Science Spotlight

Pronghorn antelope (Antilocapra americana) were once one of the most numerous large mammals in California, with populations estimated to have been as high as 500,000 prior to the Gold Rush era. In the mid-1800s, pronghorn were nearly extirpated by market-shooting to feed California’s rapidly expanding human population.

The remaining population of pronghorn has long been understudied. Prior data collected on the species have been limited to herd counts and habitat selection. In recent years, there has been growing concern over pronghorn populations, particularly in northeastern California. During the harsh winter of 1992, the number of pronghorn dropped almost 50 percent to an estimated 5,000 individuals. The northeastern portion of the state currently supports a population of approximately 4,500 animals that occur primarily in Modoc, Lassen, Siskiyou and Shasta counties and has been fairly stable, with slow declines, since about 2000. The herd’s inability to rebound has prompted scientists to try to understand the specific conditions leading to the declines.

In 2016 the Institute for Wildlife Studies (IWS) completed a two-year study, with funding from CDFW’s Big Game Management Account, which explored aspects of the pronghorn population on the Modoc Plateau. The study involved 48 does (adult females) and 42 fawns that were radio-collared and followed until their deaths or the study’s end. The researcher’s objectives were to learn more about the pronghorn use of habitat, aspects of their reproduction and factors affecting survival of does and fawns.

The researchers found that for most of the year, pronghorn used open areas with less shrubby and more herbaceous vegetation within their sagebrush-steppe habitats. But during fawning, when does need to hide their young, they shifted to spending more time in areas with greater densities of shrubs and juniper trees. The annual survival rate for does in the study was 69 percent, which is low compared to other pronghorn populations. Mountain lions accounted for 80 percent of predator-related mortalities, most of which occurred during and just after the peak birthing period when does are most vulnerable. Fawn survival averaged 44 percent, a higher-than-typical figure, with unknown causes (37.5 percent) or suspected coyote predation (21 percent) accounting for most fawn mortalities.

The adults’ increased use of shrubby areas and conifer woodlands during fawning suggests an important factor in the population’s continued decline. Juniper woodlands have been encroaching on the sagebrush-steppe habitat in the Modoc Plateau for decades, and these juniper trees provide areas of concealment for ambush predators such as mountain lions. Most ungulate studies demonstrate that adult survival plays a more critical role in population stability than juvenile survival. CDFW may be able to reduce adult pronghorn mortality through habitat restoration – the removal of encroaching junipers could help to reduce predations by lions, and potentially increase the Modoc pronghorn population.

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For some public properties, livestock grazing can be an important land management tool to help maintain specific habitat conditions, control invasive weeds and reduce fire hazards. In areas invaded by non-native vegetative species, it is necessary to control vegetation height and density in order to keep habitats functioning for certain sensitive species. For instance, the burrowing owl requires low vegetation cover in order to forage for prey effectively and prevent predators from approaching unseen. Other species that have been known to benefit from managed livestock grazing include California tiger salamander, Yosemite toad and certain sensitive butterfly species.

Controlling the height and density of non-native annual grasslands through grazing has also been shown to help increase forage efficiency for many species of raptors, and it’s used as a tool in some areas of California to maintain sensitive vernal pool habitats.

For years, local, state and federal agencies, as well as non-government organizations, have utilized livestock grazing on public lands. It is important to have many tools available for habitat management because it can sometimes be complicated. Factors such as the presence of listed or protected species, compatible public land uses and erosion -- as well as other land management practices such as herbicide application, mowing and prescribed fires -- must all be considered.

CDFW’s Rancho Jamul Ecological Reserve in San Diego County recently hosted a livestock grazing workshop presented by Rancher to Rancher (R2R) program spokesperson Kent Reeves, and UC Davis researcher Dr. Christina Wolf. The goal of the R2R program is to provide information and educational resources to private ranchers about evolving livestock management practices intended to promote native ecosystem health, and therefore, more sustainable grazing programs. More than 40 participants, including local private ranchers and representatives from about a dozen government and non-government organizations attended the workshop. Participants learned about developing progressive management practices, such as the use of increased stocking densities for shorter, more frequent durations, in order to increase carbon sequestration in the soils. This method is currently being studied as a way of encouraging native grass species as well as combating changing climate conditions.

Since 1959 the California Department of Fish and Wildlife (CDFW) has used a combination of scientific techniques to better understand fish populations and the general health of Northern California waterways. Examples include tagging sturgeon, trawling the Delta for smelt, and counting salmon carcasses. CDFW uses data from these strategies and others to help influence operations of the State Water Project and Central Valley Project, ultimately helping decision makers determine water flows. This short video highlights these operations along the Sacramento River and into the Delta, including a smelt survey conducted by Environmental Scientist Felipe la Luz.