Primary producers can use other energy sources than light. The primary producers around our deep ocean volcanic vents use heat and sulphur dioxide that are in the water. Scientists wonder if there is life under the frozen oceans on Europa or other distant moons. For animals, the problem of staying alive is more difficult. All animals are dependent on the food created by plants.
Some animals eat plants, and some eat other animals.
In order to stay alive, animals must either locate themselves where there is a steady supply of food or find ways to move to where the food is. Mobility Living things need to be able to go to where they can find food. Although we think of plants as being immobile, plants grow toward the light, and can slowly lengthen their stems and turn their leaves to take advantage of the light available. They can change form by letting one side of a stem grow faster than the other. The big mobility challenge for plants, however, is how to scatter their seeds over the widest possible area.