Sierra Nevada Network

Stressor: Habitat Fragmentation

Habitat Fragmentation and Human Use

Bighorn Sheep: Successfully reintroduced to Yosemite National Park.

Sierra Nevada parks have the potential to become functional biological islands due to future human population growth and increases in amounts and types of development on adjacent lands. Population growth for the Sierra bioregion is forecasted to increase by over 50 percent in the next 20 years, from 717,400 in 1990 to 1,110,200 by 2020. This will pose increasing challenges for preserving park ecosystems and biodiversity. Several species already have disappeared from the parks (e.g., grizzly bear, California Condor, California red-legged frog), and others survive in greatly reduced numbers (e.g., mountain yellow-legged frog, Yosemite Toad, Western pond turtle, Willow Flycatcher) (NPSpecies Database). These losses are partly due to habitat loss on adjacent lands, with park habitat being insufficient to support local populations over the long term. This problem is particularly serious for foothill species, including seasonally resident species, because most land adjacent to undisturbed foothill habitat is primarily privately owned and subject to development, grazing, agriculture, water diversions, altered fire regime, and non-native invasive species (including freeranging pets and feral animals).

Coniferous forests on lands adjacent to park boundaries are mostly within national forests, where forest ecosystems have been altered by timber harvest, grazing, water diversions, non-native invasive species, and altered fire regimes. Declines of forest mesocarnivores (e.g., wolverine, fisher, red fox), bats, and owl species are attributed to forest structure changes in the region.

Livestock grazing on other non-park public land east of the Sierra Nevada crest has prevented re-establishment of healthy metapopulations of Sierra Nevada bighorn sheep (Ovis canadensis ssp. nova) within the parks, leading to their endangerment. Animals that routinely travel outside park boundaries (e.g., mule deer, black bear, and band-tailed pigeon) thereby become part of hunted populations. Such management activities outside parks are likely to affect age structure and abundance of species within park boundaries. Non-hunted park populations are a likely reservoir for hunted and less dense populations outside the parks.

Concomitant with population growth are changes in wilderness values such as dark night sky and the natural soundscape. Dark night sky benefits many living things, and light pollution is rapidly eroding the unspoiled view of stars. Natural sounds (e.g., morning bird chorus) are integral to the park experience for visitors and essential to the health of ecosystems. Increases in anthropogenic sound such as from airline  overflight can disrupt wildlife behavior.

This article is an excerpt from the Sierra Nevada Network: Vital Signs Monitoring Plan (2007).