We, as humans, depend on vegetables, fruit and grain plants along with animals to feed us. We are omnivores - we eat both plants and animals.

Most of our food is artificially grown on farms. It is not part of the naturally occurring chain. Yet some of the food we eat is part of that chain and how we harvest that food makes a difference to the survival of many other species.

In a simple food chain, grass converts sunlight to energy, the grass is eaten by a cow, and the cow is eaten by a human.

Humans cannot convert sunlight to food - we need the grass to do that. And we cannot convert the grass to food very well - we need the cow to do that.

In the wild, the food chains are much more complex. They exist all around us, on land and in the water. Predators are continuously catching and eating their prey.

Each predator eats more than once, so there needs to be more prey than predators. Otherwise the predators will starve. This is how the pyramid is formed.

There should be fewer predators than prey. But that does not mean we should eat predators.

In fact, predators are vital to the food chain because they keep things in balance.

Those creatures near the top of the pyramid take longer to reach sexual maturity and produce fewer offspring than those further down the pyramid.

Compare how quickly rabbits breed to how quickly hawks breed. Many more rabbits are produced than hawks.

But if the hawks were no longer there, disaster would strike.

There would be millions of rabbits in very little time.

They would strip the land of all vegetation, which would mean no food for other plant eaters, like us and our cattle.

Predators increase the biodiversity of an area because they stop one grazing species becoming dominant. For example, by eating rabbits, hawks ensure that there is enough vegetation for a range of animals.

Often we eat predators without giving them a second thought.

But now the numbers of sharks, tigers and bluefin tuna are dwindling.

Used in traditional dishes and medicine, tigers and sharks are extremely endangered.

Bluefin tuna is used to make sushi. These giant fish have been hunted so badly that their population has dropped by 90 per cent since the 1970s.

The effects of overfishing drives fishermen further afield, some as far as Africa where fish populations are also under stress. In these countries, fishermen are noticing a decline in their catches, in both size and amount.

What we choose to eat affects people halfway across the world.

There are also health benefits to eating lower down the food chain. Poisons like mercury accumulate in the bodies of predators because they eat poisoned prey. So when you eat the predator you get a higher dose of poison in every bite.

To keep your body and our world in balance, choose to eat from further down the chain.

discuss

1. Should shark's fin soup be banned in Hong Kong?

2. Given that whale numbers are increasing, should the ban on whaling remain in place?

what you can do

Make your money talk for you. Don't support the trade in predators, even if they are used in traditional or exotic dishes.

Try to choose your food from further down the food chain. This has health benefits, too, because predators accumulate poisons which are passed on to us when we eat them.

Don't eat shark in any form.

Ask for stock fish only at your favourite seafood restaurant.

Do not take any traditional medicine that uses tiger or other exotic animal parts, no matter what the benefits are said to be.

When eating out, stick to the basics: pork, beef, mutton, chicken and duck.

Don't choose tuna when eating sushi. Even bluefin tuna that has been 'farmed' causes widespread damage.

Become familiar with the sea watch lists and choose eco-friendly foods.