#=========================================================
# Figure 6.1.
# R script to calculate and plot age class sizes through
# time for an age-structured wildlife population, using
# projection equations. Demographic rates for the
# Northern Spotted Owl are from Noon and Biles
# (Journal of Wildlife Management, 1990).
#=========================================================
num.times=20
p0=.11     # Age-specific survival and fecundity rates.
p1=.71     #
p2=.94     #
f=.24      #

J.t=numeric(num.times) # Vector for juveniles.
S.t=J.t                # Vector for subadults.
A.t=J.t                # Vector for adults.

J.t[1]=1200 # Initial age class sizes.
S.t[1]=800  #
A.t[1]=2000 #

for (i in 1:(num.times-1)) {
   J.t[i+1]=f*A.t[i] # Recursion equations
   S.t[i+1]=p0*J.t[i] # for projection of
   A.t[i+1]=p1*S.t[i]+p2*A.t[i] # age classes.
}

J.t     # Print the results to the console.
S.t     #
A.t     #

# Plot the age classes through time.
time.t=0:(num.times-1)
plot(time.t,J.t,type="l",lty=2,xlab="time in years",
ylab="population size",ylim=c(0,2600))
points(time.t,S.t,type="l",lty=5)
points(time.t,A.t,type="l",lty=1)