Foraging ecology and learning. Adaptive behavioural strategies and the value of information
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All animals face the challenge of acquiring resources for growth, survival, and reproduction. In environments that vary in time and space, foragers need to make apparently complex foraging decisions on which prey to select, where to forage, and for how long. Animals gain information from sampling and exploring the environment, and in this ecological context information becomes valuable. Learning provides a way for foragers to track changes in environmental conditions, but it involves costs that may often offset this advantage. Animals pay for information by spending energy and time, forgoing opportunities to gain resources elsewhere. The value of acquiring information hence depends on the benefits an individual obtains from using that information and the costs of collecting it. Early foraging models assumed that individuals had full information on resource levels and distribution patterns. Theoretical models predicted how individual foragers should allocate their time among resource patches, or how competing foragers should distribute to exploit the resource habitat most efficiently. These models emphasized the ultimate causes of behavior and did not consider the proximate mechanisms that foragers used to obtain information and to select the best behavioural option. Asking the same basic questions, my thesis explores how limited information may affect distribution patterns and the evolution of foraging strategies. The thesis includes five models on how animals may allocate their foraging effort in time and space in response to experiences of local resource conditions, and in response to predators or competitors. The presence of predators often causes prey to alter their behaviour. Confronted with several predator types such behavioural adjustments may cascade through several trophic levels. Prey susceptibility to one predator type (fish) may therefore depend on the abundance of another predator (zooplankton), as zooplankton prey manage their exposure to risk by moving vertically in the water column. This illustrates how the inclusion of flexible behavioural responses alters predictions from classical population level models. Behavioural decisions and flexible responses may also be important when considering dynamics of foraging groups. Social foragers may benefit from cooperative prey search or predator defence, but as groups increase in size resource competition intensifies. Foragers therefore often benefit from being in groups of intermediate size. Mobile individuals that sample the environment and collect information may aggregate in groups of preferred size. As the number of selective ‘learners’ increases in a population, groups become more similar and a simple sedentary ‘stayer’ strategy may prosper. The benefit of being selective hence depends on what the other foragers are doing, and such frequency dependence may facilitate coexistence between foraging strategies that differ in mobility and the way they sample information. In natural systems, decision making incurs conflicting demands on the design of learning and memory systems. Under stable environmental conditions, information stored in inherited traits may suffice, whereas animals foraging in temporally changing environments often need to continuously collect information and learn from experience. The value of learning is tightly linked to both the temporal and spatial variability of the resource environment. When foragers are able to obtain accurate local information, they should rely on recent experiences and quickly adjust to temporal change. Short time memories are, however, susceptible to spatial variation as learners rely on some persistency in the information gained from different patch samples in order to track changes in resource conditions. The trade-off between accuracy in estimates and ability to respond to temporal change varies also with ecological factors such as rate of predation. Learners generally take the costs of exploration early in life to enhance performance later on. When life time expectancy decreases, foragers should become less willing to invest in information acquisition. This thesis illustrates how low sampling activity enhances resource harvest early in season, at the cost of lower precision and accuracy of environmental estimates as time progresses. Changes in resource availability influence both the quality of information that a forager may obtain and the utility of this knowledge. Substituting assumptions of ideal omniscient individuals with more realistic and less critical assumptions of limited information and perceptual constraints yields different behavioural adaptations, which scale up to distribution patterns. This thesis illustrates how the action of individuals may themselves alter the quality of information, persistence of signals, and the value of exploring the habitat. As sampling and exploration alter resource and forager distributions, this affects the performance of learners, but may also alter fitness landscapes for other foraging strategies that interact within the same habitat. Through the formulation of realistic behavioural strategies, it is possible to interpret how environmental and ecological factors affect competition between individuals and life-history trade-offs. The thesis provides a modelling framework in which to interpret the effects of ecological factors on the evolutionary process of phenotypic diversification.
Paper I: Journal of Animal Ecology 74(3), Fisken, Øyvind; Eliassen, Sigrunn; Titelman, Josefin, Multiple predators in the pelagic: modelling behavioural cascades, pp. 423-429(7). Copyright 2005 British Ecological Society. Published by Blackwell Publishing.The definitive version is available at www.blackwell-synergy.com: http://dx.doi.org/10.1111/j.1365-2656.2005.00937.xPaper II: Evolutionary Ecology Research 8, Eliassen, Sigrunn; Jørgensen, Christian & Jarl Giske, Co-existence of learners and stayers maintains the advantage of social foraging, pp. 1311-1324. Copyright 2006 Sigrunn Eliassen.Paper III: Oikos 116(3), Eliassen, Sigrunn; Jørgensen, Christian & Jarl Giske, Exploration or exploitation: life expectancy changes the value of learning in foraging strategies, pp.513 - 523. Preprint version. Copyright 2008 Oikos. The final version is available at: http://dx.doi.org/10.1111/j.2006.0030-1299.15462.xPaper IV: Eliassen, Sigrunn; Jørgensen, Christian and Jarl Giske, When to learn: The ecological basis for learning in a foraging context.