TABLE 15-2 Re 

 (Lack 1954). 



ation of reproductl' 



nortality rates per year 



A. Local differences in same or related species. 



Young Adult 



produced mortality 



Peromyscus califomicus 

 Peromyscus iruei 

 Peromyscus maniculatus 



6.2 

 11.7 

 20.0 



275 

 190 

 152 



destruction of nests, eggs, or young by storms, wind, 

 floods, predators, accidents, and desertion of the par- 

 ents. Considerable data are available in this connec- 

 tion with birds. 



Nest failures in birds are most frequent early in 

 the nesting cycle and decrease progressively as fol- 

 lows as nesting proceeds : 2.4 per cent per day dur- 

 ing nest-building, 2.2 per cent per day during egg- 

 laying. 1.2 per cent per day during incubation, and 

 0.5 per cent per day while the young are in the nest 

 (Kendeigh 1942). Location of the nest is a factor in 

 the successful raising of young (Table 15-3). 



The relatively low percentage of nests that pro- 



TABLE 15-3 Correlation between type of nest or nest location 

 in birds and percentage of fledglings raised from eggs laid 

 (Kalmbach 1939, Nice in Spector 1956: 93-94). 



duce young successfully in many species is not a true- 

 index of annual reproductivity, since birds commonly 

 make a second or even a third attempt if earlier nest- 

 ings were failures. The ring-necked pheasant in 

 Iowa has maximum nesting success averages of only 

 41 per cent : yet, before the season is over, by mak- 

 ing repeated efforts, between 70 and 80 per cent of 

 the hens are successful in raising broods. Full repro- 

 ductive success is not assured, however, in raising the 

 young to the stage of leaving the nest. In the study 

 of the ring-necked pheasant, the average number of 

 young hatched in successful nests was 8.7; after 1 

 to 3 weeks the average size of the brood was reduced 

 to 6.7; after 4 to 5 weeks to 5.9; after 6 to 7 weeks 

 to 5.3; and after 8 to 10 weeks to only 4.9 (Errington 

 and Hamerstrom 1937). 



LIFE TABLES 



Species dififer widely in the number of 

 young produced each year, in the average age to 

 which they live, and in their average rate of mortal- 

 ity. When sufficient facts about a species are known, 

 a life table that tabulates the vital statistics of mor- 

 tality and life-expectancy for each age group in the 

 population may be formulated (Table 15-4, Pearl 

 1923). Age is usually represented by the symbol x 

 and is some convenient fraction of a species' mean 

 life span, such as a year or stage of development. 

 The life table is set up on the basis of an initial cohort 

 of 100, 1000, or 100,000 individuals: and the number 

 living to the beginning of each successive age interval 

 is symbolized as 4. Plotting these data gives a sur- 

 vivorship curve for the species. The number dying 

 within each age interval is designated as dx and gives 

 a mortality curve. The rate of mortality during each 

 age interval is commonly expressed as the percent- 

 age of the number at the beginning of the interval 

 100((/j.//,.) and as indicated as q.r. Survival rate is 

 the difference between the mortality rate and one 

 hundred per cent (100 — q^) and is expressed as Sx 

 (Hickey 1952). Life expectancy {e.r) is the mean 

 time that elapses between any specified age and the 

 time of death of all animals in the age group. 



Life tables are also useful for computing the aver- 

 age longevity of a population, for showing the age 

 composition of a population, for indicating critical 

 stages in the life-cycle at which mortality is high, 

 for showing differences between species, for showing 

 the success of the same species in different communi- 

 ties, for furnishing information of value in game and 

 fish exploitation (yield), and in control of pests 

 (Quick, in Mosby 1960). 



Information for constructing life tables may be 

 obtained from a knowledge of age at death of a ran- 

 dom but adequate sample of the population ; informa- 



212 Ecological processes and dynamics 



