Morphological variation within a species may reflect adaptive genetic variation or phenotypic plasticity, whereby individuals are able to modulate their morphology in response to foraging conditions. Food specialization in relation to morphology is thought to be a response to competitive pressures within a population favoring niche partitioning among conspecifics, and hence niche expansion. Diet-morphology relationships are mainly due to tradeoffs in either search- or handling-time efficiencies where adaptations to maximize foraging intake have long been recognized to be dependent on specific habitats or prey types. Morphological specialization is thus presumed to lead to an increase in foraging efficiency in the habitat to which they are adapted but on the other hand, it is likely to constrain their efficiency in alternate habitats. For example, the beak size in birds can be related to feeding efficiencies on different-sized seeds. Larger beak sizes are better adapted at large seed sizes but they lose their efficiency on smaller seed sizes. Smaller beak sizes on the other hand are more efficient on smaller seed sizes. In fish, species (or individuals) that are specialized for living in open water and searching for widely dispersed prey have fusiform bodies that minimize drag and allow for efficient cruising. In contrast, fish that are adapted for searching for prey in structurally complex habitats have a deep and laterally compressed body and extended fins and are well suited for slow and precise maneuverability.
Behavioral variation can also influence feeding performance and thus diet use of individuals. Behavioral variation can arise from morphological variation in the sense that morphology may determine an individual's ability to carry out a specific prey-capture behavior. However, behavioral variation may also arise from variation in cognitive ability, perception, risk aversion, which may all have a genetic and/or environmental basis. In particular, early experience may lead to learned behaviors that influence prey use. For instance, individuals that are initially exposed to different prey acquire divergent search images that influence later foraging abilities. For example, sea otters (Enhydra lutris) show extensive among-individual variation in prey preferences. These differences are maintained through cultural transmission, as mothers teach their offspring how to forage. In Pieris butterflies, individuals form a new search image for a single flower type at the start of each day, and specialize on that flower for the rest of the day. Individuals that feed on multiple flower types form less effective search images and thus forage less efficiently.
Subscript's' in WICs, BICs, and TNWs distinguishes this from the continuous index. The variable pi is the proportion of all resources used by the population that are used by individual i, qi is the proportion of the jth resource category in the population's niche, and 7 j is the proportion of the population's total use of resource j that was used by individual i, so that
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