Consequences of food distribution for optimal searching behavior: an evolutionary model

Active predators do not necessarily specialize in sedentary prey: A simulation model

Predators employ diverse foraging modes, ranging from ambush to active pursuit of prey. While ambush predators are associated with capturing mobile prey, the specialization of active predators on sedentary prey remains less understood. I examined the circumstances under which active predators preferentially capture sedentary prey. Using a spatially explicit individual-based simulation model, I manipulated the spatial patterns of sedentary prey, movement directionality, speed of mobile prey and active predators, and the presence of competing ambush predators. Key factors such as area-restricted search (ARS) by active predators, uncertain capture success of prey, and prey reappearance after capture were also considered. The results suggest that active predators do not necessarily specialize in sedentary prey. Instead, their prey preference is influenced by prey spatial patterns and competition with ambush predators: clumped spatial patterns of sedentary prey and the use of ARS by active predators as well as competition with ambush predators drove active predators to focus on sedentary prey. Conversely, nondirectional movement by predators and faster-moving prey often led to higher proportions of mobile prey being captured. These findings challenge traditional assumptions about active predator specialization and emphasize the importance of integrating spatial and behavioral dynamics into predator-prey models.

Reticulitermes flavipes (Blattodea: Rhinotermitidae) Response to Wood Mulch and Workers Mediated by Attraction to Carbon Dioxide

The eastern subterranean termite, Reticulitermes flavipes, is challenged by the significant energy expenditures of tunnel construction for resource discovery. Subterranean termites use idiothetic mechanisms to explore large spaces, while the use of resource-specific cues for localized search is disputed. Here, termite response to wood mulch, termite workers, extracts of wood mulch, and CO2 alone were tested using a bioassay design that distinguished between attraction and arrestment. Termites showed significant attraction to wood mulch with workers or to wood mulch alone. They did not respond to workers alone at the initial dose tested, but were attracted to workers at higher densities. Termites did not respond to water or the acetone extracts of wood mulch, but did show a partial response to hexane extract compared to intact wood mulch. More significantly, when CO2 was removed from the emissions of wood mulch and workers using soda lime, attraction was eliminated. Furthermore, termites showed a quadratic response to CO2 concentration that peaked at ca. 14,000 ppm. The response to CO2 alone predicted by the model matched termite response to mulch + workers when compared at the level of CO2 they emitted. The results suggest that CO2 is both necessary and sufficient to explain the attraction response of R. flavipes to mulch and workers we observed. It is argued that orientation to food cues complements the previously demonstrated idiothetic program to maximize the efficiency of resource location.

C. elegans foraging as a model for understanding the neuronal basis of decision-making

Animals have evolved to seek, select, and exploit food sources in their environment. Collectively termed foraging, these ubiquitous behaviors are necessary for animal survival. As a foundation for understanding foraging, behavioral ecologists established early theoretical and mathematical frameworks which have been subsequently refined and supported by field and laboratory studies of foraging animals. These simple models sought to explain how animals decide which strategies to employ when locating food, what food items to consume, and when to explore the environment for new food sources. These foraging decisions involve integration of prior experience with multimodal sensory information about the animal's current environment and internal state. We suggest that the nematode Caenorhabditis elegans is well-suited for a high-resolution analysis of complex goal-oriented behaviors such as foraging. We focus our discussion on behavioral studies highlighting C. elegans foraging on bacteria and summarize what is known about the underlying neuronal and molecular pathways. Broadly, we suggest that this simple model system can provide a mechanistic understanding of decision-making and present additional avenues for advancing our understanding of complex behavioral processes.

Search patterns, resource regeneration, and ambush locations impact the competition between active and ambush predators

Many predators ambush prey rather than pursue them or shift between foraging modes. Active predators typically encounter prey more frequently than ambush predators. I designed a simulation model to examine whether this always holds and how active and ambush predators fare in capturing mobile prey. Prey foraged for clumped resources using area-restricted search, shifting from directional movement before resource encounter to less directional movement afterward. While active predators succeeded more than ambush predators, the advantage of active predators diminished when ambush predators were positioned inside resource patches rather than outside. I investigated the impact of eight treatments and their interactions. For example, regeneration of prey resources increased the difference between ambush predators inside and outside patches, and uncertain prey capture by predators decreased this difference. Several interactions resulted in outcomes different from each factor in isolation. For instance, reducing the directionality level of active predators impacted moderately when applied alone, but when combined with resource regeneration it led to the worst success of active predators against ambush predators inside patches. Ambush predators may not always be inferior to active predators, and one should consider the key traits of the studied system to predict the relative success of these two foraging modes.

Intermittent Search, Not Strict Lévy Flight, Evolves under Relaxed Foraging Distribution Constraints

AbstractThe survival of an animal depends on its success as a forager, and understanding the adaptations that result in successful foraging strategies is an enduring endeavour of behavioral ecology. Random walks are one of the primary mathematical descriptions of foraging behavior. Power law distributions are often used to model random walks, as they can characterize a wide range of behaviors, including Lévy walks. Empirical evidence indicates the prevalence and efficiency of Lévy walks as a foraging strategy, and theoretical work suggests an evolutionary origin. However, previous evolutionary models have assumed a priori that move lengths are drawn from a power law or other families of distributions. Here, we remove this restriction with a model that allows for the evolution of any distribution. Instead of Lévy walks, our model unfailingly results in the evolution of intermittent search, a random walk composed of two disjoint modes-frequent localized walks and infrequent extensive moves-that consistently outcompeted Lévy walks. We also demonstrate that foraging using intermittent search may resemble a Lévy walk because of interactions with the resources within an environment. These extrinsically generated Lévy-like walks belie an underlying behavior and may explain the prevalence of Lévy walks reported in the literature.

A guide to area-restricted search: a foundational foraging behaviour

Area-restricted search is the capacity to change search effort adaptively in response to resource encounters or expectations, from directional exploration (global, extensive search) to focused exploitation (local, intensive search). This search pattern is used by numerous organisms, from worms and insects to humans, to find various targets, such as food, mates, nests, and other resources. Area-restricted search has been studied for at least 80 years by ecologists, and more recently in the neurological and psychological literature. In general, the conditions promoting this search pattern are: (1) clustered resources; (2) active search (e.g. not a sit-and-wait predator); (3) searcher memory for recent target encounters or expectations; and (4) searcher ignorance about the exact location of targets. Because area-restricted search adapts to resource encounters, the search can be performed at multiple spatial scales. Models and experiments have demonstrated that area-restricted search is superior to alternative search patterns that do not involve a memory of the exact location of the target, such as correlated random walks or Lévy walks/flights. Area-restricted search is triggered by sensory cues whereas concentrated search in the absence of sensory cues is associated with other forms of foraging. Some neural underpinnings of area-restricted search are probably shared across metazoans, suggesting a shared ancestry and a shared solution to a common ecological problem of finding clustered resources. Area-restricted search is also apparent in other domains, such as memory and visual search in humans, which may indicate an exaptation from spatial search to other forms of search. Here, we review these various aspects of area-restricted search, as well as how to identify it, and point to open questions.

The interaction between ambush predators, search patterns of herbivores, and aggregations of plants

While predators benefit from spatial overlap with their prey, prey strive to avoid predators. I used an individual-based simulation comprising sit-and-wait predators, widely foraging herbivores, and plants, to examine the link between predator ambush location, herbivore movement, and plant aggregation. I used a genetic algorithm to reach the best strategies for all players. The predators could ambush herbivores either inside or outside plant patches. The herbivores could use movement of varying directionality levels, with a change in directionality following the detection of plants. When the predators were fixed outside plant patches, the herbivores were selected to use a directional movement before plant encounter followed by a tortuous movement afterwards. When predators were fixed inside patches, herbivores used a continuous directional movement. Predators maintained within-patch positions when the herbivores were fixed to use the directional-tortuous movement. The predator location inside patches led to higher plant aggregations, by changing the herbivore movement. Finally, I allowed half of the predators to search for herbivores and let them compete with sit-and-wait predators located inside plant patches. When plants were clumped and herbivores used a directional-tortuous movement, with a movement shift after plant detection, ambush predators had the highest success relative to widely foraging predators. In all other scenarios, widely foraging predators did much better than ambush predators. The findings from my simulation suggest a behavioral mechanism for several observed phenomena of predator–prey interactions, such as a shorter stay by herbivores in patches when predators ambush them nearby, and a more directional movement of herbivores in riskier habitats.

Factors That Can Affect the Spatial Positioning of Large and Small Individuals in Clusters of Sit-and-Wait Predators

Shadow competition, the interception of prey by sit-and-wait predators closest to the source of prey arrival, is prevalent in clusters of sit-and-wait predators. Peripheral positions in the cluster receive more prey and should thus be more frequently occupied. Models predicting spatial positioning in groups, however, usually ignore variability among group members. Here, I used a simulation model to determine conditions under which small and large sit-and-wait predators, which differ in their attack range, should differ in their spatial positions in the cluster. Small predators occupied peripheral positions more frequently than large predators at the simulation beginning, while the opposite held true as time advanced. Because of the large and small attack range of large and small predators, respectively, small predators mistakenly relocated away from peripheral positions, while large predators did not relocate fast enough from inferior central positions. Any factor that moderated the frequent relocations of small predators or had the opposite effect on large predators assisted small or large predators, respectively, in reaching the more profitable peripheral positions. Furthermore, any factor elevating shadow competition led to longer occupation of the periphery by large predators. This model may explain why sit-and-wait predators are not homogenously distributed in space according to size.

The effect of maze complexity on maze-solving time in a desert ant

One neglected aspect of research on foraging behavior is that of the effect of obstacles that increase habitat complexity on foraging efficiency. Here, we explored how long it takes individually foraging desert ant workers (Cataglyphis niger) to reach a food reward in a maze, and examined whether maze complexity affects maze-solving time (the time elapsed till the first worker reached the food reward). The test mazes differed in their complexity level, or the relative number of correct paths leading to the food reward, vs. wrong paths leading to dead-ends. Maze-solving time steeply increased with maze complexity, but was unaffected by colony size, despite the positive correlation between colony size and the number of workers that searched for food. The number of workers observed feeding on the food reward 10 min after its discovery decreased with complexity level but not colony size. We compared our experimental results to three simulation models, applying different search methods, ranked them according to their fit to the data and found the self-avoiding random search to fit the best. We suggest possible reasons for the model deviations from the observational findings. Our data emphasize the necessity to refer to habitat complexity when studying foraging behavior.

Prey density, value, and spatial distribution affect the efficiency of area-concentrated search

Searching individuals need to take decisions on where and how long to search. When food is spatially aggregated, detection of a food item signals a probability for the presence of further prey items in its surrounding. Organisms can thus intensify search effort upon detecting a prey item, but after unsuccessfully searching for a while, return to the previous, extensive search, this strategy is known as 'area-concentrated-search' (ACS). Here we present results of simulations where individuals perform ACS employing a correlated random walk with variable directional persistence. Switching between intensive and extensive search (with respectively low and high directional persistence) is a function of searcher's internal state represented as 'satiety' level depending on preceding consumption of prey items. We explore the effect of this function's control parameters ('switching level' i.e., the satiety at which the switching occurs, and the switchover shape parameter) on searching efficiency in dependence of (1) prey items' spatial distribution ranging from randomly uniform to highly contagious, (2) the overall prey density, and (3) prey 'caloric' value. Our main conclusions: (1) the form of the adopted switchover exerts an effect on searching efficiency, and this effect is most pronounced in landscapes with highly aggregated resources. Except for the most homogeneous prey distributions, there was a clear optimum area within the movement parameter space, yielding highest efficiency. (2) The optimal switching level is larger in heterogeneous landscapes, but optimum switchover shape is little affected by any of the landscape attributes. In most landscapes, it is most profitable to switch gradually rather than abruptly. (3) The success and optimal switching level depend not only on the prey's spatial distribution but also on average prey density while the value of prey items has little effect on the optimal movement parameters.

Spatial structure and nest demography reveal the influence of competition, parasitism and habitat quality on slavemaking ants and their hosts

BACKGROUND

Natural communities are structured by intra-guild competition, predation or parasitism and the abiotic environment. We studied the relative importance of these factors in two host-social parasite ecosystems in three ant communities in Europe (Bavaria) and North America (New York, West Virginia). We tested how these factors affect colony demography, life-history and the spatial pattern of colonies, using a large sample size of more than 1000 colonies. The strength of competition was measured by the distance to the nearest competitor. Distance to the closest social parasite colony was used as a measure of parasitism risk. Nest sites (i.e., sticks or acorns) are limited in these forest ecosystems and we therefore included nest site quality as an abiotic factor in the analysis. In contrast to previous studies based on local densities, we focus here on the positioning and spatial patterns and we use models to compare our predictions to random expectations.

RESULTS

Colony demography was universally affected by the size of the nest site with larger and more productive colonies residing in larger nest sites of higher quality. Distance to the nearest competitor negatively influenced host demography and brood production in the Bavarian community, pointing to an important role of competition, while social parasitism was less influential in this community. The New York community was characterized by the highest habitat variability, and productive colonies were clustered in sites of higher quality. Colonies were clumped on finer spatial scales, when we considered only the nearest neighbors, but more regularly distributed on coarser scales. The analysis of spatial positioning within plots often produced different results compared to those based on colony densities. For example, while host and slavemaker densities are often positively correlated, slavemakers do not nest closer to potential host colonies than expected by random.

CONCLUSIONS

The three communities are differently affected by biotic and abiotic factors. Some of the differences can be attributed to habitat differences and some to differences between the two slavemaking-host ecosystems. The strong effect of competition in the Bavarian community points to the scarcity of resources in this uniform habitat compared to the other more diverse sites. The decrease in colony aggregation with scale indicates fine-scale resource hotspots: colonies are locally aggregated in small groups. Our study demonstrates that species relationships vary across scales and spatial patterns can provide important insights into species interactions. These results could not have been obtained with analyses based on local densities alone. Previous studies focused on social parasitism and its effect on host colonies. The broader approach taken here, considering several possible factors affecting colony demography and not testing each one in isolation, shows that competition and environmental variability can have a similar strong impact on demography and life-history of hosts. We conclude that the effects of parasites or predators should be studied in parallel to other ecological influences.