The ultimate source of almost all energy on earth is the sun. Green plants, algae and bacteria convert light energy into usable chemical energy. Animals eat them, turning their bodies into food that they can use. Other animals eat these, only to be eaten by creatures on the next level of the food chain. The energy that originally comes from the sun is passed from organism to organism. However, when one living thing eats another, most of the energy from the victim is lost. Eating a plant is about 10 times more efficient than eating an herbivorous animal (like a cow), and at least 100 times less wasteful than eating a predator (like many of the fish we eat).

The human population is expanding at an alarming rate. We need space to grow food for all the new people coming into our world. Livestock - from chickens to cows - consume enough plant food to feed billions of people. Of course the meat, eggs and dairy products that domestic animals produce provide people with food, but many more human beings could be fed if the grain used to feed livestock was made into food for our own species and we bred fewer domestic animals.

Even if livestock are fed plants that humans can't eat, they use more space than a field of crops that would supply the same amount of energy. This means that more natural ecosystems have to go.

One of the biggest threats to the rainforests of South and Central America is the clearing of land for beef cattle. The destruction is not so obvious in our own country, but more land devoted to agriculture still means less room for forest, wetlands and other wild places.

There is no getting around the fact that a huge human population will cause problems for the planet. The least we can do is try to have as little impact on natural ecosystems as possible. One way we can do this is by choosing not to eat meat and other foods that come from animals.