where the T means that we are transposing the vector, so that it is now a row, rather
than a column, vector. Stage-specific reproductive values for the loggerhead sea
turtle are calculated in Box 14.3.
If we look carefully through the elasticities for the loggerhead turtle given in
Box 14.3 we see that there is a great deal of variation. Changing some parameters obvi-
ously has more important consequences than changing others. The largest elasticities
correspond to the probabilities of survival and remaining within the adult and juve-
nile stage classes. If one has a finite amount of money, energy, and time to devote
to conserving this species, it would be most effectively spent on improving adult or
juvenile survival. Interestingly, most conservation effort before the paper of Crouse
et al. (1987) had been devoted to enhancing breeding success on the beaches. Although
such efforts were no doubt useful, the elasticity calculations suggest that this activ-
ity may not be as useful as concentrating on improvement of survival at sea. Turtle
excluder devices (TEDs) can greatly reduce mortality at sea by turtles which get caught
accidentally in fish and shrimp trawls. The elasticity calculations suggest that appli-
cation of TEDs should be the most efficient means of improving long-term viability
of the population. Let us assume that such devices improve survival of old adults
from 81% to 95%. Would this be sufficient to ensure long-term viability?
The demographic modified matrix obtained by changing adult survival to 95% leads
to a finite rate of increase λ=1.01, which is indeed just sufficient to allow sustain-
ability. Hence, strenuous enforcement of TEDs would be sufficient to allow popula-
tion recovery. Given the slender margin, however, other conservation practices are
also called for, such as improved breeding success and enhanced hatchling survival
to the juvenile stage. In practice, enhanced usage of TEDs has led to dramatic improve-
ment in loggerhead turtle survival, a genuine conservation success story!
Although the Leslie matrix and the geometric models make similar predictions in the
long term, they definitely make different predictions in the short term, before the age
distribution has had a chance to stabilize. This may be particularly important in con-
servation and management of many wildlife species that tend to be long lived. For
example, a recent study of Soay sheep on the island of Hirta demonstrated that age
structure can be crucial to understanding dynamics over time (Coulson et al. 2001a).
As we showed in Chapter 8, Soay sheep tend to have a strong threshold response to
changes in population density: as density rises, survival drops precipitously. Different
age groups vary in their degree of density dependence and sensitivity to weather con-
ditions (Catchpole et al. 2000; Coulson et al. 2001a). As a result, a population dom-
inated by young animals would have quite different population dynamics in the short
term from one with a more equitable distribution of age groups (Fig. 14.3).
Many wildlife populations have age distributions that are far from stable (Owen-
Smith 1990; Coulson et al. 2001a, 2003; Lande et al. 2002). Under these circum-
stances, it would be useful to have a reliable means of quantifying which
demographic parameters have the greatest short-term impact on population growth.
A promising approach has recently been outlined by Fox and Gurevitch (2000), based
on use of the full set of eigenvalues and eigenvectors, rather than just the dominant
set. Application of this new approach in an endangered cactus species (Coryphanta
robbinsorum) demonstrated that the key demographic parameters to improve popu-
lation growth in the short term differed considerably from those identified by
standard elasticity assessment (Fox and Gurevitch 2000).
AGE AND STAGE STRUCTURE 251
14.4 Short-term changes in structured populations
