Food sources

A typical aquatic food 

In a given ecosystem, food forms a of interlocking chains with primary producers at the bottom and apex predators at the top. Other aspects of the web include detrusors (that eat detritus) and decomposers (that break down dead organisms). Primary producers include algae, plants, bacteria and protists that acquire their energy from sunlight. Primary consumers are the herbivores that consume the pants and secondary consumers are the carnivores that consume those herbivores. Some organisms, including most mammals and birds, diets consist of both animals and plants, and they are considered omnivores. The chain ends with the apex predators, the animals that have no known predators in its ecosystem. Humans are often considered apex predators.

Humans are omnivores finding sustenance in vegetables, fruits, cooked meat, milk, eggs, mushrooms and seaweed. Cereal grain is a staple food that provides more food energy worldwide than any other type of crop. Corn (maize), wheat, and rice account for 87% of all grain production worldwide. Just over half of the worlds crops are used to feed humans (55 percent), with 36 percent grown as animal feed and 9 percent for biofuels. Fungi and bacteria are also used in the preparation of fermented foods like bread, wine, cheese and yogurt.

Sunlight

Photosynthesis is the ultimate source of energy and food for nearly all life on earth. It is the main food source for plants, algae and certain bacteria. Without this all organisms which depend on these organisms further up the food chain would be unable to exist, from coral to lions. Energy from the sun is absorbed and used to transform water and carbon dioxide in the air or soil into oxygen and glucose. The oxygen is then released, and the glucose stored as an energy reserve.

Plants

Plants as a food source are often divided into seeds, fruits, vegetables, legumes, grains and nuts. Where plants fall within these categories can vary with botanically described fruits such as the tomato, squash, pepper and eggplant or seeds like peas commonly considered vegetables. Food is a fruit if the part eaten is derived from the reproductive tissue, so seeds, nuts and grains are technically fruit. From culinary perspective fruits are generally considered the remains of botanically described fruits after grains, nuts, seeds and fruits used as vegetables are removed. Grains can be defined as seeds that humans eat or harvest, with cereal grains (oats, wheat, rice, corn, barley, rye, sorghum and millet) belonging to the Pinaceae (grass) family and pulses coming from the Fabaceae (legume) family. Whole grains are foods that contain all the elements of the original seed (bran, germ, and endosperm). Nuts are dry fruits distinguishable by their woody shell.

Fleshy fruits (distinguishable from dry fruits like grain, seeds and nuts) can be further classified as stone fruits (cherries and peaches), pome fruits (apples, pears), berries (blackberry, strawberry), citrus (oranges, lemon), melons (watermelon, cantaloupe), Mediterranean fruits (grapes, fig), tropical fruits (banana, pineapple). Vegetables refer to any other part of the plant that can be eaten, including roots, stems, leaves, flowers, bark or the entire plant itself. These include root vegetables (potatoes and carrots), bulbs (onion family), flowers (cauliflower and broccoli), leaf vegetables (spinach and lettuce) and stem vegetables (celery and asparagus).

Plants have high carbohydrate, protein and lipid content, with carbohydrates mainly in the form of starch, fructose, glucose and other sugars. Most vitamins are found from plant sources, with the notable exceptions of vitamin D and vitamin B12. Minerals are also plentiful, although the presence of phytates can prevent their release. Fruit can consist of up to 90% water, contain high levels of simple sugars that contribute to their sweet taste and have a high vitamin C content. Compared to fleshy fruit (excepting Bananas) vegetables are high in starch, potassium, dietary fiber, folate and vitamins and low in fat and calories. Grains are more starch based and nuts have a high protein, fiber, vitamin E and B content. Seeds are a good source of food for animals because they are abundant and contain fiber and healthful fats, such as omega fats.

Animals that only eat plants are called herbivores, with those that mostly just eat fruits known as frugivores, leaves, while shoot eaters are folivores (pandas) and wood eaters termed xylophages (termites). Frugivores include a diverse range of species from annelids to elephants, chimpanzees and many birds. About 182 fish consume seeds or fruit. There are many types of grasses, adapted to different locations, that animals (domesticated and wild) use as their main source of nutrients.

Humans only eat about 200 out of the worlds 400 000 plant species, despite at least half of them being edible. Most human plant-based food comes from maize, rice, and wheat. Plants can be processed into breads, pasta, cereals, juices and jams or raw ingredients such as sugar, herbs, spices and oils can be extracted. Oilseeds are often pressed to produce rich oils - sunflower, flaxseed, rapeseed (including canola oil) and sesame.

Many plants and animals have coevolved in such a way that the fruit is good source of nutrition to the animal who then excretes the seeds some distance away allowing greater dispersal. Even seed predation can be mutually beneficial as some seeds can survive the digestion process. Insects are major eaters of seeds, with ants being the only real seed dispersers. Birds, although being major dispersers, only rarely eat seeds as a source of food and can be identified by their thick beak that is used to crack open the seed coat. Mammals eat a more diverse range of seeds as they are able to crush harder and larger seeds with their teeth.