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PDI <- function(web, normalise=TRUE, log=FALSE){
# function implementing the Paired Difference Index proposed by Poisot et al. 2011:
# log: this option was my own addition, since often the absolute values are extremely skewed
# author: Carsten F. Dormann 10 Aug 2011
web <- as.matrix(web)
N <- NCOL(web)
H <- NROW(web)
PDIj <- web[1,] # to automatically get the names, too.
if (log) web <- log(web+1)
for (i in 1:N){
Ps <- web[,i]
if (normalise) Ps <- Ps/max(web[,i]) # compute relative abundances
sort.Ps <- sort(Ps, decreasing=TRUE) # sort by value
P1 <- sort.Ps[1] # P1 is maximum value
PDIj[i] <- sum(P1 - sort.Ps[-1]) / (H - 1)
}
return(PDIj)
}
#data(Safariland)
#PDI(Safariland) # specialisation of pollinators
#PDI(t(Safariland)) # specialisation of plants
#PDI(t(Safariland), log=TRUE)
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