Ebook Intraspecific competition affects the strength of individual specialization: an optimal diet theory method
Many apparently generalist populations are in fact composed of relatively specialized individuals (Bolnick et al., 2003). For example, the Cocos finch, Pinaroloxias inornata, uses a broad range of resources including insects, fruits and flower nectar (Werner and Sherry, 1986). This diet diversity spans a range of resources normally characteristic of disparate families of birds. In contrast, individual finches tend to specialize on one particular foraging strategy (Werner and Sherry, 1986). Such individual specialization has important implications for both ecological interactions and evolutionary processes, including polymorphism and speciation (Bolnick et al., 2003). However, very little is known about the mechanistic basis for diet variation. In particular, why would a group of conspecific individuals, faced with the same set of available resources, select different diets?.
To effectively address the basis of intra-population diet variation, one must first consider the factors determining diet selection of a single individual. This classic topic in individual ecology is most often addressed via optimal diet theory (ODT) (Schoener, 1971; Pulliam, 1974; Werner, 1974), which wasdeveloped to explain why an individual forager might select a restricted diet, when a range of potentially profitable resources is available. The theory proposes that an individual selects a particular foraging strategy to maximize some currency related to fitness, such as the rate of energy income. This rate depends on the energetic content of alternative prey, encounter rates, and how effective the forager is at detecting, capturing, handling and digesting the prey. The forager should drop any food type whose energetic value is less than the expected energetic return for foregoing that item to search for more profitable prey. Optimal diet theory has been fairly successful, allowing qualitative and often even quantitative predictions of individuals’ diets in many systems (Sih and Christensen, 2001), though it has also been subjected to extensive criticism (Pierce and Ollason, 1987; Bunnell and Harestad, 1990; Ward, 1992; Sih and Christensen, 2001).
By identifying parameters that affect individuals’ diet breadth, optimal diet theory provides a framework for thinking about inter-individual variation. If encounter rates, handling times and energetic value really can determine individuals’ diets, then individuals that differ in their ability to find, handle or digest alternative prey may have different diets. For instance, diet variation among individuals of the cabbage butterfly, Pieris rapae, can be attributed to individuals having search images for different species of flowers, which affects encounter rates (Lewis, 1986). Individual specialization in the medium ground finch, Geospiza fortis, is tied to variation in bill morphology that affects individuals’ handling times for different seeds (Price, 1987). A given prey type can also have different energetic value for different individual foragers. Yellow-rumped warblers, Dendroica coronata, vary in their capacity to digest particular prey as a result of variation in digestive enzyme production (Afik and Karasov, 1995), leading to inter-individual variation in the energetic value of a given prey type.
Optimal diet theory can thus provide a biologically realistic way to model individual specialization, particularly if biomechanical or cognitive models allow one to mechanistically link phenotypic traits to ODT parameters. In contrast, previous theoretical models of niche variation have taken a simple phenomenological approach to individual specialization (Roughgarden, 1972; Taper and Case, 1985; Dieckmann and Doebeli, 1999), in which individuals vary for a continuous trait that determines what subset of the prey axis an individual will consume. The breadth of this subset is either held constant
(Dieckmann and Doebeli, 1999; Bolnick and Doebeli,2003), or is itself allowed to evolve (Taper and Case, 1985). However, neither the phenotype–diet mapping nor the niche width is given a clear biological basis. In contrast, optimal diet theory can explain niche breadth and between-individual differences in terms of biologically measurable traits, and more closely mimics the process by which foragers select their diet.
In this paper, we show that optimal diet theory can explain diet variation when individuals differ in their handling times or attack rates. We outline three distinct patterns of rank-preference variation that could produce individual specialization. We then apply this approach to investigate how resource competition can modify the degree of individual specialization. A number of studies have found that the level of diet variation can vary with the strength of intraspecific competition. For example, in the Eurasian perch, Perca fluviatilis, diet variation is more pronounced at higher population densities (Svanbäck and Persson, 2004). The models discussed in this paper can account for this interaction between competition and individual specialization.
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