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.     

Small mammal community ecology

Summary The abundance of food in natural 1-ha plot of shortgrass prairie was artificially manipulated by adding alfalfa pellets and whole oats on a regular schedule. The small mammal species naturally inhabiting the manipulated food plot did not respond in either their density or weight to the supplemented food. However, a new specialized seed-eating species, Dipodomys ordii, invaded the food plot and persisted in relatively high density. As a result of this colonization, species diversity was significantly higher on the food plot relative to the unmanipulated control plot. In a second experiment, application of water and nitrogen to two 1-ha plots of shortgrass prairie resulted in increased productivity. However, the increased production on this latter treatment was associated with vegetation growth and thus major changes in habitat structural characteristics relative to the control treatment. Two new species, Microtus ochrogaster and Reithrodontomys megalotis, colonized the nitrogen+water treatment, but other small mammal species resident to the shortgrass prairie largely avoided this treatment. As a result of this manipulation, species diversity was significantly lower than the species diversity on the control treatment. These two results agree with the MacArthur prediction regarding how species diversity responds to an increase in production of scarce resources without changes in other habitat variables, and to an increase in productivity associated with changes in other habitat variables.     

The effect of water availability on fuel deposition of two staging Sylvia warblers

In order to succeed in crossing extensive ecological barriers, migratory birds usually deposit fuel en route. High rates of fuel deposition may enable birds to shorten their total migration time and are therefore advantageous for time‐minimizing migrants. Several studies have suggested that water provision may increase food utilization in non‐migratory birds. The goal of this study was to test the influence of water availability on the fuel deposition of en route migratory passerines. We studied fuel deposition of blackcaps Sylvia atricapilla and lesser whitethroats S. curruca staging in a plantation of Mount Atlas gum‐tree Pistacia atlantica in the northern Negev desert, Israel, during the autumns of 2000 and 2002. We manipulated water availability at the site and measured the effect of water supplementation on fuel deposition of birds of both species. We found that when water was available, blackcaps had higher fuel loads and higher fuel deposition rates than during control trials. However, water availability had no effect on fuel deposition of lesser whitethroats. Species‐specific differences in adaptations to arid conditions, reflected in the species’ winter habitat preferences, may be responsible for the between‐species dissimilarity in responding to water provision. We suggest that water availability may have strong ecological and evolutionary consequences for birds migrating through arid environments, by its possible effect on bird behavior and physiology.     

Effects of food abundance, age, and flea infestation on the body condition and immunological variables of a rodent host, and their consequences for flea survival

Temporal variation in body condition and immunological variables of animals that harbor parasites may explain patterns of variation in infestation, as well as parasite impact on the host. We emulated such variability in Sundevall's jirds by manipulating food availability and flea infestation in juveniles and adults and examining how these changes affect survival of fleas on their hosts. Body condition of food-restricted jirds deteriorated, but there was no change in their immunological variables. Adult jirds were in better body condition and had higher immunocompetence than juveniles, however there were no significant effects of flea infestation on any of the variables examined. The main effects of flea infestation were a decrease in the response to phytohaemagglutinin injection, and an increase in the negative effects of food restriction on body mass. Flea survival was higher on juveniles, but fleas did not respond to temporal variability in body condition and immunocompetence of the jirds. We concluded that changes in body condition and immune responses due to growth or variability in food abundance are more important than changes caused by the fleas themselves. Flea infestation is more detrimental to jirds when they are not able to compensate for mass loss through increased food consumption.     

Can models of density-dependent habitat selection be applied for trap-building predators?

Density-dependent habitat selection theory was mainly tested on active foragers and therefore its applicability to trap-building predators is poorly understood. The high sensitivity of trap-building predators to changes in their physical environment, combined with their limited movement capability, can lead to habitat specialization, reducing their utilization of alternative habitats. We studied density-dependent habitat selection in two pit-building antlions, differing in their habitat utilization spectrum. The habitat generalist, Myrmeleon hyalinus, inhabits and performs equally well in both sand- and loess-derived soils, although preferring the former, more productive coarse-grained soils. In contrast, the habitat specialist, Cueta lineosa, only inhabits fine-grained soils such as loess, while showing reduced foraging performance in coarse-grained sandy soils. We allowed larvae to select between these two soils, while manipulating con-specific density and initial stocking position. Irrespective of the initial stocking position, the number of M. hyalinus pits in the sand was positively correlated with the number of con-specific pits in the loess, implying that this species is a density-dependent habitat selector. Furthermore, these patterns of density-dependence were consistent with the expectations of ideal pre-emptive distribution (i.e., strong non-linearity in the distribution of antlions between soils with increased total density), suggesting that interference competition largely dictates habitat selection in this species. In contrast, the habitat specialist showed constant habitat selectivity, as neither con-specific density nor initial stocking positions influenced its habitat preference. Although mainly tested on active foragers, habitat selection theory can be applicable for trap-building predators, demonstrating how mechanisms operating at the individual level influence spatial distribution patterns.     

Bird predation alters infestation of desert lizards by parasitic mites

Theory predicts that predators can reduce parasite abundance on prey by reducing prey density and through disproportionate predation on heavily infested individuals. We experimentally tested this prediction by examining the effects of bird predation on parasitic mite infestation of the prey lizard Acanthodactylus beershebensis. We manipulated predation by adding perches to arid scrubland, allowing avian predators to hunt for lizards in a habitat the birds would not normally use. Host density influenced parasite abundance in hatchlings, but not in older aged individuals and parasite abundance did not affect lizard host survival. Contrary to expectation mite abundance on adult lizards increased under low predation intensities. We explain these results by suggesting a novel hypothesis based on the assumption that the two components of predation, i.e. actual removal of prey and risk, exert contradictory effects on macroparasite abundance.     

Large scale systemic control short-circuits pathogen transmission by interrupting the sand rat (Psammomys obesus)-to-sand fly (Phlebotomus papatasi) Leishmania major transmission cycle

Systemic control uses the vertebrate hosts of zoonotic pathogens as “Trojan horses,” killing blood-feeding female vectors and short-circuiting host-to-vector pathogen transmission. Previous studies focused only on the effect of systemic control on vector abundance at small spatial scales. None were conducted at a spatial scale relevant for vector control and none on the effect of systemic control on pathogen transmission rates. We tested the application of systemic control, using Fipronil-impregnated rodent baits, in reducing Leishmania major (Kinetoplastida: Trypanosomatidae; Yakimoff & Schokhor, 1914) infection levels within the vector, Phlebotomus papatasi (Diptera: Psychodidae; Scopoli, 1786) population, at the town-scale. We provided Fipronil-impregnated food-baits to all Psammomys obesus (Mammalia:Muridae; Cretzschmar, 1828), the main L. major reservoir, burrows along the southern perimeter of the town of Yeruham, Israel, and compared sand fly abundance and infection levels with a non-treated control area. We found a significant and substantial treatment effect on L. major infection levels in the female sand fly population. Sand fly abundance was not affected. Our results demonstrate, for the first time, the potential of systemic control in reducing pathogen transmission rates at a large, epidemiologically relevant, spatial scale.