Foraging responses of predators to novel toxic prey: effects of predator learning and relative prey abundance

Learning to avoid toxic prey items may aid native predators to survive the invasion of highly toxic species, such as cane toads Bufo marinus in tropical Australia. If the predators’ initial aversion is generalized, native prey that resemble the toxic invader may receive a benefit through accidental mimicry. What ecological factors influence the acquisition of learned avoidance (and hence, the impact of invasion on both predators and native prey)? We conducted laboratory experiments to evaluate how the relative abundance of toad tadpoles compared to palatable native tadpoles (Litoria caerulea and L. rubella) affected the ability of native aquatic predators to discriminate between these two prey types. Both fish (northern trout gudgeon, Mogurnda mogurnda) and frogs (Dahl's aquatic frog, Litoria dahlii) learned to discriminate between toads and frogs within an eight‐day period. Higher abundance of toad tadpoles relative to frog tadpoles enhanced rates of predator learning, and thus reduced predation on toads and increased predation on native tadpoles. In the field, spatial and temporal variation in the relative abundance of cane toads compared to native frogs may influence the rates at which these novel toxic items are deleted from predator diets, and the duration of predator protection afforded to natives that resemble the invader.     

Correlation between environmental parameters and behaviour during high tides in Periophthalmus modestus

Migratory behaviour in relation to tides was investigated in the mudskipper Periophthalmus modestus at a small estuary in southern Japan. Fish remained and fed on the tideland at low tides, but migrated along the water edge and clung to the wall of the bank above high tidemark at high tides. The number of the mudskippers clung to the wall of the bank at high tides increased from spring to summer with an increase in air temperature, but then decreased as air temperature became lower. The seasonal fluctuation of the number of clinging fish was positively correlated to that of air temperature, and negatively correlated to that of dissolved oxygen. Since metabolic rate increases with the increase of environmental temperature, the migratory and clinging behaviours may reflect behavioural adaptation to terrestrial life in terms of oxygen respiration.     

The effects of prey relative abundance and chemical cues on prey selection in rainbow trout

An optimal foraging model was used to predict prey selection based on both energy maximization and number maximization strategies. The influence of chemical cues and relative abundance on rainbow trout diet selection was examined under laboratory conditions.
In most fish, diet composition was strongly influenced by chemical cues. No fish followed an energy maximization strategy, and selection of prey based on taste persisted despite large caloric penalties associated with these choices. In the absence of chemical cues, diet composition was based on relative abundance of prey (a number maximization strategy). Within the feasible constraints of the optimal foraging model lie a large number of possible diet combinations which would provide sufficient energy for growth and reproduction. This provides a wide scope for feeding flexibility. Response to prey chemical cues may be the basis for observations of individual diet variability in trout.     

Distribution of intestinal parasitoses in relation to environmental and sociocultural parameters in La Plata,Argentina

Environmental parameters influencing the distribution of parasite species in three neighbourhoods of differing socioeconomic conditions in La Plata, Argentina were analysed. Coproparasitological screenings were performed in children up to 14 years old from a marginal zone (100), a suburban neighbourhood (101), and an urban area (91) in 1999--2000. The presence of parasite species in environmental samples (water and soil) and the degree of association among parasite communities was documented and evaluated. The prevalence of infection in each population was 73.0%, 54.4% and 35.2%, respectively. The frequencies of helminths and pathogenic protozoa were both higher in the marginal zone, where sanitary and environmental conditions were significantly inferior compared with the other zones. The high prevalence of intestinal parasites in this infantile population was related to parasitic contamination of the soil and water sources in addition to deficient sanitary and sociocultural conditions. Calculation of an equitability index revealed that the specific richness was less equitable once socioeconomic conditions and hygienic practices were improved. This study demonstrates the need to implement management practices for the control of intestinal parasitoses in accordance with the environmental and sociocultural characteristics of a given ecosystem.     

Orientation behaviour of the predatory hemipteran Perillus bioculatus to plant and prey odours

The pentatomid predator P. bioculatus responded by positive odour-conditioned anemotaxis when exposed to airborne volatiles emitted by potato plants damaged by Colorado potato beetle larvae, whereas intact potato plants and non-feeding larvae as odour sources failed to elicit anemotaxis. Walking tracks of adult predators had higher values of straightness and upwind fixation when odours emanating from mechanically damaged plants were encountered than tracks registered in response to air carrying volatiles from intact plants, but these parameters returned to control values within 1–2 h after damage was caused. In contrast, air led over plants damaged by beetle larvae elicited orientation responses at least 3 hours after feeding damage ceased. The combination of chemical data on headspace composition, olfactory sensitivity established in electro-antennogram studies and behavioural data presented here imply a role of sesquiterpenoid plant volatiles as odorous cues eliciting attraction of this predator to damaged potato plants.     

Foraging behaviour and prey size of the painted stork

The foraging behaviour of painted stork Mycteria leucocephala was studied during 2004–2006 at 14 different sites in the Delhi region, India. Observations were recorded on 131 individuals, including 29 juvenile birds using a video camera. Recordings were also made at the nesting colony in Delhi zoo to study the prey sizes regurgitated to nestlings. The results confirm that the painted stork is a tactile forager and exclusively piscivorous. Foraging group size ranged from 1 to 18 individuals. Per 5 min foot stirring rates in the vegetated habitats were significantly higher than in non-vegetated habitats. The attempt rate and feeding rate in the breeding season were significantly higher than that in the non-breeding season. Prey sizes taken in the breeding season were significantly smaller than those taken in the non-breeding season. About 80% fish fed to the chicks were smaller than 10 cm. Young chicks were offered smaller prey compared with older chicks. The variations in foraging parameters are discussed in relation to habitats and their conservation in the Delhi region.     

Consumption by carabid beetles of three cereal aphid species relative to other prey types

The cereal aphidRhopalosiphum padi has previously been found to be a low quality prey for a range of generalist arthropod predators. The aim of this study was to reveal, using food consumption experiments whether this applies to other cereal aphids. The question of whether predator feeding capacity increased when several aphid species were offered relative to a single aphid species was also addressed by measuring food consumption on a mixed aphid diet relative to single aphid diets. Food consumption by five carabid beetles of the three cereal aphid speciesRhopalosiphum padi, Sitobion avenae andMetopolophium dirhodum was determined relative to fruit fliesDrosophila melanogaster and the collembolanIsotoma anglicana. Feeding rate was measured as food consumption over 24 hour both for previously satiated and beetles starved for 7 days. Generally the largest aphid consumption was ofM. dirhodum and the lowest ofR. padi, withS. avenae in between. The mixed aphid consumption experiments did not reveal a higher feeding rate on mixed aphid diets relative to single aphid diets. The results indicate low preference forR. padi andS. avenae.     

Cranial postnatal growth of the rat relative to environmental variance

Neurocranial growth during the first 40 days of postnatal development was biometrically investigated relative to the specific parameters of calvarium length and width; basioccipital length and width; basisphenoid length; biparietal width; frontal bone length and cranial vault length. Specimens were obtained from rat litters subject to moderate nutritional suppression by raising the litter numbers above the norm via the cross-fostering technique and causing the overtaxing of the lactational capacity. Alterations in growth rate occurred throughout the pre- and postweaning periods with clearly demarcated phases of growth. Male and female growth in the preweaning period was uniform within each litter type but the phasic growth changes in the experimental litters differed from the control litters with regression line breakage emerging at different points and altering the phasic lengths. The preweaning phasic spectrum in both control and experimental litters was evidently linked to the maternal lactation capacity--relative to quantity and duration. Weaning was insidious in both litter types but occurred earlier in the experimental litters although the continuity of growth was maintained up to the end of the definitive preweaning period (day 20). The definitive postweaning period (day 21-40) showed considerable heterogeneity--unsatisfactory scatter--with differing regression line breakage points and phasic lengths. Analysis based on the total period (day 1-40) brought compensatory adjustments and indicated more acceptable phasic patterns in the postweaning period with confirmation of catch-up growth in the experimental litters. Allometric analysis of the parameters revealed varying rates of growth, line breakage points and differing phasic lengths indicating that each neurocranial parameter and the skeletal unit forming it had an individual characteristic response spectrum mostly attributable to the functional matrix with emphasis on the capsular rather than the periosteal.     

Assessing prey provisioned to Common Buzzard Buteo buteo chicks: a comparison of methods

Capsule: Methods of assessing raptor diet carry significant inherent biases which can vary over time.

Aims: To compare methods of assessing Common Buzzard Buteo buteo diet composition and assess how any differences vary between years.

Methods: Diet was assessed at 32 Common Buzzard nests on an area of upland heather moorland in Britain, managed for Red Grouse Lagopus lagopus scotica, over three years. Data obtained from nest cameras were compared with data from prey remains and regurgitated pellets.

Results: Diet composition differed between methods in all years. Methodological differences varied between years in relation to an almost twelve-fold change in Field Vole abundance, a key prey of Common Buzzards, while abundances of alternative prey changed little. Small mammals were underestimated by prey remains in all three years, while herpetofauna were underestimated by prey remains and pellets in two years. Large birds were overestimated by prey remains, significantly so in one year. Pellets overestimated invertebrates in all years. By combining prey remains and pellets, significant yearly variations in biases were eliminated, although the combined measure overestimated large birds and invertebrates.

Conclusion: Our results suggest that future studies should consider not only how chosen methods may affect results, but also how effects can differ between years.     

The pattern and range of movement of a checkered beetle predator relative to its bark beetle prey

Theoretical studies of predator‐prey population dynamics have increasingly centered on the role of space and the movement of organisms. Yet, empirical studies have been slow to follow suit. Herein, we quantified the long‐range movement of a checkered beetle, Thanasimus dubius, which is an important predator of a pernicious forest pest, the southern pine beetle, Dendroctonus frontalis. Adult checkered beetles were marked and released at five sites and subsequently recaptured at traps baited with pine and pine beetle semiochemicals and located at distances up to 2 km away from the release point. While the pattern of recaptures‐with‐distance at each site provided a modest fit to a simple random‐diffusion model, there was a consistent discrepancy between observed and expected recaptures: a higher than expected proportion of beetles were recaptured at the more distant traps. To account for this deviation, we developed a model of diffusion that allowed for simple heterogeneity in the population of marked beetles; i.e., a slow and fast moving form of the checkered beetle. This model provided a significantly better fit to the data and formed the basis for our estimates of intra‐forest movement. We estimated that on average, one half of the checkered beetles dispersed at least 1.25 km, one third dispersed>2 km, and 5% dispersed>5 km. The source of the heterogeneous dispersal rates were partially due to differences in beetle size: smaller beetles (for both males and females) were more likely to be recaptured away from the release site than larger beetles. The southern pine beetle (prey for the checkered beetle) exhibited no significant heterogeneity in dispersal ability and provided a very good fit to the simple diffusion model. The only difference in dispersal between these two species was that checkered beetles were undergoing greater long‐distance dispersal than the pine beetles (the radius containing 95% of the dispersing individuals was 5.1 km for the checkered beetle and 2.3 km for the pine beetle). Data on the movement of these two species is used to evaluate a general model of spatial pattern formation in a homogeneous environment, and the potential of the checkered beetle as a biological control agent for the southern pine beetle.     

Adaptive allocation of resources and life-history trade-offs in aphids relative to plant quality

The need to allocate a limited amount of energy between different life-history traits is a fundamental assumption in life-history theory. However, it has often turned out to be extremely difficult to measure the competing processes that contribute to costs or benefits for individual organisms. The present investigation begins by analysing how an aphid clonal lineage adapts its reproductive investment to moderate changes in host plant quality (e.g. during the life cycle of its host). Using Centaurea jacea and Uroleucon jaceae as a model plantaphid system, I show that reproductive investment can be far more complex than indicated by dry or wet mass of the gonads alone. The number of embryos of a particular size class or developmental state present in the reproductive system of an aphid is highly flexible and is influenced by the quality of the host plant. Next, the effects of a particular reproductive investment on survival during periods of food deprivation are analysed for aphids originating from host-plants of different qualities. When food stress is severe the ability to rapidly resorb and reallocate resources committed to offspring is important for survival. However, this ability is limited. I argue that, in periods of food stress, young, unsclerotized embryos might serve as a kind of energy buffer similar to a fat body and are therefore not relevant to cost-benefit calculations. However, embryos that are beginning to sclerotize within the ovarioles are not available for resorption and resource reallocation. They compete for nutrients with their mother and contribute to costs. Therefore, it is suggested that the reproductive investment of an aphid should not be equated with reproductive costs in a general al way. The dynamics of adaptive resource allocation and resorption are a key feature of an aphid's life history, and the implications for life-history theory are discussed.     

Spatial dynamics of lions and their prey along an environmental gradient

Establishing the ecological determinants of the spatial dynamics of large African savanna mammals is necessary for understanding the cumulative impacts on the suitability and resilience of their natural habitats, of progressive habitat fragmentation and temporal shifts in climate, fire regimes and elephant browsing. Systematic directional gradients and small‐scale spatial dependence were evident in rainfall, herbivore biomass and lion density in the Masai Mara National Reserve during September 1990 to July 1992. Lion density, the biomass of resident and all herbivore species were autocorrelated within 4.3, 4.1 and 3.3 km, respectively, and peaked in areas of relatively low rainfall but higher microtopographic and vegetation heterogeneity and complexity and more diverse drainage systems. Rainfall influenced herbivore biomass and hence lion density non‐linearly and that influence was apparently modified by forage mineral nutrients, structural habitat complexity, degree of drainage and distance to waterpoints. Lion density was related to the biomass of resident herbivores, even after controlling for rainfall and spatial trends. Persistence of the recent decline in Mara woodlands and increasing human population along the reserve–ranch boundary would likely reduce the reserve's lion population because most lions spent the daytime within the vegetation mosaic fringing drainage lines, away from humans and domestic livestock.     

Temperature dependence of predation depends on the relative performance of predators and prey

The temperature dependence of predation rates is a key issue for understanding and predicting the responses of ecosystems to climate change. Using a simple mechanistic model, we demonstrate that differences in the relative performances of predator and prey can cause strong threshold effects in the temperature dependence of attack rates. Empirical data on the attack rate of northern pike (Esox lucius) feeding on brown trout (Salmo trutta) confirm this result. Attack rates fell sharply below a threshold temperature of +11°C, which corresponded to a shift in relative performance of pike and brown trout with respect to maximum attack and escape swimming speeds. The average attack speed of pike was an order of magnitude lower than the escape speed of brown trout at 5°C, but approximately equal at temperatures above 11°C. Thresholds in the temperature dependence of ecological rates can create tipping points in the responses of ecosystems to increasing temperatures. Thus, identifying thresholds is crucial when predicting future effects of climate warming.     

Feeding rates in the chaetognath Sagitta elegans: effects of prey size, prey swimming behaviour and small-scale turbulence

The gut contents of Sagitta elegans were sampled twice daily(noon and midnight) during 9 days in October at an anchor stationin the northern North Sea. Observations of the ambient preyfield and of turbulent dissipation rates were collected simultaneously.The average number of prey per chaetognath was among the highestever recorded, 0.57 ± 0.10. Total gut content was independentof ambient prey concentration, suggesting that feeding ratewas saturated. Clearance rates were estimated from gut contentsand ambient prey concentrations and a literature-based estimateof digestion time. The clearance rate to prey size showed adome-shaped relationship. The maximum clearance rates, about100–300 ml h^-1, were observed for prey sizes correspondingto 6–10% of Sagitta length. Clearance rates varied notonly with prey size, but also with prey type. For example, copepodmales were cleared at rates up to an order of magnitude higherthan similarly sized females, probably owing to differencesin swimming behaviour. Sagitta elegans is an ambush predatorthat perceives its prey by hydromechanical signals. Faster swimmingprey generates stronger signals and is, hence, perceived atlonger distances. We develop a simple prey encounter rate modelby describing the swimming prey as a ‘force dipole’and assuming that a critical signal strength is required toelicit an attack. By fitting the model to the observations,a critical signal strength of 10^-2 cm s^-1 is estimated; thisis very similar to estimates for copepods that also perceiveprey by mechanoreceptory setae. Gut contents were independentof turbulent dissipation rate. Because feeding rates were saturated,we did not expect to see positive effects of turbulence. However,the strong wind-generated turbulent dissipation rates observedduring the study (10^-3–10^-1 cm² s^-3 in the upper mixedlayer) could lead to negative effects by interfering with preyperception. At a dissipation rate of 10^-2 cm² s^-3 a 10-mm longS. elegans would experience fluid signals of order 0.3 cm s^-1due to turbulence, 30 times stronger than the signal strengthfrom the prey. It is, therefore, suggested that S. elegans isable to separate prey signals from turbulence signals due totheir different spatial characteristics.     

Adaptive phosphate uptake behaviour of phytoplankton to environmental phosphate fluctuations

When phytoplankton growth in lakes is limited by the available phosphate, the external phosphate concentration fluctuates around a threshold value at which available energy is insufficient to drive phosphate incorporation into a polyphosphate pool. As a result, occasional increases in the external concentration are experienced by phytoplankton as a series of phosphate pulses. Based on [(32) P] phosphate uptake experiments with lake phytoplankton, we show that a community is able to process information about the experienced pattern of phosphate pulses via a complex regulation of the kinetic and energetic properties of cellular phosphate uptake systems. As a result, physiological adaptation to alterations of ambient phosphate concentration depends on the pattern of phosphate fluctuations to which the community had been exposed during its previous growth. In this process, the entire community exhibits coherent uptake behaviour with respect to a common threshold value. Thereby, different threshold values result from different antecedent pulse patterns, apparently unrestrained by the amount of previously stored phosphate. The coherent behaviour observed contradicts the basic assumptions of the competitive exclusion principle and provides an alternative perspective for explaining the paradoxical coexistence of many phytoplankton species.     

Invertebrate prey and predatory behaviour of the omnivorous African tortoise Kinixys spekii

Invertebrates form an important part of the diet in the omnivorous African tortoises of the genus Kinixys. Millipede prey of Kinixys spekii in Zimbabwe had a mean volume of 0.9 ml, and made up 64.7% of invertebrate food by volume; beetles made up most of the remainder and had a mean volume of 1.4 ml. The mean mass of invertebrate prey was 0.19% of tortoise mass, a similar value of relative prey mass to many insectivorous lizards. Tortoises preferentially attacked moving millipedes at or just behind the head, but predatory behaviour was otherwise unspecialized, with tortoises killing millipedes by ingesting them in pieces. Handling times of millipede prey varied significantly with relative prey mass, defensive behaviour, and direction of ingestion. The profitability (mass intake/handling time) of millipedes was maximal at a relative prey mass of 0.2%; the basis on which prey are selected is discussed.