In today's world there are few that truly respect the environment and keep it clean. Buddhists believe that the up keeping of the environment is very important and they work hard to maintain it. Their aim is to reduce all forms of pollution (even visual and noise). Although Siddhartha wanted to reduce worldly attachment - he also wanted to keep the world clean and this is what Buddhists do amongst their other practices.
Ahimsa is respect for life, the belief that all animals have a soul - they call living things 'Sentients'. When the Dali Lama won the "Nobel Prize For Peace", he said:
"As long as there are sentient beings about in this world, I will be there to help them"
This quote started up 'Engaged Buddhism' - which is taking things one-step further and doing something about important issues as well as meditating about them. For example: if a Buddhist notices a problem with the amount of waste generated by a certain area, then they may write to the local authorities to suggest a recycling scheme; which they would then help promote.
Buddhists also practise collective Karma, which is when several Buddhists get together and do something beneficial - like recycling. This improves the Karmas of everyone involved and it is improving the world for the future.
Because of samsara (the cycle of life) Buddhists may also keep the environment clean because they won't want to be born into a dirty world.
Right thoughts, living, effort and action are the four things from the eight-fold path that Buddhists use when helping the environment and animals. Right action is not polluting or littering. Right thought is thinking about the environment before doing anything that may harm it. Right living is not getting a job which promotes pollution, or similar (like a car sales man). Right effort is putting your back into it and really trying to improve things.
The majority of Buddhists are vegetarian. This is because they respect the life of other sentients and don't believe in killing them for food. They also firmly believe that animals have rights too - and shouldn't be tested on for various products. They think that if they go about killing animals, they might kill their friends or relatives in other lives. This all works towards respecting the rights of animals and not harming them.
All in all, Buddhists care very much for the world they live in and the sentients that they live with and they try to help keep the world a clean place to live in - like all should do.