Ecological and anthropogenic covariates ... - GANGAPEDIA

We compared Zero-Inflated Poisson (ZIP) and Zero-Inflated Negative Binomial
(ZINB) that accounts for over-dispersion which can account for both over-dispersed
and under-dispersed counts. To these models I assumed a Conditional Auto-
Regressive (CAR) normal distribution as an uninformative prior distribution for
spatial random effects. The inverse of the precision parameter (spatial variance) for
the CAR normal models was calculated and compared between models. The CAR
prior provides spatial smoothing of parameter estimates.
The value of Gharial count per segment is influenced by the probability that it takes a
value conditional upon the gharial count in the neighbouring segment. The CAR
model used here has four terms: the number of neighbours of each site, (2 for all
segments except terminal sites which have only one neighbour), adjacency matrix
based on the IDs of neighbouring sites and spatial weights which we assign as 1 for
all areas. The spatial precision (1/variance) parameter tau is the important parameter
for the model, as it gives us an estimate of spatial effect. Higher the parameter tau,
lower is the spatial variation or spatial effect.
We used the ZINB and ZIP models to estimate the slope and intercept parameters, as
well as spatial variance parameter for relationship between Gharial abundance and
basking site presence, as well as between abundance and depth of river channel in that
segment. The parameters slope, intercept, spatial variability (variance of CAR
Normal), over-dispersion parameters, and other parameters of the respective
distributions (ZIP, ZINB) were estimated in each model. Deviance was compared for
model selection. All statistical analyses were conducted using the software R 2.11.1
(R Development Core Team 2010) and WinBUGS (Spiegelhalter et al. 2007). For
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