Resource quality is the net energy and nutrient value of food resources after accounting for an individual's ability (and energetic or nutrient cost) to digest the resource. The energy and nutrient value of organic molecules is a product of the number, elemental composition, and bonding energy of constituent atoms.
However, organic resources are not equally digestible into useable components. Some resources provide little nutritional value for the expense of acquiring and digesting them, and others cannot be digested by common enzymes. Many organic molecules are essentially unavailable, or even toxic, to a majority of organisms. Vascular plant tissues are composed largely of lignin and cellulose, digestible only by certain microorganisms. Nitrogen is particularly limiting to animals that feed on wood or dead plant material. Some organic molecules are cleaved into toxic components by commonly occurring digestive enzymes. Therefore, acquiring suitable resources is a challenge for all animals.
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