Hi,
I am using Jacobs' Index (D) to study habitat preferences. The references said that D value range from -1 to 1. I am getting values lower to -1. Does anybody have information of D values out of suggested? Thanks.
Yes, Andrew is right. Values for Jacob's Index can't fall out of that range. An alternative approach, similar but different, is Duncan's index of habitat preference: HPI = (Oh/Ot)/(Ah/At) with 'Oh' being observations in habitat 'h', 'Oh' is all observations in all habitats, 'Ah' is area covered by habitat 'h' and 'At' is the total area.
Note that this is an unbounded index ranging from zero (total avoidance), to one (use in proportion to availability), to unbounded upper values. Usage values are intuitive - value of 2 is used twice as much as available, for example - but avoidance values are compressed.
Publication attached.
It's impossible to obtain values > 1 or < –1 if you use the formula: D = (r - p)/(r + p – 2rp), where r is the proportion of habitat used and p the proportion of habitat available lol :)
See:
Folia Zool. – 59(1): 16 – 25 (2010)
Estimating habitat selection of badgers - a test between different methods.
Yes, Andrew is right. Values for Jacob's Index can't fall out of that range. An alternative approach, similar but different, is Duncan's index of habitat preference: HPI = (Oh/Ot)/(Ah/At) with 'Oh' being observations in habitat 'h', 'Oh' is all observations in all habitats, 'Ah' is area covered by habitat 'h' and 'At' is the total area.
Note that this is an unbounded index ranging from zero (total avoidance), to one (use in proportion to availability), to unbounded upper values. Usage values are intuitive - value of 2 is used twice as much as available, for example - but avoidance values are compressed.
Publication attached.
Hi
Did you try with the Manly selectivity measure (selection ratio = used/available) usin R??
It is just an idea
best
César
Andrew, Campbell,
Thanks for your input. I am using home range data of eight Andean Condor and we want to understand habitat preferences. I really do not know why I am getting values from -126 to 10. According to Morrison et al. Wildlife-Habitat Relationships, Jacobs Index "Values usually range from –1 to 0 for avoidance to 0 to infinity for preference" (page 246). I will check again the proportions.
Cesar, I havent tried Manly selectivity measure. I will. Thanks for the advice.
All the best,
Instead you can use resource selection function (RSF) for use-availability data, e.g., package “ResourceSelection” in R.
Best,
Andrzej
I think like Andrzej, but considering if you have variable and covariable data and dont forget the design
Cheers
C
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