Fig_03

#---- Nicholson & Bailey
# fig_03

# variables
N <- numeric()
P <- numeric()

# coeficientes
lambda <- 1.5
c <- 3
a <- 0.1

# condiciones simulación
ngen <- 30     # generaciones
N[1] <- 3      # N inicial
P[1] <- 4      # P inicial

# simulación
for (t in 1:ngen) {
  f <- exp(-a * P[t])
  N[t + 1] <- lambda * N[t] * f
  P[t + 1] <- lambda * c * N[t] * (1 - f)
}

# equilibrio
Peq <- log(lambda) / a
Neq <- Peq / (c * (lambda - 1))

# gráfico (dos en la misma figura)
pdf("fig_03.pdf", width = 5, height = 9)
colorN <- "olivedrab"
colorP <- "coral"
par(mfrow = c(2, 1),
    pty = "s",
    bty = "n",
    mar = c(5, 2, 1, 1),
    xaxs = "i",
    yaxs = "i",
    las = 1)
plot(0:ngen, N,
    type = "b",
    col = colorN,
    cex = 0.7,
    pch = 16,
    ylim = c(0, 20),
    ylab = "N, P",
    xlab = "Generación")
lines(0:ngen, P,
    type = "b",
    col = colorP,
    cex = 0.6,
    pch = 21)
legend("topleft",
    bty = "n",
    c("hospedador (N)", "parásito (P)"),
    col = c(colorN, colorP),
    lty = 1,
    pch = c(19, 21),
    cex = 0.8,
    pt.bg = "white")
plot(N, P,
    type = "b",
    lty = 2,
    cex = 0.8,
    pch = 16,
    col = "royalblue",
    xlim = c(0, 10),
    ylim = c(0, 20))
abline(h = Peq,
    col = "orange",
    lwd = 0.5)
abline(v = Neq,
    col = "orange",
    lwd = 0.5)
dev.off()