Foraging Habitat Selection

The issue of finding and exploiting food is crucial. In heterogeneous environments, foraging habitat patches differ in quality depending on the availability of food resources and their intrinsic quality (e.g., nutritive value), but also on the costs associated to exploit these resources, involving access to food items in terms of energy, time, and injury risk when competing, risk of being preyed upon, of getting scrounged by con- or heterospecific competitors, etc. Spatial and temporal scales involved in foraging habitat selection can vary (e.g., from a few seconds up to days in large predators), but in most species, foraging habitat choices are made by individuals a large number of times over their entire lives. Decisions linked with foraging habitat selection include the choice of patches where to start foraging and duration of patch exploitation following progressive resource depletion over time, thus decision to depart to another foraging patch (Figure 3).

Theoretical models have been built and tested empirically to investigate which conditions affect these two types of decisions. Classical optimal foraging theory addresses the patch-time allocation that maximizes an individual's fitness by referring to the marginal value theorem (Figure 4), which states that the optimal strategy is to leave a foraging patch when the instantaneous fitness gain rate from the current patch falls below the average gain rate that can be achieved in the environment. The model predicts that individuals will stay longer (1) in a more profitable patch, (2) as the distance between patches (and thus travel time) increases, and (3) when the environment as a whole is less profitable (Figure 4). The marginal value theorem has been a useful tool but has however been criticized on the grounds that it makes simplified assumptions, in particular that foragers are optimal and have a complete knowledge of resource abundance and distribution in the environment, which is unrealistic. Linking the optimality predictions of the model with proximate mechanisms of patch departure decisions involved is necessary. Simple mechanistic rules for patch-leaving decisions have therefore been proposed and tested experimentally (Figure 5):

(a) Incremental rule. The probability to stay in the current patch decreases with unsuccessful search time spent in the patch, but increases each time a resource is found; individuals will find more resources items, and thus stay longer, in rich patches.

(b) Decremental rule. The probability to stay decreases each time a resource is found; individuals will thus stay shorter in high-quality patches.

(c) Giving-up-time rule. The tendency to stay decreases with unsuccessful search time spent on the patch, but each time a resource item is found, the tendency to stay is reset to a maximum level; individuals leave after a fixed unsuccessful search time (giving-up time).

(d) Fixed-number rule. The individual leaves the patch after a given, fixed number of resource items have been found.

(e) Fixed-time rule. The individual forages for a fixed period of time in each patch and leaves the patch independently of the number of resource items found.

Which decision rule will be adaptive depends on (1) the spatial distribution of resource items in the environment, which conditions the information about patch quality that

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Patch containing 6 larvae

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Figure 3 Foraging decisions in an insect parasitoid, Aphidius rhopalosiphi. Females of this parasitic wasp lay eggs in grain aphids Sitobion avenae, which are spatially distributed in discrete patches. Parasitic wasps have to adjust their searching time within a given host patch and allocate their foraging time among the different patches available in the habitat, to maximize their fitness. These decisions have to be based upon information on patch quality, obtained through both host encounter rate in the patch and previous searching experience by the wasp on the same or other patches. Patch-leaving decisions in this species depend on (a) host patch size, (b) quality of the last patch visited, and (c) previous experience in the current patch. Females spent more time in a patch when it contains more resources, when they just visited a high-quality patch, and during their first visit in the patch. From Outreman Y, Le Ralec A, Wajnberg E, and Pierre J-S (2005) Effects of within- and among-patch experiences on the patch-leaving decision rules in an insect parasitoid. Behavioral Ecology and Sociobiology 58: 208-217.

Number of previous visits on the patch:

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0 1000 2000 3000 4000 Patch searching duration time (s)

Figure 3 Foraging decisions in an insect parasitoid, Aphidius rhopalosiphi. Females of this parasitic wasp lay eggs in grain aphids Sitobion avenae, which are spatially distributed in discrete patches. Parasitic wasps have to adjust their searching time within a given host patch and allocate their foraging time among the different patches available in the habitat, to maximize their fitness. These decisions have to be based upon information on patch quality, obtained through both host encounter rate in the patch and previous searching experience by the wasp on the same or other patches. Patch-leaving decisions in this species depend on (a) host patch size, (b) quality of the last patch visited, and (c) previous experience in the current patch. Females spent more time in a patch when it contains more resources, when they just visited a high-quality patch, and during their first visit in the patch. From Outreman Y, Le Ralec A, Wajnberg E, and Pierre J-S (2005) Effects of within- and among-patch experiences on the patch-leaving decision rules in an insect parasitoid. Behavioral Ecology and Sociobiology 58: 208-217.

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Figure 4 The marginal value theorem yields the 'giving-up time' when an individual should leave its current foraging patch. (a) As an individual forages, its cumulative fitness gain gradually slows down as food becomes scarcer in the patch and food items take longer to find. An individual should aim at maximizing the net rate of fitness (or energy) gain, including time during which it cannot feed because it travels between patches. The rate of fitness gain corresponds to line A-B. The steepest slope line (gain/time), which maximizes the rate of energy gain, corresponds to the tangent line to the gain curve. When time on the patch reaches the point of contact between line A-B and the gain curve (Topt), the individual should leave the patch. (b) An individual that leaves too early (T1) will gain less energy per unit of time relative to the maximum (line A-B'). Similarly, there is no benefit in staying too long (T2) as food items are running out. (c) When an individual should stop exploiting its current patch and leave will depend on the travel time between patches, even though the gain curve once on the patch does not change. When travel time is long, individuals should leave the patch after spending more time (Topt2). Adapted from Charnov EL (1976) Optimal foraging: The marginal value theorem. Theoretical Population Biology 9: 129-136.

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Travel time Time spent in patch

Figure 4 The marginal value theorem yields the 'giving-up time' when an individual should leave its current foraging patch. (a) As an individual forages, its cumulative fitness gain gradually slows down as food becomes scarcer in the patch and food items take longer to find. An individual should aim at maximizing the net rate of fitness (or energy) gain, including time during which it cannot feed because it travels between patches. The rate of fitness gain corresponds to line A-B. The steepest slope line (gain/time), which maximizes the rate of energy gain, corresponds to the tangent line to the gain curve. When time on the patch reaches the point of contact between line A-B and the gain curve (Topt), the individual should leave the patch. (b) An individual that leaves too early (T1) will gain less energy per unit of time relative to the maximum (line A-B'). Similarly, there is no benefit in staying too long (T2) as food items are running out. (c) When an individual should stop exploiting its current patch and leave will depend on the travel time between patches, even though the gain curve once on the patch does not change. When travel time is long, individuals should leave the patch after spending more time (Topt2). Adapted from Charnov EL (1976) Optimal foraging: The marginal value theorem. Theoretical Population Biology 9: 129-136.

can be derived from finding a resource item, and (2) the individual's a priori knowledge about the environment (Table 1).

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