According to Don Neal, a research scientist with the Forest Service, Pacific Southwest Forest and Range Experiment Station (PSW) (now retired), stationed in Fresno, California, a recent cooperative study by PSW and the California Department of Fish and Game has shed light on a situation where these theories appear to break down. In the study area on the west slope of the Sierra Nevada, apparently mountain lion numbers have increased while deer numbers declined to about one-eighth their peak numbers in the 1950s. Neal, along with George Steger (also with PSW), studied the California mule deer in the Sierra Nevada from 1970 to 1985 as part of an interagency effort to reverse the decline. This effort showed that the decline was primarily due to loss of fawns during the first 6-8 months of life.
The focus of the study was the North Kings deer here, a population of California mule deer. This herd declined from an estimated 17,000 animals in 1950 to about 2,000 animals in 1988. While the initiation of the decline was probably a result of overpopulation in the 1940s and 1950s, the lack of recovery seems to be related to heavy predation.
All the fawns were healthy at time of capture, and their size and weight were comparable to those of fawns from other mule deer herds. During the 7 years of the study, fawn survival ranged from 13% to 42% and averaged 38%. Two percent were killed in accidents, 9% died from disease or birth defects, and predators were responsible for the deaths of 51% of the fawns. Of those taken by predators 3% were killed by bobcats, 22% by bears, 27% by coyotes, and 49% by mountain lions.
Neal and his team were at first surprised by these results, because the general perception was that mountain lions were in very low numbers in California--the State Legislature had placed them under protection in 1971. It was obvious that it would require a healthy mountain lion population to be responsible for the death of an average of 25% of all the fawns born each year, as was the case in the North Kings deer herd.
First, they selected a 215-square-mile area within the 800-square-mile range of the North Kings deer herd and set out to capture as many lions as time and funding allowed. Over a period of 3 years, they captured, radio equipped, and tracked 22 mountain lions. During the study they discovered 15 adult mountain lions that were using the area but were not radio-equipped, yet were known to be different individuals.
The lion locations determined by radio triangulation were computer plotted onto large-scale maps and aerial photos. This gave a good picture of daily and seasonal movements of mountain lions in the study area.
However, detailed examination of the data revealed that several of the lions remained at low elevation in the foothills and valley edges throughout the year. They were found on ranches and among the rural communities. These lions occupied territories below most of the migrating deer in the winter, and these areas had no deer in the summer. This leaves only small mammals, livestock, and pets for a diet--a good way for a mountain lion to get into trouble.
Of the 22 lions captured and radio equipped within the 215-square-mile study area, not all were alive with operating radios during the entire study period. Therefore, one date was selected, January 1, 1987, and only the 14 lions alive and being monitored on that date were used to estimate density. This of course, underestimates the lion density because it does not include lions without radios using the area , or those with radios that have quit transmitting.
The team recognized that the number of lions using an area and lion density are not the same thing. Every radio-equipped lion used some area outside of the 215-square-mile study area. They calculated the proportion of each animal's home range that was within the study area and used that to estimate density. In other words, if a lion's home range was 50% within the study area, it was counted as 0.5 cat. Therefore, the 14 adult cats using the area on January 1, 1987, adjusted to a total of 6.3 lions, or 2.9 per 100 square miles. When the scientists added in the known cats that were not radio-equipped, making a similar adjustment to allow for only partial use of the study area, they calculated the density of adult mountain lions in the study area at 6.1 per 100 square miles.
This can only mean overlap and home-range sharing.
Extensive home-range overlap was found between females, between males, and between females and males. One female shared parts of her home range with five other radio-equipped females and an unknown number of males and unradioed lions.
The magnitude of the problem can be understood when we consider that the ratio of deer to mountain lions has apparently declined from an estimated 750:1 in 1950 to about 30:l in 1988. Deer populations cannot meet the needs of the mountain lions and maintain their numbers with the heavy predation that these ratios bring. This is especcially true when you consider the additional predation from coyotes, bears, and bobcats.
Livestock losses to mountain lions have become a serious concern of this team. The number of permits to take mountain lions that are killing livestock reached an all-time high in 1988, with 145 issued and 62 lions taken. Neal, Steger, and Bertram expect livestock predation to continue at a high level or even increase, and deer to continue to decline in all but the most favorable years.
To learn more about this subject, contact PSW Station and request Research Note PSW-392, titled Mountain Lions: Preliminary Findings on Home-Range Use and Density in the Central Sierra Nevada.