Genetic differences within a species can mean differences in shape, size, smell, and color. Diversity of populations are "measured in both the number of individuals within a local group--the size of the loon population in northern Wisconsin--and the distribution of a species' geographic range--the presence of loon populations from
Alaska to Maine" (How Do Scientists Define Biodiversity). The variety of species within a natural community means all the various species in a particular habitat. "A wide array of natural communities and ecosystems throughout the world, from tropical
rainforests to tall grass prairies to boreal forests" make up the biodiversity of the world. Biodiversity is the natural make up of plants and animals of the world, and it should be valued highly. (How Do Scientists Define Biodiversity) Humans play a tremendous role in wildlife extinction. Habitat loss, degradation, and fragmentation are the leading factors that cause wildlife extinction to occur. The second largest threat is from introducing nonnative species into ecosystems. Many species are accidentally introduced. Many are introduced because of their value as a food source and as pest control. The problem with these nonnative species is that they have no "natural predators, competitors, parasites, or pathogens" that control their population.