The food you eat has the greatest impact on the environment and meat production is one of its greatest threats.

Deforestation and species extinction
In order to produce meat we need land. When no more land is available to grow the cereals needed to feed the livestock, we have to make land. How do we make land? We destroy our forests by cuting down and we are losing our forest. This means we are losing wildlife species (animal and plant) too. A large part of the Central American rainforest has already been cleared to create pastureland for cattle. Approximately 1,000 species a year become extinct because of the destruction of the rainforests.
It is same problem with fishes, advances in fishing methods is bringing ocean species, one after another, to the brink of extinction.

When we eat meat we ignore that we are also harming the earth and its ecosystem in a way that is not reparable.

Water
It takes 15 more time of fresh water to produce 100 g of beef than 100 g of cereals. The meat production requires more pesticides and other chemical products than the production of cereals.
We want to think that water is not a problem since we are surrounded by it. It is not true. All the chemicals we use are spoiling the water.Water is treated and can be used again but unfortunately it is not 100% efficient and there is water we will not be able to use again.

"It takes less water to produce the food that a pure vegetarian needs for one year than to produce the food that a meat eater needs for a month." ("Becoming Vegetarian" Melina, Davis and Harrison.)

Fuel
It takes a lot more calories of fossil fuel to produce one calorie of meat than 1 calorie of cereals.
Three times more fossil fuels must be burned to produce a meat-centered diet than for a meat-free diet.

Erosion
To produce 100 g of beef, 3500 g of topsoil is eroded and the nature needs many years to produce a few centimeters of topsoil. It is depleted more quickly than it is formed.
In America the topsoil is becoming thinner and thinner and once again we are destroying our ecosystem.

Pollution
The pesticides, herbicides and fertilizers used in large amount to produce the animal's food go into the soil and reaches our water supply, and the animals which live underground.