Tourism presents some challenges in such an environmentally sensitive area, as it creates mostly seasonal employment, results in a large share of local houses being used only in the summer, and puts additional strain on environmental resources, thus exacerbating the effect of a significant increase in the number of houses in some areas during the 1990s and 2000s. This has been a thread in the development and preservation activities previously discussed, and has driven the growth of local ecotourism, and the "Leave No Trace" local campaigns. Concerns have also arisen about the level of day-tourism, especially by coaches, as opposed to overnight-staying tourists, who generate significantly more revenue.