Burt P. Kotler

Risk of Predation and the Structure of Desert Rodent Communities

Communities of granivorous desert rodents may be influenced by either predation risk or resources. To examine the influence of these factors, I manipulated illumination, using lanterns, and resources, using seeds. Foraging behavior is responsive to changes in predation risk; increased illumination reduces foraging in open areas without cover. Foraging behavior is also affected by resource enrichments. Differences among species in habitat selection are correlated with specific abilities to detect and avoid predators. The least vulnerable species, Dipodomys deserti, foraged heavily in the open and was largely unaffected by treatments; the other species of kangaroo rats and kangaroo mice (Dipodomys merriami, Dipodomys microps, and Microdipodops pallidus) also prefer the open, but responded to both risk and resource manipulations; highly vulnerable Peromyscus maniculatus was restricted to bushes, even under the best of circumstances; Perognathus longimembris was displaced from preferred microhabitats by the presence of kangaroo rats. A correlation between auditory bullar volume and use of open habitat by the various species in this community suggests that predation risk provides an axis along which habitat segregation occurs. Predation can structure communities of mobile prey when risk differs among habitats. Animals specializing in predator avoidance and in exploitation of risky environments have reduced interactions with superior competitors; this promotes coexistence.

Factors Affecting Gerbil Foraging Behavior and Rates of Owl Predation

We experimented on how illumination, habitat structure, and three different species of owls affected the foraging behavior of Gerbillus allenbyi and G. pyramidum, two gerbil species that coexist on sand dune habitats in the Negev Desert, Israel. We also tested how illumination and habitat structure affected rates of predation by owls on the two gerbil species. In a large aviary, we manipulated presence and absence of owls, owl species, presence and absence of illumination, and shrub cover. In response to the presence of owls or to increased illumination, gerbils foraged less, shifted foraging activity to the bush microhabitat, and quit patches at a higher giving-up density of resources. In accord with moonlight avoidance, both gerbil species suffered higher predation rates under illumination than in the absence of illumination. In addition, G. pyramidum distinguished among owl species, as indicated by changes in patch use and habitat selection. Habitat structure also affected foraging behavior and rates of predation. Gerbils foraged less in the open than in the bush microhabitat, foraged less when there was no cover present, and foraged less in the bush microhabitat when patches were encumbered by entangling branches. In accord with avoidance of open areas, both gerbil species suffered higher rates of predation when shrub cover was 0% than when shrub cover was 10%. With 0% cover, G. allenbyi suffered higher predation rates than G. pyramidum, but with 10% cover, rates of owl predation did not differ between gerbil species. Rates of owl predation on the two species corresponded to their natural patterns of macro- and microhabitat partitioning; relative to G. allenbyi, G. pyramidum predominates on open sand dunes and biases its behavior toward the open microhabitat. The results suggest that predation interacts with resource competition to determine the distribution and habitat separation of G. allenbyi and G. pyramidum.

The effects of owl predation on the foraging behavior of heteromyid rodents

SummaryResearchers have documented microhabitat partitioning among the heteromyid rodents of the deserts of North America that may result from microhabitat specific predation rates; large/bipedal species predominate in the open/risky microhabitat and small/quadrupedal species predominate in the bush/safer microhabitat. Here, we provide direct experimental evidence on the role of predatory risk in affecting the foraging behavior of three species of heteromyid rodents: Arizona pocket mouse (Perognathus amplus; small/quadrupedal), Baileys pocket mouse (P. baileyi; large/quadrupedal), and Merriams kangaroo rat (Dipodomys merriami; large/bipedal). Both kangaroo rats and pocket mice are behaviorally flexible and able to adjust their foraging behavior to nightly changes in predatory risk. Under low levels of perceived predatory risk the kangaroo rat foraged relatively more in the open microhabitat than the two pocket mouse species. In response to the presence of barn owls, however, all three species shifted their habitat use towards the bush microhabitat. In response to direct measures of predatory risk, i.e. the actual presence of owls, all three species reduced foraging and left resource patches at higher giving up densities of seeds. In response to indirect indicators of predatory risk, i.e. illumination, there was a tendency for all three species to reduce foraging. The differences in morphology between pocket mice and kangaroo rats do appear to influence their behavioral responses to predatory risk.

Short-Term Apparent Competition

Interspecific interactions reflect the cumulative consequences of individual behavioral acts. The foraging decisions made by predators influence the way in which predation shapes the structure of prey communities. Alternative prey species co-occurring in a patch embedded in a matrix of many similar patches may interact through a shared mobile predator in two distinct ways. First, the functional response by an individual predator foraging in the patch to one prey species may be affected by the density of a second prey species in the patch (e.g., any time spent handling one prey reduces the time available for capturing other prey). Second, the presence of a second prey species may alter the propensity of predators to aggregate or remain in a given patch. We argue that this aggregative numerical response can in many circumstances generate -, - interactions (apparent competition) between prey species that otherwise would not interact. This is most likely if predators use a simple optimality criterion for prey selectivity within patches and the marginal-value theorem for deciding when to enter and leave patches. By contrast, if predators have suboptimal diets within patches but leave in accord with the marginal-value theorem, alternative prey may experience a +, - interaction; and, if predators use patches independently of prey availability, a +, + interaction between alternative prey can occur. Hence, the qualitative character of the interaction between alternative prey in a patchy environment depends on the degree to which predators do, or do not, match the canonical predictions of optimal foraging theory.

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)

Habitat Selection: An Experimental Field Test with Two Gerbil Species

We provide experimental evidence for the isoleg theory of habitat selection in a pair of psammophilic gerbil species. Gerbillus allenbyi (mean mass: 26 g) and G. pyramidum (mean mass: 40 g) coexist in Israels Negev desert in areas that may contain three distinct sandy habitats: stabilized sand fields, semistabilized dunes, and drifting dunes. When all three habitat types are available, coexistence between the two species has been explained by a centrifugal model of community organization that has been untested until now. To begin testing it, we conducted a field experiment in six 1 ha enclosures, each containing similar proportions of two of the sandy—habitat types (stabilized sand and semi—stabilized dune). This experiment tested the following hypotheses concerning the coexistence of the two species: (1) both species prefer the same primary habitat type; (2) G. allenbyi and G. pyramidum exhibit intraspecific density—dependent habitat selection; (3) habitat preference of both G. allenbyi and G. pyramidum...

The effects of morphology and body size on rates of owl predation on desert rodents.

Predation has been implicated in promoting microhabitat partitioning among desert rodents, but little direct evidence exists on how microhabitat or rodent morphology affect predation rates. We measured rates of predation by barn owls (Tyto alba) on heteromyid rodents which differ in body size (Perognathus amplus versus P. baileyi) and morphology (P. baileyi versus Dipodomys merriami). The experiments were conducted in a large, semi-natural enclosure. Owls captured the larger species at a significantly higher rate than the smaller species. We were unable to detect a consistent effect of rodent morphology on rates of predation. Also, rates of predation were higher for full moon levels of illumination than for starlight levels. In a separate experiment, we demonstrated that rates of predation are higher on rodents foraging in the open than on rodents foraging with cover. The results support the hypothesis that desert rodents represent a community where predation and competition interact directly to determine rodent foraging behavior and species interactions.

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.

Titrating food and safety in a heterogeneous environment: when are the risky and safe patches of equal value?

A forager should exploit a depletable resource patch until the marginal benefits of patch exploitation fall to equal the marginal costs, and it should allocate effort to different patches in order to equalize marginal value across patches. Foraging decisions are therefore titrations of marginal costs and marginal benefits within patches and of marginal value across patches. Often, a forager must balance conflicting demands for food acquisition and safety. Thus, insights into foraging can be gained through titration experiments involving food and safety. Two types of behavioral titrations are possible : (1) equalizing marginal costs and marginal benefits within patches and marginal value across patches and (2) equalizing time, energy harvested, or some other quantity across patches and habitats. The first type of titration is performed by the animal, and the second type can be performed by the experimenter ; the first type of titration makes the second possible. We conducted titration experiments involving food and safety with gerbils subjected to predation by owls in a large aviary. We examined patch use by gerbils in manipulated resource patches (seed trays) placed in both the bush and the open microhabitats. Rodents took both food and safety into consideration when deciding how long to stay in a patch : giving-up densities of resources in seed trays (GUDs) were higher in the open than in the bush microhabitat. Also, enrichment of resource patches in the open microhabitat revealed that resource patches in the open needed to be 4 to 8 times richer than patches in the bush microhabitat in order to be equally valuable. These results are consistent with field data collected elsewhere indicating that foraging costs are comprised mostly of costs arising from the risk of predation.

Oviposition habitat selection by the mosquito, Culiseta longiareolata: effects of conspecifics, food and green toad tadpoles

Abstract. 1. Ovipositing females should choose egg‐laying sites which maximize the growth and survivorship of their offspring. Accordingly, they should avoid habitats with high densities of competitors and predators and should choose sites with adequate food. We tested experimentally the oviposition response of the mosquito, Culiseta longiareolata Macquart, to extra food and two species which are both potential competitors and predators of its offspring: conspecific larvae and green toad (Bufo viridis) tadpoles.