Jeff,
I am not sure why you need 100 random numbers for r and K, but if your goal
is to get stochastic state-space model, you need to define the error term
as a separate parameter and run the loop 100 times with the *same* fixed
parameter values. When you do this, then you need to be aware of
parametrization (e.g. Normal error on the log scale is what people usually
use). Here is logistic Ricker state-space model:
K <- 10000
r <- 0.3
sigma <- 0.2
T <- 50
N0 <- 10
N <- numeric(T)
N[1] <- N0
for(i in 2:T) {
N[i] <- exp(r * (1 - N[i - 1]/K) + log(N[i - 1]) + rnorm(1, 0, sigma))
}
plot(N, type="b")
Wrapped in a function, you get:
Ricker <- function(r, K, sigma, T, N0) {
N <- numeric(T)
N[1] <- N0
for(i in 2:T) {
N[i] <- exp(r * (1 - N[i - 1]/K) + log(N[i - 1]) + rnorm(1, 0,
sigma))
}
N
}
set.seed(1234)
NN <- replicate(1000, Ricker(r=0.3, K=10^4, sigma=0.2, T=50, N0=10))
matplot(NN, type="l", col=1, lty=1)
lines(rowMeans(NN), col=2, lwd=2)
abline(h=K, col=4, lty=2)
HTH,
Peter
--
Péter Sólymos
780-492-8534 | soly...@ualberta.ca | http://psolymos.github.com
Alberta Biodiversity Monitoring Institute http://www.abmi.ca
Boreal Avian Modelling Project http://www.borealbirds.ca
On Sun, Feb 23, 2014 at 9:44 PM, Stratford, Jeffrey <
jeffrey.stratf...@wilkes.edu> wrote:
> Hi everyone,
>
> I would like to create stochastic population models for an undergraduate
> course.
>
> The goal would be to have students run models, record, results, change
> parameters, and make inferences on changing the effects.
>
> I understand how to draw from distributions, I hit a knowledge wall with
> loops.
>
> Here's what I have for a logistic model to predict N_t (not dN/dt)
>
> K <- rnorm(100, 10000, 400) # fluctuating environment
> r <- rnorm(100, 1, 2) # variable reproductive potential
> t <- 50
> N0 <- 10 # start with 10 individuals
>
> for (i in 1:t) { N[,i+1] <- (N0*K)/(N0+((K-N0)^exp(-r*t)))}
> plot(t, Nt)
>
> This doesn't work but I think I'm getting close.
>
> Any help would be much appreciated.
>
> Thanks,
>
> Jeff
>
> --
> ********************************************************
> Jeffrey A. Stratford, PhD
> Department of Biology and Health Sciences
> Wilkes University, PA 18766 USA
> 570-332-2942
> http://web.wilkes.edu/jeffrey.stratford/
> ********************************************************
>
> [[alternative HTML version deleted]]
>
> _______________________________________________
> R-sig-ecology mailing list
> R-sig-ecology@r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
>
>
[[alternative HTML version deleted]]
_______________________________________________
R-sig-ecology mailing list
R-sig-ecology@r-project.org
https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
