We therefore adopted a population model without density regulation writing Nt for the population size at time t so that:
E(Nt+1 | Nt) = rNt eqn 1a
and var(Nt+1 | Nt ) = σ2dNt + σe2Nt . eqn 1b
Here r is the specific population growth rate, σ2d is the demographic variance and σe2 is the environmental variance. The first order approximation of the mean and variance in Xt+1 = ln Nt +1 is then:
E(Xt+1 |Xt ) = r − σe – e tσd
2 2
and
var(Xt+1 |Xt ) = σ2d e−Xt + σe2. eqn 2a
eqn 2b
1 2 1 −X 2