Sampling Plans for Mule Deer Sex and Age Ratios

Abstract

Validity of models for sample observations from each of two sampling plans for estimating fawn: doe and buck : doe ratios of Rocky Mountain mule deer (Odocoileus hemionus hemionus) are examined. Standard-error formulas useful for bionomial random variables can give misleading results when applied to fawn : doe Standard-ratio estimators with route and quadrat sampling plans yielded nearly identical estimates of buck:doe and fawn:doe However, for equivalent levels of precision, route counts were more efficient. J. WILDL. MANAGE. 48(2):500-509 Management of mule deer is routinely dependent on sample ratios of sex and age to: (1) estimate the proportion of bucks, does, and fawns to assess the effects of sport hunting regulations on a population (Connolly 1981b:261), and (2) provide an index of fawn production and survival (Connolly 1981a:294-298). Sex and age ratios are also the basic data for the change-in-ratio method of estimating the abundance of many vertebrate populations, including deer (Hanson 1963, Paulik and Robson 1969, Seber 1982). Based on computer simulations, Caughley (1974:562) concluded that age ratios of wildlife populations cannot be interpreted without a knowledge of rate of increase, and if we have an estimate of this rate we do not need age ratios. He conceded (p. 562), however, that a sudden change in an age ratio ... indicates that something has happened-but more information is needed to find out what has happened. Since an estimate of the rate of increase requires annual population estimates (Connolly 1981a:301) and most wildlife management agencies do not invest in such estimates, the more easily obtained age (fawn: doe) ratios, cautiously interpreted, provide a useful index of the dynamics of mule deer populations. Despite widespread use of sex and age ratios in the management of mule deer, the associated sampling variation, sample s ze requirements, and statistical measures of reliability have received little specific study. Published exceptions include estimates of sample size requirements (Leop ld et al. 1951:108, Robinette et al. 1977: 79) and procedures for calculating approximate confidence limits (Riney 1956). Random sampling plans for estimating sex and age ratios of North American deer have not been published. The objectives of our study were to: (1) present an example of sex and age ratio estimates with relevant standard errors for a mule deer population, (2) examine assumptions or models for sample observations which permit development of measures of reliability of estimates of sex and age ratios, and (3) present implementable random sampling plans for sex and age ratio estimates of deer populations. We thank D. L. Baker, T. H. Pojar, L. H. Stelter, and particularly C. Wallmo and E. MacConnell-Yount for field assistance. E. G. Johnson examined probability models describing field counts of mule ' Present address: 206 South 5th, Montrose, CO 81401. 500 J. Wildl. Manage. 48(2):1984 This content downloaded from 157.55.39.223 on Wed, 24 Aug 2016 05:51:30 UTC All use subject to http://about.jstor.org/terms SEX AND AGE RATIOS OF MULE DEER * Bowden et al. 501 deer, and E. E. Remmenga provided initial advice. This is a contribution from Colorado Fed. Aid Proj. W-105-R and W38-R. ENVIRONMENT AND SAMPLE POPULATION Fieldwork was done on a 647.5-km2 portion of mule deer winter range within the Cache la Poudre River drainage, Roosevelt National Forest, on the east slope of the Front Range in north-central Colorado. The Cache la Poudre River flows easterly through the Front Range, a generally rugged, often precipitous region characterized by massive granitic outcrops and dendritic drainage patterns. That portion of winter range sampled extended from the eastern edge of the Front Range from about 1,646 to 2,896 m elevation. Mountain shrub and ponderosa pineDouglas-fir (Pinus ponderosa-Pseudotsuga menziesii) plant communities characterize the lower and upper elevational limits, respectively, of the Cache la Poudre winter range (Costello 1954). Mountain park, quaking aspen (Populus tremuloides), and big sagebrush (Artemisia tridentata) communities occur at higher elevations within the ponderosa pineDouglas-fir vegetation zone. Selected abiotic and biotic factors of the Cache la Poudre winter range and apparent responses of mule deer to those factors have been characterized by Loveless (1967) and Anderson et al. (1972a,b,c). The mule deer population is largely migratory. No estimates of the total population of wintering deer are available, but average deer pellet group-derived densities on three representative portions of the Cache la Poudre winter range varied from 10.8 to 23.9 deer/km2, 1962-65 (Anderson et al. 1972c). METHODS