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The Habitable Planet: A Systems Approach to Environmental Science 

Ecosystems Interview with Stuart Davies

Interviewer: What is your position and who do you work for?

STUART: I am the Director of the Center for Tropical Forest Science, which is a group who study the rainforests of the world. We are interested in long term studies of how rainforests function and how they are being maintained.

Interviewer: How many different sites around the world are working on this study?

STUART: We have a very large network now. We work in fourteen different countries. We have seventeen research sites. We have two in Africa, nine in Asia, and another five or six in the neo tropics. In each of those countries we partner with researchers, partner with students, forest departments, and we work together as one very large network to try to understand how rainforests function and how they are going to change in the future.

Interviewer: How much tropical rainforest is involved in your global network of researchers?

STUART: Our basic research program is to study large pieces of forest. That means we set up these research plots which are fifty hectares in area, typically – that is a kilometer by half a kilometer – in which we study every single tree species that occurs in those areas. It is a very intensive way to study rainforests and we hope, by sampling representative biodiversty in forests around the world, we will be able to understand the major processes that affect forests.

Interviewer: Why is the tropical rainforest important?

STUART: It is hugely important because as you probably know, the vast majority of biodiversity in the world is in tropical rainforests. Tropical rainforests have a very, very important function and that is to keep our climate functioning. They produce a lot of water, store a lot of carbon, and help modulate or protect the climate of the globe. So it is a hugely important ecosystem.

Interviewer: How many different scientists are working on research for CTFS (The Center For Tropical Forest Science)?

STUART: Well over a hundred scientists are now working with CTFS throughout the world. We have scientists from the United States, we have scientists from Europe and Australia, but we also have many, many scientists now – and this is a very important mission of CTFS – from within the tropical countries where we work. We also have an increasing number of students and post-doctoral researchers and faculty researchers doing all kinds of studies in their own forests.

Interviewer: What are the big research questions that everybody is trying to work on?

STUART: The major research question that motivates our program and has since the start, is why there are so many species of trees in tropical rainforests. We have, in some of our research plots, more species of trees than the whole of North America or the whole of Europe and that is a phenomenal number of species. A thousand species or twelve hundred species in one plot and we want to know why, how that is maintained, and how that is potentially going to be changing with the future changes in climate.

Interviewer: Describe the worldwide census project.

STUART: What we do is we set up these large plots and we monitor them every five years. We go and re-measure all the trees. The idea is to try to work out for hundreds and hundreds of species the population biology, that is the rate at which they grow, the rates they die at, to try to understand the whole biology of the species. And to do that we need large numbers of individuals monitored over a long time period. So for each of these seventeen plots, we go in, we measure all the trees. That can be three hundred and fifty thousand trees in one plot. Then we come back five years later, do it again, and that gives us a very detailed set of information on how the forest is changing and what species are becoming more common and what species are becoming more rare. We do that in every place. The idea of doing it right across the whole world is that then we will be able to come to some generalizations about what is happening to tropical rain forests. We couldn’t do it in one place because it wouldn’t give us the generality we need. And so we are doing it right across the world to try to understand what we call global processes.

Interviewer: How many tree species and individual trees are you studying?

STUART: Across the seventeen sites we are monitoring about 6300 species of trees. At any one time, we have about 2.5 million trees living in the plots. Since the start of the program, we started doing this in 1980, we have monitored something in the order of 7 million, 7.5 million trees. So this is an extreme, this is a huge data set; no one else in the world is doing, this kind of research.

Interviewer: Does a person actually measure every single tree?

STUART: We do it all one by one. We go out. We start at the corner of these big plots and we tag and map and measure every tree bigger than a centimeter in diameter. That is as big as a little sapling in your garden. And we monitor everything. We measure the big trees. We measure the little trees. Every single tree. So we have, as I said, in one plot, three hundred and fifty thousand stems and it takes a team of about twenty five people. They are usually students or research assistants or volunteers in some cases, and we work as a big team and we just count our way through the forest, measuring every single tree.

Interviewer: Why are you concerned with tree species instead of animals when studying why the rainforests are so diverse?

STUART: One of the famous ecologists said a couple of years ago that trees in tropical rainforests are like,the ecosystem engineers; what he meant by that is they provide the structure. They provide the habitat on which all of these organisms depend. So monkeys that live up in canopies of trees, go extinct if you take the trees away. Ants and termites are going to be absent if the trees disappear. So we are very concerned about forests because they create the structure. The trees create the structure in forests and therefore, they are central to this whole ecosystem.

Interviewer: What are you researching now in tropical forest science that will tell us things that we didn’t already know?

STUART: There are a lot of different things we don’t know. Possibly one of the most important issues we are researching at the moment is what is the impact of increasing carbon dioxide levels in the atmosphere on the functioning of tropical forests? We all know that human activities are producing increased levels of carbon dioxide in the atmosphere. There is a hope that rainforests and other kinds of forests will take up that carbon, take up that additional carbon, and that will reduce the impact of that carbon on the rest of our world. We have been doing research on these forests by measuring trees. Trees are fifty percent carbon, you have to remember. Trees take up the carbon through photosynthesis. We want to know if the elevated carbon is being taken up by the trees. And we are doing some work right now that shows that may not be the case. We haven’t got strong evidence for it but we don’t think that forests are actually acting as what they call a sink, that is they take up this additional carbon. And this is a very important area of research that we will be putting a lot more of our effort into in the future.

Interviewer: What is being studied in tropical forest science now that will tell us things we didn’t already know about diversity?

STUART: We are doing a lot of work at the moment on trying to understand the various factors that explain the coexistence of species. That is what we call the maintenance of diversity in rainforests. There are a couple of very important studies going on in these plots that are going to give us a lot of information in this area. One of them is to study in great detail the soils of these forests. Different plant species require different soil resources, soil nutrients in which to live. We want to know to what extent all the different species use different sets of soil nutrients. So for ten of our big plots right across the world we are doing very, very intensive studies of the soil. That means measuring nitrogen and phosphorous and these important nutrients for plants. Then we map the distribution of all the tree species to understand the correlations between the trees and the soils and then we get at understanding diversity.

Interviewer: At all your different sites across the world, are you studying the entire tropical forest that is within that area?

STUART: Tropical rainforests are extremely diverse. They vary from coastal forests up through mangrove forests through lowland forests, up mountains, and then on specialized kinds of geologies, kinds of rocks like limestone and so on. We decided at the start of this program to study the most representative, the most common type of rainforest and that is lowland forests. Lowland forests are the most important in several different ways. They are used by people extensively throughout the tropics for a whole range of services, timber, food products, fire wood. We wanted to understand how forests function in the way that will impact people the most or people will benefit from the most.

Interviewer: So all of your sites are in lowland forests? How well does that cover the diversity of trees in the world?

STUART: Yes. All of our sites are in the lowlands and we have 6300 species in our research network, which is about fifteen percent of the total expected tree diversity of the tropics. So we are now getting information on a very large fraction of the whole diversity of the tropics and that is an undertaking that has not been done before. One of our major supporters says it is like putting a man on the moon. He says you want to do something in science that is creative, that is completely novel, and he says well, let’s go out and learn about a quarter of the diversity of the tropics. And so that is our ambition in the next five years or ten years. We hope to have twenty five percent of the tree diversity of the world, of the tropics, in our research plots.

Interviewer: What percentage of the rainforest are you studying?

STUART: In each of our research areas, we work in a national park or a forest reserve, which is a protected forest. That is important because we want to have these forests monitored for a long time, but we only sample a small part of that. The key is that we sample a very large number of individuals so that we learn about each of the species inside that plot. But a single plot may be less than one percent of the whole forest area. That is not so important as getting a large number of samples of each species.

Interviewer: If there is so much diversity, why do you have to study it? Things seem pretty healthy on their own.

STUART: Well, you know that over the last three decades, the loss of tropical rainforests has been significant. And so if we don’t understand the biology of these species in their natural habitats, I don’t think we have any chance of understanding how they are going to respond to disturbances, to human modifications of those habitats. So it is crucial. We take the biology of these species from the natural forest and we are doing a lot of work now. It is another important area of our program to apply that knowledge in degraded lands, to apply that knowledge in human used lands, to provide foresters in many of these countries. They want to know how to grow these trees and the place to start is their natural conditions.

Interviewer: What if tropical forests just went by the wayside? We would be all okay, wouldn’t we?

STUART: My personal point of view is that I am deeply committed to this environment. I think you guys, in coming here to film us, see what a beautiful place this is. We have now many, many, many, many people coming into these forests and enjoying them for what they are. I think that is a very basic human requirement, to have these places available for us to visit. That is a sort of personal view. At a global view, I think we have a responsibility to protect these ecosystems because they are so fundamentally important to regulating global climates. We already see changes in the climate that are affecting the United States and other developed countries. And we need to do what we can to understand these rainforests.

Interviewer: Are there roughly the same amount of individuals of each tree species in tropical rainforests, or is there variation?

STUART: In each of our sample plots, these big areas of plots we are monitoring, there are species that are quite abundant. They may account for ten or twenty percent of the total species in the plot. And then there are species that are extremely rare. Let’s take this forest for example. Within the Barro Colorado Island fifty hectare plot we have a common species which accounts for about fifteen percent of the individuals. But then, of the three hundred species in the plot, about fifteen or twenty of them are represented by just a single individual. Our research is interested in both ends of this. We are interested in what controls the common species from becoming more common. Why aren’t they more abundant? Why don’t they just take over the whole forest? And on the other end, how do these individual species with just one tree, how do they survive? How do they reproduce? Both of those questions are very important and we are doing work on both of them.

Interviewer: How much of the rainforest do common species of trees represent?

STUART: Common species in these very, very diverse forests- there is no common species in the way that a red maple is a common species in a New England forest. But the common species on BCI (Barro Colorado Island) accounts for maybe fifteen percent of the stems.

Interviewer: What percentage do rare species make up of tropical forests?

STUART: In very diverse forests rare species are very important in terms of the amount of diversity. Within the Barro Colorado Island plot, about twenty percent of the species, that is about sixty species, have fewer than ten individuals in the whole plot, fewer than ten individuals in a hundred and twenty acres of forest.

Interviewer:: Which have a better chance of survival, common species or rare species?

STUART: Our concern is what happens to rare species. To reproduce they need to find their mate. They need to receive pollen from a mate of the same species and the question is if they are very, very rare, if their individuals are very far apart, how does that happen? And if they don’t reproduce, do they become extinct within this ecosystem? At the moment, there are two hypotheses about how rare species get maintained. One theory is that the pollinators may be very mobile. They may be able to travel large distances to find their mates, to find the individuals of the same species to reproduce. The second one is that they may be self-fertile. That means they may just reproduce themselves and they don’t need any other individuals nearby. But we have just started research on trying to understand that question.

Interviewer: What are some interesting results you have found in your census?

STUART: One of the things we have found in our Indian plot is that one of the very common species, which had about fifteen thousand stems at the start of the census, has gone to only a couple of hundred stems fifteen years later. The question is wow, that is a very big change in abundance- Why? And it appears in that particular forest, fire is a very important ecological driver. It is a natural ecological phenomenon. Fire kills these trees and unless the interval between fires is long enough, the recruits, the new individuals can’t re-spread, can’t regenerate. It appears that over the recent past, fire frequencies have been very regular and this species hasn’t been able to regenerate.

Interviewer: Are rare species all found in the same area?

STUART: No, quite the contrary. Rare species are often—not always—but often distributed sparsely throughout the forest. And so, one, they are hard to find. And two, often they are very poorly known. And something about plots, we have twenty percent of the species that we are not completely sure what the names are. That means that a lot of species in these plots, this is a rainforest in Borneo, in the Amazon, a lot of these species are new to science. They haven’t been described yet. These rare species are the difficult ones. They are the ones you don’t have many samples of.

Interviewer: If all rare species died out, would things be okay?

STUART: We don’t know the answer to that. We don’t know the answer and so our view is that one needs to be cautious in that kind of argument because they could be very important parts of an ecosystem. In addition, they could have very important potential uses from a forest. Local people may use them for different things. We don’t know anything about their biology. So we don’t know anything about their chemistry or their potential uses or their potential benefits to human society in the future. So, it is in our interest, in my opinion, to protect them and to continue to understand their biology.

Interviewer: Describe the plot here on Barro Colorado Island?

STUART: The plot here on BCI is a very large plot. It sits up on the top of the island on a flat plateau. It is as I said, one hundred and twenty acres in area. That is a kilometer or a thousand meters by five hundred meters in area. Every single tree in sight is mapped, tagged, measured, and identified to species.

Interviewer: How is this plot here at BCI representative of all of the rainforest sites?

STUART: A very important axis of variation in rainforests is how wet they are. Rainforests are not all completely wet all the time, which maybe a lot of people might think oh, rainforests are always wet. That is actually not the case. Rainforests go from being extremely wet with ten meters of rainfall a year to quite dry with one meter or one point five meters of rainfall a year. The rainfall is also distributed across the year in different ways, in different forests. Some forests, like the Amazon, are every month wet. In BCI, we actually have a four month dry season, four months in which there is very little rainfall, and then the rest of the year is very wet like you experienced today.

Interviewer: Is it just one person going up there and doing all this work, just taking data down on each tree, and coming back?

STUART: When we do these censuses, we have a team, usually about twenty people who go out and work in a group right across the plot. They survey all the trees, they collect the specimens, they tag the trees, they tie a little piece of aluminum on the trees.

Interviewer: Can you describe what other data you are collecting?

STUART: In addition to measuring the trees, we take a lot of climate information. We want to know how much rain is falling. We want to know the conditions of humidity. We want to know the temperature variation. All those variables affect forests and how they are functioning. We also do very detailed soil maps right across all these plots because we want to understand where the species or specialists of high nitrogen soils or high phosphorous soils are. So we have a very detailed soil map. We also map the topography very carefully. That is, the little hills and valleys across the plot are all mapped in great detail.

Interviewer: Why are you studying seeds and seedlings?

STUART: Some of our research involves looking at the very, very small plants, that is the seeds and the seedlings, because we want to know what the future forest is going to look like, and the future forest is the seeds and seedlings that are germinating. We have a number of studies where we map very small patches of the forest to monitor these little seedlings. We want to know if the seedlings have the same composition or the same species diversity as the canopy because we want to understand how the development of a forest occurs.

We also want to see if a particular species produces a lot of seeds- millions and millions of seeds – and they all fall to the forest floor, why don’t they all germinate? Why don’t they all grow up and become the dominant species in the forest? And so we are doing quite a bit of research to understand what controls the next generation. What kills the seedlings, because only a tiny fraction, less than one percent, less than point one percent of the seedlings,` become big trees. Why doesn’t the abundant species become, like a pine forest? Why doesn’t one species win out? Why doesn’t the common species just become the dominant? And then all the rare ones would go away. Why doesn’t that happen?

Interviewer: So, why don’t the common species just win out? Why is there diversity in the first place?

STUART: One of the very important areas of work we have been doing over the last twenty years, and that has been driven by work here at BCI, is to ask why don’t common species take over. And the reason is that common species, when they produce seedlings, they produce more seedlings than rare species. But the common seedlings, get affected by pathogens and pests at a very heavy rate. You can imagine a specialist pathogen or predator that likes a particular species will come into a little patch and it will eat all the seedlings. It will kill them all. We call this density dependent processes. That means if you are at a high density, you suffer very high mortality. You die more often than if you are rare. And so this balances diversity. No species can become common because if it tries to become common, which they are all trying to do, its seedlings suffer very high death rates.

Interviewer: Are there any other reasons why you think the forest is so diverse?

STUART: I think that this idea of high density species suffering high mortality is part of the story. There is also the issue that we spoke before about species being specialized to individual habitats within the rainforest, that is they are specialists of gaps. We see an open gap here. There are species that specialize in those gaps and they don’t grow anywhere else. Then there are species that are specialized on wet ground and species that are specialized on high phosphorous areas. That also promotes diversity because there are diverse areas within a forest where different species can live. So that is another area of our research.

Interviewer: It seems that you already have a good idea about why tropical forests are so diverse, so is there room for another generation of tropical forest scientists?

STUART: There is a huge, huge amount that we still don’t know. We don’t know a lot about the biology of most of these species. We don’t know a lot about the animals that eat or feed or live on most of these species. So there is a vast amount to know. We have programs to engage students in the research. We have a lot of volunteers that come down to BCI. We have a lot of volunteers in all of the different countries where we work who come and work with us, who learn scientific techniques, and we run also a range of courses to bring students in so they learn about rainforest biology.

Interviewer: How does the tropical forest research affect the surrounding communities?

STUART: That is a good question. Many of our forests where we work are surrounded by people, in some cases, people live within the forests. They extract resources like they take firewood or they take timber or they take fruits or they take animals for subsistence. It is in their interest to make sure that that extraction is sustainable in the long run. So we are doing research to try and understand how much food, how much wood, how much firewood these people can take but still not affect the long-term sustainability. So it is very much in the local country’s interest to understand that. Within CTFS we have two programs that I think also benefit our local partners very, very well. One of them is an educational program. We do a lot of work with in-country researchers and students to train them in the latest techniques in science of tropical forests. The other thing we do is we have a very important program to take the knowledge from the primary forest, this forest we are standing in, take that to degraded lands or heavily utilized lands so that people can restore and regenerate these forests or parts of these forests, for example: how to grow the most important timber species in an open area.

Interviewer: Can you talk a bit about the diversity of plant life and animals?

STUART: In addition to the trees, tropical forests house a huge diversity of other plants and animals. There are all kinds of plants that grow on these trees. There are little orchids that grow up on the top of the canopies. But then, for every plant species in this forest, there is at least ten or fifty species of insects, different kinds of bugs and butterflies and other insects that grow on the trees. So there are thousands and thousands of species of insects. And there are also hundreds of species of birds. There are many, many species of primates, monkeys, all kinds of different organisms. You have seen probably I think the agouties, these small animals that run around the forest floor and eat seeds, very important part of this rainforest structure.

Interviewer: Who are the different scientists you work with?

STUART: One of the nicest things about our network of scientists is that we are international. We are very, very diverse, and come from all over the world. We come from Africa, Asia, and the Americas. My PhD supervisor Peter Ashton at Harvard, Stephen Hubbell, a professor at Georgia, were the founders of this program and continue to be very much involved in training and giving ideas about what we should be doing, what kinds of science we should be doing. And similarly, my students and the researchers who work with me, are taking on that tradition. So it is a very nice network of researchers.

Interviewer: Within the tropical forest, what species are you most enthused about studying?

STUART: When I first started working in rain forests, the thing that really excited me was what we call gap phase dynamics. That means a tropical rainforest is not like a pine plantation. It is not this kind of even balance of pine trees all about the same size. It is a very jumbled up canopy of different size trees and every now and again, a big tree will fall over and create a huge opening. And in that opening, a rush of trees germinates. We call those trees—they are very fast growing—we call them pioneers. They pioneer the new gap. I have been working on them a lot myself ever since I started in biology and I am interested in what causes those species to come in. It is a very important process because when you cut down a forest, when you lose a forest, the pioneer species are the ones that can start a forest again. And so, we are very interested in why, what controls these species, and how can we grow them.

Interviewer: Can you describe one of the pioneer species?

STUART: Yes. I have also done a lot of work on one of these pioneer trees. It has a very specialized relationship with ants. Ants live inside the stems of this tree and they actually defend the tree from herbivores. Herbivores are the little insects that eat leaf tissue. And these ants come along and whenever a herbivore lands on a leaf surface, the ants run up and they kill or cut off the little insect herbivore. I have been doing a lot of work trying to understand how this system came about because we don’t see it in the temperate zone. We don’t see it in North America – ants living inside tree stands and the plants producing little food bodies that feed the ants. They have got this nice symbiosis and we are doing a lot of work to try and understand that.

Interviewer: What got you interested in science?

STUART: I got interested in science when I first started traveling around Australia and then outside of Australia. Eevery place you go there are all these interesting birds and these interesting forests and trees and this enormous diversity and I started to just think about it. I love hiking and going and visiting different forests and I started to get interested in rainforests.

Interviewer: What is it like actually doing research in tropical forests?

STUART: It is sweaty. You feel it now I think. It is hot. There are bugs but there is a great thrill to being in the rainforest, to being in the middle of Borneo where I did much of my early work. It is a great thrill to be out in the middle of the wild of probably one of the least known places in the world. It is for me incredibly exciting. It is like you are on a different planet almost. Every student we take out to those places, every student that comes to BCI itself, really enjoys it. They just have I think a life changing experience.

Interviewer: Are tropical rainforests changing?

STUART: Yes. There is very strong evidence coming out from our plot network that rainforests are undergoing certain changes. Firstly, there appear to be changes in forest structure. There is significant evidence that there is a change towards more pioneer species in tropical rainforests over the last couple of decades. There is also a change in climbing plants; these plants that climb on trees are also becoming more abundant. Those two are both very important because this could lead to a reduced structure of the whole forest.

Interviewer: Given the current state of the environment, what do you see looking forward?

STUART: I am very optimistic about the future of tropical rainforests because I believe that people within the countries we are working have a growing concern about their natural heritage. Their individual countries are growing more and more concerned about the loss of biodiversity, the loss of forest cover just like we have in the U.S. And so I see that concern growing and if that is growing, then there is more pressure on governments. There is more pressure on states to make changes, to conserve more of their natural heritage. I am optimistic about the future because I believe that people will make the right decisions in the future.

Interviewer: What is it like actually doing research in tropical forests?

STUART: It is sweaty. You feel it now I think. It is hot. There are bugs but there is a great thrill to being in the rainforest, to being in the middle of Borneo where I did much of my early work. It is a great thrill to be out in the middle of the wild of probably one of the least known places in the world. It is for me incredibly exciting. It is like you are on a different planet almost. Every student we take out to those places, every student that comes to BCI itself, really enjoys it. They just have I think a life changing experience.

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