Organic lawns contribute to biodiversity, by definition, when they contain more than one or two grass species. Examples of additional lawn and grasslike species that can be encouraged in organic lawns include dozens of grass species (eight for ryegrass alone, sedges, mosses, clover, vetches, trefoils, yarrow, ground cover alternatives, and other mowable plants[11]).[12] Biodiversity increases the functioning and stress tolerance of ecosystems.[13] Lack of biodiversity is a significant environmental issue brought up by the use of lawns with grassroots groups emerging to promote this method of lawn care.[14] Certain low-growing grass species can also eliminate the need for mowing, thus also being environmentally friendly. Clover is often mixed with grasses for its ability to fix nitrogen into the soil and fertilize the lawn.