Non-native Invasive Plants
Throughout the Sierra Nevada, invasive plants have transformed plant communities and contributed to the decline of native species in a variety of ecosystems. Foothill oak woodlands and riparian communities, so vital to the maintenance of wildlife diversity, have been particularly affected by invasions of non-native grasses and shrubs.
In many instances, non-native invasive species were introduced by pioneers for landscaping purposes, as in the case of Scotch broom. Other species, such as non-native grasses, were introduced in livestock feed. Many invasive plants are pioneer species that are adapted to take hold of freshly disturbed sites without competition from firmly established mature species. Ground disturbance is the hallmark of our time, and the plant communities which result are symptomatic of human activities.
The best way to prevent establishment of non-native, invasive plants is to reduce disturbance to mature forest ecosystems. This is even more important as our region undergoes rapid climate change. While we cannot predict exactly what will happen or when, science has demonstrated that mature plant communities present a suite of resistance adaptations to invasion, provided they are not disturbed with events outside of their evolutionary experience.
Wildlife species can be significantly impacted by abrupt changes in plant communities because they are co-evolved with and adapted to the native species in their habitats. Invasive plants may offer inferior nutrition, or may be less useful as nesting habitat. For predators, the increase in non-native plant species can impact prey habitat, thus reducing the prey densities in the impacted areas. In many areas, invasive shrubs and grasses increase the frequency and intensity of fires due to increased fuel load and greater susceptibility to fire leading to further ecological damage.
The impacts from invasive species was identified as one of the five most pressing environmental issues in need of attention, in the 2001 Sierra Nevada Framework plan. In spite of new efforts by the Forest Service to identify areas where non-native species occur prior to logging projects, such as through the use of flagging on the ground and increased environmental documentation, follow-up efforts to halt the spread of non-native species have been practically non-existent and have had little effect.
Scotch broom was one of the first species to be introduced by Gold Rush era pioneers for landscaping purposes, and has become naturalized in vast areas surrounding the Mother Lode counties of the Sierra. The plant is a serious threat to the ecological health of forest communities throughout the Sierra Nevada because of its ability to rapidly recolonize after wildfire, displacing native species. Like other invasive species, it can become a monoculture that can hold a site for a century or more, depending upon the disturbance regime of the area.
The California Department of Fish and Game estimates that at present it occupies over 500,000 acres in the Sierra Nevada foothills. Scotch broom has many negative effects on plant communities and wildlife. It is a robust plant and can out-compete and dominate a plant community, forming a dense stand impenetrable by native species. Its seeds are toxic to ungulates, its mature shoots are unpalatable and not used for forage, and its foliage has been known to cause digestive disorders in horses. Since Scotch broom grows more rapidly than many tree species, its impact on the structure of a forest is substantial. Its quick growth can shade out tree seedlings of native species. Scotch broom also increases the fire threat because it burns easily and hotly and can quickly spread a fire from the understory into the canopy. Its seeds are adapted to frequent fire and rapidly capture sites which have recently burned. The soil disturbance associated with logging, along with the opening up of the canopy frequenlty results in explosive growth of the plant afterwards, even in areas where only small amounts of broom existed prior to the logging.
Cheatgrass, also known as downy brome, has significantly altered the areas in which it has infested and it has a variety of effects on both ecosystem function and native species diversity. One major problem caused by the invasion of cheatgrass is its impact on the fire regime. The presence of cheatgrass allows fires to burn at higher frequencies and across wider areas and typically results in communities dominated by cheatgrass in the future. Cheatgrass has been observed in most Sierra Nevada counties and has caused an interesting dilemma for national park managers in particular. The use of prescribed fire as an ecological tool is more widely accepted and utilized in national parks, but prescribed burning in areas with cheatgrass present can increase its spread following a fire event. Low elevation yellow pine forests, such as those in Sequoia and Kings Canyon National Park have been particularly impacted by cheatgrass. Cheatgrass is known to out-compete native tree seedlings and poses a significant threat to forest dynamics in these systems.
Yellow starthistle is one of the most serious invasive plants found in the Sierra Nevada. It is estimated to have invaded in areas contained in 15 million acres in California and has a profound impact on native plant diversity, species health, and aquatic systems. Dense infestations of yellow starthistle not only displace native plants and animals, but also threatens natural ecosystems and nature reserves by fragmenting sensitive plant and animal habitat. Recent scientific studies indicate that yellow starthistle significantly depletes soil moisture reserves in annual grasslands and can significantly dry-out areas that previously served as secure water sources for wildlife. Yellow starthistle can also be toxic to horses if ingested, causing a serious neurological disorder.
Non-native Aquatic Species
The introduction, either intentionally or accidentally, of aquatic species such as non-native fish and amphibians into the lakes and streams of the Sierra Nevada has had a profound effect on many wildlife species. A perfect example of the enormous detrimental impact that a species can have on entire ecosystems and the very survival of individual species is the introduction of non-native trout species into the Sierra.
Historically, trout were absent above the approximately 5,200 feet elevation level in the Sierra Nevada. In the mid-1800's, however, widespread trout introductions were begun to move fish into formerly fishless lakes and streams to enhance recreational fishing. Trout stocking is still conducted today by the California Department of Fish and Game, and the current program is intended to supplement and maintain existing populations of non-native trout. As a result of past and current trout stocking, the proportion of trout-containing lakes in the Sierra Nevada has increased from less than 1% of all lakes to approximately 63%. The national forests of the Sierra Nevada have a much higher proportion of lakes containing non-native trout than national parks, with trout in at least 85% of lakes on national forest lands. Only 7% of national forest lakes are know to be fishless.
The greater number of fishless lakes in the national parks than national forests is due in part to the termination of fish stocking in park lakes in the 1970's. Recent surveys in the national parks of the central and southern Sierra Nevada show that trout have disappeared from 29-44% of previously stocked lakes.
The impacts of these introduced trout on native aquatic species have been substantial. Introduced trout are affecting the distribution and viability of a wide range of native aquatic species in the Sierra Nevada, including native fishes, amphibians, zooplankton, and macroinvertebrates. The introduction of non-native trout has caused widespread declines of native trout species such as golden trout as a result of hybridization, competition, and predation. The decline of at least one amphibian species, the mountain yellow-legged frog, has been attributed largely to predation by introduced trout. Predation by introduced trout has also caused dramatic changes in zooplankton and benthic invertebrate species composition in lakes, shifting the dominant species in these communities from large-bodied to small-bodied forms.