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April 27, 2024 20:14
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# Bioeconomic model from Milner-Gulland & Rowcliffe, p. 163-... | |
# define constants | |
K = 1000 # carrying capacity | |
r = 0.2 # intrinsic rate of increase | |
P = 105 # price | |
a = 200 # constant for cost calculation | |
b = 0.2 # constant for cost calculation | |
s = 10 # SD of the distribution for the cost | |
N1 = 500 # initial pop size | |
Years = 30 # run the model for 30 years | |
Hunters = 200 # nb of hunters | |
Pop <- rep(0,Years+1) # pop size | |
Harvest <- rep(0,Years) # harvest | |
Prod <- rep(0,Years) # productivity | |
Pop[1] <- N1 | |
for (t in 1:Years){ | |
PrF <- r * Pop[t] * (1 - Pop[t] / K) # logistic productivity | |
Prod[t] <- round(PrF) # nb of births must be a whole number | |
Cost <- a - b * Pop[t] # mean hunter cost | |
for (i in 1:Hunters){ | |
zval <- rnorm(1,0,1) | |
IndC <- Cost + zval * s # hunter cost | |
if (IndC < 0) IndC = 0 # ensure no negative costs | |
B = P - IndC # profitability | |
if (B > 0) Harvest[t] <- Harvest[t] + 1 # hunting decision | |
} | |
if (Harvest[t] > Pop[t] + Prod[t]) Harvest[t] <- Pop[t] + Prod[t] # can't harvest more than is there | |
Pop[t+1] = Pop[t] + Prod[t] - Harvest[t] | |
} | |
Pop | |
Prod | |
Harvest | |
par(mfrow=(c(3,1))) | |
plot(1:Years,Pop[-1],xlab='Years',ylab='Pop size',type='o') | |
plot(1:Years,Prod,xlab='Years',ylab='Productivity',type='o') | |
plot(1:Years,Harvest,xlab='Years',ylab='Harvest',type='o') | |
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