One of the primary threats to biodiversity today are habitat loss and habitat fragmentation, both of which are rooted in expanding human populations and excessive consumption of natural resources. For instance, many species living in tropical forests, freshwater ecosystems, the marine environment, and seasonal drylands are at risk of extinction due to habitat loss. To predict the number of species that will go extinct due to this phenomena, the theory of island biogeography and the species-area relationship can be used. Both these theories predict that large habitat patches are better able to maintain wildlife populations because they accommodate populations better buffered against extinction. Habitat fragmentation describes the process when once large and widespread habitats (and hence wildlife populations) are divided into several increasingly smaller and isolated units. This process leads to extinctions because it impedes dispersal, colonisation, foraging, and reproduction. Edge effects also reduce the functional size of habitats because it alters microclimates and exposes habitat specialists to displacement by invasive species, predators, and other disturbances.