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PubMed2004

Pattern formation and nonlocal logistic growth.

Logistic growth process with nonlocal interactions is considered in one dimension. Spontaneous breakdown of translational invariance is shown to take place at some parameter region, and the bifurcation regime is identified for short and long-range interactions. Domain walls between regions of different order parameter are expressed as soliton solutions of the reduced dynamics for nearest-neighbor interactions. The analytic results are confirmed by numerical simulations.

Animals↗
PubMed2006

Self-organized packs selection in predator-prey ecosystems.

We present a lattice model of a system of predators of five kinds, competing for prey. The predators are grouped in packs and characterized by two parameters-the energy spent on hunting and energy gained by the kill. The success of hunting depends on the actual competition among predators found near a prey. We determine via Monte Carlo simulations the numbers of predators of each kind as a function of time and the distribution of the size of their packs. We show that the ratio of the energy spent by the competing predators determines their fate. The energy gain plays only a secondary role. We show also that the system self-organizes itself into groups of predators living in well defined packs, which size depends on the energy spent. The most preferred size dependence on the energy spent follows a very simple power law. We present also a mean-field-type approach to the problem and we discuss the differences in the results obtained by the two methods, showing in particular, that the simulation approach produces more reliable results.

Animals↗
PubMed2006

Fisher waves and front roughening in a two-species invasion model with preemptive competition.

We study front propagation when an invading species competes with a resident; we assume nearest-neighbor preemptive competition for resources in an individual-based, two-dimensional lattice model. The asymptotic front velocity exhibits an effective power-law dependence on the difference between the two species' clonal propagation rates (key ecological parameters). The mean-field approximation behaves similarly, but the power law's exponent slightly differs from the individual-based model's result. We also study roughening of the front, using the framework of nonequilibrium interface growth. Our analysis indicates that initially flat, linear invading fronts exhibit Kardar-Parisi-Zhang (KPZ) roughening in one transverse dimension. Further, this finding implies, and is also confirmed by simulations, that the temporal correction to the asymptotic front velocity is of O(t(-2/3)).

Adaptation, Physiological↗
PubMed2006

Validation of a randomization procedure to assess animal habitat preferences: microhabitat use of tiger sharks in a seagrass ecosystem.

1. Tiger sharks Galeocerdo cuvier are important predators in a variety of nearshore communities, including the seagrass ecosystem of Shark Bay, Western Australia. Because tiger sharks are known to influence spatial distributions of multiple prey species, it is important to understand how they use habitats at a variety of spatial scales. We used a combination of catch rates and acoustic tracking to determine tiger shark microhabitat use in Shark Bay. 2. Comparing habitat-use data from tracking against the null hypothesis of no habitat preference is hindered in Shark Bay, as elsewhere, by the difficulty of defining expected habitat use given random movement. We used randomization procedures to generate expected habitat use in the absence of habitat preference and expected habitat use differences among groups (e.g. males and females). We tested the performance of these protocols using simulated data sets with known habitat preferences. 3. The technique correctly classified sets of simulated tracks as displaying a preference or not and was a conservative test for differences in habitat preferences between subgroups of tracks (e.g. males vs. females). 4. Sharks preferred shallow habitats over deep ones, and preferred shallow edge microhabitats over shallow interior ones. The use of shallow edges likely increases encounter rates with potential prey and may have profound consequences for the dynamics of Shark Bay's seagrass ecosystem through indirect effects transmitted by grazers that are common prey of tiger sharks. 5. Females showed a greater tendency to use shallow edge microhabitats than did males; this pattern was not detected by traditional analysis techniques. 6. The randomization procedures presented here are applicable to many field studies that use tracking by allowing researchers both to determine overall habitat preferences and to identify differences in habitat use between groups within their sample.

Animals↗
PubMed1994

Differential fitness returns in relation to spatial position in groups.

1. A general overview of position-related fitness differences in group-living animals is given for the first time. 2. Differential fitness returns in groups are often related to competition for resources among group members due to resource limitation. If resources are very scarce, then competition may finally lead to the disbandment of groups. The encounter-dilution effect predicts that grouping confers a benefit if the conspicuousness of large groups does not outweigh the dilution effect and each predator or parasite attacks only one (or a few) prey per encounter (Uetz & Hieber, 1993). While the encounter-dilution effect limits the number of predators and parasites a group is faced with at a given time, marginal predation and communal defence are probably the most important factors for position-related differences in predation risk. 3. Limitation of resources increases competition between groups members and should widen the gap between fitness returns of dominant and subordinate group members. In some group-living animals such as schooling fish, however, predator pressure has apparently selected for uniformity in morphology and behaviour which presumably restricts the potential for resource competition (Magurran & Seghers, 1991). 4. A general problem of assessing individual fitness returns in that different short-term strategies may achieve the same long term goals (Magurran, 1993). The use of feeding rates and predation risks as currencies for fitness returns over short time periods is therefore questionable and can only be a first step. So far, hardly any information is available on long-term effects of differences in positioning behaviour. 5. Food availability and food intake rates tend to be higher in edge positions, whereas energy expenditure does not differ significantly with group position in non-roosting mobile groups. This suggests that edge positions achieve a higher net-energy pay-off and should therefore be preferred by individuals with low energy reserves. 6. Differential predation risks are well documented for stationary and colony-breeding species, but there is a lack of data for mobile, non-breeding species. The reason for the latter is that predation is rarely observed (in the field) and often difficult to relate to group position in fast-moving species such as flocks of birds. Further studies, especially in the field, are needed. 7. Vigilance rates and attack rates do not necessarily give a true reflection of mortality risk. Vigilance is not exclusively related to predation and can be confounded by competition effects and hunger. Attack rates are difficult to translate into per capita risks if hardly any mortality is observed and therefore little information is available about the ratio of successful attacks/total attacks. 8. Most of the literature up to date consists of descriptive studies. There is a need for experimental work that investigate (a) the influence of predator attack mode on differential predation risks; (b) the influence of internal states (such as hunger) on the tradeoff between predation risk and foraging behaviour and its consequences for the positioning behaviour in groups; (c) the 'egalitarian' nature of groups with respect to group position. Do all individuals continuously rotate positions? Or do dominant individuals monopolize certain positions?, and (d) that separate the effects of nutritional state, vigilance and food availability on feeding rates in different group positions. 9. Reproductive success is often related to group position in breeding colonies and lekking animal species. Females probably prefer the safer territories in the group centre, but more empirical data are required to test this hypothesis. 10. Position-related differences in parasitism rate between group members are well documented with per capita rate of parasitism being lower in the centre of groups than at the periphery.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2001

Recolonizing carnivores and naïve prey: conservation lessons from Pleistocene extinctions.

The current extinction of many of Earth's large terrestrial carnivores has left some extant prey species lacking knowledge about contemporary predators, a situation roughly parallel to that 10,000 to 50,000 years ago, when naive animals first encountered colonizing human hunters. Along present-day carnivore recolonization fronts, brown (also called grizzly) bears killed predator-naive adult moose at disproportionately high rates in Scandinavia, and moose mothers who lost juveniles to recolonizing wolves in North America's Yellowstone region developed hypersensitivity to wolf howls. Although prey that had been unfamiliar with dangerous predators for as few as 50 to 130 years were highly vulnerable to initial encounters, behavioral adjustments to reduce predation transpired within a single generation. The fact that at least one prey species quickly learns to be wary of restored carnivores should negate fears about localized prey extinction.

Alaska↗
PubMed2006

Escape responses in juvenile Atlantic cod Gadus morhua L.: the effects of turbidity and predator speed.

We examined the effect of turbidity (0.5-14 beam attenuation m(-1)) and predator attack speed (150 and 296 cm s(-1)) on escape responses of juvenile cod Gadus morhua in the laboratory. We triggered escape responses using a predator model and measured escape timing, direction and locomotor performance. We also measured responsiveness and estimated the likelihood of fish escaping the ;predator attack' (putative escape success, PES). Turbidity affected both PES and the type of escape response used by the fish, but these effects depended on predator speed. PES for the fast predator attack declined from 73% in clear water to 21% in highly turbid water, due to decreased responsiveness and poorly timed escapes. Intermediate turbidity enhanced PES and responsiveness to the slow predator attack. Locomotor performance was reduced by turbidity, whereas predator speed had the opposite effect. Our results suggest that both predator attack speed and turbidity have important roles in determining the vulnerability of fish attacked by piscivorous predators.

Animals↗
PubMed1991

Mutual inhibition among neural command systems as a possible mechanism for behavioral choice in crayfish.

Mutual inhibition among behavioral command systems frequently has been suggested as a possible mechanism for switching between incompatible behaviors. Several neural circuits in crayfish that mediate incompatible behaviors have been found to interact through inhibition; this accounts for increased stimulus threshold of one behavior (e.g., escape tailflip) during performance of others (eating, walking, defense). To determine whether mutual inhibition between command systems can provide a mechanism that produces adaptive behavior, I developed a model crayfish that uses this mechanism to govern its behavioral choices in a simulated world that contains a predator, a shelter, and a food source. The crayfish uses energy that must be replaced by eating while it avoids capture by the predator. The crayfish has seven command systems (FORAGE, EAT, DEFENSE, RETREAT, ESCAPE, SWIM, HIDE) that compete through mutual inhibition for control of its behavior. The model crayfish was found to respond to changing situations by making adaptive behavioral choices at appropriate times. Choice depends on internal and external stimuli, and on recent history, which determines the pattern of those stimuli. The model's responses are unpredictable: small changes in the initial conditions can produce unexpected patterns of behavior that are appropriate alternate responses to the stimulus conditions. Despite this sensitivity, the model is robust; it functions adaptively over a large range of internal and external parameter values.

Animals↗
PubMed2001

Combined effects of predatory fish and sublethal pesticide contamination on the behavior and mortality of mayfly nymphs.

We evaluated the potential interaction of pesticide effect and predatory fish on behavior and mortality of a stream mayfly. Experiments in laboratory stream microcosms compared the activity, drift, and mortality of Baetis mayfly nymphs in the absence of fish with that in the presence of Cape galaxias (Galaxias zebratus), both species inhabiting the same stream environments in the Western Cape of South Africa. These two predator treatments were combined with exposure either to no pesticide or to 0.2 microg/L of the organophosphate insecticide azinphos-methyl (AZP) or 0.2 microg/L of the pyrethroid insecticide fenvalerate (FV). Such pesticide levels are known from transient spraydrift- or runoff-related pesticide input into running waters. Each of the six combinations was replicated six times as 30-min trials during the day and effects were analyzed using 2 x 2 factorial analysis of variance (ANOVA). A single exposure to either fish or pesticide significantly increased the absolute activity of mayflies, measured as number of animals visible on top of stones, and the absolute mayfly drift in the fish treatment and in the FV treatment but did not increase the mortality above 0.8%. The combination of predator presence and sublethal pesticide exposure resulted in a significant increase in the mortality rate, to about 9% in the AZP x fish and 25% in the FV x fish treatment, although the activity and drift rates were not increased compared with the single-stressor treatments. Two-by-two factorial ANOVA and the comparison of expected and measured responses indicated that the mortality resulted from a synergistic interaction of the two stressors. The observed mortality was without exception caused by predation of the fish on drifting mayflies. The relative drift rate in the FV x fish treatment was decreased, again due to a synergistic interaction, which suggests an active drift avoidance reaction of the mayflies exposed to the combined pesticide x fish treatment, in contrast with those exposed just to FV. We conclude that the drift response of mayflies to transient sublethal pesticide exposure results in a synergistically increased adverse effect in the presence of predatory fish.

Animals↗
PubMed2002

Comparison of responses to alarm calls by patas (Erythrocebus patas) and vervet (Cercopithecus aethiops) monkeys in relation to habitat structure.

We studied responses to alarm calls of sympatric patas (Erythrocebus patas) and vervet (Cercopithecus aethiops) monkeys in relation to habitat structure, with the intention of understanding the relationship between the environment and predator avoidance. Patas and vervet monkeys are phylogenetically closely related and overlap in body size. However, while patas monkeys are restricted to nonriverine habitats at our study site, vervets use both nonriverine and riverine habitats, allowing us to "vary" habitat structure while controlling for effects of group size, composition, and phylogeny. Patas monkeys in the nonriverine habitat responded to mammalian predator alarm calls with a greater variety of responses than did vervets in the riverine habitat, but not when compared with vervets in the nonriverine habitat. Ecological measurements confirm subjective assessments that trees in the riverine habitat are significantly taller and occur at lower densities than trees in the nonriverine habitat. Despite the lower density of trees in the riverine habitat, locomotor behavior of focal animals indicates that canopy cover is significantly greater in the riverine than the nonriverine habitat. Differences in responses to alarm calls by the same groups of vervets in different habitat types, and convergence of vervets with patas in the same habitat type, suggest that habitat type can be a significant source of variation in antipredator behavior of primates.

Animals↗
PubMed2004

Remarks on antipredator behavior and food chain dynamics.

When consumers feeding on a resource spend time in avoiding high risks of predation, the predator functional response declines with predator density. While this is well established, less attention has been paid to the dependence of the consumer functional response on predator density. Here we show how the separation of behavioral and ecological timescales allows one to determine both responses starting from an explicit behavioral model. Within the general set-up considered in this paper, the two functional responses can tend toward Holling type II responses when consumers react only weakly to predation. Thus, the main characteristics of the standard Rosenzweig-MacArthur tritrophic food chain (logistic resource and Holling type II consumer and predator) remain valid also when consumers have weak antipredator behavior. Moreover, through numerical analysis, we show that in a particular but interesting case pronounced antipredator behaviors stabilize the system.

Animals↗
PubMed2002

Intraguild predation, invertebrate predators, and trophic cascades in lake food webs.

The top-down and bottom-up properties of model food webs that include intraguild predation and self-limiting factors such as cannibalism are investigated. Intraguild predation can dampen or even reverse the top-down effects predicted by food chain theory. The degree of self-limitation among the intraguild prey is a key factor in determining the direction and strength of the top-down response. Intraguild predation and self-limiting factors can also substantially alter the bottom-up effects of enrichment. These results can help explain the disparate results of trophic cascade experiments in lakes, where cascades are usually seen when large Daphnia are the primary herbivores, but not when smaller-bodied herbivores are dominant. Top-down manipulations should cascade at least modestly to phytoplankton in those lakes whose food web can be reasonably approximated by a chain (typically, those where Daphnia is the dominant herbivore), as predicted by food chain theory. On the other hand, smaller-bodied zooplankton are often preyed upon heavily by invertebrate predators as well as by planktivorous fish, thereby introducing elements of intraguild predation into these food webs. In this case, conventional food chain theory is likely to give incorrect predictions. Very large cascade effects may be due primarily to regime shifts between intraguild predation-dominated food webs and those that more resemble food chains, rather than due to the simple food chain cascade usually considered.

Animals↗
PubMed2002

A double test of the parasite manipulation hypothesis in a burrowing bivalve.

The parasite manipulation hypothesis predicts that parasites should be selected to manipulate host behaviour to facilitate transmission to the next host. The bivalve Macoma balthica burrows less deep when parasitized by the trematode Parvatrema affinis. Shallow burrowing increases the likelihood of ingestion by birds, their final hosts, and therefore this has been interpreted as manipulation by the parasite. When unparasitized, M. balthica displays seasonal changes in burrowing depth, becoming less accessible to predators in winter. If shallow burrowing of parasitized individuals is due to direct manipulation by the parasite, the availability of parasitized individuals should be high throughout the year, or at least especially in the season when most birds are present and potential transmission rates are highest. We compared burrowing depths of parasitized and unparasitized individuals in a single population during seven consecutive years. Parasitized individuals showed reduced burrowing depths but, in contrast to the prediction, the effect of parasites on availability to predators was smallest, not largest, in the season with the highest bird numbers. The parasite P. affinis competes for energy with the host, and M. balthica with low energy stores are known to reduce depth of burrowing. When we included size-corrected somatic ash-free dry mass (as an estimate of the energy stores) in our statistical analysis, the effect of infection on burrowing depth disappeared. Thus the effect of infection on burrowing depth is likely to be an unavoidable, indirect effect of the channelling of energy towards the parasite, causing the starving individual to take greater risks in the acquisition of food. Since both the seasonal pattern and the magnitude of increased availability of parasitized individuals are inadequate, the increased exposure of parasitized M. balthica to the final host does not seem to represent an example of adaptive host manipulation by the parasite.

Animals↗
PubMed1991

Effects of stimulus intensity and quality on discrimination of odorant mixtures by spiny lobsters in an associative learning paradigm.

The Florida spiny lobster, Panulirus argus, can behaviorally discriminate between members of a set of four artificial odorant mixture types: crab, mullet, oyster, and shrimp. The present experiments were designed to examine the effects of both intensity and quality on discrimination of odorant mixtures. This was accomplished by conditioning lobsters to only one concentration (0.5 mM) of shrimp mixture and testing them with four concentrations (0.005, 0.05, 0.5 and 5.0 mM) of shrimp mixture and two concentrations (0.05 and 0.5 mM) of oyster mixture. For the two appetitive behaviors examined, lobsters did not discriminate the conditioned mixture (0.5 mM shrimp) from any other of the same-type nonconditioned mixtures. For a third behavior, active avoidance, lobsters discriminated, to a significant degree, the 0.5 mM shrimp mixture from all of the nonconditioned mixtures. However, aversion values for the nonconditioned shrimp mixtures were markedly and consistently higher than those for the nonconditioned oyster mixtures for all three behaviors. Thus, when spiny lobsters are forced to use intensity as a cue (as in an aversive conditioning situation), they have the ability to discriminate between mixtures of the same quality but different intensity. Nevertheless, based on examination of post-conditioning decreases in appetitive behaviors, it is most likely that the intensity of an odorant mixture has a relatively minor effect on the discrimination of the quality of that mixture by these animals, at least over a 1000-fold concentration range.

Animals↗
PubMed1990

The effect of unilateral anteromedial cortex lesions on prey-catching and spatio-motor behaviour in the rat.

The neglect which follows a unilateral lesion of anteromedial cortex in rat was examined using two tasks. In the first task animals had to press the left bar of a two-bar Skinner box for food reward. In the second task animals captured live locusts presented in either their right or left visual field. The results showed that in the Skinner box there was a transitory spatial neglect contralateral to the lesion, which disappeared after 2-3 days. In the prey-catching task animals failed to target or track locusts when they were in the field contralateral to the lesion throughout the 4-day testing period. Capture and consummatory responses were normal when the locust was presented (or moved independently) into the ipsilateral (non-neglected) field. 60 days later the asymmetrical response to locusts in the contralateral field had disappeared.

Animals↗
PubMed2006

Enrichment and stability: a phenomenological coupling of energy value and carrying capacity.

Simple predator-prey models with a prey-dependent functional response predict that enrichment (increased carrying capacity) destabilizes community dynamics: this is the 'paradox of enrichment'. However, the energy value of prey is very important in this context. The intraspecific chemical composition of prey species determines its energy value as a food for the potential predator. Theoretical and experimental studies establish that variable chemical composition of prey affects the predator-prey dynamics. Recently, experimental and theoretical approaches have been made to incorporate explicitly the stoichiometric heterogeneity of simple predator-prey systems. Following the results of the previous experimental and theoretical advances, in this article we propose a simple phenomenological formulation of the variation of energy value at increased level of carrying capacity. Results of our study demonstrate that coupling the parameters representing the phenomenological energy value and carrying capacity in a realistic way, may avoid destabilization of community dynamics following enrichment. Additionally, under such coupling the producer-grazer system persists for only an intermediate zone of production--a result consistent with recent studies. We suggest that, while addressing the issue of enrichment in a general predator-prey model, the phenomenological relationship that we propose here might be applicable to avoid Rosenzweig's paradox.

Animals↗