Sardines are small fish, but their abundance has made them valuable for humans and for marine ecosystems. For humans, they are commercially important and are an excellent source of nutrition, including high amounts of various vitamins and minerals, and a rich source of omega 3 fatty acids. Overall, they are an exceptional source of vitamin B12, a rich source of niacin, vitamin D, protein, calcium, and selenium, and a good source of vitamins B2, B6, iron, copper, and zinc, as well as a source of iodine (Bender and Bender 2005).
Ecologically, they are important in food chains, consuming copepods and zooplankton and being eaten by larger fish (sharks, salmon, swordfish, etc.), spiny lobsters, birds, and marine mammals. These values of sardines reflects the principle that while animals advance their own survival, reproduction, and maintenance—that is, an individual purpose—the various taxa also provide a purpose for the whole (for the ecosystem and humans). Often packed tightly in cans for sale, they also provide an apt metaphor for crowded conditions: "packed like sardines in a can."