Sierra Nevada Network
Stressor: Non-native Invasive Species
Plants
Some of the most widespread invasive grasses first arrived in California
during the 16th century as propagules hitchhiking on explorers; their
spread was subsequently exacerbated by grazing, drought, and burning
by Native Americans.
Numerous non-native invasive vascular plant species are present in Sierra Nevada parks. Despite management efforts, many are spreading and new invasions continue: at least 180 species now occur in Yosemite, 200 in Sequoia and Kings Canyon, and eight in Devils Postpile. Herbaceous biomass of foothill grasslands in Sequoia is 99% invasive species, and altered fire regime (i.e., a particular fire frequency, intensity, or seasonal distribution) may be one of the reasons. Fire suppression has likely inhibited plant invasion into montane landscapes because closedcanopy forests are not generally favorable sites for invasive plants. However, reintroduction of fire onto the landscape may promote establishment of invasive species, particularly in resultant light gaps or areas of high fire severity. Because plant species evolve–not in association with fire per se–but within a particular fire regime, some highly fire-adapted plant communities (e.g., chapparal) may be vulnerable to invasive competition. Also, the invasion process is affected by (1) the extent to which fires and fire management practices encourage establishment and spread, and (2) the degree to which such practices inhibit or reverse the invasion process. concomitantly, the presence of invasive plant can lead to altered fire regimes, including increased fire frequency.
Invasive plants can severely alter ecosystems. They can alter soil water dynamics, thereby stressing native species and perhaps increasing the potential for invasion by noxious species such as yellow star-thistle. Parts of Sequoia National Park that have been severely grazed by cattle (trespassing) now harbor numerous invasive species.
Animals
At least 30 invasive vertebrate species have been reported in Sequoia
and Kings Canyon, and 21 have been reported in Yosemite (NPSpecies
Database https://science1.nature.nps.gov/npspecies/). Many of these
species (e.g., trout, bullfrog) are of concern to management because
they may have deleterious effects on native wildlife populations.
The widespread introduction of brown, rainbow, and brook trout into
high elevation lakes and streams has altered ecosystems, which were
naturally without fish. Introduced fish and chytrid fungus are suspected
of being leading factors in declines of native amphibian species
in the Sierra Nevada, including the precipitous decline of the mountain
yellow-legged frog. Bullfrogs are voracious predators, and carriers
of chytrid fungus. The full impact of bullfrogs on native species
in the parks is unknown, but extirpation of California redlegged
frog (federally threatened) from Yosemite is attributed to
bullfrog presence. Domestic animal species (e.g., free-ranging house
and feral cats) consume native species and compete with native wildlife
for resources.
This article is an excerpt from the Sierra Nevada Network: Vital Signs Monitoring Plan (2007).
Learn More
A Growing Problem: Invasive Non-native Plants Learn why we shold be concerned about non-native plants becoming established in the Sierra ecosystem.