pcount {unmarked} | R Documentation

## Fit the N-mixture model of Royle (2004)

### Description

Fit the N-mixture model of Royle (2004)

### Usage

    pcount(formula, data, K, mixture=c("P", "NB", "ZIP"),
        starts, method="BFGS", se=TRUE, engine=c("C", "R"), ...)

### Arguments

`formula` | Double right-hand side formula describing covariates of detection and abundance, in that order
`data` | an unmarkedFramePCount object supplying data to the model.
`K` | Integer upper index of integration for N-mixture. This should be set high enough so that it does not affect the parameter estimates. Note that computation time will increase with K.
`mixture` | character specifying mixture: "P", "NB", or "ZIP".
`starts` | vector of starting values
`method` | Optimization method used by `optim`.
`se` | logical specifying whether or not to compute standard errors.
`engine` | Either "C" or "R" to use fast C++ code or native R code during the optimization.
`...` | Additional arguments to optim, such as lower and upper bounds

### Details

This function fits N-mixture model of Royle (2004) to spatially replicated count data.

See `unmarkedFramePCount` for a description of how to format data for `pcount`.

This function fits the latent N-mixture model for point count data (Royle 2004, Kery et al 2005).

The latent abundance distribution, f(N | theta) can be set as a Poisson, negative binomial, or zero-inflated Poisson random variable, depending on the setting of the `mixture` argument, `mixture = "P"`, `mixture = "NB"`, `mixture = "ZIP"` respectively. For the first two distributions, the mean of N_i is lambda_i. If N_i ~ NB, then an additional parameter, alpha, describes dispersion (lower alpha implies higher variance). For the ZIP distribution, the mean is lambda*(1-psi), where psi is the zero-inflation parameter.

The detection process is modeled as binomial: y_ij ~ Binomial(N_i, p_ij).

Covariates of lamdba_i use the log link and covariates of p_ij use the logit link.

### Value

unmarkedFit object describing the model fit.

### Author(s)

Ian Fiske and Richard Chandler

### References

Royle, J. A. (2004) N-Mixture Models for Estimating Population Size from Spatially Replicated Counts. Biometrics 60, pp. 108–105.

Kery, M., Royle, J. A., and Schmid, H. (2005) Modeling Avaian Abundance from Replicated Counts Using Binomial Mixture Models. Ecological Applications 15(4), pp. 1450–1461.

Johnson, N.L, A.W. Kemp, and S. Kotz. (2005) Univariate Discrete Distributions, 3rd ed. Wiley.

### See Also

`unmarkedFramePCount`, `pcountOpen`, `ranef`, `parboot`

### Examples

    # Simulate data
    set.seed(35)
    nSites <- 100
    nVisits <- 3
    x <- rnorm(nSites)               # a covariate
    beta0 <- 0
    beta1 <- 1
    lambda <- exp(beta0 + beta1*x)   # expected counts at each site
    N <- rpois(nSites, lambda)       # latent abundance
    y <- matrix(NA, nSites, nVisits)
    p <- c(0.3, 0.6, 0.8)            # detection prob for each visit
    for(j in 1:nVisits) {
      y[,j] <- rbinom(nSites, N, p[j])
      }

    # Organize data
    visitMat <- matrix(as.character(1:nVisits), nSites, nVisits, byrow=TRUE)

    umf <- unmarkedFramePCount(y=y, siteCovs=data.frame(x=x),
        obsCovs=list(visit=visitMat))
    summary(umf)

    # Fit a model
    fm1 <- pcount(~visit-1 ~ x, umf, K=50)
    fm1

    plogis(coef(fm1, type="det")) # Should be close to p

    # Empirical Bayes estimation of random effects
    (fm1re <- ranef(fm1))
    plot(fm1re, subset=site %in% 1:25, xlim=c(-1,40))
    sum(bup(fm1re))         # Estimated population size
    sum(N)                  # Actual population size

    ## Not run:

    # Real data
    data(mallard)
    mallardUMF <- unmarkedFramePCount(mallard.y, siteCovs = mallard.site,
    obsCovs = mallard.obs)
    (fm.mallard <- pcount(~ ivel+ date + I(date^2) ~ length + elev + forest, mallardUMF, K=30))
    (fm.mallard.nb <- pcount(~ date + I(date^2) ~ length + elev, mixture = "NB", mallardUMF, K=30))

    ## End(Not run)

[Package unmarked version 0.12-3 Index]