Beissinger 



Chapter 37 



Population Trends Projected from Demographic Analyses 



Simplified Marbled Murrelet Demographic Stage Life Cycle 



Figure 1 A simplified life cycle diagram for the Marbled Murrelet used in developing 

 predictions of demographic trends: P = Probablity of annual survival for fledglings that 

 have reached the oceans; P, = Probablity of annual survival for subadults; P 2 = 

 Probablity of annual survival for adults; and F 2 = annual fecundity, i.e., the number of 

 young reaching the ocean per pair. 



(Caswell 1989, McDonald and Caswell 1993) represent the 

 stage classes: juveniles (0), subadults (1), and adults (2). P is 

 the probability of annual survival for fledglings that have 

 reached the ocean. P, is the annual survivorship of subadults. 

 Note that this stage may take several years for birds to mature 

 and additional nodes would need to be added for each year 

 that the age of first breeding exceeded 2 years old. The 

 annual rate of adult survival is given by P 2 . By definition, 

 only adults breed and their average annual fecundity (i.e., the 

 number of young reaching the ocean per pair) is given by F 2 . 

 I explored only the simplest deterministic version of the 

 model because no data yet exist on the magnitude of 

 fluctuations of demographic characteristics from year to year. 

 The model assumed: (1) survivorship and fecundity would 

 change little from year to year; (2) populations were near a 

 stable age structure; (3) a 1:1 sex ratio, supported by Sealy 

 (1975a); (4) no density dependence; and (5) no senescence 

 occurs and adult birds have no maximum life span. Such 

 assumptions, although sometimes violated to varying extents 

 in real populations, are typical for models of this nature 

 (Lande 1988, Noon and Biles 1990). Usually such models are 

 constructed only for females, since it is often difficult to 

 know much about male fecundity. Thus, all rates needed for 

 figure 1 were expressed on a per female basis. Since there are 

 little data available for murrelets, the model was evaluated 

 for a range of feasible demographic values. 



Methods 



Survivorship estimates were derived from the literature, 

 because there have been no studies of individually-marked 



murrelets. A comparative analysis of survivorship of auks 

 was conducted by Nadav Nur (1993). Allometric relationships 

 and multiple regression models between body size (32-8000 

 g), reproductive rate (which is clutch size [1-2 eggs] times 

 brood number [1-2 broods per year]), and annual survival 

 were developed for 10 species of Alcidae. Estimates of 

 annual survival for adult murrelets were then made assuming 

 an adult body size of 222 g (Sealy 1975a) and a clutch size of 

 1 egg. Estimates of annual survival for juveniles and subadults 

 were assumed to be proportional to adult survival as revealed 

 from the literature survey of other seabird species. 



Fecundity values indicate the average number of female 

 young produced annually by a female that has reached or 

 exceeded the age of first breeding. The only murrelet 

 demographic data that I have found pertains to the 

 reproductive potential of the species: ratios of juveniles to 

 after-hatch-year birds (adults and subadults) in the ocean 

 (hereafter called the "juvenile ratio"), and an estimate of 

 nesting success (the number of young produced per nesting 

 pair). Information on nesting success was derived from 

 Nelson and Hamer (this volume b). 



Arguably the best data on reproductive potential are 

 ratios of juveniles from at-sea surveys. If measured at the 

 end of the breeding season, these ratios act like a "snapshot" 

 census of recruitment rates because they implicitly 

 incorporate all of the parameters needed to estimate 

 fecundity: clutch size, the proportion of nests fledging young, 

 the proportion of birds nesting, the number of nesting 

 attempts per year, and the survivorship of fledglings to the 

 sea until the time of census. Because this "snapshot" is 

 taken immediately near the end of the breeding season, a 



386 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



