TABLE 1. — Life history statistics used in the derivation 

 of the Leslie matrix for Mya arenaria (data from Brous- 

 seau 1978a, b). 



'Fecundity = number of female eggs produced per individual 

 assuming a 1:1 sex ratio. 



Mya arenaria to changes in the settlement rate 

 (r s ) can be calculated. Results are summarized in 

 Table 2 for a range of values of A. . As expected, r s 

 increases as the population growth rate increases, 

 while the sensitivity of A to changes in r s de- 

 creases as A increases. The population growth 

 rate, A. , is far more sensitive to changes in r s than 

 it is to changes in any other single life history 

 parameter. A further discussion of this point is 

 given below. 



TABLE 2. — Sensitivity of various population growth rates (A.) 

 to changes in the settlement rate (r s ). The intrinsic growth 

 rate = log A . 



Population 



growth rate Intrinsic Settlement rate Sensitivity of 



(a) growth rate (rs) A to rs 



V s = Equilibrium settlement rate, rs e q 



Fecundity and Other Survivorship Rates. — The 

 sensitivity of A. to changes in fecundity are illus- 

 trated in Figure 1. Under equilibrium conditions 

 (A = 1.0), sensitivity to fecundity changes over the 

 reproductive life span of the individual are slight. 

 If the population is actually growing (A > 1.0), the 

 magnitude of the sensitivity to changes in the 

 fecundity decreases with increasing age, while 

 the reverse is true if the population is actually 

 declining (A < 1.0). This follows from Statement 

 (13). For declining populations this is probably 

 due to the combined effects of an increasing 

 reproductive value with increasing age and a 

 shift in the age structure to older individuals as 

 the population declines. 



The sensitivity of A to changes in survivorship 

 parameters other than r s is illustrated in Figure 

 2, where it is evident that A is more sensitive to 

 changes in bi, the survivorship of an individual 

 from 2 mo to 1 yr of age, than to other values of bi 

 for i > 1. As above, these curves illustrate a 

 general result. Since bi is >6i for i > 1 in the Mya 

 arenaria model, dkldbi is >dkldbi using State- 

 ment (14). 



By comparing Figures 1 and 2, it seems evident 

 that the population growth rate is more sensitive 

 to changes in survivorship than to changes in 

 fecundity. This result may be made precise if the 

 population is actually growing (A > 1), since using 

 Statements (13) and (15) it follows that dk/dbi is 

 >dk/da { for all values of i. Finally, by examining 

 Statements (10) and (11), it is clear that A is more 

 sensitive to r s than to b\ for the Mya arenaria 

 model as long as r s is <6i. Hence, A is more 

 sensitive to r s than to all other survivorship 

 parameters, and, at least for growing populations, 

 more sensitive to changes in r s than to any other 

 fecundity parameter as well. 



Discussion 

 Fisher (1958) in his fundamental theorem of 



3 4 5 6 7 8 9 



AGE-CLASS 



10 11 12 



FIGURE l.— Sensitivity of a range of As (0.25-3.0) to changes in 

 the fecundity (a,) of Mya arenaria in each age class. The first 

 age class is not included since Mya arenaria are not mature 

 until after the first year of age. 



539 



