The effect of intra- and interspecific aggression on patch residence time in Negev Desert gerbils: a competing risk analysis

We observed patch-use behavior by two gerbil species in a fieldsetting and investigated how aggression and intrinsic decision-makinginteract to influence patch residence times. Results were interpretedby using a competing risk analysis model, which uniquely enabledus to estimate the intrinsic patch-leaving decisions independentlyof external interruptions of foraging bouts by aggression. Theexperiment was conducted in two 1-ha field enclosures completelysurrounded by rodent-proof fences and included allopatric (onlyGerbillus andersoni allenbyi) and sympatric (G. a. allenbyiand G. pyramidum) treatments. We predicted that increased foodpatch quality (i.e., habitat quality) should decrease intrinsicpatch-leaving rates and increase rates of aggressive interactionsinvolving the forager feeding in the patch (i.e., the occupantindividual). We also anticipated that increasing populationdensity should result in an increase in the rate of aggressiveinteractions involving the occupant individual. Our resultssupported the first two predictions, indicating a trade-offbetween foraging and aggression. However, the third predictionwas realized only for G. a. allenbyi in allopatry. Furthermore,in allopatry, occupant G. a. allenbyi individuals with highcompetitive ranks were involved in aggressive interactions atlower rates than those with low competitive ranks. However,in sympatry, patch-use behavior of occupant G. a. allenbyi individualswas mainly influenced by aggressive behavior of G. pyramidum,which did not respond to their competitive rank. Thus, it shouldpay less for G. a. allenbyi to be aggressive in sympatric populations.The observed reduction in intraspecific aggression among individualG. a. allenbyi in the presence of G. pyramidum supports thisassertion. We suggest that this reduction likely weakens thenegative effect of intra- and interspecific density on the percapita growth rate of G. a. allenbyi. Because this would changethe slope of the isocline of G. a. allenbyi, it could be animportant mechanism promoting coexistence when habitat selectionis constrained.     

Foraging in the subterranean social highveld mole-rat (Cryptomys hottentotus pretoriae): an investigation into mass-dependent geophyte use and foraging patterns

The foraging behaviour of captive colonies of the highveld mole-rat Cryptomys hottentotus pretoriae was investigated in an artificial soil-filled burrow system provided with four trays (patches) that varied in geophyte density and mass. An initial trial involving empty trays (only soil) revealed that there was no preference for any specific tray. There were no statistically significant preferences for excavating in any of the patches of different geophyte density. No preferences were evident for excavation in patches containing geophytes of different mass classes. Empty patches seemed to be preferred over patches containing geophytes when combinations of geophyte density/mass were investigated. The duration of handling and the rate of consumption of geophytes were recorded for 23 individuals of two mass classes. Handling time of geophytes was not related to mole-rat sex, but was strongly linked to mole-rat mass class. Handling time of geophytes was related to geophyte mass class. Small geophytes were less profitable to consume. These findings are considered in light of optimal foraging theory and the situation in the field. It was concluded that the mole-rats generally followed the qualitative predictions of optimal foraging theory, although falling short of being energy maximizers.     

捕食风险及其对动物觅食行为的影响

对捕食风险的涵义及其对猎物动物觅食行为的影响、猎物动物面对捕食风险时的反应进行了论述。捕食风险可以简单地理解为一定时间内猎物动物被杀死的概率。当捕食风险存在时 ,动物会选择相对安全但觅食效益较低的地点觅食 ;由于死亡率和消化方面的限制 ,一般都会产生食谱收缩 ;觅食活动方式的时间格局也会因捕食风险而发生改变 ,如水生动物的昼夜垂直迁移、某些陆生动物昼行性与夜行性活动的转换、月光回避等。在与捕食者发生遭遇时 ,猎物动物的主要反应是 :①发出某些信号以阻止捕食者的追捕 ;②靠近并注视捕食者 ;③逃逸 ;④在一定的时间恢复觅食活动。在以往的研究中 ,对捕食者种类已经有了较多的了解 ,而对猎物如何判断捕食者丰富度信息、估计风险程度等方面则知之甚少 ;同时 ,对捕食风险水平的调控、对多种因素的综合分析也较少涉及。在今后的研究中 ,还应该考虑研究的尺度问题 ,因为在不同尺度的环境条件下 ,猎物动物对于捕食风险的反应可能大相径庭。     

Predatory responses of selected lines of developmental variants of ladybird,Propylea dissecta (Coleoptera: Coccinellidae) in relation to increasing prey and predator densities

Individuals of the same species, population and generation frequently exhibit sub-maximal and significant genetic and phenotypic variation in their rate of development, showing slow and fast developers. Fast developers commonly have higher foraging and predation rates than slow developers. The consequence of such differences and foraging for the efficacy of biocontrol species remains under-explored. Slow and fast developers from a population of the ladybird, Propylea dissecta were separated and selected experimentally for F15 generations, and the predatory response of fourth instar larvae of control and experimentally selected slow and fast developers was then assessed at differing levels of prey (pea aphid, Acyrthosiphon pisum) and conspecific predator abundance. All individuals, whether slow or fast developers, showed a Type-II functional response, decrease in proportion of prey consumed with increasing prey biomass and an increase in proportion of prey consumed with increasing predator density. The proportion of prey consumed was highest in experimental fast developers and lowest in experimental slow developers. Attack rate was highest and handling time longest in slow developers of control/experimental groups. Mutual interference was least while area of discovery was highest in experimental fast developers. Thus, selection of fast developers for F15 generations led to higher functional responses, slower attack rates and faster prey consumption. This lower mutual interference and high searching efficiency indicates that they can be experimentally selected and used for better control of the pea aphids. This study is the first attempt to evaluate predatory responses of selected lines of an aphidophagous ladybird.     

Time-budgeting and foraging strategy of the stoplight parrotfish Sparisoma viride Bonnaterre,in Jamaica

Quantitative data on the ways in which the different phases of the stoplight parrotfish (Sparisomaviride Bonnaterre) distribute their time among various activities in different habitats are presented. Individuals spent from 84–97% of their diurnal time swimming, feeding, and hovering. Additionally, large adults spent a significant amount of time sheltering among crevices. Phase-related differences in these activities are statistically significant, as are differences in duration and rates of change of the activities. Large individuals spent more time swimming, while small individuals spent more time hovering. In addition, large individuals performed longer bouts of activity and switched activities less frequently than small individuals. Adult males and females spent approximately equal proportions of time in each of the activity states. Stochastic analyses of behavioural sequences show second order Markov chain dependencies, suggesting that preceding activity states affect subsequent behaviour. Possible relationships between behavioural sequencing and the species foraging strategy are discussed, and it is suggested that the sequence of behavioural activities can provide an estimation of the distribution of food resources in the environment.     

Individual constancy of local search strategies in the giant tropical ant,Paraponera clavata (Hymenoptera: Formicidae)

Paraponera clavata workers engage in a period of local search after encountering a small amount of artificial nectar. Giving-up times from local search are not distributed normally; there is a strong skew to longer times. There is no statistically significant relationship between the amount of time required to collect the food and the subsequent search time. Giving-up time in response to the first reward presented to an ant is positively correlated with that ant's response to a second such reward. However, giving-up times diminish when an ant is presented with a series of rewards. Local search is a function of individual strategies, which remain relatively constant in the short term.     

Optimal foraging and fitness in Columbian ground squirrels

Summary Optimal diets were determined for each of 109 individual Columbian ground squirrels (Spermophilus columbianus) at two sites in northwestern Montana. Body mass, daily activity time, and vegetation consumption rates for individuals were measured in the field, along with the average water content of vegetation at each ground squirrel colony. I also measured stomach and caecal capacity and turnover rate of plant food through the digestive tract for individuals in the laboratory to construct regressions of digestive capacity as a function of individual body mass. Finally, I obtained literature estimates of average daily energy requirements as a function of body mass and digestible energy content of vegetation. These data were used to construct a linear programming diet model for each individual. The model for each individual was used to predict the proportion of two food types (monocots and dicots) that maximized daily energy intake, given time and digestive constraints on foraging. Individuals were classified as optimal or deviating, depending on whether their observed diet was significantly different from their predicted optimal diet. I determined the consequences of selecting an optimal diet for energy intake and fitness. As expected, daily energy intake calculated for deviators (based on their observed diet proportion) was less than that for optimal foragers. Deviating foragers do not appear to compensate for their lower calculated energy intake through other factors such as body size or physiological efficiency of processing food. Growth rate, yearly survivorship, and litter size increase with calculated energy intake, and optimal foragers have six times the reproductive success of deviators by age three. Optimal foraging behavior, therefore, appears to confer a considerable fitness advantage.     

Warming up the system: higher predator feeding rates but lower energetic efficiencies

Predictions on the consequences of the rapidly increasing atmospheric CO₂ levels and associated climate warming for population dynamics, ecological community structure and ecosystem functioning depend on mechanistic energetic models of temperature effects on populations and their interactions. However, such mechanistic approaches combining warming effects on metabolic (energy loss of organisms) and feeding rates (energy gain by organisms) remain a key, yet elusive, goal. Aiming to fill this void, we studied the metabolic rates and functional responses of three differently sized, predatory ground beetles on one mobile and one more resident prey species across a temperature gradient (5, 10, 15, 20, 25 and 30 °C). Synthesizing metabolic and functional‐response theory, we develop novel mechanistic predictions how predator–prey interaction strengths (i.e., functional responses) should respond to warming. Corroborating prior theory, warming caused strong increases in metabolism and decreases in handling time. Consistent with our novel model, we found increases in predator attack rates on a mobile prey, whereas attack rates on a mostly resident prey remained constant across the temperature gradient. Together, these results provide critically important information that environmental warming generally increases the direct short‐term per capita interaction strengths between predators and their prey as described by functional‐response models. Nevertheless, the several fold stronger increase in metabolism with warming caused decreases in energetic efficiencies (ratio of per capita feeding rate to metabolic rate) for all predator–prey interactions. This implies that warming of natural ecosystems may dampen predator–prey oscillations thus stabilizing their dynamics. The severe long‐term implications; however, include predator starvation due to energetic inefficiency despite abundant resources.     

Decay rates of attractive and repellent pheromones in an ant foraging trail network

Pharaoh’s ants (Monomorium pharaonis) use at least three types of foraging trail pheromone: a long-lasting attractive pheromone and two short-lived pheromones, one attractive and one repellent. We measured the decay rates of the behavioural response of ant workers at a trail bifurcation to trail substrate marked with either repellent or attractive short-lived pheromones. Our results show that the repellent pheromone effect lasts more than twice as long as the attractive pheromone effect (78 min versus 33 min). Although the effects of these two pheromones decay at approximately the same rate, the initial effect of the repellent pheromone on branch choice is almost twice that of the attractive pheromone (48% versus 25% above control). We hypothesise that the two pheromones have complementary but distinct roles, with the repellent pheromone specifically directing ants at bifurcations, while the attractive pheromone guides ants along the entire trail. Received 15 November 2007; revised 7 March 2008; accepted 18 March 2008.     

Simulation of the evolution of root water foraging strategies in dry and shallow soils

Background and Aims

The dynamic structural development of plants can be seen as a strategy for exploiting the limited resources available within their environment, and we would expect that evolution would lead to efficient strategies that reduce costs while maximizing resource acquisition. In particular, perennial species endemic to habitats with shallow soils in seasonally dry environments have been shown to have a specialized root system morphology that may enhance access to water resources in the underlying rock. This study aimed to explore these hypotheses by applying evolutionary algorithms to a functional–structural root growth model.

Methods

A simulation model of a plant''s root system was developed, which represents the dynamics of water uptake and structural growth. The model is simple enough for evolutionary optimization to be computationally feasible, yet flexible enough to allow a range of structural development strategies to be explored. The model was combined with an evolutionary algorithm in order to investigate a case study habitat with a highly heterogeneous distribution of resources, both spatially and temporally – the situation of perennial plants occurring on shallow soils in seasonally dry environments. Evolution was simulated under two contrasting fitness criteria: (1) the ability to find wet cracks in underlying rock, and (2) maximizing above-ground biomass.

Key Results

The novel approach successfully resulted in the evolution of more efficient structural development strategies for both fitness criteria. Different rooting strategies evolved when different criteria were applied, and each evolved strategy made ecological sense in terms of the corresponding fitness criterion. Evolution selected for root system morphologies which matched those of real species from corresponding habitats.

Conclusions

Specialized root morphology with deeper rather than shallower lateral branching enhances access to water resources in underlying rock. More generally, the approach provides insights into both evolutionary processes and ecological costs and benefits of different plant growth strategies.     

A life in the fast lane: energetics and foraging strategies of the great cormorant

Body insulation is critically important for diving marine endotherms. However,cormorants have a wettable plumage, which leads to poor insulation. Despitethis, these birds are apparently highly successful predatorsin most aquatic ecosystems. We studied the theoretical influenceof water temperature, dive depth, foraging techniques, and preyavailability on the energetic costs of diving, prey search time,daily food intake, and survival in foraging, nonbreeding greatcormorants (Phalacrocorax carbo). Our model was based on fieldmeasurements and on data taken from the literature. Water temperatureand dive depth influenced diving costs drastically, with predicted increasesof up to 250% and 258% in males and females, respectively. Changes inwater temperature and depth conditions may lead to an increaseof daily food intake of 500-800 g in males and 440-780 g infemales. However, the model predicts that cormorant foragingparameters are most strongly influenced by prey availability,so that even limited reduction in prey density makes birds unableto balance energy needs and may thus limit their influence onprey stocks. We discuss the ramifications of these results withregard to foraging strategies, dispersal, population dynamics,and intraspecific competition in this avian predator and pointout the importance of this model species for our understandingof foraging energetics in diving endotherms.     

Behavioral interference and facilitation in the foraging cycle shape the functional response

Individual forager behaviors should affect per capita intakerates and thereby population and consumer-resource properties.We consider and incorporate conspecific facilitation and interferenceduring the separate foraging-cycle stages in a functional responsemodel that links individual behavioral interactions with consumer-resourceprocesses. Our analyses suggest that failing to properly considerand include all effects of behavioral interactions on foraging-cyclestage performances may either over- or underestimate effectsof interactions on the shape of both functional responses andpredator zero-growth isoclines. Incorporation of prey- and predator-dependentinteractions among foragers in the model produces predator isoclineswith potentials for highly complex consumer-resource dynamics.Facilitation and interference during the foraging cycle aretherefore suggested as potent behavioral mechanisms to causepatterns of community dynamics. We emphasize that correct estimationsof interaction-mediated foraging-cycle efficiencies should beconsidered in empirical and theoretical attempts to furtherour understanding of the mechanistic link between social behaviorsand higher order processes.