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Wildlife Overview
With habitats ranging from thick foothill chaparral to expanses of alpine rock, Yosemite National Park supports over 250 species of vertebrates, which include fish, amphibians, reptiles, birds, and mammals. This high diversity of species is also the result of habitats in Yosemite that are largely intact, compared to areas outside the park where various human activities have resulted in habitat degradation or destruction. Along much of Yosemite's western boundary, habitats are dominated by mixed coniferous forests of ponderosa pine, sugar pine, incense cedar, white fir, and Douglas fir, and a few stands of giant sequoia, interspersed by areas of black oak and canyon live oak. A relatively high diversity of wildlife species are supported by these habitats, due to relatively mild, lower-elevation climate, and the mixture of habitat types and plant species. Wildlife species typically found in these habitats include black bear, bobcat, gray fox, mountain kingsnake, Gilbert's skink, white-headed woodpecker, brown creeper, spotted owl, and a wide variety of bat species. In the case of bats, large snags are important as roost sites. Going higher in elevation, the coniferous forests become purer stands of red fir, western white pine, jeffrey pine, and lodgepole pine. Fewer wildlife species tend to be found in these habitats, due to their higher elevation, and lower complexity. Species likely to be found include golden- mantled ground squirrel, chickaree, marten, Steller's jay, hermit thrush, and northern goshawk. Reptiles are not common, but include rubber boa, western fence lizard, and alligator lizard. As the landscape rises, trees become smaller and more sparse, with stands broken by areas of exposed granite. These include lodgepole pine, whitebark pine, and mountain hemlock that, at highest elevations, give way to vast expanses of granite as treeline is reached. The climate in these habitats is harsh and the growing season is short, but species such as pika, yellow-bellied marmot, white-tailed hare, Clark's nutcracker, and rosy finch are adapted to these conditions. Also, the treeless alpine habitats are the areas favored by Sierra Nevada bighorn sheep. This species, however, is now found in the Yosemite area only around Tioga Pass, where a small, reintroduced population exists. At a variety of elevations, meadows provide important, productive habitat for wildlife. Animals come to feed on the green grasses and use the flowing and standing water found in many meadows. Predators, in turn, are attracted to these areas. The interface between meadow and forest is also favored by many animal species because of the proximity of open areas for foraging, and cover for protection. Species that are highly dependent upon meadow habitat include great gray owl, willow flycatcher, Yosemite toad, and mountain beaver. Despite the richness of high-quality habitats in Yosemite, three species have become extinct in the park within historical time, and another 37 species currently have special status under either California or federal endangered species legislation. The most serious current threats toYosemite's wildlife and the ecosystems they occupy include loss of a natural fire regime, exotic species, air pollution, habitat fragmentation, and climate change. On a more local basis, factors such as road kills and the availability of human food have affected some wildlife species. Amphibians and Reptiles Twelve amphibian and 22 reptile species inhabit Yosemite National Park. They are often considered together because both are poikilotherms, animals that lack the ability to generate their own body heat, relying instead on their environment to regulate body temperature. Amphibians spend the early part their life cycle in water as larva before metamorphosing into adults that live on land. Toads are a good example. As tadpoles, they are restricted to aquatic environments, but spend almost all of their time on land as adults. In contrast, reptiles are generally terrestrial throughout their life cycle. Amphibians are also characterized by moist, highly vascularized skin while reptiles are typically covered by scales. Yosemite’s amphibians can be divided into two groups - anurans and caudates. Anurans are frogs and toads. Caudates include newts and salamanders. They have tails and resemble lizards in appearance. Caudate amphibians spend most of the year hidden to escape Yosemite’s hot dry summers. They live under rocks and rotten logs or underground in damp crevices and burrows and typically emerge in autumn after the first soaking rain. Yosemite’s lone newt species, the California newt, is the park’s most visible caudate. They move like a lizard in slow motion and are often observed in early spring crossing roads or trails on their way to breed in rivers and streams. Of the four salamander species inhabiting Yosemite, one, the Lyell salamander, is endemic to the region. Yosemite’s anurans include the western toad; its close relative, the Yosemite toad; and five species of frogs. The Yosemite toad is endemic to the region. The most common frog species is the ubiquitous Pacific treefrog. Found at nearly all elevations in nearly all habitats, this diminutive frog is the species most often heard calling around meadows and ponds in the spring. The bullfrog is an non-native introduced species restricted to a few locations in Yosemite Valley and near Hetch Hetchy. The other three frog species are the red-legged frog, the foothill yellow-legged frog, and its cousin, the mountain yellow-legged frog. Yosemite has very little suitable habitat for the red-legged and foothill yellow-legged frog, thus these species are extremely rare within the park and may be extinct. Because they have also disappeared from many other locations around California, red-legged and foothill yellow-legged frogs are being considered for endangered species status. The mountain yellow-legged frog inhabits higher elevations and was once numerous in Yosemite’s lakes and streams. However, in recent years, its numbers have declined dramatically and it is also being considered for protection under the Endangered Species Act. The cause of this frog’s disappearance, and that of both toad species, is currently uncertain. Evidence strongly suggests that trout, introduced to Yosemite’s lakes and streams, have played a role in amphibian population declines. Pesticides drifting into the park from California’s Central Valley and diseases have also been implicated. In marked contrast to the amphibian’s preference for moist, damp habitats, reptiles prefer dry, rocky places. Yosemite’s reptile fauna can be divided into three groups - turtles, lizards, and snakes. The lone turtle species, the western pond turtle, is an uncommon resident of mid-elevation ponds and slow moving streams. The eight lizard and thirteen snake species are seen at varying degrees of frequency. The western fence lizard is the most numerous and readily observed lizard species in Yosemite. They are often seen darting across granite slabs or heard skittering through dry leaves. They are easily recognizable as the lizard doing “pushups,” a common display behavior. The northern and southern alligator lizards are almost as common as the western fence lizard. However, their habit of hiding under rocks and logs makes them much less obvious. The coast horned lizard is extremely rare and has only been recorded once or twice within the park. Yosemite has a diverse snake fauna with thirteen species recorded in the park. The most common are the garter snakes with three species. They frequent ponds and wet meadows hunting for small fish and frogs. The mountain king snake with red, black, and cream colored bands encircling its body is the park’s most beautiful snake. The rubber boa looks like a giant earthworm. Light gray in color with a blunt tail, this gentle snake primarily eats insects and soil invertebrates. The remaining species are uncommon to rare, mainly inhabiting the warm, dry lower elevations of the park. Of the thirteen species of snakes found in Yosemite, only the western rattlesnake is venomous. Fortunately, the likelihood of encountering one is relatively low. Pay attention when hiking or climbing in dry, rocky places. Avoid putting your hands in holes or on ledges where snakes may be sunning themselves. If you do see or hear one, simply detour around it or let the snake crawl away. Rattlesnakes are an important part of Yosemite’s ecosystem, as they help keep the park’s rodent numbers in check. With the exception of the mountain yellow-legged frog and Yosemite toad, relatively little is known about the amphibian and reptile fauna of the park. Ongoing investigations into the causes behind declining frog and toad populations are our best source of information. Bears American black bears (Ursus americanus) are an integral part of the Sierra Nevada ecosystem and are only one of the many animal species protected in Yosemite National Park. Black bears forage on a wide variety of natural foods, including grasses, insects, berries, and acorns. The bears, however, are intelligent and adaptable, and will readily accept human foods when they are available. Bears that are exposed to human food often change their behavior and begin seeking it in campgrounds, parking lots, and from backpackers. This results in property damage and dangerous confrontations between humans and bears. The ecological role of such bears is also changed - their use of natural foods diminishes, they become more nocturnal, and the elevation range of habitat use changes. When a bear's search for human food makes them aggressive toward humans, it poses an unacceptably high threat and must be killed. As a result, black bears have been the subject of intense management efforts in Yosemite for many years, to protect both people and the bears. Bear Biology Black bears (Ursus americanus) in Yosemite have long been of intense interest to both park visitors and park managers. For visitors, the sight of a bear can evoke a mixture of excitement, awe, and fear; all of which can mark the highlight of a vacation. For park managers, black bears offer the challenge of preserving an ecologically-important species that, by its nature, can easily be "corrupted" by human influences in Yosemite. When Euro-Americans arrived in California, grizzly bears (Ursus arctos) inhabited most of the state, including the area that is now Yosemite National Park. The grizzlies, however, were seen by the settlers as a dire threat to life and property and were killed in large numbers. By the early 1900s, few grizzlies and little of their prime habitat in the Central Valley remained. The last known grizzly bear in California was killed in the Sierra foothills south of Yosemite in the early 1920's. In contrast, black bears have fared much better. Much of their preferred habitat in the foothills and mountains remains suitable, and their more timid behavior has limited persecution by humans. Black bears have proven to be adaptable to many human-caused changes to their habitat in the Sierra Nevada and eagerly take advantage of food sources these changes sometimes provide - such as garbage cans and dumps, orchards, and domestic beehives. These food sources, however, often bring the bears into conflict with humans, for which the bears can pay with their lives. Nonetheless, there are an estimated 20,000 to 24,000 black bears in California, despite a fall hunting season that annually removes up to 1,500 bears from the population. No scientific census of bears has occurred in Yosemite, but it is estimated that there are roughly 300 to 500 black bears in the park. Despite their name, most black bears in Yosemite are not black in color. Most are some shade of brown, ranging from almost blond, to reddish brown, to a dark chocolate color. Truly black black bears are relatively rare here. In other areas of the country, such as the eastern United States, and the Pacific Northwest, black bears with black fur are the most common. One common question is how big are Yosemite's bears? This is a difficult question to answer because bears, like people, can vary greatly in size. Also, an individual bear's weight can change greatly throughout the year. Before entering winter hibernation, a bear's weight can be double what it was when it emerged from its den the previous spring, if food sources are rich enough. In general, however, the weight of an average, adult male Yosemite black bear in summer is 300 to 350 pounds (136 to 159 kg). Females are smaller, with typical weights ranging from 200 to 250 pounds (91 to 113 kg). Much bigger bears, however, do occur. The largest black bear ever captured in Yosemite weighed 690 pounds (375 kg)! Bears are classified as carnivores, in the same taxonomic order as dogs and cats, but a majority of a black bear's diet is made up of vegetable matter. In the spring, after emerging from winter dens, the bears feed largely on meadow grasses, which are relatively low in nutrition, but sustain the bears until more nutritious foods become available. As berries of various plant species ripen in the summer, the bears shift to these higher-calorie foods. Animal matter that is eaten consists primarily of ants, termites, and insect larvae ripped out of logs or dug from the ground. Black bears also sometimes kill young deer or scavenge the kills of other predators, such as mountain lions and coyotes. In the fall, black bears gorge on acorns, which are especially important to the bears as they fatten before going into winter dens for hibernation. Winter dens are typically established in hollow trees or logs, under the root mass of a tree, or in caves formed by the jumble of large rocks on a talus slope. Here, the bears enter a state of reduced body temperature, pulse rate, and respiration that enables them to conserve energy. They neither defecate nor urinate while hibernating, and even have unique metabolic pathways that enable them to extract energy from body wastes. Their "sleep," however, is not a deep one, and bears may leave the den periodically. Cubs are born in the winter den, weighing less than 1/2 pound (0.23 kg) at birth, and typically number from one to three cubs in a litter. The fast-growing cubs will remain with its mother through another winter, before leaving her in the spring at about 16 to 17 months of age. By spring, a hibernating bear can lose as much as half of its body weight. This is especially true of females nursing cubs. Human-Bear Management Program The history of interactions between humans and black bears in Yosemite is a long one, marked by some periods that we now look upon as shameful. Early in the park's history, little was done to keep bears from becoming conditioned to human food. Garbage was readily available in developed areas, and little was done to discourage visitors from feeding bears. Indeed, the National Park Service maintained several "bear pits" in the park where bears were fed garbage in an attempt to keep them out of park campgrounds and lodging areas, and to provide visitor entertainment. Human injuries were common, and many bears were killed in the name of public safety. Thankfully, times have changed, and the emphasis is now on managing the behavior of humans rather than the behavior of bears. All outdoor garbage cans and dumpsters are bear-resistant. All campsites, parking lots, and major trailheads are equipped with bear-proof food storage lockers that allow visitors to remove food from their cars and store it safely away from bears. In recent years, increased staffing has allowed more patrols to detect and correct food storage problems and to provide visitor education. Also, all park employees - of the National Park Service, the concessioner, and other park partners - have accepted larger roles in protecting the bears, whether it is diligence in emptying trashcans or dispensing information to visitors. As a result, human-bear incidents and property damage have declined by nearly 90% over the last three years. To continue this success and protect Yosemite's bears, strict compliance with the park's food storage regulations is necessary. The bear management program still involves some actions that deal directly with the bears. Bears are hazed out of developed areas, while others are captured, tagged, and released . Some of these bears are relocated to other areas within the park, although most of them just return. Despite all the improvements in facilities and education, some bears become dangerously aggressive and must be killed, usually two to three per year. These sad events indicate that further progress must be made in making human food unavailable to bears in Yosemite. To learn more about how to avoid dangerous encounters with bears, see our bear safety page. The Hornocker Wildlife Institute, a program of the Wildlife Conservation Society, recently examined and evaluated factors influencing human-bear interactions in order to accurately identify methods to improve bear management, reduce the number of human-bear incidents, and provide for the continued, long-term existence of bears in Yosemite National Park. Specific recommendations included maintaining personal contacts by park staff to remind visitors of food storage regulations, increasing the level of human interest in the messaging used to educate visitors about bears and bear-related regulations, stronger law enforcement efforts, more effective waste management, more aggressive aversive conditioning techniques on bears visiting developed areas, and research into the transmission of the damage behavior throughout the bear population. Additional information regarding the 2000-2003 research effort and copies of the final report and scientific publications can be found here. Birds At nearly 750,000 acres (303,525 ha), and elevations that range from 2,000 feet (610 m) to over 13,000 feet (3,962 m), Yosemite National Park provides habitats for many bird species. Over 150 species regularly occur in the park, with around 80 additional species that have been seen in Yosemite only a few times (see species list). Of the regularly- occurring species, approximately 80% are known or suspected to breed in the park. Most of these bird species migrate to lower elevations or latitudes in the late summer and fall. For example, of the 84 species known to nest in Yosemite Valley, 54% are rare or absent in winter. Fewer yet remain at higher elevations. Noticeable population declines have been detected in numerous bird species in the Sierra Nevada, including Yosemite. Possible causes for these declines include grazing, logging, fire suppression, development, recreational use, pesticides, habitat destruction on wintering grounds, and large-scale climate changes. Nest parasitism by brown-headed cowbirds (Molothrus ater), a non-native species, has also been identified as a significant factor in the declines of certain species. Cowbirds lay their eggs in the nests of other birds, usually songbirds. When the cowbird eggs hatch before those of their host species, the larger, more vigorous cowbird young eject the eggs or young of the host species or compete the host’s young for food. One example of population decline is the willow flycatchers (Empidonax trailii). These birds are known to breed in only three locations in Yosemite, and there are only and estimated 200 breeding pairs in all of California. Factors such a destruction of their willow-studded meadow habitat, grazing, and brown-headed cowbirds are suspected causes. While some meadow habitats have been affected in the park (especially in Yosemite Valley) and cowbirds are present in the park, the decline of willow flycatchers in Yosemite is thought to be largely a reflection of losses across the whole of the Sierra Nevada. Much suitable habitat remains in Yosemite, but the factors that have decimated this species across the state have ultimately affected the number in the park. To help protect birds in Yosemite, there have been studies and monitoring efforts to collect essential data. This has included monitoring of peregrine falcons (Falco peregrinus), a species whose numbers were decimated in the 1940s through 1960s from the effects of the pesticide DDT, and have only now staged a recovery after the banning of this pesticide in 1972. Great gray owls (Strix nebulosa) and their habitats have been surveyed, and several studies have mapped the distribution of California spotted owls (Strix occidentalis occidentalis) in the park. For the last 10 years, the Institute for Bird Populations (IBP) has operated MAPS (Monitoring Avian Productivity and Survivorship) bird-banding stations in Yosemite, yielding valuable data on long-term population trends. IBP also conducted a parkwide survey of bird species in 1999-2000 that provided extensive information about species-habitat relationships in Yosemite. This work also provided an assessment of meadows, with recommendations for some to be designated Important Bird Areas. Research on northern goshawks (Accipiter gentilis) has shown the importance of old-growth forests to this species, and how low-intensity fires can have a beneficial effect on the goshawks. Great gray owls are of special interest in Yosemite because here they reach the furthest southern extent of their global range, and they are isolated by hundreds of miles from the next closest population in far northern California. Besides brown-headed cowbirds, there are two other non-native bird species in Yosemite that are of concern. White-tailed ptarmigan (Lagopus leucurus) were introduced as a game bird into the east side of the Sierra in the 1960s and have since expanded their range into Yosemite, occupying wide areas of alpine habitat in the park. Along the park’s western boundary, wild turkeys (Meleagris gallopavo) have invaded. This species, also, was also introduced in California for hunting, but may be having adverse effects on native plants and animals. Fire and Wildlife Wildlife in Yosemite National Park is diverse and abundant, reflecting a wide range of Sierra Nevada habitats in relatively intact condition. Areas of concentrated human use in Yosemite have affected wildlife, primarily by removing, fragmenting, and degrading habitat, which affects the diversity and abundance of some species in those areas. These effects, however, are limited to relatively small areas of the park. On the other hand, a history of over 80 years of fire suppression in Yosemite has adversely affected wildlife habitat over a much larger area of the park. Restoration of natural fire regimes in Yosemite is perhaps the most important action that can be taken to restore and protect the natural abundance, diversity, and distribution of wildlife in the park. Under natural conditions, fire maintains habitat heterogenity and structural diversity in vegetation, which provide an abundance of ecological niches for wildlife. For example, fires under natural conditions most often burn in a mosaic of intensities, ranging from areas of light burning of surface fuels to areas of stand-replacing fires that create gaps in the forest canopy, while adjacent areas are untouched. Under such a pattern, a wide range of wildlife species, which are adapted to take advantage of different habitat conditions created by a fire, can exist in a relatively small area. Overlaying this spatial dimension is the temporal change in habitat that occurs following a fire. Over time, a succession of plant species and forest structure occurs, with a concurrent succession of wildlife species adapted to take advantage of the different stages of habitat change. The frequency of fire in an area is related to its plant community, and determines how often this process of succession is reset. Fish Most fish in Yosemite National Park have been introduced. Prior to trout stocking for sport fishing, native fish were limited in both range and number of species (see species list). The series of glaciations that covered much of the area that is now the park eliminated all fish from the high country. After the glaciers retreated, the waterfalls and steep gradients that were created on the rivers and streams by glaciation prevented repopulation of fish by upstream migration. Only the lower reaches of the Tuolumne and Merced Rivers were populated by fish when Euro-Americans first arrived in Yosemite in the mid-1800s. Because of severe climatic conditions, low nutrient availability associated with snowmelt over granitic watersheds, and a lack of spawning habitat, fish introduced in many of Yosemite's lakes have not survived. Fish surveys conducted in 1977 at 102 lakes that have a history of fish planting found that about 55% of these lakes contained self-sustaining fish populations, while 22% of the surveyed lakes had reverted to a natural, fish-free state, and another 24% were expected to achieve this state. A re-survey of some of these lakes in 1996, however, found a number of the lakes expected to go fishless still contained fish. It is estimated that approximately 550 miles of streams in Yosemite support fish. Beginning in 1978, a park policy was implemented that, by 1991, ended almost 100 years of fish stocking in Yosemite. This policy recognized that non-native fish were having an adverse effect on native aquatic ecosystems that had evolved in the absence of fish. Predation by non-native fish is identified as one of the primary reasons for the precipitous decline of mountain yellow-legged frogs (Rana muscosa) in the Sierra Nevada, including Yosemite. This species is currently under consideration by the U.S. Fish and Wildlife Service for listing as endangered. Non-native fish have also likely had profound effects on aquatic invertebrate communities. Scientists from the Sierra Nevada Aquatic Research Lab (SNARL) surveyed over 3,000 lakes and ponds in 2000 and 2001 evaluated fish, amphibian, and invertebrate populations to shed more light on the possible effects of non-native fish. The data from this research is currently being analyzed. Introduction of non-native strains of rainbow trout have probably affected the genetic integrity of the native rainbow trout found in the lower reaches of the Merced River (including Yosemite Valley) and the Tuolumne River. Mammals Yosemite is home to 90 species of mammals in seven orders, including marsupials, insectivores, bats, lagomorphs, rodents, carnivores and hooved animals. The Virginia opossum is the lone marsupial. Seven species of shrews and one mole comprise insectivores. Lagomorphs include one rabbit, the pika, and three hares. Bats and carnivores number 17 and 19 species, respectively. Rodents form the largest group with 39 species. The two hooved mammals inhabiting the park are the California mule deer and the Sierra Nevada bighorn sheep. One species, the Mount Lyell shrew is endemic to the Yosemite region while two others, the Virginia opossum and beaver are non-native introductions. The Order Rodentia includes many common genera. Mice, chipmunks, squirrels, and gophers are its best known members. The order also includes lesser known taxa like voles. Mice and their allies (8 species), chipmunks (8 species), and squirrels (6 species) constitute more than half of the rodent species in the park. Most of the remaining genera are represented by only a single species like marmots, aplodontia, and porcupines. Carnivores are perhaps the most widely recognized group of mammals in Yosemite. Its members include the American black bear, bobcat, mountain lion, raccoon, coyote, foxes, weasels, and skunks. Lesser known species like the American marten, fisher, and ringtail are also present. Although grizzly bears once roamed the area, they were extirpated from Yosemite when the last individual was shot in the early 1920's. Although often overlooked because of their nocturnal habits, bats represent a large proportion of the park’s mammalian fauna. The mobility of these remarkable flying mammals enables them to occupy a wide range of habitats. They are found from the lowest elevations in the park to over 10,000 feet. They roost in rock crevices and caves, under loose bark and bridges, in attics and tree cavities. North America’s largest bat species, the western mastiff, is a Yosemite resident, as is the spotted bat with its huge ears and vivid white spots. These are the only two species in Yosemite whose echolocation calls are audible to the human ear. Many of Yosemite’s mammals, like mule deer and gray squirrels, are fairly common and can be readily seen every day. Others, like the wolverine and Sierra Nevada red fox, are extremely rare and might be sighted only once a decade. Of the 90 mammal species on the park’s fauna list, 17 are considered "special status" by either the federal or California state government due to declining population numbers or a lack of information about their distribution and abundance. Currently, Sierra Nevada bighorn sheep are the only park mammal on the U.S. Fish and Wildlife Service Endangered Species list. Several mammal species have been the subject of research within the park. Graber (1980) investigated black bear ecology and population dynamics. Sherman and Holmes (1985) conducted long term studies of belding ground squirrel social behavior. Todd (1990) examined the distribution, abundance and habitat requirements of Sierra mountain beaver. And Chow (1991) studied the population dynamics and movement patterns of Sierra Nevada bighorn sheep while Moore (1992) examined the foraging ecology of those sheep. Additional work on forest carnivores and the interactions between mountain lions and human beings is also in progress. |
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