PONE-D-20-24701

How widespread was the Tapanuli orangutan and what led to its decline?

PLOS ONE

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3. Thank you for stating the following in the Competing Interests section:

"Several of the authors are engaged in programs focused on the conservation of the Tapanuli orangutan. Erik Meijaard and Serge Wich have actively advocated against a hydropower project that is being developed in the current range of the Tapanuli orangutan but have not received funding for this. These activities could be perceived to bias our research."

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Reviewers' comments:

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Comments to the Author

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Reviewer #1: Yes

Reviewer #2: Partly

Reviewer #3: Yes

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: N/A

Reviewer #3: N/A

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Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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5. Review Comments to the Author

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Reviewer #1: This manuscript uses historical data to better understand the current distribution of the newest and most endangered great ape, Pongo tapnuliensis. This is a species of high interest and interesting results are presented using a new approach. I suggest publication after making the following revisions.

Did the authors look at museum records, specifically the catalog cards that show where specimens came from? These provide solid documentation of individual animals and where they came from. Many such records are available on-line or at least by contacting the museum. I think this would provide useful source of data that should be accessed for this paper that won't require too much additional work.

I also wonder how the authors distinguished between records of orangutans vs. gibbons/siamangs? "Orang pendek" (short person in Indonesian/Malay), who the authors reference in lines 164-169, is commonly thought to be a gibbon walking on the ground. I think the presence of gibbons/siamangs should at least be referenced as a confounder in this study.

It was a little unclear to me why some records were rejected and others weren't. For example, it would be hard to mistake a pig-tailed macaque from an orangutan (line 265). So, why were the records not accepted? The results thus seem a bit cherry-picked. It would be helpful to know that there was a systematic process to reject or accept the particular records.

One thing I thought was missing was an answer to the important question posed at the beginning of whether Pongo tapanuliensis is really a highland specialist. This wasn't really answered by the article.

It is thought that all species south of Lake Toba would historically have been Pongo tapnuliensis? This should be explained more.

The first figure should be labeled as Current Orangutan Distribution.

A few small errors:

Line 179: Figure 2 legend should be "orangutans 'were' historically..."

Line 187: No need to say 'as the crow flies'

Line 266: Should be 'were' summarized

Line 294: Explain why it was though that these nests were thought to be eagle nests.

Line 466: should be "a" clear, science-based plan

Reviewer #2: The manuscript reports historical evidence regarding the original range/distribution of the Tapanuli orangutan and discusses implications for conservation and management of the extant populations of this species. The historical occurrence information is of interest, but is limited in scope and quality. Because of the nature of the information sources, the authors had to speculate to some degree regarding which species was observed and the exact location of the observations. This limits inference. The discussion of the causes of range contraction are somewhat speculative. One would like to see additional information on human population growth and/or increased access to the forest which would support the apparent conclusion that hunting has been a contributor (major?) to population declines. The management implications discussed do not really depend upon the historical information; one would come to these conclusions simply understanding the current and recent trends in habitat loss and population declines of many Sumatran species. This species is clearly highly endangered and effective action is needed immediately. I am unclear to what extent the historical information adds to this argument.

The manuscript is generally well written but there are a few typos and inconsistent word usage. Sometimes the authors use Lake Toba while in other instances it is Toba lake. I prefer the word "elevation" to "altitude" in the context of heights on the Earth's surface. The authors use "extend" instead of "extent" at one point; clearly just a typo.

The maps could be more clear, particularly Figure 1. And do the numbered locations in Table 1 correspond to the locations in Figure 2?

Regarding the discussion of historical accounts, I recommend that the publication date be mentioned each time a new publication is discussed. That will give the reader immediate historical context and eliminate the need for the reader to turn to the literature cited to determine publication date.

The Tapanuli orangutan is critically endangered, and concerted conservation action is needed now. However, I believe that the information and analysis in the manuscript are not of sufficient interest or rigor to warrant publication in PLOS ONE. The authors could attempt to revise the manuscript to conduct a more rigorous temporal analysis of habitat loss and human hunting pressure, or they might seek publication in a different journal.

Reviewer #3: This article investigated the potential historic range of the recently described and critically endangered Tapanuli orangutan, including the likely habitat types historic populations occupied. They asked if the current range of this species is indicative of its historic range, and if not, is it possible to assess the reasons for the decline in range area. This topic is of particular immediate importance because all orangutan species, though especially those found on the island of Sumatra, are experiencing rapid population declines due to anthropogenic pressures. The authors used a historical ecology approach that combined written historical accounts from the past 130 years with a modern approach to spatial description. While the methods are overall qualitative, they are still appropriate for the question, and the scope of the submission fits within the requirements of PLOS ONE. They found that Tapanuli orangutans did indeed occupy a much larger range and potentially overlapped with the Sumatran orangutan. Additionally, the authors found that a combination of habitat destruction/fragmentation and hunting pressure resulted in the reduction of Tapanuli home range over the past century and a half.

In general, the manuscript is easy to read, uses a combination of methods in a unique, meaningful way, and conveys the importance of the topic. This project excites me and I believe it is a crucial addition to the literature on Tapanuli orangutans. Still, the manuscript requires revisions to better connect the aims with the results/discussion and to clarify several main points of the discussion.

Major:

• In the final paragraph of the Introduction (lines 105-108), the authors should rework their aims statement. In its current state the statement is a method and not a research aim. Throughout the introduction, the main points of the project are mentioned, so these points should be explicitly stated as individual research aims.

• In line 111 of the Methods, clarify which databases were used.

• In lines 133 and 136-137 of the Methods, the authors describe designating vegetation types on a digitized historic map. However, vegetation types to this point have not been mentioned in the manuscript. The authors then continue to use vegetation types going forward. Therefore, the authors need to be more explicit and clarify how this method is used to address the states aims of the project.

• In lines 90-94 and 107 of the Introduction, the authors ask the question if Tapanuli orangutans are adapted to highland habitats and state it as a main aim of the paper, yet this is only limitedly mentioned in the discussion. The authors need to explicitly address this aim in the Discussion.

• In lines 359-360 of the Discussion, the authors state that historic populations of Tapanuli orangutans became extinct because of “…a combination of habitat loss and fragmentation, and mortality rates that exceeded reproductive replacement rates.” However, the authors in line 402 state the “… Tapanuli orangutan was hunted to extinction…”, but then again switch back to the main cause of extinction being habitat loss and fragmentation in lines 408-409. I believe the authors are saying that habitat fragmentation came first, and then the orangutans were hunted to extinction within these fragments. If this is the relationship the authors were trying to convey, then it needs to be more explicitly stated. If this is not the relationship they were trying to convey, then the authors need to reassess what the true cause of extinction was.

Minor:

• Throughout, fix “Tapanuli Orangutan” to “Tapanuli orangutan”

• Line 18 in Abstract: “…other literature” is very broad. Please be more explicit.

• Line 86-87 in Introduction: The authors state that Tapanuli orangutans are found mainly around higher elevations (~834m ± 219m), but Nater et al. 2017 (reference 12 from manuscript) described the range from 300m-1300m. Need to clarify.

• Lines 125-127 in Methods: Clarify that the first official forest cover map is the 1950 Indonesia forest cover map.

• Table 1: In Vegetation column, clarify if “(logged)” is historic or current and approximately when. If possible, include altitudes on Tall Swamp Forest habitats. Additionally, Point 13 has no value in Vegetation column.

• Line 355-357: It is not clear what the 48% forested in the 1930s is being compared to. Please clarify.

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

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The historical range and drivers of decline of the Tapanuli orangutan

PONE-D-20-24701R1

Dear Dr. Meijaard,

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Kind regards,

Bi-Song Yue, Ph.D

Academic Editor

PLOS ONE