The effect of tourism on the environment can be both positive and negative. Each person or group of people will impact the area they visit in a different way. Whether it is in a negative or positive manner is entirely up to the visitor.
Let's consider the NEGATIVE impacts first:
POLLUTION - This is probably the biggest negative factor concerning tourism. People have to travel to their destination. It doesn't matter if it's by plane to another country or by car to the local national park, traveling puts emissions into the air.
WASTE - People typically use disposable items when they pack for a trip, after all, it's a holiday and who wants to be bothered with washing dishes or saving scraps for the compost? Landfill here it comes!
PHYSICAL DAMAGE - especially to the local fauna and animal habitat. Tromping carelessly through the forest, smashing down delicate flowers and grasses or burning deadfall and destroying habitat for local ground dwelling animals. We may not even notice what we are doing but the local area does.
NOISE - may not seem like a big deal, but loud noises can scare animals out of the area and possibly even disrupt the local ecosystem.
There are some POSITIVE impacts too:
AWARENESS - when people visit an area, they notice things a local person may not. A tourist is more likely to notice a problem like pollution or habitat encroachment and bring it to the attention of local authorities or even the world!
EDUCATION - tourists provide an endless supply of people from around the world that can be targeted and educated on everything from forest stewardship to local history and plans for the future.
FUNDRAISING - a tourist provides an easy target for fundraising. Tourists pay fees for everything from camping sites and park entry fees, to licenses for fishing and hunting. The more tourists in a controlled area, means the more money that can be collected for things like education, policing and maintenance of