I don’t particularly enjoy reading scientific papers, but I always enjoy finding out how they were made. Last year, an international research team led by Trevor Price published a paper in the journal Nature about bird diversity in the Himalayas. One of the members of the team was Mousumi-Ghosh Harihar, then a PhD student at the Wildlife Institute of India. I spoke to Mousumi about the making of this paper and how she got involved in this project.
Here is a link to the paper:
Hari: Mousumi, what was your contribution to this study?
Mousumi: Figure 4b.
H: Intriguing! Tell us a little more.
M: I was not part of this study from the beginning. Yes, I was hired to do my PhD under this larger project at the Wildlife Institute of India (WII), looking at determinants of bird diversity in the Himalayas. And Trevor was my co-supervisor. But I wasn’t involved in the main piece of work presented in this paper. My PhD was focused only on warblers. What happened was that, Trevor, after analysing the data he and his colleagues had collected, found that species richness of songbirds in the Eastern Himalayas peaked at mid-elevations, around 1800 m. He was trying to figure out what explained this pattern, for example, did rates of species formation vary across elevation? I, at the same time, was analysing my data on arthropod abundances across the same elevational gradient. Since warblers are primarily insectivorous, I wanted to see if their diversity was linked to availability of food, i.e. arthropods. I found that arthropod abundances too peaked at mid-elevations (what is shown in Figure 4b). When I showed it to Trevor he had just made the plot for bird species richness, and he was testing all these other explanations, and they were all wondering what might explain the pattern since nothing seemed to fit the observed pattern. And then there it was, this arthropod abundance graph, and it matched perfectly with the bird species richness pattern!
H: And then you became part of the team?
M. Yes. I made the graph that appears in the paper and also commented on drafts of the manuscript. But before that, Trevor went back to field, to the lowest elevations, which I hadn’t sampled in my warbler work, and sampled arthropods just like I had done, to get the full gradient in arthropod abundance.
H: I can understand that arthropod abundance influences warbler diversity, but why should it be related to the diversity of all songbirds?
M: We censused birds in a number of five hectare plots at each elevation. Interestingly, we found that a major percentage of songbirds at all elevations were insect-eating species. That’s why it is not surprising that diversity of all songbirds is also influenced by arthropod abundances.
H: Do we know why arthropod abundances are highest at mid-elevations?
M: This result is a little surprising because the mid-elevation is not even the place which gets the maximum temperature and rainfall. What we believe is that the mid-elevations are the lowest elevations that freeze regularly in the winter, and thereafter experience a large flush of leaves and arthropods in the spring.
H: How important was this particular result – the link between songbird diversity and arthropod abundance – to the study?
M: Quite important I think. It suggested a role for competition in determining the diversity of songbirds and lends support to the idea that niche-filling is the process that has slowed down the rate of new species formation in the Himalayas. The question of what determines and controls diversity is a central one in ecology and evolution, with many explanations proposed. Our study provides evidence in support of one of these explanations, namely niche-filling.
H: Stepping back a bit, tell us a little more about the origins of this project? How did you get involved?
M: It started with my Master’s thesis project at WII, in which I worked on three species of wintering warblers in Himachal Pradesh and looked at how changes in resource abundance influenced the pattern of ecological segregation among them. During this time, I got in touch with Trevor and benefited a lot from discussions with him.
H: Why Trevor?
M: Trevor has been interested in warblers in the Indian region for a long time. He came to India, for the first time, back in 70s as a student taking a gap year from school and worked with the Bombay Natural History Society on a project looking at greenish warblers wintering in Andhra Pradesh. After that he went onto work in the Galapagos islands on Darwin’s finches with the famous husband and wife pair of Peter and Rosemary Grant. Trevor saw a lot of parallels in these two groups – the finches and the Himalayan warblers – and thought it will be interesting to compare whats happening on an island with whats happening in a continental system. Warblers are a particularly speciose group in the Himalayas with over 20 breeding species, and all of them are similar in the way they look and behave. Trevor thought it will be interesting to find out how this diversity came about. After he completed his doctoral studies he came to India – to Kashmir – to work on breeding warblers. That project met an unfortunate end when he and two of his students were kidnapped by a militant group in Kashmir, in 1991 I think. Fortunately, they managed to escape. Later Trevor found two other suitable sites – near Manali in Himachal Pradesh and Mahabaleshwar in Maharashtra – and has been monitoring wintering warblers there for over 25 years now.
H: How did the project on songbird diversity, in collaboration with WII, start?
M: Over this twenty-five year period, Trevor has had a string of graduate students and post-docs working on warblers and other birds in the Indian region. One of these students – an intern actually – carried out a project in which she gridded all of India into 50*50 km grids and calculated the richness in each. Interestingly, she found that, in the Himalayan grids, bird richness doubled from west to east. That motivated Trevor to write-up a proposal to study this gradient in diversity in the Himalayas. After some back and forth with reviewers of the proposal, it was finally sanctioned in 2006 and started in 2007 in collaboration with WII. Around the same time, Dhananjai Mohan and Pratap Singh, both Indian Forest Service officers and avid birdwatchers, joined WII on deputation and became co-investigators on the project. So things sort of just fell in place. For me too. I was just finishing my master’s at WII and Trevor asked me if I would like to join the project as a research fellow. I thought it was a great opportunity and said yes without much hesitation. Right from the beginning, Trevor encouraged me to work independently and develop my own ideas for my PhD dissertation. I decided to continue work on warblers and examine the influence of climate, ecology and historical factors on breeding warbler assemblages at different altitudes in western and eastern Himalayas.
H: How did the rest of the team come together?
M: Many of them are, like me, Trevor’s graduate students and post-docs who have contributed different kinds of data – field data, sequence data, measurements of specimens etc. Then there are other contributors like Per Alström and Farah Ishtiaq who contributed field samples and specimens for sequencing.
H: Give us a sense of the fieldwork and effort involved in collecting the data. What was a typical day in field like?
M: I used to get up at around 4.30 in the morning, have a glass of tea and some biscuits and head into the field. For 2-3 hours I would walk along a pre-chosen trail and count warblers. Then I would head back to my camp, have a quick breakfast and then head out again to make behavioral observations of warblers along the same trail. After finishing behavioural observations, on the way back, I would sample for arthropods along the trail.
H: How did you do that?
M: Every 50 m along the trail, I would select a shrub or bush, select a branch on that bush which was within the range of heights at which warblers feed, put a 15 litre black garbage bag around that branch, tie it, and break it off. In this way I would collect 10-12 branches on my way back to the camp. Once back at the camp, I would drop a ball of cotton soaked in chloroform into each bag, tie it up and let it remain for two hours. In those two hours I would go back to field to do more behavioural observations on warblers. After two hours, I would come back, open the bags and, using a pair of forceps, I would search every leaf and bark and carefully spread all arthropods on a sheet of paper. Then using field keys I would identify the order to which each arthropod belongs and get counts for each order. This information on orders was not used for the Nature paper – we only used total counts of arthropods here. I also measured the length of each arthropod using a scale and got a crude estimate of the volume of foliage on the branch, using the dimensions of the branch and the lengths and breadths of five leaves. The information on arthropod orders and measurements was not used in the Nature paper. We only used the information on total arthropod abundance in each site, here.
I did my field work over four summer seasons and sampled 16 sites across the Himalayas across nine elevational zones. In each site I would spend 3-4 days on a trip, but also visit it a second time to sample to check if patterns are consistent. In total I collected 535 branches and counted over 6000 arthropods. I have even written up a paper focusing only on the arthropods, although that wasn’t my primary interest. The motivation came after hearing a scientist say, at a panel discussion on ethics in research and conservation, that he felt terrible about collecting lots of arthropod samples during his fieldwork on birds and never doing anything with it. I didn’t want to end up feeling that way and so decided to write-up this arthropod paper.
H: That is a lot of work! Can you give a list of all the different kinds of data collection and analysis that went into making this paper?
M: Trail walks to count birds, five hectare plots for spot mapping of birds, mist netting to get feather and blood samples for sequencing, call playbacks to attract birds to the mist nets, measurements of specimens in museums in USA, UK and Sweden, and lab work to obtain sequences.
H: What did you enjoy the most about being part of this study and the ensuing paper?
M: It is not often, especially early in your career, that you get a chance to be involved in a big global collaborative project. I think I am lucky that I got this opportunity, to collect so much data, play around with it, and see how it can be used to meaningfully test hypothesis. In that sense it was very useful. Also, it was a great learning experience to see the paper evolve; watching the process was exciting.
H: Among all the papers you have published is this your favourite?
M: No. My favourite would be the paper that appeared in the Journal of Biogeography. I wrote that up all by myself. I like it mainly because it is based on work that I had not planned to do, initially. It was an idea that I got after reading some papers, and it turned out to be quite exciting.