Our analysis of mule deer population trends suggests that fawn production in the Trans-Pecos is most influenced by rainfall received in spring months.

Population Trends

Mule deer populations ebb and flow with precipitation but the relationship isn’t always simple. To investigate the influence of precipitation on mule deer populations in west Texas, we obtained long-term survey data from Texas Parks and Wildlife Department from 1977-2009. Individual deer were identified by gender and age (adult or juvenile) which produced information on densities, sex ratios, and fawn production.

Mule deer herds in the Trans-Pecos averaged 150,000 individuals and varied considerably (98,000-222,000). Fawn production (fawn:doe) was also highly variable with an average of 48% and ranged from 12-87%. Rainfall levels in the Trans-Pecos ranged from 6 to 21 inches and averaged 13 inches. Precipitation levels could explain 42% of the variation in mule deer numbers.

Trends of mule deer populations in Trans-Pecos, Texas.

In general, long-term drought eventually takes a toll on the west Texas mule deer herd. The strongest influence drought has on mule deer occurs in the winter and spring months, when rainfall is especially sparse. For west Texas, most rainfall occurs in summer and fall resulting in a flush of forbs that are critical to mule deer nutrition. As winter approaches, the availability of forbs decreases and mule deer increase their use of browse. In drought years, forbs are even more rare.

Fawn production was also affected by rainfall where precipitation levels accounted for 40% of the fawn production. From our analysis, fawn production was most influenced by droughty conditions that occurred in spring. Spring is usually the driest season for the Trans-Pecos and has a significant impact on pregnant does. Other studies have demonstrated that poor nutrition in spring can affect the ability of does to carry fawns to term, their ability to produce twins, and affect the weight of fawns (and ultimately fawn survival). Late winter and/or early spring precipitation will generally produce above average fawn crops.

One way to curtail the effects of drought is to ensure that valuable forb and browse resources are available for your deer through proper habitat management. This may mean conducting habitat improvements to promote forbs and quality browse or minimizing competition with livestock and other wildlife. Conservative stocking rates can also help ensure adequate fawning cover which is critical to fawn survival. Creating more permanent water sources will allow also mitigate your losses. Lastly, harvest rates may need to be modified to allow mule deer to recover.