In 1799 Alexander von Humboldt went to see the world. The Sun fell straight down in front of his ship's bow, and moonlight rose all around him. He watched great pods of whales jump from the sea and surveyed the beauty of night skies bright with migrating stars. More striking to Humboldt than the beauty of the world, however, was the bounty of life it held. And more specifically, the patterns he saw in the distribution of life. The nearer he approached the tropics, he later wrote in Ansichten der Natur (Views of Nature), the greater "the variety of structure, grace of form, and mixture of colors, as also in perpetual youth and vigor of organic life." Humboldt had discovered the latitudinal gradient in biological diversity. All it took to see the pattern was traveling south for a few years. But as the next 200 would show, that was to be the easy part.

Time has added both detail and exclamation points to Humboldt's initial observations. We can now map the patterns of diversity in mammals, birds, amphibians, and reptiles for the entire world. Nearly all groups of organisms, from foraminifera to frogs, are most diverse in the tropics. And that gradient has great implications. There are not only more species in the tropics, but there are also more potential medicines (and conversely, more diseases), fruits, cultures, and languages. The gradient in diversity that Humboldt detected shapes human life, from our economies to our well-being.

In the article, "The Trouble with Biodiversity (link)" published in Seed Magazine I explore whether we will ever be able to say with certainty and consensus what governs large-scale patterns of biological diversity. As part of research for the article, each scientist below was asked a series of questions about their experience with and thoughts on patterns of biological diversity. Citation: Dunn, R. R. 2008. The Trouble with Biodiversity. Seed Magazine. October. If you study large-scale patterns of species diversity and would like to add your response, please email me at Rob_Dunn "at" ncsu.edu.

"I also work on questions that one may regard as more pressing, related to the conservation of species and restoration of ecosystems. However, I think there are important lessons to learn on how to conserve biodiversity if we understand the processes that shaped biodiversity in the first place. For example, taxa may be differently vulnerable to human threats depending on how they evolved and the challenges they have experienced in the past, in terms of e.g. climate, competition and predation. This is how I rationalize my occupation with something I find immensely interesting. I always found the fact that we don’t have a satisfying explanation to the most obvious biodiversity gradient (the latitudinal gradient in species richness) a fascinating challenge and a cause for curiosity. It was on a study trip to South Africa as PhD students that I and my colleague Mats Dynesius started to contemplate the role of climatic variability. We compared the floras of Scandinavia and the Cape Region and pondered over the reason for the dramatic difference in plant diversity between the regions to the advantage of the Cape."—Roland Jansson, Associate Professor, Department of Ecology and Environmental Science, Umeå Universitet, Sweden.

" I went to school in Arizona and did my Ph. D. in Costa Rica, so I experienced both ends of the global diversity gradient first hand. By the time I was a global change postdoc, I knew that most of what we knew about diversity gradients came from trees and vertebrates, and that you could get more good data on ants in day than you could get on birds in a month. When my proposal to survey the New World ant diversity got funded, I was able to travel (pre-911) from Oregon to Panama, dropping into habitats I'd only seen pictured in books, and processing the dirt and litter in grimy hotels while watching HBO. And when I got home, I would have long delightful hours at the microscope staring at ants. It continues to be a lot of fun."

—Michael Kaspari, Professor, Department of Zoology, University of Oklahoma

"I was interested in evolutionary biology, but didn't want to study a detailed aspect of one particular species, but rather try to understand the big picture. In the big picture, species richness gradients are quite prominently present. "— Folmer Bokma, Department of Ecology and Environmental Sciences, Umeå University

"Well, because that is what ecology is about: it boils down to two questions: how many individuals and how many species? And when you think about all the contexts in which you can ask those questions, the global context is the most exciting because of size: it is very obvious and therefore from an inquiry perspective would need to be understood. Understand the big picture first, or, to quote donald trump, if you're going to be thinking anyway, think big." Peter Mayhew, Department of Biology, University of York.

"I started out as a paleontologist interested in historical patterns of diversification but realized that I needed to understand the ecological and spatial patterns first in order to make sense of the temporal dynamics. That was what initially got me into this. But over the last decade or so my interest in diversity gradients is closely tied to our attempts to better understand the biological effects of climate change as well as how human exploitation of marine species (i.e. fisheries) is affecting biodiversity patterns in the ocean. These days I spend more time on those two (more depressing) questions than I do on the lat gradient work but obviously they are all closely tied together. Clearly effective conservation and management strategies cannot be developed unless we understand the processes that produce biodiversity patterns." –Kaustuv Roy, Professor of Biology, University of California, San Diego.

"Again two parts:

1) I like to tackle challenges, especially longstanding ones that have eluded explanation by many previous scientists for long periods. This is one of the greatest challenges an ecologist - or any scientist - could address.

2) Ever since I was a kid, I have always been enthralled by biodiversity. So I am privileged to be studying what I love."

—James Brown, Distinguished Professor, University of New Mexico.

"I spent over six years at the University of Malaya and was amazed about the huge diversity in tropical rainforests, compared with what one finds in Germany, where I came from. Later, I spent three years as Director of the Heron Island Research Station on the Great Barrier Reef, where I was impressed by the enormous diversity on coral reefs. I asked myself: why? At the time, I worked on parasites of marine fishes and found that fish gills are an ideal model to study the causality of biodiversity. Fish gills from low and high latitudes do not differ in size, heterogeneity and "productivity". Nevertheless, gills of tropical fish harbour vastly richer parasite communities than those of cold water fish. This eliminates many of the hypotheses supposedly explaining diversity gradients, at least for fish parasites." --Klaus Rohde, Professor Emeritus, University of New England, Armidale, Australia.

"My interest in spatial gradients originally comes from a very local perspective. While analysing factors affecting local diversity in my PhD thesis, I became interested in the role of dispersal and regional species pools for these interaction (kind of a metacommunity perspective). From that I started to investigate the processes affecting diversity turnover in space and time. The ultimate driver of this research, however, is the notion of rapidly changing diversity worldwide and the inability to adequately answer the question whether this actually matters for ecosystem functioning." –Helmut Hillebrand, Associate Professor, Institute for Botany, University of Cologne.

"I have been always fascinated by sites with high diversity, and the differences in species richness between different sites is clearly very conspicuous pattern, which itself asks for an explanation. Ecology studies the reasons of differences observed in nature (e.g. differences between sites, biomes, species etc.) and differences in diversity seem to be extremely important. Also, diversity study is related to the question of species coexistence, which is a crucial problem apparently overreaching pure ecology. Last but not least, diversity is where ecology and evolution meet each other. Personally, I have been strongly influenced by Rosenzweig's book Species diversity in space and time, because thanks to this book I have realized that there are many fascinating questions so far unresolved."—David Storch, Associate Professor, Department of Science, Charles University.

"Because it's interesting! And a major unresolved question about the diversity of life."

—Tim Blackburn, Head of the Institute of Zoology, London.

“It is the 'biggest' question in ecology (broadly defined). It is also the longest and best known pattern in nature and is probably the only pattern known by non-scientists. Big questions interest me.”

—Bradford Hawkins, Professor, Ecology & Evolutionary Biology School of Biological Sciences, University of California, Irvine.

"My primary interest is in diversity: how does it vary in space and time, and what controls local species number. So, the latitudinal gradient is a natural extension of those questions to large spatial scales."

—Nick Gotelli, Professor, Department of Biology, University of Vermont.

[Prof. Nettle studies and is referring explicitly to human cultural and language diversity.]

"I got interested in language diversity gradients because social scientists don't think about ecology. They think about political and historical factors, but don't link these to the broader ecological context. Language gradients are a nice example of how distribution of humans is patterned in similar ways to distributions of other creatures."

—Daniel Nettle, Reader in Psychology in the Centre for Behaviour and Evolution, Newcastle University

"Paleontologists like to interpret the depth at which ancient sediments were deposited. The farther back you go in time, the fewer modern species you have to act as indicators. However, if you can establish a pattern of diversity that is not dependent on particular taxa, you can make an interpretation. So I studied modern faunas to look at diversity gradients."

—Marty Buzas, Senior Geologist in the Department of Paleobiology at the National Museum of Natural History, Washington, D.C., U.S.

"My long-term study site in the Great Victoria Desert of Western Australia supports 55 species of lizards, an almost unbelievable species richness. Similar flatland desert study sites in North America have only 5-10 species whereas those in the Kalahari Desert in Africa support only 11-16 species. Understanding these differences constitutes a challenge I simply cannot ignore. When I began studying these systems in the early 1960's, I knew little and expected easy answers. Now, after devoting my life to trying to understand lizard diversity, I have many more questions than answers. Science is counterproductive in that, the more you know, the more acutely aware you become of what you do NOT know."

—Eric Pianka, University of Texas

"Perhaps May (Sir Robert) put it best when he simply asked "How many species are on Earth?" The answer seems so deceptively simple! It is because diversity is so complex yet so intriguing an academic question that I continually revisit it. My background is as a deep-sea biologist. The two most conspicuous patterns in the deep sea are elevated species richness (rivaling that of tropical rain forests) and gigantism/miniaturization. My research focuses on both these in part because of this and part because we don't know anything of how they arise. I believe that body size and biodiversity are the two most fundamental factors of our field (at least that's what I always say in the introductions of my papers!). All other factors somehow relate to them or are umbrellaed under them."

—Craig R. McClain, Postdoctoral fellow, Monterey Bay Aquarium Research Institute, California.

"Why do I study diversity? You might as well ask why I so love my grandchildren! Passions cannot be rational, I guess."

—Michael Rosenzweig, Professor of Ecology and Evolutionary Biology, University of Arizona.

"In my PhD, I was quite focused on population genetics, evolution and multivariate morphometrics, and was basically interested in the application of spatial analyses to biological data, as started by R. R. Sokal. Then I found a permanent position to teach ecology and slowly started to look for research in this field. Because I had a direct interest in macroevolution and phylogenetic comparative methods, I was attracted to macroecology and then it was a short jump to diversity gradients. But this occurred mainly because my colleague Brad Hawkins from University of California asked me to help him with spatial analysis, in summer 2001. After that, I completely shifted my interests to this new field."

—Alexandre Felizola Diniz Filho, Professor, Universidade Federal de Goiás, Brazil.

"This goes back a long way for me. I graduated in Geology from the University of Wales at Aberystwyth and just knew that I wanted to be a paleontologist. But although I knew all the graptolite zones of the Rheidol Gorge and trilobites from the Harlech Dome I knew absolutely nothing about the larger scale patterns of life on Earth. And then I went to the Natural History Museum in London to do my PhD and found a whole new world of excitement. They asked me to work on coral reefs and I had simply never seen so many different types of plants and animals. My Advisor made sure that I tackled some of the basic questions about tropical high diversity and I knew I was hooked. Even when I moved to BAS I never lost my interest in largescale patterns and how they may have formed. I looked into the evolution of bipolar taxa and how they may have differentiated through time. These also involve some form of dispersal-assembly mechanisms, and possibly shifts in major climate belts. I built up quite a database of bipolar taxa and it seemed to me that, no matter how far back in time you went, high-latitude and polar biotas were always of lower diversity than tropical and low-latitude ones. This brought me right back to gradients and with the advent of powerful databases the possibility of compiling the sorts of inventories that are necessary to look at them in a rigorous way. You’re looking at a problem here that affects so many aspects of biology and geology; you can go back to the fundamentals of both rates of evolution and plate tectonics, and combine modern observation with ancient theory. A molecular phylogeneticist needs the help of palaeoclimatologist, and the person studying angiosperms needs to compare patterns with colleagues studying snails. This is interdisciplinary science at its best, and the chance to get to grips with some truly global-scale processes. Darwin, Wallace and Lyell all knew that this pattern was important and it has been a cornerstone of biogeography (macroecology, if you will) ever since."

—Alistair Crame, Division Head Geological Sciences Division Cambridge University.

"A complex question, of course. I have always been fascinated by Earth's past and present biodiversity. This devloped from an initial childhood interest in dinosaurs etc to an ever increasing interest in present-day biodiversity and its near-past background (Cenozoic and especially the Quaternary). Combined with a family tradition for working internationally and especially in tropical South America, this inspired me to do a PhD and subsequent postdoc in tropical plant community ecology; I still work in this field, but based on my longer-term interests in combination with what I learnt from these tropical studies, plus an interest in making a difference in terms of nature conservation and people's view on nature, my research have increasingly been focusing on macroecology as well as broadened its organismic and geographic scope."

—Jens-Christian Svenning, Associate Professor, Department of Biological Sciences, Aarhus Universitet, Denmark.

"As the child of ecologists, I always have been around science and have been fascinated by organisms as long as I can remember. The more I traveled while growing up, the more I become aware that the environment and organisms changed as I rode across the U.S. on long car trips, and that there were particular floras and faunas in different regions. As I proceeded through college and graduate school, my interests in the relationship between organisms and their environments developed. At the same time, I was introduced to the concepts of biogeography. I become more intrigued by the way that species, their traits, and their distributions were mapped across space and the environment; later, I became interested in how they were “mapped” over time as well. From these general ideas arose several major questions on which my current research focuses. I study these gradients to try to understand the fundamental principles of ecology and evolution, as well as to understand what ecological rules underlie patterns of distribution and diversity; how ecological communities and continental biotas are assembled; how diversity has changed through paleontological time until now; and how ongoing human-caused changes—such as global climate change—may affect the future of diversity and the world in which we live."

—Dawn Kauffman, Assistant Professor, Joint Science Department, the Claremont Colleges, Claremont, CA

"I started out as an ecologist in the 1990's, when niche-theory explanations of diversity had largely fallen out of favour, and large-scale diversification patterns represented an exciting new frontier. Two books that had a strong influence on my thinking were James Brown's Macroecology and Bob Ricklefs and Dolph Schluter's Species Diversity in Ecological Communities. I was drawn to big-picture ecology as my PhD topic, and it remains a primary interest, although my research is now moving more towards classical community ecology."

—Marcel Cardillo, ARC QEII Fellow, School of Botany & Zoology, ANU

"I find this to be a differently phrased problem - this could be, for example, be posed about patterns of richness as a function of island isolation, or it could be about richness of sets of pitfall traps in a series of woodlands of varying character, or it could be about the so-called latitudinal gradients in diversity. I don't really have an answer to why I think these phenomena to be interesting other than to say that I think that measurements of diversity are intended to capture some fundamental emergent properties of ecological systems, and that patterns of diversity reveal the operation of important ecological and biogeographical processes."— Robert Whittaker, Professor of Biogeography, School of Geography and the Environment, University of Oxford.

"I'm working on latitudinal diversity gradients because geography is part of the bigger picture of global diversity, which is my main focus. If you know local diversity in each region and know how well differentiated the regions are, you know global diversity (i.e., gamma = alpha x beta). I'm also interested in this topic exactly because so many other people care about it, and because the paleontological literature on this topic is so thin. So, I'd like to get some more reliable results out there for the ecologists to use." — John Alroy, National Center for Ecological Analysis and Synthesis