Beissinger 



Chapter 37 



Population Trends Projected from Demographic Analyses 



estimate an annual survival rate of alcids on the basis of 

 body mass and clutch size. This resulted in an estimate of 

 0.845 for the Marbled Murrelet. Two standard errors of the 

 estimate for the prediction, encompassing 95% of the likely 

 values for typical murrelet survivorship (Steel and Torrie 

 1960), fell between 0.811 and 0.880. I used 0.85 for adult 

 survival and also explored the possibility that the annual 

 probability of survival might be as high as 0.90, a value 

 typical for larger Atlantic alcids (Hudson 1985). Values of 

 survivorship as low as 0.81 were not considered because 

 they would have required extremely high fecundity values 

 for populations to persist. 



Annual survival for juveniles and subadults of most bird 

 species is usually less than adult survival. Survival for juvenile 

 and subadult alcids is not as well known as adult survival. 

 These values are hard to estimate and can often be 

 underestimated due to emigration. Frequently these values 

 are simply given as the probability of surviving to the age of 

 first breeding. Hudson (1985) gives a range for the probability 

 of surviving to first breeding of 13-53 percent, with a mean 

 close to 30 percent, but this is for large-bodied birds with 

 late ages of first breeding. Nur (1993) suggested that survival 

 of juveniles and subadults could be considered to be 

 proportional to adult survival. Using data from Hudson (1985) 

 for five populations of murres, Nur calculated that juveniles 

 survive their first year of life at about 70 percent the rate of 

 adult survival, first year subadults survived slightly less well 

 than adults (0.888), and that after 2 years of age survivorship 

 was approximately equal to adult survivorship. I used these 

 proportions for juvenile and subadult survival estimates in 

 the model. 



Predicted Murrelet Population Trends 



Figure 3 shows the possible combinations of adult survival 

 and fecundity for populations experiencing no growth (lambda 

 equal to 1) for different possible ages of first breeding. 

 Combinations above the lambda isobar result in increasing 

 populations and combinations below the lambda isobar result 

 in declining populations. For the Marbled Murrelet, fecundity 

 may not exceed 0.5 because females are thought to lay only 

 1 egg per year and, on average, only half of the young that 

 fledge would be females. Note that the lambda isobars for 

 different ages of first breeding converge as survivorship 

 increases and fecundity declines. As fecundity values drop 

 below 0.20 and survivorship rises above 0.90, our assumption 

 of the age of first breeding will have little effect on the 

 predicted population trends. 



Likely combinations of adult survivorship and fecundity 

 are shown for the murrelet in the box on figure 3. These 

 estimates are well below the lambda isobars, and indicate 

 that murrelet populations are likely to be declining. Given an 

 annual survivorship of 0.85-0.90, murrelet fecundity would 

 have to range from 0.20 to 0.46 to result in stable populations 

 for different ages of first breeding. Such values would result 

 in adjusted juvenile ratios of 15 percent to 22 percent, well 

 below the values currently observed. Fecundity at these levels 



I 



on 



1.OO 



0.95 



O.9O 



T3 0.85 



0.80 



5 

 4 

 3 

 2 



0.0 0.1 0.2 0.3 0.4 



0.5 



Fecundity 



Figure 3 Sets of isobars where lambda equals 1 (i.e. populations are 

 neither increasing or decreasing) for different combinations of fecundity 

 and annual survivorship. Above the isobars populations should in- 

 crease and below the isobars populations should decline. Isobars are 

 shown for ages of first breeding from 2 to 5 years. Survivorship of 

 juveniles and subadults was set at 0.700 and 0.888 times adult 

 survivorship, respectively. Likely Marbled Murrelet values for survivorship 

 and fecundity are delimited within the box. See text for details. 



is typical for other auks, which generally experience nesting 

 success in excess of 70-80 percent (Hudson 1985, Nur 1993). 

 For example, if murrelets experienced nesting success similar 

 to other seabirds (75 percent), nests were attempted by 90 

 percent of the potential breeding population each year, and 

 90 percent of the young survived to reach the ocean (i.e., 

 fecundity = 0.30), then murrelet populations would grow 

 when adult survivorship exceeded 0.862-0.894. These values 

 fall well within the expected range of survivorship values. 

 Unfortunately, even the most favorable estimate of fecundity, 

 conceivable from current field data for the Marbled Murrelet 

 (i.e., uncorrected nesting success = 36 percent), would require 

 survivorship values to exceed 0.908-0.924 for populations to 

 grow. Such survivorship values may occur during some years, 

 but seem likely to be higher than the long term average 

 expected for this species (Nur 1993). 



The above analyses suggest a predicted rate of decline 

 for the murrelet population that is substantial. Using the 

 estimates of survival and fecundity obtained above, likely 

 combinations of demographic rates and their resulting annual 

 change in population size are compiled (table 3). It appears 

 that murrelet populations are likely to be declining 2-4 percent 

 per year and it is conceivable that the decline may even be 2- 

 3 times larger. 



A sensitivity analysis (table 4) indicated that estimates of 

 lambda were most strongly affected by adult survivorship. 

 Changes in fecundity had about half the effect on lambda that 

 changes in adult survivorship had. Neither juvenile survivorship 

 nor adult survivorship had strong effects on lambda. 



390 



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



