Herbivores are the primary consumers, obtaining all nutrients from plants.
Some herbivores may be killed and eaten by secondary consumers who, in turn,
could provide meals for higher order consumers.
Most species of animals are at the mercy of their environment since they
must obtain essential nutrients from their surroundings. However, many can
migrate if local conditions become unfavorable and alternatives are accessible.
Humans, in contrast to most other animals, have the mental capacity and
manual dexterity to modify environments. They exploit plants, animals and the
physical world to satisfy their basic requirements plus many additional