Optimal Foraging Theory Research Articles

Optimality modelling in the real world

In a recent paper, Potochnik (Biol Philos 24(2):183–197, 2009) analyses some uses of optimality modelling in light of the anti-adaptationism criticism. She distinguishes two broad classes of such uses (weak and strong) on the basis of assumptions held by biologists about the role and the importance of natural selection. This is an interesting proposal that could help in the epistemological characterisation of some biological practices. However, Potochnik’s distinction also rests on the assumption that all optimality modelling represent the selection dynamic involved in the system of phenomena being considered. Since this assumption does not hold for models belonging to optimal foraging theory (OFT)—one of behavioural ecology’s important modelling traditions—Potochnik’s proposal has to be critically reappraised. In this paper, we briefly discuss what is optimality modelling and what it means for a model to represent a dynamic of selection or of evolution. Then, we demonstrate that OFT modelling is unable to represent either past or contemporary selection dynamics. In order to make this point, we carefully delineate the theory’s rationale. This allows us to identify and analyse the assumptions on which the theory is built, and to circumscribe precisely the role that natural selection plays in it. Next, we show that the distinction of weak and strong uses of optimality modelling is seriously weakened when OFT modelling is taken into account. More precisely, the distinction is either irrelevant (if the assumption that selection dynamics are represented in all optimality modelling is held) or of a modest utility (if the assumption is dropped). However, we suggest that Potochnik’s original proposal could be saved, and that it even constitutes a tool to appraise the marks left in the literature by the evolution of optimality modelling practices in the last four decades, provided that it is made into a tripartite distinction.

Habitat use and selection by the vicuña (Vicugna vicugna, Camelidae) during summer and winter in the High Andean Puna of Argentina

Habitat use and selection by vicuñas and particularly by individuals of different social categories were evaluated during the summer and winter of two years (2006 and 2007) that differed in their precipitation pattern. Both, use and selection, were analyzed within the framework of the optimal foraging theory (OFT) that predicts a high proportion of individuals using habitats where the most nutritional food items are present, and if other things are equal, habitats should be chosen if they yield the highest average rate of energy intake, which fits with this theory. Indeed, a prediction of the habitat selection theory (HST) was evaluated during winter (dry season). Over the study period high proportions of vicuñas occupied habitats with availability of high quality food, which supports the OFT. However, habitat selection differed between seasons and years due to the decrease in plant resource availability in the winter of both years and in the summer of 2007 (with drought conditions). In the dry season of the good year (2006), vicuñas (familial individuals, particularly) were more homogeneously distributed among habitats than in summer, which is consistent with the HST. High proportions of individuals at the wetland (called vega) were more evident during the bad year (2007) than in the good one. Therefore, habitat use (and selection) by vicuñas in this sector of the Puna region is influenced by the availability of high quality food resources and by the presence of water, and the relevance of the latter increased with drought conditions. An assessment of the interactions between individuals of different social categories will contribute to better understanding the distribution of animals among different habitats.

The application of foraging theory to the information searching behaviour of general practitioners

BackgroundGeneral Practitioners (GPs) employ strategies to identify and retrieve medical evidence for clinical decision making which take workload and time constraints into account. Optimal Foraging Theory (OFT) initially developed to study animal foraging for food is used to explore the information searching behaviour of General Practitioners. This study is the first to apply foraging theory within this context.Study objectives were:1. To identify the sequence and steps deployed in identifiying and retrieving evidence for clinical decision making.2. To utilise Optimal Foraging Theory to assess the effectiveness and efficiency of General Practitioner information searching.MethodsGPs from the Wellington region of New Zealand were asked to document in a pre-formatted logbook the steps and outcomes of an information search linked to their clinical decision making, and fill in a questionnaire about their personal, practice and information-searching backgrounds.ResultsA total of 115/155 eligible GPs returned a background questionnaire, and 71 completed their information search logbook.GPs spent an average of 17.7 minutes addressing their search for clinical information. Their preferred information sources were discussions with colleagues (38% of sources) and books (22%). These were the two most profitable information foraging sources (15.9 min and 9.5 min search time per answer, compared to 34.3 minutes in databases). GPs nearly always accessed another source when unsuccessful (95% after 1st source), and frequently when successful (43% after 2nd source). Use of multiple sources accounted for 41% of searches, and increased search success from 70% to 89%.ConclusionsBy consulting in foraging terms the most 'profitable' sources of information (colleagues, books), rapidly switching sources when unsuccessful, and frequently double checking, GPs achieve an efficient trade-off between maximizing search success and information reliability, and minimizing searching time. As predicted by foraging theory, GPs trade time-consuming evidence-based (electronic) information sources for sources with a higher information reward per unit time searched. Evidence-based practice must accommodate these 'real world' foraging pressures, and Internet resources should evolve to deliver information as effectively as traditional methods of information gathering.

Generalizing foraging theory for analysis and design

Foraging theory has been the inspiration for several decision-making algorithms for task-processing agents facing random environments. As nature selects for foraging behaviors that maximize lifetime calorie gain or minimize starvation probability, engineering designs are favored that maximize returned value (e.g. profit) or minimize the probability of not reaching performance targets. Prior foraging-inspired designs are direct applications of classical optimal foraging theory (OFT). Here, we describe a generalized optimization framework that encompasses the classical OFT model, a popular competitor, and several new models introduced here that are better suited for some task-processing applications in engineering. These new models merge features of rate maximization, efficiency maximization, and risk-sensitive foraging while not sacrificing the intuitive character of classical OFT. However, the central contributions of this paper are analytical and graphical methods for designing decision-making algorithms guaranteed to be optimal within the framework. Thus, we provide a general modeling framework for solitary agent behavior, several new and classic examples that apply to it, and generic methods for design and analysis of optimal task-processing behaviors that fit within the framework. Our results extend the key mathematical features of optimal foraging theory to a wide range of other optimization objectives in biological, anthropological, and technological contexts.

Foraging behaviour of Brazilian riverine and coastal fishers: How much is explained by the optimal foraging theory?

Optimal Foraging Theory (OFT) is here applied to analyse the foraging behaviour of Brazilian artisanal fishers of the Atlantic coast (Itacuruca and Sao Paulo Bagre villages) and of the inland Amazonian region (Jaraua and Ebenezer villages). Two OFT predictions are tested. Hypotheis1: A fisher who travels to more distant sites should return with more fish, and Hypothesis 2: The further a fisher goes, the longer s/he should stay fishing in a patch. OFT did not explain fishers' behaviour (non-significant regressions for coastal villages) or explain it in specific seasons (low water season for one Amazonian village: H1 r 2 =24.1; H2 r 2 =37.2) and in specific habitats (e.g., lakes and backwaters in Jaraua village, Lakes: H1 r 2 =13.5; H2 r 2 =24.0; Backwaters: H1 r 2 =34.4; H2 r 2 =46.5). The findings can indicate areas or seasons that are under higher fishing pressure, when fishers try to get the best out of a situation without any concern about resource conservation. By knowing the variables that influence fishers' decision-making processes, management initiatives may be more fine-tuned to the local reality and are thus more likely to succeed.

Predicting trophic relations in ecological networks: A test of the Allometric Diet Breadth Model

Few food web theory hypotheses/predictions can be readily tested using likelihoods of reproducing the data. Simple probabilistic models for food web structure, however, are an exception as their likelihoods were recently derived. Here I test the performance of a more complex model for food web structure that is grounded in the allometric scaling of interactions with body size and the theory of optimal foraging (Allometric Diet Breadth Model—ADBM). This deterministic model has been evaluated by measuring the fraction of trophic relations it correctly predicts. I contrasted this value with that produced by simpler models based on body sizes and found that the quantitative information on allometric scaling and optimal foraging does not significantly increase model fit. Also, I present a method to compute the p-value for the fraction of trophic interactions correctly predicted by the ADBM, or any other model, with respect to three probabilistic models. I find that the ADBM predicts significantly more links than random graphs, but other models can outperform it. Although optimal foraging and allometric scaling may improve our understanding of food webs, the ADBM needs to be modified or replaced to find support in the data.

Ecology of Plant and Free-Living Nematodes in Natural and Agricultural Soil

Nematodes are aquatic organisms that depend on thin water films to live and move within existing pathways of soil pores of 25-100 mum diameter. Soil nematodes can be a tool for testing ecological hypotheses and understanding biological mechanisms in soil because of their central role in the soil food web and linkage to ecological processes. Ecological succession is one of the most tested community ecology concepts, and a variety of nematode community indices have been proposed for purposes of environmental monitoring. In contrast, theories of biogeography, colonization, optimal foraging, and niche partitioning by nematodes are poorly understood. Ecological hypotheses related to strategies of coexistence of nematode species sharing the same resource have potential uses for more effective biological control and use of organic amendments to foster disease suppression. Essential research is needed on nematodes in natural and agricultural soils to synchronize nutrient release and availability relative to plant needs, to test ecological hypotheses, to apply optimal foraging and niche partitioning strategies for more effective biological control, to blend organic amendments to foster disease suppression, to monitor environmental and restoration status, and to develop better predictive models for land-use decisions.

Hierarchical movement decisions in predators: effects of foraging experience at more than one spatial and temporal scale

Animal movements in heterogeneous environments play a crucial role in a variety of ecological processes. Although a hierarchical structure to the scale of movements has been observed in many animal species, few studies to date have revealed what causes such multi-spatial scale movements within the framework of optimal theories. Using detailed information on movement paths and prey captures by Intermediate Egrets in rice fields, this study explored the effects of individual experience at multi-temporal scales on subsequent movement parameters. The results supported three predictions based on optimal foraging theories: (1) movement distances at small and large spatial scales are determined by past foraging experiences over short (one minute) and long (seven to eight minutes) temporal scales, respectively; (2) responses to prey encounters vary between temporal scales, e.g., prey attacks in the previous one minute caused egrets to walk away (i.e., area-avoided search), whereas those in the preceding eight minutes caused egrets to stay around (i.e., area-restricted search); and (3) the probability of patch departure by flying increases with decreasing intake rates in the previous seven minutes, and the quitting intake rate is mostly lower than the long-term average, not including travel time. These results suggest that egrets make different adaptive decisions at different spatial scales: in response to a clumped prey distribution at a large spatial scale, and to a dispersed distribution or behavioral depression of prey at a small spatial scale. This study showed that the framework of optimal foraging theories is useful for comprehensively explaining the different movement modes in animals. It should also be emphasized that considering both area-avoided and restricted searches at multi-temporal scales is important for understanding movement decisions, particularly in organisms searching for hierarchically distributed resources.

Optimal search behavior and classic foraging theory

Random walk methods and diffusion theory pervaded ecological sciences as methods to analyze and describe animal movement. Consequently, statistical physics was mostly seen as a toolbox rather than as a conceptual framework that could contribute to theory on evolutionary biology and ecology. However, the existence of mechanistic relationships and feedbacks between behavioral processes and statistical patterns of movement suggests that, beyond movement quantification, statistical physics may prove to be an adequate framework to understand animal behavior across scales from an ecological and evolutionary perspective. Recently developed random search theory has served to critically re-evaluate classic ecological questions on animal foraging. For instance, during the last few years, there has been a growing debate on whether search behavior can include traits that improve success by optimizing random (stochastic) searches. Here, we stress the need to bring together the general encounter problem within foraging theory, as a mean for making progress in the biological understanding of random searching. By sketching the assumptions of optimal foraging theory (OFT) and by summarizing recent results on random search strategies, we pinpoint ways to extend classic OFT, and integrate the study of search strategies and its main results into the more general theory of optimal foraging.

Wolf, <em>Canis lupus</em>, Visits to White-tailed Deer, <em>Odocoileus virginianus</em>, Summer Ranges: Optimal Foraging?

We tested whether Wolf (Canis lupus) visits to individual female White-tailed Deer (Odocoileus virginianus) summer ranges during 2003 and 2004 in northeastern Minnesota were in accord with optimal-foraging theory. Using GPS collars with 10- to 30-minute location attempts on four Wolves and five female deer, plus eleven VHF-collared female deer in the Wolves' territory, provided new insights into the frequency of Wolf visits to summer ranges of female deer. Wolves made a mean 0.055 visits/day to summer ranges of deer three years and older, significantly more than their 0.032 mean visits/day to ranges of two-year-old deer, which generally produce fewer fawns, and most Wolf visits to ranges of older deer were much longer than those to ranges of younger deer. Because fawns comprise the major part of the Wolf's summer diet, this Wolf behavior accords with optimal-foraging theory.

The use of metapopulation and optimal foraging theories to predict movement and foraging decisions of mobile animals in heterogeneous landscapes

Metapopulation and optimal foraging theories predict the presence of animals and their duration, respectively, in foraging patches. This paper examines use of these two theories to describe the movements and patterns of foraging in patches used by Caspian gulls (Laruscachinnans) at inland reservoirs during the chick-rearing period. We assumed that birds would move differently across diverse habitats, with some types of land cover less permeable than others, and some landscape features acting as corridors. We also expected larger and less isolated patches, and patches that were close to corridors, to have a higher probability of the presence of foraging birds, and that they would be more abundant, forage for a shorter time, and hunt smaller prey than in small, more isolated patches surrounded by barriers. Forests seem to be a much less permeable type of land cover, whereas rivers became corridors for Caspian gulls during foraging trips. Probability of bird presence was positively related to the size of foraging patches and negatively linked with distance to the nearest river, distance to the nearest foraging patch, and the presence of forests in the vicinity. The same factors significantly affected bird abundance. Contrary to expectations, the duration and success of foraging were not influenced by any variable we measured, suggesting that although larger patches contain a higher abundance of fish, their density and the probability of capturing prey were relatively stable among the various patches. However, gulls that foraged in more isolated ponds that were located further from the river and the colony, and also surrounded by forest, captured larger fish more often than birds that foraged near the colony in less-isolated patches. Pooling metapopulation and optimal foraging concepts seems to be valuable in describing patch use by foraging animals.

MOVEMENT DECISIONS BY INTERMEDIATE EGRETS

An Intermediate Egret (Egretta intermedia) catching a tadpole in a flooded rice field in Japan. Animal movements in heterogeneous environments play a crucial role in a variety of ecological processes. But few studies have revealed what causes animals to show multispatial scale movements within the framework of optimal theories. Using detailed information on movement paths and prey captures by Intermediate Egrets in rice fields, we explored the effects of individual experience at multitemporal scales on subsequent movement parameters. A mobile GIS system (left), which consists of a portable GPS receiver, a laser rangefinder, and an electronic compass, allowed information about a target location to be recorded from a distant observation point (right). Simultaneously, we recorded the timing, duration, and type of behavior of the focal egret. This study showed that the framework of optimal foraging theories is useful for explaining comprehensively the different movement modes in animals. We also concluded that considering both area-avoided and restricted searches at multitemporal scales is important for understanding movement decisions, particularly in organisms searching for hierarchically distributed resources. The results of the study suggested that egrets make different adaptive decisions at different spatial scales. Prey attacks in the previous 1 minute caused egrets to walk away (area-avoided search, left) at a small spatial scale, whereas those in the preceding 8 minutes caused egrets to stay around (area-restricted search, right) at a large spatial scale. Also, the timing of patch departure by flying seemed to follow predictions by the marginal value theorem. These photographs illustrate the article “Hierarchical movement decisions in predators: the influence of foraging experience at more than one spatial and temporal scale” by Tatsuya Amano and Naoki Katayama, tentatively scheduled to appear in Ecology in 2009.

Characteristics of the acorns selected by free range Iberian pigs during the montanera season

Acorn weight, size, shape and composition vary a great deal among evergreen oaks ( Quercus ilex rotundifolia); however these characteristics remain constant for a tree throughout the fruiting period. The number of consumed acorns under the canopy of a tree could be an indicator of preference for its acorn characteristics. This paper analyses the characteristics of the acorns selected by free-ranging Iberian pigs at the start (November) and end (February) of the montanera or acorn mast-feeding season, when these are sustained fattened. At these two stages, acorn samples were taken from 20 oak trees ( Q. ilex rotundifolia) that had been rejected by the pigs (only eating between 2 and 9 acorns from the ground underneath the tree) and from 20 oak trees that were actively sought out by the pigs (eating at least 40 acorns). The differences observed between the sought out and rejected acorns at the start and end of the montanera season are too great to be only a matter of chance, suggesting that Iberian pigs must form associations between variables when choosing to eat or reject the acorns from a specific oak tree. The results show that pigs tend to select heavier acorns at the start of the montanera season, while at the end their selection is based more on the composition of the acorns. The greatest number of significant differences between the groups of acorns were found in the sought out acorns in November and February, owing to all the variables related with the composition of the acorns: crude fat (10.8 ± 0.3 vs. 7.4 ± 0.3), starch (58.3 ± 1.0 vs. 50.3 ± 0.4), sugars (6.8 ± 0.4 vs. 13.2 ±0.3); most of the fatty acids; dimensions: length (38.1 ± 0.8 vs. 31.9 ± 0.7) and diameter (16.6 ± 0.4 vs. 13.9 ± 0.1); acorn weights: fresh whole acorn (6.9 ± 0.3 vs. 3.4 ± 0.1) and kernel DM (2.5 ± 0.2 vs. 1.7 ± 0.1). Pigs prefer acorns with larger kernel. This selective consume goes along with the theory of Optimal Foraging and suggest that pigs learn to use visual stimuli when grazing. In conclusion, this study shows a constant selective acorn consumption and a variation in acorn preferences throughout the montanera period; consequently it is proven that acorn diet is not homogeneous and varies over the fattening period.

Generalist foraging of pollinators: diet expansion at high density

Summary Plant–pollinator interactions are crucial for the reproduction of most angiosperms. When faced with perturbations, plant–pollinator networks might be robust mainly due to the presence of highly generalist species. Perturbations can, in turn, affect how pollinators exploit their food sources and therefore their degree of generalization. In particular, generalization of pollinators might vary with forager density. At high densities floral resources available in plant communities would be more rapidly depleted than at low densities. According to optimal foraging theory (OFT), this decrease in resources should lead to an increase in diet breadth. We investigated the impact of pollinator density on diet breadth of Bombus terrestris. We recorded the individual foraging behaviour of bumblebees from eight colonies. They foraged in presence of either one or several conspecifics on experimental plant communities composed of five plant species. Diet breadth was calculated as the number of plant species visited, as well as by the Levins index. We analyzed the effect of forager density on diet breadth at both the individual and the colony levels. We found that forager density affected degree of generalization at individual and colony levels. A more competitive environment increased individual generalization, as predicted by OFT, and decreased the variability in generalization across colonies. Moreover, we found that bumblebee size was positively related to the amplitude of diet breadth change. Synthesis. Our study sheds light on an additional level of complexity of plant–pollinator systems, showing that pollinator diet breadth is a flexible trait which results from behavioural adaptation to resource availability. More generally, changes in foraging insects’ diet via competition for resources are likely to ensue from anthropogenic ecosystem disturbance and to impact upon the functioning of plant–pollinator networks.

Search in External and Internal Spaces

There is compelling molecular and behavioral evidence that goal-directed cognition is an evolutionary descendant of spatial-foraging behavior. Across animal species, similar dopaminergic processes modulate between exploratory and exploitative foraging behaviors and control attention. Consequently, we hypothesized that spatial-foraging activity could prime attentional cognitive activity. We examined how searching in physical space influences subsequent search in abstract cognitive space by presenting participants with a spatial-foraging task followed by a repeated Scrabble task involving search for words that could be made from letter sets. Participants who searched through clumpier distributions in space behaved as if words were more densely clumped in the Scrabble task. This was not a function of arousal, but was consistent with predictions of optimal-foraging theory. Furthermore, individual differences in exploratory search were conserved across the two types of tasks. Along with the biological evidence, our results support the idea that there are generalized cognitive search processes.

The assumption of optimality in foraging models: A simulated experiment with dairy cows grazing grass pasture

The “patch model” of optimal foraging theory (OFT) assumes that animals choose and exploit patches of food in order to maximise their food intake rate and derives the foraging rules (patch selection and exploitation) to achieve this. While evidence from field studies has been found to be broadly consistent with the predictions of OFT, this does not imply that animals actually forage using OFT rules, however, as the same observations could possibly arise as the result of non‐optimising foraging rules (e.g., avoidance of low food intake rate). Because OFT is potentially very useful for the purpose of managing animal production systems (e.g., for predicting and manipulating the intake of grazing dairy cows), it is of interest to know to what extent OFT remains useful if animals actually behave sub‐optimally. This paper examines this question by simulating animals foraging in a two‐dimensional spatially heterogeneous environment (New Zealand dairy cows grazing grass pasture) using six different foraging rules with varying degrees of optimality. The results showed that both optimal patch selection and optimal patch residence time rules significantly increase intake rate in this type of environment, but differences resulting from use of different variations of the patch selection rules are small. Therefore, although OFT is highly simplified, and animals’ senses and knowledge are imperfect, this need not preclude the use of OFT in practical applications, as the predictions of OFT are likely to be similar to the predictions of other, more realistic, foraging strategies, even when these are quite sub‐optimal in formulation.

Meat in the human diet: An anthropological perspective

Human ancestral diets changed substantially approximately four to five million years ago with major climatic changes creating open grassland environments. We developed a larger brain balanced by a smaller, simpler gastrointestinal tract requiring higher-quality foods based around meat protein and fat. Anthropological evidence from cranio-dental features and fossil stable isotope analysis indicates a growing reliance on meat consumption during human evolution. Study of hunter-gatherer societies in recent times shows an extreme reliance on hunted and fished animal foods for survival. Optimal foraging theory shows that wild plant foods in general give an inadequate energy return for survival, whereas the top-ranking food items for energy return are large hunted animals. Numerous evolutionary adaptations in humans indicate high reliance on meat consumption, including poor taurine production, lack of ability to chain elongate plant fatty acids and the co-evolution of parasites related to dietary meat.

Effect of breed, season and pasture moisture gradient on foraging behaviour in cattle on semi-natural grasslands

The objectives of this study were to determine the effects of breed, season and pasture moisture gradient on the extent and location of grazing and ruminating behaviour in cattle kept on heterogeneous semi-natural grasslands. Two groups of heifers, 12 of the traditional Swedish breed Väneko and 12 of the commercial Continental breed Charolais, were allocated into three enclosures per breed. Each enclosure consisted of heterogeneous semi-natural grasslands dominated by Deschampsia cespitosa (tufted hairgrass) and each contained dry, mesic and wet areas. In spring, summer and autumn, behaviour was recorded using automatic behaviour recorders, and positioning and activity were recorded using GPS receivers for 24 h for every heifer. The Väneko heifers had a higher activity than the Charolais heifers ( P = 0.006), which supports the theories of resource allocation and contrafreeloading, but there were no differences between breeds in location of grazing, ruminating or idling. The heifers spent more time grazing in autumn (42.5% of the day) than in spring (38.5%; P = 0.006) and summer (38.9%; P = 0.014) and the efficiency of grazing ( i.e. proportion of eating during grazing bouts) increased over the grazing period ( P < 0.001). The results indicate the heifers avoided grazing in darkness which is consistent with the theory that predation risk affects foraging. Herbage in wet areas had a lower concentration of crude protein ( P = 0.036) and a higher concentration of neutral detergent fibre ( P = 0.011) than herbage in dry areas. At the same time, on average over seasons, 28% of the herbage mass was found in the wet areas, whereas only 8% of the grazing occurred there. Furthermore, the proportion of eating during grazing bouts was lower (73.9%; P < 0.001) in wet areas than in dry (80.1%) and mesic (79.6%) areas. The results indicate the heifers avoided grazing in wet areas where forage had a low nutritional value, supporting the theory of optimal foraging. In conclusion, both breed and season affects foraging behaviour of cattle on semi-natural grasslands, as the heifers of the traditional breed had a higher activity than the commercial breed and the grazing time was longer and more effective in late than in early grazing period. The cattle avoided foraging in wet areas, which may impact on the management of these areas.

Short‐Term Foraging Costs and Long‐Term Fueling Rates in Central‐Place Foraging Swans Revealed by Giving‐Up Exploitation Times

Foragers tend to exploit patches to a lesser extent farther away from their central place. This has been interpreted as a response to increased risk of predation or increased metabolic costs of prey delivery. Here we show that migratory Bewick's swans (Cygnus columbianus bewickii), though not incurring greater predation risks farther out or delivering food to a central place, also feed for shorter periods at patches farther away from their roost. Predictions from an energy budget model suggest that increasing metabolic travel costs per se are responsible. Establishing the relation between intake rate and exploitation time enabled us to express giving-up exploitation times as quitting harvest rates (QHRs). This revealed that net QHRs were not different from observed long-term net intake rates, a sign that the birds were maximizing their long-term net intake rate. This study is unique because giving-up decisions were measured at the individual level, metabolic and predation costs were assessed simultaneously, the relation with harvest rate was made explicit, and finally, short-term giving-up decisions were related to long-term net intake rates. We discuss and conceptualize the implications of metabolic traveling costs for carrying-capacity predictions by bridging the gap between optimal-foraging theory and optimal-migration theory.

OPTIMAL MOVEMENT STRATEGIES FOR SOCIAL FORAGERS IN UNPREDICTABLE ENVIRONMENTS

Spatial movement models often base movement decision rules on traditional optimal foraging theories, including ideal free distribution (IFD) theory, more recently generalized as density-dependent habitat selection (DDHS) theory, and the marginal value theorem (MVT). Thus optimal patch departure times are predicted on the basis of the density-dependent resource level in the patch. Recently, alternatives to density as a habitat selection criterion, such as individual knowledge of the resource distribution, conspecific attraction, and site fidelity, have been recognized as important influences on movement behavior in environments with an uncertain resource distribution. For foraging processes incorporating these influences, it is not clear whether simple optimal foraging theories provide a reasonable approximation to animal behavior or whether they may be misleading. This study compares patch departure strategies predicted by DDHS theory and the MVT with evolutionarily optimal patch departure strategies for a wide range of foraging scenarios. The level of accuracy with which individuals can navigate toward local food sources is varied, and individual tendency for conspecific attraction or repulsion is optimized over a continuous spectrum. We find that DDHS theory and the MVT accurately predict the evolutionarily optimal patch departure strategy for foragers with high navigational accuracy for a wide range of resource distributions. As navigational accuracy is reduced, the patch departure strategy cannot be accurately predicted by these theories for environments with a heterogeneous resource distribution. In these situations, social forces improve foraging success and have a strong influence on optimal patch departure strategies, causing individuals to stay longer in patches than the optimal foraging theories predict.