitored owl location, activity, and hunting attempts, viper activity and ambush site location, and the foraging behavior of the gerbils in bush and open microhabitats. Owls directed more attacks towards patches with vipers, and vipers were more active in the presence of owls. Owls and vipers facilitated each other's hunting through their combined effect on gerbil behavior, especially on full moon nights when vipers are more active. Owls forced gerbils into the bushes where vipers preferred to ambush, while viper presence chased gerbils into the open where they were exposed to owls. Owls and vipers took advantage of their indirect positive effect on each other. In the foraging game context, they improve each other's patch quality and hunting success.

14Unlike desert rodents from North America, Allenby's gerbil (Gerbillus andersoni 15 allenbyi) from the Negev Desert, Israel has evolved with snakes that do not have heat-sensitive 16 sensory pits that enhance night vision. Does this history affect their ability to assess and respond 17 to a snake that has this ability? As a test, we exposed gerbils to risk of predation from various

The biodegradation capacity of indigenous microbial populations was examined in a desert soil contaminated with crude oil. To evaluate biodegradation, soil samples supplemented with 5, 10 or 20% (w/w) of crude oil were incubated for 90 days at 30 degrees C. The effect of augmentation of the soil with vermiculite (50% v/v) as a bulking agent providing increased surface/volume ratio and improved soil aeration was also tested. Maximal biodegradation (91%) was obtained in soil containing the highest concentration of crude oil (20%) and supplemented with vermiculite; only 74% of the oil was degraded in samples containing the same level of crude oil but lacking vermiculite. Gas chromatograms of distilled fractions of crude oil extracted from the soil before and after incubation demonstrated that most of the light and part of the intermediate weight fractions initially present in the oil extracts could not be detected after incubation. Monitoring of microbial population densities revealed an initial decline in bacterial viable counts after exposure to oil, presumably as a result of the crude oil's toxicity. This decline was followed by a steep recovery in microbial population density, then by a moderate increase that persisted until the end of incubation. By contrast, the inhibitory effect of crude oil on the fungal population was minimal. Furthermore, the overall increased growth response of the fungal population, at all three levels of contamination, was about one order of magnitude higher than that of the bacterial population.