[R-sig-eco] [AniMov/R-sig-eco] adehabitat HR: Problem with BRB and sample size (number of fixes)

[Mathieu Basille]

Hey Dagmar,

What do you mean by "testing if the UD size is related to the number of
fixes"? Did you model UD size as a function of number of fixes using all
fixes for all animals? If that's what you did, you could also check if it's
true by animal too. One way to do it would be to compute UDs for each
animal over samples of fixes for instance every 10, 20, 30, 40, 50 and 60
minutes, and see if there is any relation here. If there is, you may not
have other option than subsampling at the hour scale to compare all animals
(but what does the UD mean if there is such variation related to sampling?
I'm not familiar enough with BRBs to comment here). If there is no
relationship per animal, then you simply have animals with high number of
fixes AND large UDs!

Hope this helps,
Mathieu.


On 08/23/2017 03:59 PM, Dagmar wrote:
> Dear all,
> 
> I want to compare the homerange size between animals based on GPS data but
> it seems not to work.
> 
> Here is what I did:
> 
> I calculated the homeranges using the Biased random bridges from adehabitat
> HR.
> 
> Animal_ltraj <- as.ltraj(animalxy, animaltime, id=TierID)
> D_Animal <- BRB.D(Animal_ltraj, Tmax=21600, Lmin=36)
> 
> Animal_Grid <- ascgen(allanimals_xy, cellsize=32)  # I chose 32 because it
> was the smallest cellsize chosen automatically. I thought to compare
> homerange size between animals I must use the same grid for all of them.
> 
> Animal_BRB <- BRB(Animal_ltraj, D=
> D_Animal,type=c("UD"),Tmax=21600,Lmin=36, hmin=100,grid=Animalgrid)
> kernel.area(Animal_BRB, unout=c("km2"))
> 
> My problem:
> 
> I have very different sample sizes (i.e. number of fixes): Most animals
> were located hourly but two animals were located about each 5 min. This
> results in sample sizes of about 150 fixes per animals and some animals do
> have almost 10.000 fixes for the same periods of time.
> 
> Because I want to compare the resulting home range sizes I tested if the UD
> size is related to the number of fixes and unfortunatly (!!) they are.
> 
> That means that I cannot compare homerange sizes between my animals
> 
> My question:
> 
> - did I do something wrong?
> 
> - is that problem known and is there a way to solve it?
> 
> The only way that I find is to reduce the number of fixes of my frequently
> located animals artificially / randomly to start with the equal number of
> fixes for all animals. This would be a shame though!
> 
> Help would be very, very, very much appreciated!!!
> 
> 
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-- 

Mathieu Basille

basille at ufl.edu | http://ase-research.org/basille
+1 954-577-6314 | University of Florida FLREC

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