Study of Monkeys in Bali

[Aymeric Hoyois]

Dear All,

I'm a Master degree student currently working on a project of estimating the abundance of Long-tailed Macaque on the whole island of Bali. 

For this purpose, we collected data using Point Distance Sampling and recorded all monkeys on a 50m radius. We visited different sites across the island (5 lines of 5km and 9 points per line so 45 points per site in total). 

I tried different analysis and followed your very detailed and well explained online course but I can't seem to find what would be the most appropriate way of analysing this data. I tried analysing it as single observations but as macaques live in groups, cluster analysis seems more appropriate to me. 

Any advices ? 

PS: Please find attached the data for better understanding of our project (Written for distance and global data recorded on the field)

[Eric Rexstad]

Aymeric

I have had a brief look at the radial detection distances in the spreadsheet you provided. You will have difficulty fitting a detection function to the data for a few reasons:

• point transect data are inherently difficult to which to fit detections function models,
• the number of detections per site is small (presumably you wish to produce estimates at the site level) and
• there is considerable round of distances to favoured distances (10, 15, 20, 25m) see below
• 
• I note that not all of your sites had 45 points: Bakas and Tegallalang  had 46, Hengkulak and Ubud had 36, Bali Barat and Uluwatu had 9, Candidasa had 11 and Mekori had 1.
• It is difficult to know (because of the huge number of detections around 10m) whether detectability is perfect to that distance or whether rounding is influencing the shape of the fitted detection function.
• Estimated numbers of macaques from these models is extremely large (in the tens of thousands for some sites); you must determine if those numbers are biologically plausible.

As for your comment about analysing data as detections of clusters or detections of individuals, the results produced should be similar from the two approaches. Using detections of individuals, there is not the challenge of estimating the distance to the centre of the group, nor estimating the group size.

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From: distance...@googlegroups.com <distance...@googlegroups.com> on behalf of Aymeric Hoyois <aymeric...@gmail.com>
Sent: 18 July 2023 10:42
To: distance-sampling <distance...@googlegroups.com>
Subject: [distance-sampling] Study of Monkeys in Bali

 

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From: distance...@googlegroups.com <distance...@googlegroups.com> on behalf of Aymeric Hoyois <aymeric...@gmail.com>
Sent: 18 July 2023 10:42
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Subject: [distance-sampling] Study of Monkeys in Bali

 

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[Aymeric Hoyois]

Dear Eric,

Thank you for your complete answer ! We will work on that and try to improve our data and analysis 

Best,

Aymeric

[Aymeric Hoyois]

Dear Eric,

I discussed with my Professor about the data and we would like to know how we could still use the data in distance. What would in your opinion be the best way of making it work if possible ? 

Please note that the distances were recorded one metre at the time because we didn't manage to get a Range Finder before going on the field 

Best, 

Aymeric 

[Eric Rexstad]

Aymeric

I don't think there is a simple answer to your question; as I noted yesterday, not only is there rounding in the distance measurements, but there is also a small number of detections per site and considerable variability in the number of detections per point.

I will details these matters in a private email to you (after I finishing teaching today) and if a resolution is found, you can report back to the list.

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From: distance...@googlegroups.com <distance...@googlegroups.com> on behalf of Aymeric Hoyois <aymeric...@gmail.com>

Sent: 19 July 2023 12:39
To: distance-sampling <distance...@googlegroups.com>
Subject: Re: [distance-sampling] Study of Monkeys in Bali

 

To view this discussion on the web visit https://groups.google.com/d/msgid/distance-sampling/930cecbf-195c-4b9a-ba3c-af260fe2b2afn%40googlegroups.com.

[ABRAHAM EUSTACE]

Dear Eric

I'm curious on the number of detections, what is the recommended number of detections in a strata or in an area without a strata.

Regards

Abraham 

To view this discussion on the web visit https://groups.google.com/d/msgid/distance-sampling/DBAPR06MB6694A573F7BEE0DA52282BD6EA39A%40DBAPR06MB6694.eurprd06.prod.outlook.com.

[Eric Rexstad]

Abraham

Have a look at slides 2-3 in our introductory workshop materials regarding numbers of detections to reliably estimate the parameters of a detection function:

https://workshops.distancesampling.org/online-course/lecturepdfs/Ch4/L4-3%20Sample%20Size.pdf

Note the guidance of ~70 detections, but the subsequent slide indicates more detections are likely needed when fitting detection function models to point transect data.

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From: ABRAHAM EUSTACE <abr...@gmail.com>
Sent: 19 July 2023 13:44
To: Eric Rexstad <Eric.R...@st-andrews.ac.uk>
Cc: Aymeric Hoyois <aymeric...@gmail.com>; distance-sampling <distance...@googlegroups.com>