The total population of bighorn sheep in the Sierra Nevada has made large gains in the past decade. Yet evidence suggests that population growth in numerous herds is slowing and may be reversing.
The total number of female bighorn sheep in the Sierra has essentially quadrupled from its low of about 50 in 1995 to about 200, but has shown no gains for two years. Within that larger pattern, however, are a variety of patterns at the level of individual herd units, varying from herds that have remained static at low numbers to herds that have grown as much as 7.8-fold and account for much of the overall gain. Population growth has been driven primarily by the Mount Langley and Wheeler Ridge herd units, which in recent years have accounted for about half of the total population, and secondarily by the Mount Baxter and Sawmill Canyon herd units, which together account for another 30%. The Mount Langley and Wheeler Ridge herd units reached peak sizes in 2007, and both have exhibited declining trends since then. The Mount Baxter herd has not gained any females for four years. Meanwhile, the adjacent Sawmill Canyon herd has been found to be three times the size recorded just three years ago. This increase can probably be attributed to the deployment of more radio collars in this herd, leading to greatly-improved data collection.
For the past two years, total numbers of adult and yearling females in six of eight herd units exhibit a lack of gains. This pattern is certainly representative of the total population. Four years ago it was already possible to project that the total Sierra bighorn population exceeded 400 animals, but in recent years the population appears to have declined to 400 or fewer.