SPECIES SPOTLIGHT: Colombian black-tailed deer

By Joshua Bush

Mule deer

Photo credit: Charles Krebs/Corbis

Mule deer (Odocoileus hemionus) are found throughout California; CDFW  recognizes 6 subspecies that occur in the state.  The different subspecies are mainly distinguished through coat, color marking, body size, and other physical characteristics of the animal.  The Northern California Interior Coast Range Ecoregion (NCICR) is home to one of the subspecies known as the Colombian black-tailed deer. Although not a special status species, this deer can be utilized as a barometer to give insight on ecosystem health. Deer populations can often infer the relative conditions of other largely unstudied species, especially in forested areas like the Blue Oak woodlands found in the NCICR.

Colombian black-tailed deer are found throughout the NCICR where their life expectancy is between 8-11 years. Their body weight is typically between 130-180 lbs., for males and 70-120 lbs., for females.  Deer are crepuscular (mainly active at dusk and dawn), and prefer to lie in brushy cover during the middle of the day. Mature bucks live isolated from doe and fawn groups and will seek out mates during the “rut”. Mating occurs in mid-fall and fawns are born in the spring.  Healthy females will often produce two fawns.  Less than optimal health reduces births to one or possibly no young. Newborn fawns and their mothers will spend the majority of their time in riparian areas where hiding cover, water and food are available.

mule deer

Photo credit: National Park Service, 2009

Deer are browsing animals, as opposed to grazers (like cows), relying on a diet of mainly shrubs – with grasses, forbs or wildflowers, and other plant material mixed in. Deer are specialists in their preferred habitat which consists of young, new or disturbed growth as opposed to old growth. As fawns become older and less susceptible to predators, both mother and fawn will venture from the safety of the riparian areas and utilize the brush and trees of the landscape.

This region has a Mediterranean climate with hot dry summers and wet cold winters.  Because of this, food availability is seasonal. Spring is when food is most bountiful as rain brings new growth. Deer utilize new growth of shrub species including poison oak, buck brush, and redberry as well as new forbs.  They continue to utilize these sources throughout summer but due to lack of rain, the plants become less palatable and the nutritional value decreases. Blue-oak acorns can be a significant food source in the winter and fall months.  High in nutritional value, they can often help sustain deer until the rains come and the plants begin to grow once again.

mule deer

Photo credit: Charles Krebs/Corbis

There are many threats to deer in the NCICR.  They face predation from mountain lion, golden eagles, black bear, and wild pigs.  They are also a game animal – and bucks are harvested in the late summer and fall.  However, habitat degradation is the single most severe impact that affects not only the deer species but all other native species found in the NCICR, including the Species of Greatest Conservation Need (SGCN). Incompatible grazing,  noxious weeds infestation, trees and brush removal, habitat fragmentation, reduced native biodiversity, degraded fire regime, increased soil compaction, and decreased water retention are all negatively impacting the NCICR and deer in the area.

The State Wildlife Action Plan is developing regional strategies to address those factors negatively affecting SGCN.  These strategies are aimed at helping increase the functionality and health of the ecoregion.  One strategy includes conserving habitats through Conservation Easements (CEs). By purchasing CEs, willing landowners and conservation groups will be able to reduce property parcelization, helping create large contiguous corridors of protected land for wildlife movement. Another strategy promotes outreach and education to the public and interested parties about the benefits of implementing grazing Best Management Practices (BMPs).  This strategy proposes to create a program that will systematically inform the public about the incentive programs regarding the implementation of grazing BMPs, habitat restoration, and invasive species control.  By working on an ecosystem level within the ecoregion, we hope that the NCICR’s SGCN receive the greatest bang for the “buck.”

As mentioned above, deer is considered an indicator of habitat health especially in forested ecosystems because they are sensitivity to changes in habitat conditions. The species is actually an efficient indicator because they are rather common and easy to count as opposed to other species. Population estimates based on long-term studies together with harvest data provide evidence that deer populations have declined, suggesting poor habitat conditions are affecting survivorship. As the strategies are implemented, it will be important to monitor selected indicators such as deer populations. Using deer as proxy, analysis of the gathered data will give insight into the habitat’s overall health – and ensure that the blue-oak forest ecosystem conditions are improving.