CHAPTER 11 



POPULATION DYNAMICS 



The population data obtained on our study area depict a 

 mule deer population which, following recovery from near 

 extirpation in the 1930s, has fluctuated widely for at least 

 the past 30 years and presumably since the initial peak in 

 numbers during the late 1940s (Fig. 4.7). Specific data for 

 the past 28 years (Fig. 4.1) indicate declines spanning 2 to 

 5 years occurred during the mid 1960s, the early-mid 1970s, 

 and the mid 1980s. Relative peaks occurred in the early 

 1960s, 1970-1971, 1983, and 1987. The major fluctuations 

 involved 2- to 4.4-fold decreases or increases between low 

 spring and high autumn numbers . 



In addition to year-to-year and periodic fluctuations, 

 large within-year changes in numbers of mule deer occurred 

 during all years. Because of this, mule deer numbers were 

 seldom stable from year to year during the same time period. 

 Spring populations, measured after most annual mortality had 

 occurred, were most stable but the "normal" range of 

 fluctuation remained large. 



All fluctuations were the result of changes in 

 reproduction and mortality. For this discussion, immigration 

 and emigration were included as special cases of reproduction 

 and mortality, respectively, because consequences to 

 population numbers were similar. Understanding the dynamics 

 of regulation in Missouri River breaks mule deer thus requires 

 that we address factors influencing variation in reproduction 

 and mortality and how they interact to ultimately determine 

 deer numbers over time. 



Variation in Reproductive Rate 



Although reproductive output varies to some extent in all 

 populations, it has seldom been found to vary enough to 

 indicate significant "self-control" by a population (Lack 

 1954, Caughley 1977). Mule deer on our study area were no 

 exception. Fawn production varied from a high of 1.48 fawns 

 per female to a low of 0.99 fawns per female within the 

 extremes in environmental conditions and deer densities that 

 prevailed during the study. Reproductive effort was at the 

 low observed levels during and following drought, especially 

 if deer had been very productive for several years prior to 

 the drought . 



Thus, in the most "restrained" situation, about 50 female 

 fawns were added for every 100 adult (> 1 year) females in the 

 population during June, and about 50 male fawns were added for 



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