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Figure 5.4. Graphical results ofWhelan and Brown's (2005) foraging model incorporating both external (preconsumptive) and internal (postconsumptive) handling of food. (A) Harvest rate as a function of resource abundance when both external and internal food handling operate (h = gB); when only external food handling operates (h); and when only internal food handling operates (gB). (B) Proportion of gut filled as a function of increasing resource abundance as external handling time (h) increases from 1 to 100. The proportion of the gut filled rises monotonically with decreasing slope. Note that the proportion of gut filling declines sharply with longer external handling times. (C) Proportion of gut filled as a function of increasing resource abundance as internal handling time (T) increases from 1 to 100. The proportion of the gut filled rises monotonically with decreasing slope. Note that the proportion of gut filling declines sharply with shorter internal handling times.

Figure 5.4. Graphical results ofWhelan and Brown's (2005) foraging model incorporating both external (preconsumptive) and internal (postconsumptive) handling of food. (A) Harvest rate as a function of resource abundance when both external and internal food handling operate (h = gB); when only external food handling operates (h); and when only internal food handling operates (gB). (B) Proportion of gut filled as a function of increasing resource abundance as external handling time (h) increases from 1 to 100. The proportion of the gut filled rises monotonically with decreasing slope. Note that the proportion of gut filling declines sharply with longer external handling times. (C) Proportion of gut filled as a function of increasing resource abundance as internal handling time (T) increases from 1 to 100. The proportion of the gut filled rises monotonically with decreasing slope. Note that the proportion of gut filling declines sharply with shorter internal handling times.

Resource 2

Figure 5.5. Harvest rate surface for two resources, 1 and 2, as a function of their abundance. Resource 2 has half the energetic value and 10 times the bulk volume of resource 1. Note that because the forager has greater external handling efficiency on resource 1, and because resource 1 is richer than resource 2, the total harvest rate is maximal when the forager consumes only resource 1.

Resource 2

Figure 5.5. Harvest rate surface for two resources, 1 and 2, as a function of their abundance. Resource 2 has half the energetic value and 10 times the bulk volume of resource 1. Note that because the forager has greater external handling efficiency on resource 1, and because resource 1 is richer than resource 2, the total harvest rate is maximal when the forager consumes only resource 1.

gut fullness. These different properties ofexternal and internal handling time result directly from their batch reactor—like and continuous flow reactor—like properties, respectively.

Referring to the food type with the greater profitability e/h as resource 1, there can be two relationships between resources 1 and 2 in terms of bulk properties. First, both profitability (external handling efficiency) and food richness (ratio of energy to bulk) are greater for resource 1 (e1/h1 e2/h2 and e1/&1 > e2/b2). Second, profitability is greater for resource 1, but food richness is greater for resource 2 (e1/h1 > e2/h2 and e2/&2 > e1/b{). These relationships between the food types lead to different expectations regarding diet selectivity. Under the first relationship (e1/h1 > e2/h2 and e1/b1 > e2/b%), the forager will exhibit either complete selectivity for resource 1, partial selectivity for resource 2, or complete opportunism, depending on the relative abundances of the two foods (fig. 5.6). Interestingly, the relative food abundances that result in partial preferences depend on the ratio of the richness of resource 1 to that of resource 2: the larger the ratio, the greater the range of partial preferences. Under the second relationship (e1 /h1 > e2/h2 and e2/b2 > e1/b\), the forager will either show complete selectivity for resource 1 or take both foods opportunistically, depending again on resource abundances.

Food Preference Reconsidered

When gut capacity is not limiting, food preference is determined by a descending ranking of external handling efficiency, ei /hi. Whether a food item is included in the diet is determined by the position ofthe (vertical) preference isoleg (see fig. 5.6). Once gut capacity limitations enter the equation, however,

Figure 5.6. Selectivity isolegs in state space of abundance of resources 1 and 2. In the region ofthe state space to the right ofthe solid vertical isoleg, the forager is selective on resource 1. In the region ofthe state space between the vertical isoleg and the curved isolegs, the forager is selective on resource 1, but takes only some of encountered resource 2. The degree of spread between the two isolegs is determined by the ratio ofthe richness (e/b) of resource 1 to that of resource 2. As this ratio increases, the spread between the isolegs increases. To the left ofthe curved isoleg, the foragertakes both resources opportunistically. Forthis figure, a1 = 0.1; e1 = e2 = hi = b1 = b2 = 1; b2 = 1 to 5; T = 1; k =2; R1 = 0.01; R2 = 0,1 50.

Figure 5.6. Selectivity isolegs in state space of abundance of resources 1 and 2. In the region ofthe state space to the right ofthe solid vertical isoleg, the forager is selective on resource 1. In the region ofthe state space between the vertical isoleg and the curved isolegs, the forager is selective on resource 1, but takes only some of encountered resource 2. The degree of spread between the two isolegs is determined by the ratio ofthe richness (e/b) of resource 1 to that of resource 2. As this ratio increases, the spread between the isolegs increases. To the left ofthe curved isoleg, the foragertakes both resources opportunistically. Forthis figure, a1 = 0.1; e1 = e2 = hi = b1 = b2 = 1; b2 = 1 to 5; T = 1; k =2; R1 = 0.01; R2 = 0,1 50.

the world of food preference gets more complicated and interesting. Now preference is determined by the relative ranking of {ei/[hi + bi(T/k)B]}, where B = (YibjHj)(T/k), the extent to which the gut is "bulked up" from previous consumption. Now let

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