
Istat
computes the I similarity statistic of Warren
et al. 2008. It is a method for defining niche overlap from
predictions of species' distributions. NOTE:
it is assumed the input data are of the same extent and
cellsize, and all values are positive.
Istat(x, y, old = FALSE)
NOTE: updated to correct equation but not to worry about old... see explanation at http://enmtools.blogspot.com.au/2010_09_01_archive.html.
#create some simple objects of class 'asc'
tasc = as.asc(matrix(1:50,nr=50,nc=50)); print(tasc)
#modify the asc objects so that they are slightly different
tasc1 = tasc + runif(n = 2500, min = -1, max = 1)
tasc2 = tasc + rnorm(n = 2500, mean = 1, sd = 1)
#ensure all data is positive
tasc1 = abs(tasc1)
tasc2 = abs(tasc2)
#calculate the I similarity statistic
I = Istat(tasc1,tasc2)
print(I) #high niche overlap
#using a more variable map
tasc2 = tasc + rnorm(n = 2500, mean = 25, sd = 15);tasc2 = abs(tasc2)
I = Istat(tasc1,tasc2)
print(I) #lower niche overlap
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