dsims questions

[PABLO PALENCIA MAYORDOMO]

Dear list,

I'm playing around with 'dsims' R package and a couple of questions emerged.

Briefly, I'm working in the sampling design for a project in which I will use CTDS to estimate the density of a low-abundant species.

Thus, I'm simulating a 4km2 in which I expect an average population density of 1ind/km2 (i.e. 4 individuals inhabiting the study site).

When I create the density surface with 'make.density' function, I coded the argument 'constant = 1'

Then, when creating the population description with 'make.population.description' function, I saw that 'N' arguments refers to 'the number of individuals / clusters in a population with one value per strata', but I'm confused.

In my case, the number of individuals in the population is 4 (multiplying 4km2 x 1ind/km2); but I think that 'N' argument in 'make.population.description' refers to a location were an animal was, rather than an animal. Thus, each animal in the population is associated with multiple (>> 1) points/locations, right?

Considering that, and assuming snapshot moments 1 second appart when applying CTDS, I estimated 'N' by multiplying:

4·12·60·60·30=5184000, where:

4 -> n individuals in the population

12 -> expected activity level (12h per day)

60 -> minutes in an hour

60 -> seconds in a minute

30 -> survey length (days)

Is that right? I misudestood something?

Finally, I think that as long as I coded 'fixed.N' = T in 'make.population.description', the underling density fixed when creating not longer applies, and the reference 'truth density' against simulation results are compared is estimated from the N argument in 'make.population.description'. Right?

Thanks a lot for your help

Best

Pablo

[Laura Marshall]

Dear Pablo,

Firstly, I have not yet put a lot of thought into using dsims for camera trap survey simulation and I wonder what specific question you are trying to answer via the simulations? I suspect there are a number of challenges to overcome to do this successfully using dsims which might require a bit of additional coding. 

That is correct, the N you provide is the population size so when the survey is simulated if fixed.N is true then this many animals or clusters will be populated throughout the survey region in proportion to the values in the density grid. So a density grid cell with a value double that of another cell on average will end up with double the number of animals but the resulting density values are probably different to the values specified in the density grid. The true density in the simulation is calculated as the value of N supplied divided by the area of the study region. The alternative is to say fixed.N is false and then the number of animals will be generated as per the values in the density grid and the N argument is not used.

For your camera trap simulations I can see that you are trying to use the population of animal locations through time as your study population. This is an interesting idea although running simulations with sample sizes of over 5 million is not current feasible on most machines. I suppose your density surface would need to represent the frequency with which the animals were likely to visit each area. 

I wonder if it is an issue that the simulations will assume that animals can be detected at any angle from the point rather than just within an angle of view from the camera. Would you have multiple cameras covering the 360 degrees at each point?

Finally, the analyses in dsims assume a conventional distance sampling analysis so you would also have to code the analysis part.

Send me a message off list and we can have a chat to see if anything might work,

Best wishes, Laura

Dr. L Marshall

Centre for Research into Ecological & Environmental Modelling

University of St. Andrews

Tel: +44 (0)1334 461808

Please note that my core working hours are now concentrated between 9am and 2pm Mon to Fri. There may be a delay in my response to emails out-with these times.

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From: distance...@googlegroups.com <distance...@googlegroups.com> on behalf of PABLO PALENCIA MAYORDOMO <palencia...@gmail.com>
Sent: 14 May 2024 2:37 PM
To: distance-sampling <distance...@googlegroups.com>
Subject: [distance-sampling] dsims questions

 

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