Tourism can have a range of impacts on the natural environment. Some of these impacts are direct, such as the construction of new roads, airports and hotels. Other impacts are indirect, such as the development of infrastructure to support tourism, which can often involve clearing land or draining wetlands. Tourism can also affect natural areas by increasing the risk of invasive species. In some cases, invasive species have been deliberately introduced by tourists who bring plants and animals from their home countries. In other cases, people visit areas that have been damaged by human activities and introduce invasive species accidentally through soil erosion or contaminated water supplies. Invasive species can harm local ecosystems by competing with native species for food and habitat, killing native plants and animals through predation or disease transmission, or damaging crops grown by local farmers. In addition to these direct effects on plants and animals, invasive species can also reduce biodiversity in an area by changing its vegetation structure or composition so dramatically that it no longer resembles its original state (known as ecosystem simplification).