Birds of the Sierra Nevada. Ted Beedy
Читать онлайн книгу.
of years before European settlement. Native peoples used fire as a management tool to clear brush, maintain grasslands and meadows, make travel easier, and improve browse for game animals. Fire management, hunting, fishing, and gathering by Native Americans affected plant and animal communities in the Sierra and likely altered the relative abundance and distribution of some bird species long before the first Europeans arrived.
Changes caused by native peoples over a 10,000-year span prior to the 19th century paled in comparison to the impacts of European settlers and their large-scale, aggressive use and abuse of Sierra resources. Forests were logged extensively, with the largest trees targeted, and sheep and cattle swarmed over mountain meadows that had never experienced such intense grazing pressure. At lower elevations exotic annual grasses were introduced and dominated the grasslands and savannas so quickly that no naturalist was ever able to observe or describe the pre-European plant communities. Some speculate that native bunch grasses dominated, but the true nature of these original landscapes and most Sierra plant communities remains unknown.
The discovery of gold in the Sierra foothills in 1848 dramatically accelerated these impacts and added to them demolition of hillsides (typically with powerful water cannons) and diversions and excavations of nearly every stream on the West Side from Butte County south to Madera County, all in a frantic search for gold. The top predators, grizzly bears and wolves, were systematically exterminated. To encourage railroad companies to invest in the transcontinental railroad in the late 19th century, the United States gave large swaths of Sierra land to these companies by granting them every other section (a one-mile by one-mile square) of land along the proposed route. The result was a checkerboard of public-private ownership that has inhibited rational conservation planning ever since. One of the most direct effects on Sierra birds including ducks, geese, quail, and grouse came from “market hunting” to feed burgeoning gold rush populations as well as rampant shooting and persecution of raptors.
We can only speculate about the impacts on birds from all these changes because no comprehensive, large-scale studies of Sierra wildlife were conducted until the early decades of the 20th century. Logging of large trees almost certainly decreased the available habitat for species such as Northern Goshawks, Spotted Owls, and Pileated Woodpeckers. Grazing of mountain meadows degraded these habitats and must have reduced habitat for Willow Flycatchers, Yellow Warblers, and Lincoln’s Sparrows. Gold-mining degraded streamside habitats for riparian-associated birds, including a diversity of migrating neotropical songbirds such as flycatchers, vireos, warblers.
Changes in public attitudes about nature and wildlife over the past few decades have eliminated or modified most of these historically damaging practices, and state and federal agencies are now tasked with managing public lands and wildlife resources using science-based approaches that help protect and enhance habitat for Sierra birds. Hunting is now highly regulated, and raptors are protected and mostly revered by the public. Logging, grazing, and mining practices are all regulated in an attempt to balance resource extraction with protection of natural resources, and riparian and wetland areas are being protected and enhanced as never before. The U.S. Forest Service and numerous nonprofit conservation organizations have actively worked to acquire land to eliminate the checkerboard pattern of ownership in the Sierra and strive to restore and enhance ecosystem processes and habitats where possible.
FIRE AND CURRENT FORESTRY PRACTICES
Fire regimes in the Sierra prior to European settlement likely consisted of frequent, low-intensity fires started by lightning or native people, maintaining an open-forest understory and supporting development of large areas of mature forest. Stand-clearing fires certainly also occurred and the resulting landscape was probably characterized by a mosaic of many different plant communities. Frequent chaparral fires maintained broad swathes of vegetation in early successional conditions and fire in grasslands and oak savanna created patches of open, treeless expanses. All these conditions would have produced a variety of habitats supporting a correspondingly high diversity of birds. Aggressive fire suppression in the Sierra began in the early 1900s and became more widespread and effective through the 1950s. Ironically, the major legacy of those years of fire suppression is a landscape that has higher densities of younger trees and denser understory, providing fuels that increase the frequency and size of large-scale, high-intensity fires.
More recently, forestry practices have recognized the important role of fire in the Sierra ecosystem. National Parks like Yosemite and Sequoia now have policies that allow most lightning-caused fires to burn, and also include controlled burns to reduce fuel levels. On publicly owned lands, fire-suppression policies have also been modified, and controlled burns are part of the management strategy. However, consideration given to commercial timber harvesting priorities, air quality, and nearby human residential areas all make implementation of these policies difficult. Experiments using selective logging to change fire behavior to reduce the risk of large stand-clearing fires are being conducted throughout the Sierra. It remains to be seen whether these practices are effective or economically practical. Their impact on wildlife is also difficult to predict. In any case, to the extent that birds have adapted to current conditions, any changes are likely to be detrimental to some species and beneficial to others.
In spite of the fact that old growth forests occupy less than one-fifth of their historical extent, these mature forests are still being logged both on private land and to a lesser extent on National Forests, further stressing birds that require this forest type. Even where protected from logging, the historical lack of fire has increased the density of shrub understory and smaller trees and increased the risk that this dwindling forest type could be lost to high-intensity fire. Clear-cutting and planting of even-aged, single-species tracks continues on private lands with a corresponding decrease in habitat diversity for birds and other wildlife.
One of the most problematic ongoing forestry issues is postfire salvage logging. When stand-clearing fires occur, the resulting landscape looks like a wasteland to the general public and represents an opportunity to harvest many large trees for commercial timber interests. However, a large and rapidly growing body of research (much of it conducted by the U.S. Forest Service) shows that removal of all or most of the largest standing dead trees (snags) is detrimental to a wide variety of cavity-nesting birds such as woodpeckers and bluebirds as well as other species that make extensive use of these landscapes for foraging (e.g., Olive-sided Flycatchers and Black-backed Woodpeckers). Much needs to be done to bring salvage logging policies in line with the best conservation science. In addition, forestry practices like herbicide application to suppress shrub growth and to accelerate regrowth of trees may not only be counterproductive, they may alter the succession of different habitat types that support diverse bird communities.
MOUNTAIN MEADOWS
Historical, unregulated overgrazing was so pervasive throughout the range that most experts in this field believe that not a single pristine Sierra meadow system remains. Many areas of these habitats have been altered right down to the basic hydrology that supports the entire meadow system. Eroded streams incise and cut deep channels, lowering water tables such that even when grazing is removed, the system cannot return to its prior wetland state without active intervention by restoration ecologists. However, grazing is now much reduced and more highly regulated, and efforts to restore these meadows are producing encouraging results. The U.S. Forest Service has partnered with other federal and state agencies and nonprofit conservation organizations to implement multiple restoration projects throughout the Sierra.
DAMS AND WATER DIVERSIONS
No major river system in the Sierra has been spared the impacts of dams or water diversions to provide water for human uses and to control flooding. While reservoirs have destroyed hundreds of miles of riparian habitat and drowned thousands of acres of meadows, likely contributing to the decline of the Harlequin Ducks and many meadow-dependent species, they have also created habitat that many species of water birds such as ducks, geese, and grebes have been quick to exploit. More recently, recovering populations of Bald Eagles and Ospreys are taking advantage of these human-made lakes to the point where they could be more numerous now in the Sierra than historically. Similarly, the massive diversion of foothill streams and the resulting canal systems that began with gold mining have led to the accidental creation of perennial wetlands in the north-central Sierra that have enabled the state-Endangered Black Rail to successfully colonize