Because the amount of energy that is available increases the further down the food chain you go. Photosynthesis provides a small amount of energy, hence why pla

Because the amount of energy that is available increases the further down the food chain you go. Photosynthesis provides a small amount of energy, hence why plants and plantlike organisms tend to be either totally stationary or don't move much on their own. Organisms that consume plants are capable of more complex behaviour and growth because the amount of energy and nutrients etc. is greater that they receive by conauming things that photosynthesize.