Shorebirds and the Dispersal of Bipolar Plant Species to South America

Round 1

Reviewer 1 Report

The paper is interesting and shows the relationship between bipolar birds and plants. However, I have three suggestions that authors might consider before publishing the text: 1. The article is based on published papers and is not an original research paper. I propose to highlight this fact from the very beginning of the article. The reader may have the wrong impression that the opinion relates to the authors' research; 2. In the era of global climate change affecting the poles, bird migrations and plant ranges, I was waiting for the conservation aspect that concerns this. I propose to add a few sentences on this subject; 3. The English language is very well written, but sometimes it bothered me that the paragraphs are very short as if they just signaled some arguments. I propose to expand the text more, it is better to connect adjacent paragraphs with each other.

Minor remarks:

L81 - spring migrants rather than sprong migrants?

Table 1 - first species name should be in italics

Author Response

The paper is interesting and shows the relationship between bipolar birds and plants. However, I have three suggestions that authors might consider before publishing the text: 1. The article is based on published papers and is not an original research paper. I propose to highlight this fact from the very beginning of the article. The reader may have the wrong impression that the opinion relates to the authors' research; Reply - The article states in the discussion and abstract that the data came from public data bases. It is original research in the sense that we use this data to come to our conclusions. The paper is a deductive framework.

2. In the era of global climate change affecting the poles, bird migrations and plant ranges, I was waiting for the conservation aspect that concerns this. I propose to add a few sentences on this subject;  Reply - We appreciate this suggestion, but are not sure how to incorporate it into the manuscript. The paper is historical in that it describes how the transfer of seeds occured in the past, and not what might happen in the furture.

3. The English language is very well written, but sometimes it bothered me that the paragraphs are very short as if they just signaled some arguments. I propose to expand the text more, it is better to connect adjacent paragraphs with each other. Reply - We will go through the manuscript and see where we can link paragraphs better. Many of them are stand alone, as they are presenting relevant but stand-alone material.

Minor remarks:

L81 - spring migrants rather than sprong migrants? Reply - We believe sprong is correct, it describes the type of migrant not when it happens.

Table 1 - first species name should be in italics. Reply - We will correct.

Reviewer 2 Report

Please find an attached document for the result of reviewing the opinion paper entitled "Shorebirds and the dispersal of bipolar plant species to South America" submitted by James F. Hancock and Harold Prince to your journal.

Overview:

This interesting opinion paper illustrated the importance of migratory birds as a vector of bipolar distributed plant species. Several hypotheses regarding bipolar distributed species have been proposed and discussed in the past. However, in this manuscript, the authors discussed one of the most realistic hypotheses, the migratory bird hypothesis, focusing on two introduction pathways: long-distance dispersal (LDD) and mountain hopping (MH). The authors argued which pathway is more important and suggested the potential candidate for long-distance dispersal using the distribution records from GBIF for plant species and bird migration patterns, breeding and migration ranges, and habitats from various sources. The authors found that LDD is more important than MH, as six species showed intermediated distribution while 17 species showed disjunct distributions in both polars. Furthermore, based on breeding and migration ranges overlapping with the plant species distribution, the authors suggested Hudsonian Godwit could be a major vector of those species.

General comments.

Although this opinion paper discussed an interesting topic, I found the logic presented by the authors is driven by their conclusion (LDD generates bipolar distributions through jumping migratory birds) rather than deduced from observed patterns. Although I agree with the conclusion, it would be nice if the authors broadened the discussion further to eliminate other explanations. For example, the authors failed to address the local extinction of bipolar species. The authors discussed the lack of records in GBIF, but what would happen if those species were once distributed over the mountain areas of north and south Americas but then disappeared due to climate change in the past. This could create the current distribution pattern without considering dispersal by birds, or it is also possible that 17 species the authors considered LDD could be MH, but the population in the mountains disappeared. The authors mainly argued the LDD by prong (jump) species, but hink (hop) or strap (skip) species can carry seeds from one polar region to another polar region, even if they stop over several places and take a longer time. Surely prong (jump) species can carry seeds faster and more successful than other species, but it is a matter of the amount of propagules. The authors discussed the abundance of prong birds in the discussion, but how about the abundance of hink or strap bird species? If their populations are much larger than prong species, the chances they could carry seeds increase.

Again, I like this opinion paper, but the authors focus too much on LDD by prong species. It would be nice to briefly introduce some other hypotheses and why they can be rejected. It is also important to consider other explanations in line with the author's logic and propose how to eliminate other explanations as well.

Minor comments

I just wonder about the seed traits of Carex species. As discussed in Villaverde (2017), bipolar Carex species do not have seeds that are suitable for attaching to animals. Of course, the absence of such traits does not mean those species cannot carry by birds, but I wonder how those seeds attached to birds and carried such a long-distance?

The number of bipolar species: the authors identified 23 plant species as bipolar distributed, but some other authors identified about 30 bipolar species. Why did you choose these 23 species? Because of enough records in GBIF? Or eliminate some species that may be introduced by human? Please clarify it in the MS.

Line-by-line comments

L136: Eskimo Curlew is probably extinct, but the species was last seen in the wild around 1980 as well. You probably want to say the species went extinct in South America (1939)? If so, please clarify it here.

L169 "South America ha" what is this "ha"? were?

L170 add "." after "[21]"

Figure 1 "B) Circumpolar minus Central US and Rockies"> but it looks like they are distributed in the Rockies?

Reviewer 3 Report

This is an interesting study that explored the association between bipolar plants species and long-distance migratory shorebirds. The authors identified 23 bipolar distributed plant species, and found 11 birds that might have spread the seeds of the plants, based on publicly available databases. The analysis part is not strong, yet the discussion section is well organized. This work is valuable for interpreting the distributions of bipolar plants species and the roles of shorebirds in seed dispersal. I have some comments listed below.

1.     Figure 1 shows the occurrences of three representative bipolar distributed plants. I suggest the authors provide the distribution maps of all 23 plants, ideally by a supplementary figure with 20 panels. I guess some plant may have wider ranges, which are not typical bipolar distributed. Anyway, readers need to know such information. In Figure 1 panels A and B, the location of lat/long (0, 0) have occurrences. I think this is an error.

2.     In Table 2, I suggest to add one column showing the number of occurrences in GBIF or other information representing the abundance of the species. I think the species abundance is the key information for plant seed dispersal. It is more important than flyways.

3.     The physical characteristics of plant seeds should be introduced, at least discussed. Those seeds should be easily attached on the feathers.

4.     The distribution maps of the 10 shorebirds (excluding the one that had gone extinct) should be provided, using the occurrences as in Figure 1.

5.     Comparisons between plants and birds based on distributions are encouraged, which can provide quantitative values rather than Y/N.

6.     In Table 3, what “L” means?

7.     Line 169. Replace “ha” by “are”.

Author Response

1. Figure 1 shows the occurrences of three representative bipolar distributed plants. I suggest the authors provide the distribution maps of all 23 plants, ideally by a supplementary figure with 20 panels. I guess some plant may have wider ranges, which are not typical bipolar distributed. Anyway, readers need to know such information. In Figure 1 panels A and B, the location of lat/long (0, 0) have occurrences. I think this is an error. Reply - These three examples do represent very well all the observed patterns. We feel showing all 20 panels would be excessive. We are not sure what the second comment is about.
2. In Table 2, I suggest to add one column showing the number of occurrences in GBIF or other information representing the abundance of the species. I think the species abundance is the key information for plant seed dispersal. It is more important than flyways. Reply - these are the only locations where they are found. The flyway is important to show where they might migrate over the distributional pattern of the bipolar species. 
3. The physical characteristics of plant seeds should be introduced, at least discussed. Those seeds should be easily attached on the feathers. Our Reply - We will add a sentence to the text.
4. The distribution maps of the 10 shorebirds (excluding the one that had gone extinct) should be provided, using the occurrences as in Figure 1. Reply - The flyways listed in Table 2 represent the shorebirds distribution maps. The citation describes these in detail.
5. Comparisons between plants and birds based on distributions are encouraged, which can provide quantitative values rather than Y/N. Reply - We cannot do better than Y/N, because numerical data do not exist. We acknowledge this in the discussion. We have used descriptive data on habitats to make predictions about what bird migrations and plant species distributions overlap. Hopefully over time, researchers will use our predictions to gather this data.
6. In Table 3, what “L” means? Reply - limited. Reply - We will describe this in the Table.
7. Line 169. Replace “ha” by “are”. Reply - it was meant to be 'have'. We will correct it.