ARIZONA ANTELOPE FOUNDATION

Winter Precipitation and
Pronghorn Fawn Survival
In the Southwest

DAVID E. BROWN, ArizonaStateUniversity, P. O. Box 87501, Tempe, AZ 85287
WILLIAM F. FAGAN, Department of Biology, University of Maryland, College Park, MD 20742.
RAY LEE, Contract Biologist, 808 Aspen Drive, Cody, WY 82414
HARLEY G. SHAW, P. O. Box 402, Hillsboro, NM 88042
BEAU TURNER, Turner Foundation Inc., One CNN Center, Suite 1090, South Tower, Atlanta, GA 30303

 Abstract: Using consistent observer and survey procedures in an arid area of southern New Mexico, we found a significant correlation between October through March precipitation and pronghorn fawn survival the following August ( = 0.67, P<0.02). Further linear regression analysis indicated less robust but still significant relationships between winter precipitation and subsequent fawn survival rates in a semidesert grassland area in central Arizona (r2=0.26, P<0.05). Neither of the areas tested showed a significant relationship between summer precipitation and the succeeding year’s fawn survival, and we postulate that forb production, as affected by winter rainfall, is more important than grass production and hiding cover in determining fawn survival in semidesert grassland habitats. Because significant relationships have also been reported for winter rainfall and pronghorn fawn survival in Arizona’sSonoranDesert (r2=0.51) we consider forb production to be the most important variable limiting pronghorn fawn production in arid areas. Although both winter rainfall and April Palmer Drought Severity Indices correlated with fawn survival rates in other arid and semiarid areas, neither winter rainfall nor drought indices correlated with fawn survival rates on the more mesic Anderson Mesa in northern Arizona (r2 = 0.03, n.s.). These data suggest that fawn survival rates following winters having less than 5 to 8 cm (2 to 3 inches) of precipitation are insufficient to maintain pronghorn population levels, and that other limiting factors have greater influence over fawn survival in higher elevation, more mesic situations.

 Key words:AndersonMesa, Armendaris Ranch, fawn survival, forb production, Palmer Drought Severity Index, pronghorn, semidesert grassland, SonoranDesert, summer precipitation, winter precipitation.

   That pronghorn fawn survival rates are highly variable in the arid and semiarid Southwest is well known (Arizona Game and Fish Department 2000). Biologists are uncertain, however, as to what determines this variation, some believing that the most important factor influencing fawn survival is the production of nutritious forbs, others believing that the primary cause of fawn mortality is predation by coyotes due to insufficient hiding cover (Neff  and Woolsey 1979).

   Because forb production has been shown to be strongly correlated with October through March rainfall and mule deer fawn survival in the Southwest (Smith and LeCount 1979, Logan and Sweanor 2001), we wanted to determine if winter rainfall had a similar influence on pronghorn fawn survival. To test the alternate hypothesis, we compared pronghorn fawn survival with the amount of rainfall during the preceding April through August as Cable (1975) showed that these precipitation amounts determined the growth of semidesert grassland grasses. Other studies have shown summer precipitation and summer grass production to have an influence over the annual survival of grassland-adapted animals (Brown 1978, 1979).

STUDY AREAS

Armendaris Ranch

            We selected the Armendaris Ranch in southern New Mexico as our primary study site as this area has on site rain gauges and has used the same observer and pronghorn survey procedures each year since 1994.

            A former land grant, the privately owned Armendaris Ranch is now managed by Turner Enterprises. Elevations range between 1375 and 1525 m, and the mean annual precipitation is <250 mm.  Approximately 74,600 ha of the ranch are classified as pronghorn habitat, in which the primary vegetation is semidesert grassland characterized by such grasses and forbs as black grama (Boutelouaeriopoda) and palmilla (Yucca elata) (Brown 1994). Most of the remaining vegetation is Chihuahuan desertscrub and the climate is warm-temperate with an average of 213 frost-free days per year (Truth or Consequences). Bison (Bison bison) are the only permitted grazing animals, other large herbivores being restricted to gemsbok (Oryx gazella) and small populations of mule deer (Odocoileus hemionus) and desert bighorn sheep (Ovis canadensis).  Most wildfires are allowed to burn.

            The pronghorn population is subject to climate-induced variations but was estimated to number more than 1000 animals in 2000. Pronghorns are hunted conservatively by archers in late August and by rifle hunters in September. Relatively few permits are issued each year and the pronghorn harvest never exceeds 10% of the available bucks.  

Horseshoe Ranch

             The second area selected was the 26,300 ha Horseshoe Ranch within the Agua Fria National Grasslands in south-central Arizona. Although slightly lower in elevation, (1050-1250 m) than the Armendaris Ranch, this area is also semidesert grassland with tobosa (Hilaria mutica) and mesquite (Prosopis velutina) being important participants. Although the average number of 213 frost-free days is the same as on the Armendaris Ranch, the Horseshoe Ranch is generally wetter, averaging ca. 380 mm per annum. The land status of the 19,425 ha that is classified as pronghorn habitat is divided between the TontoNational Forest and Bureau of Land Management. The ranch is managed for cattle grazing with mule deer, white-tailed deer (Odocoileus virginianus) and javelina (Pecari tajacu) present in low numbers. Brush encroachment is a problem, and the ranch participates in a burning program in which approximately 2500 to 2850 ha are scheduled for burning each spring. The estimated pronghorn population in 2000 was ca. 140 animals.

 Anderson Mesa

             Anderson Mesa is a high (2170 m) elevation intermountain grassland located east of Flagstaff, Arizona. Annual precipitation averages just over 500 mm and the climate is cold temperate with an average of 99 frost-free days a year (Flagstaff). Anderson Mesa is under the management of the CoconinoNational Forest with lower elevation habitats being owned by private ranchers and the state of Arizona.  Pronghorn habitat encompasses approximately 211,000 ha, which is shared with cattle, bison, and elk (Cervus elaphus) along with lesser numbers of mule deer. The pronghorn population, once estimated to exceed 2000 animals is now down to less than 500 due to very low fawn survival rates, and the status and management of this “herd” is a matter of much concern.

METHODS

             Pronghorn herd composition surveys have been conducted on the Armendaris Ranch each September since 1994 in accordance with procedures described by Lee et al. (1994). The same grid pattern was flown by a light aircraft and the same observer was present on all surveys. Personnel of the Arizona Game and Fish Department conducted similar surveys during the summer in the game management units containing the Horseshoe Ranch and Anderson Mesa.   Those animals observed in the game management unit containing the Horseshoe Ranch were separated into northern and southern populations, the latter comprising the Horseshoe Ranch population.  Surveyed pronghorn were classified as bucks, does, or fawns, and the number of fawns per 100 does was calculated.

             Monthly precipitation totals were collected for all three locations. In 1994 and 1995 precipitation data for the Armendaris Ranch were taken from the adjacent Cain Ranch; after 1995 monthly precipitation totals for the Armendaris Ranch were the means of two or three gauges located on the ranch. Monthly precipitation has been collected at the Horseshoe Ranch headquarters since 1985. Rainfall and other climatic data for Anderson Mesa were taken from national weather service summaries for the Flagstaff station. Precipitation totals for each location were summarized as summer precipitation (the April through August period preceding the spring fawning season) and as winter precipitation (the October through March period prior to fawning) and compared to the succeeding year’s fawn survival rate.

             Because drought has been shown to affect fawn recruitment in white-tailed deer and other ungulates in the Southwest, we also compared each year’s fawn survival rate with the previous April’s regional Palmer Drought Severity Index (PDSI) (Brown 1984).

 RESULTS

             Seasonal precipitation summaries are shown with annual survey results for Armendaris Ranch in Table 1.

Table 1. Pronghorn precipitation and survey information for Armendaris Ranch, 1994-2002.

 

 

 

 

 

 

 

 

 

Average

Seen

Year

Prev. Apr.-Aug.

Prev. Oct-Mar.

Bucks

Does

Adults

Fawns

Total

B: D: F

Herd Size

Per hour

 

Precipitation

Precipitation

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1994

 

0.00

179

279

458

16

474

64:100:06

3.0

11.8

1995

1.60

3.95

111

210

321

122

443

53:100:58

3.7

4.5

1996

4.28

0.55

171

309

480

26

506

55:100:08

4.0

5.1

1997

7.73

2.10

88

158

246

88

334

56:100:56

2.5

4.7

1998

9.56

0.78

113

244

357

72

429

46:100:30

3.3

4.7

1999

5.50

2.30

136

284

420

54

474

48:100:19

3.6

4.5

2000

8.00

1.06

103

175

278

45

323

59:100:26

3.2

4.6

2001

4.40

3.69

90

209

299

107

406

43:100:51

4.4

4.4

2002

5.67

1.03

102

187

289

32

321

55:100:17

4.4

3.5


These data indicate a significant positive relationship (r ²=0.68; P<0.006) between October-March precipitation and fawn survival rates the following fall (Fig. 1).  Logistic regression offered no statistical improvement over the linear fit, but predicts a slight increase (~ 3 fawns per 100 does) in fawn survival at very low precipitation levels.
 

 

Fig.1. October-March precipitation vs. fawn survival for Armendaris Ranch, 1994-2002.


Comparing late summer fawn survival rates with the previous summer’s rainfall showed no relationship between these two variables (Fig. 2).
 

  

 

Fig. 2. Summer precipitation vs. fawn survival for Armendaris Ranch, 1995-2002.

  Fawn survival rates on the Horseshoe Ranch also showed a significant positive relationship with October through March rainfall (r²= 0.26; P<0.05; Fig. 3).

  

 

Fig. 3. Winter precipitation vs. fawn survival for Horseshoe Ranch 1986-2001.

And, as with the Armendaris Ranch, there was a significant positive relationship between fawn survival and the April PDSI on the Horseshoe Ranch (r²=0.26; P<0.05) but no significant relationship between summer precipitation and fawn survival (r²=0.03, n.s.).

   We found no relationship between any of the variables tested, winter precipitation, summer precipitation and April PDSI, with fawn survival on Anderson Mesa. October through March precipitation in this unit even showed an insignificant negative relationship with fawn survival (Figure 3).




Fig. 4. Winter precipitation vs. fawn survival on Anderson Mesa, 1951 – 2001.

  
   To test if aridity was a controlling factor in the relationships between winter precipitation and fawn survival, we compared winter rainfall totals from an average of four climatic stations (Ajo, Gila Bend, Organ-pipe Cactus National Monument, and Tacna) with December fawn survival data collected by John Hervert of the Arizona Game and Fish Department for  pronghorn (A. a. sonoriensis) in southwestern Arizona. These data also indicated a positive relationship (P <0.07) between October-March rainfall and fawn survival (Fig. 5).  Once again, relationships between fawn survival and summer rainfall were nonsignificant (r²=0.21; n.s.).
 

       

 

    Figure 5. Winter precipitation vs. fawn survival for            Sonoran Pronghorn 1995-2001.

 
DISCUSSION

   The above analyses indicate that winter precipitation and forb production may be more important to fawn survival in the arid and semiarid regions of the Southwest than summer precipitation and fawn survival. To further test the significance of winter precipitation in such areas we compared 1990-2001 October through March precipitation totals for a plains grassland locality (Chino Valley, Arizona), with fawn survival rates on the nearby K-4 Ranch (W. Ough, pers. com.). This relationship was highly significant (r² 0.54; P<0.05). More surprising was the relationship obtained by comparing October through March rainfall totals for a semi-arid Great Basin locality (Kanab, Utah) with pronghorn fawn survival rates on the Arizona Strip to the south (r²=0.28; P<0.10). These significant relationships suggest that fawn survival rates following winters having less than 5 to 8 cm (2-3 in) of precipitation are insufficient to maintain pronghorn population levels, and that prediction equations can be formulated for fawn survival rates in at least some areas in the Southwest.  Fawn productivity was most variable (i.e., least predictable) for intermediate levels of rainfall (~5 cm) suggesting that other factors may interact with winter rainfall in such years to determine pronghorn recruitment.

     Neither winter nor summer rainfall appeared to influence fawn survival rates on Anderson Mesa in northern Arizona.  Linear regression analyses comparing October through March rainfall totals from other high elevation grassland areas (Springerville and Williams, Arizona) with surrounding fawn survival rates since 1990 also showed insignificant relationships (r²=0.22 and r²=0.00, respectively) at the 90% confidence level. These analyses indicated that variables other than annual forb production might be affecting fawn survival in these relatively mesic areas.  More analyses will be conducted next year in an attempt to determine what some of these variables might be.

ACKNOWLEDGMENTS 

   T. Waddell of the Armendaris Ranch was instrumental in the initiation and conduct of the study.  He also supervised the annual surveys and recorded the precipitation data for the Armendaris Ranch.

    Others who made important contributions to the study by assisting with data collection and analyses were  D. Bayer, Arizona Game and Fish Department, Mesa, AZ; J. E. Brown, Phoenix, AZ; P. Fenner, Tonto National Forest, Carefree AZ; J. Goodwin, Arizona Game and Fish Department, Flagstaff, AZ; J. Hanna, Arizona Game and Fish Department, Mesa, AZ; J. Kyl, Arizona Game and Fish Department, Mesa, AZ.: A. McKee, Arizona State University, Tempe, AZ; D. McPhee, Prescott National Forest, Camp Verde, AZ and W. Ough, Arizona Game and Fish Department, Prescott, AZ.

LITERATURE CITED

ARIZONA GAME AND FISH DEPARTMENT. 2000. Hunt Arizona. Arizona Game and Fish Dept., Phoenix.

BROWN, D. E. 1978. Grazing, grassland cover, and game birds. Trans. N. Amer.Wildl. Nat. Resour. Conf. 477-485.

BROWN, D. E. 1979. Factors influencing reproductive success and population densities in Montezuma quail. J. Wildl. Manage. 43:522-526

BROWN, D. E. 1984. The effects of drought on white-tailed deer recruitment in the arid Southwest Pp. 7-12 inP. R. Krausman and N. S. Smith, eds. Deer in the Southwest: a workshop. AZ Coop. Wildl. Res. Unit and Univ. of Arizona, Tucson.

BROWN, D. E., ed. 1994. Biotic communities: southwestern United States and northwestern Mexico. University Utah Press, Salt Lake City. 

CABLE, D. R. 1975. Influence of precipitation on perennial grass production in the semidesert Southwest. Ecology 56: 981-986.

LEE, R. M., J. D. YOAKUM, B. M. O’GARA, and R. L. OCKENFELS. 1994. Pronghorn management guidelines.  Pronghorn Antelope Workshop, Arizona Game and Fish Department, and Arizona Antelope Foundation.

LOGAN, K. A. and L. L. SWEANOR. 2001. Desert puma. Island Press, New York.   

NEFF, D. J., and N. WOOLSEY. 1979. Effect of predation by coyotes on Antelope fawn survival on Anderson Mesa. Arizona Game and Fish Department. F.A. Project W-78-R, Special Report 8:1-36.

SMITH, R. H., and A. LECOUNT 1979.  Some factors affecting survival of desert mule deer fawns. J. Wildl. Manage. 43:657-665