Comparative analyses of many taxonomic groups have shown a strong covariation among different life-history traits. At one end of this 'slow-fast' continuum of life-history variation, we find species that mature early and have large litter sizes, but short life expectancy. At the other end of this continuum, species are located that start to reproduce first after several years, have a small litter size (often just a single offspring), but have high adult survival rates, which may exceed 95%.

The sensitivity of A to a small change in a demographic trait can be derived by implicit differentiation of the Euler-Lotka equation trait, giving

0mx T 0sx T

where vx is the reproductive value:

Similarly, the elasticity of ln A to a change in the logarithm of a life-history parameter is

The elasticity index can be used to compare the relative contribution of the trait to the population growth rate A because elasticities sum up to 1. We can then examine how the same relative change in a trait, for example, in adult survival, will affect A when we move along the 'slow-fast' continuum of life-history variation.

In birds, some clear patterns appear in the interspecific distribution of elasticities. (1) The mean elasticity across species of adult survival rate was significantly larger than the mean elasticity of fecundity rate. (2) The distribution of the elasticities of fecundity rate was skewed against small values, whereas the elasticities of adult survival rate values x

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0.05 0.15 0.25 0.35 0.45 0.55 0.65 0.75 0.85 0.95 e(s)

Figure 1 The distribution of the elasticities of the (a) fecundity rate e(B) and (b) adult survival rate e(s) calculated for the asymptotic population growth rate A estimated from the Leslie-matrix (solid columns) and when the juvenile survival rate sjuv is chosen to give A = 1 (shaded columns).

0.05 0.15 0.25 0.35 0.45 0.55 0.65 0.75 0.85 0.95 e(s)

Figure 1 The distribution of the elasticities of the (a) fecundity rate e(B) and (b) adult survival rate e(s) calculated for the asymptotic population growth rate A estimated from the Leslie-matrix (solid columns) and when the juvenile survival rate sjuv is chosen to give A = 1 (shaded columns).

were approximately normally distributed around the mean (Figure 1). (3) The elasticity of adult survival increased with adult survival rate (Figure 2 a) and decreased with clutch size (Figure 2c). In contrast, the elasticity of fecundity rate decreased strongly with adult survival rate (Figure 2b), and hence age at maturity, but increased with clutch size (Figure 2d). Thus, relative changes in adult survival of birds have larger impact on the population growth rate than changes in fecundity rate. Furthermore, the growth rate of species at the slow end of the 'slow-fast' continuum of life-history variation is more sensitive to changes in adult survival rate than high-reproductive species. Similar patterns also seem to be present in other vertebrate taxa as well.

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