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 noticed 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 think history is hugely important.  Diversity at large scales results from speciation minus extinction, so factors that promote speciation have to be main drivers, for example, things like time and area and habitat heterogeneity.  Because speciation takes time, large, old areas with lots of potential barriers (either climatic or topographic -- anything that promotes reproductive isolation between populations/species) should harbor higher diversity than smaller, younger, more homogeneous areas."  –Paul Fine, Assistant Professor, Department of Integrative Biology, University of California, Berkeley.

Relevant publication: Paul V. A. Fine, and Richard H. Ree. 2006. Evidence for a time-integrated species-area effect on the latitudinal gradient in tree diversity. American Naturalist 168(6):796-804.

"The main spatial gradient in species diversity is the latitudinal gradient, and so many factors vary across latitude that I don’t think we should expect latitudinal differences in species richness to be caused by a single factor valid at all times and for all taxa. I think that a large part of the gradient can be explained by the fact that tropical biomes have existed for longer periods of time compared to higher-latitude ones, allowing lineages to accumulate over time, and because climatic oscillations are less severe closer to the equator than the poles. Climatic shifts caused by variation in the shape of earth’s orbit andaxial tilt have been a feature of earth’s entire history, and have higheramplitude towards the poles. Higher climatic variability leads to higherrisk of population extinction and more population interbreeding as speciesadjust their geographical distributions in response to the changing climate. This means there is less opportunity for speciation and higher risks of species extinction. In contrast, in low-latitude environments, with low amplitude of climatic shifts, species may accumulate."—Roland Jansson, Associate Professor, Department of Ecology and Environmental Science, Umeå Universitet, Sweden.

Relevant publication: Jansson R. Global patterns in endemism explained by past climatic change Proceedings of the Royal Society, London Series B 2003 270 583-590. PDF

"Diversity tends to accumulate where speciation rates are high or extinction rates are low. Between extinction paroxysms, the fossil record suggests regions tend to accumulate species. This means speciation must, on the whole, be adding species faster than things like invasive (or dispersing) species are eliminating them. If so, species-rich continents must have higher speciation rates, or have been habitable longer, than species poor continents. I'm betting the everwarm environments, with their combination of lotsa breeding, and lotsa metabolism causing lotsa free radicals, generate species at a faster clip. "History" shows its hand when it terrascapes regions with glaciers or causes others to slip below the sea (good for coral diversity, not so much for ants)."—Michael Kaspari, Professor, Department of Zoology, University of Oklahoma

Relevant publication: Kaspari M, P Ward & M Yuan (2004). Energy gradients and the geographic distribution of local ant diversity. Oecologia 140: 407-414.PDF

"To me it seems that productivity increases diversity, simply because the more food there is, the more individuals you can get, and with lots of individuals you can get increased diversity. Actually, we should perhaps abandon descriptive investigations into the relationship between productivity and diversity, and focus more on the mechanisms. History is important in the sense that even if you have a productive but pristine environment, it will take some time to get diversity. Thus, the question how important history is depends probably quite much on the system under study, and actually, we don't really know much about history's role."— Folmer Bokma, Department of Ecology and Environmental Sciences, Umeå University

Relevant publication: Bokma, F. Bokma, J., & Mönkkönen, M. Random processes and geographic species richness patterns: why so few species in the north? Ecography 24, 43-49. PDF

"When you say spatial gradients, I imagine latitudinal ones, longitudinal ones (e.g. east-west in Europe, east being more diverse), and the association, less well known, between altitudinal variation and richness at large spatial scales which is associated with mountains and plate boundaries. This is often quantified as endemism. My personal attitude to this question is that there are different levels of explanation, or to put it differently, you have hidden several types of question in one here. Ultimate explanations would be abiotic proxies for the spatial variation itself e.g. temperature, precipitation. Probably those two variables account for most of the spatial variation in richness on earth (plate boundaries probably just create fine scale spatial variation in those two variables). Those variables affect ecological characteristics (e.g. productivity, interactions between species, community assembly), and those then affect evolutionary processes. It is the web of interactions linking abi  otic variables to ecology to evolution where the question gets interesting, and that is where there is a deficit of information. Certainly we are able to associate gradients with evolution, or with ecology, or with abiotic variables, but the complete chain of information is, I think it is fair to say, not available for any group of organisms for any taxon. To give you an example of the former, Monika Boehm and I showed that in a group of old world primates (baboons, macaques and their kin), that the place where the group originated (tropical africa) is important because it means that the group has an extensive history of occupation there: speciation has had more time to unfold. In addition, tropical regions have experienced higher net rates of speciation, but are source regions for higher latitude clades: they do not appear to receive new species by immigration. But why these trends exist for this group is an ecological question, and ultimately must be explicable in terms of abiotic variables, probably temperature and precipitation, but we haven't demonstrated those links yet." Peter Mayhew, Department of Biology, University of York.

Relevant publication: Mayhew PJ, Jenkins GB & Benton TG (2008) A long-term association between global temperature and biodiversity, origination and extinction in the fossil record. Proceedings of the Royal Society B 275: 47-53. PDF

"I don't think there is a single explanation. I know that the literature is largely about tests of single hypotheses - energy, area (insert your favorite here) etc. - but in reality the gradient has to reflect both evolutionary and ecological dynamics. That is why these single explanations won't work. I am beginning to separate the two components of the dynamic - the diversification rates that determine how many species are around globally at any given time and the ecological/biogeographic component that determines where they are on the landscape. The gradient obviously is a function of both of these. The historical component includes the kinds of things Bob Ricklefs has so nicely articulated, not just speciation/extinction but also the history of regional climates and environments while the other component for me boils down largely to the question of what determines species range limits. As we tried to point out in the 2006 Science paper, in order to understand what produces the lat gradient we need to understand both of these aspects." –Kaustuv Roy, Professor of Biology, University of California, San Diego.

Relevant publication: Roy, Kaustuv, David Jablonski, and James W. Valentine (2000). Dissecting latitudinal diversity gradients: Functional groups and clades of marine bivalves. Proc. Royal Soc. London B 267:293-299 PDF

"Two parts here:
1) I am convinced that no single mechanism or theory will explain all the gradients or even just the latitudinal gradient. There are multiple ecological and historical factors at play, and they have different magnitudes of effect in different gradients.
2) The 'kinetic' effect of temperature (a version of Klaus Rohde's idea of "evolutionary speed") is the primary cause of the gradients of increasing species (and human cultural) diversity in environmental gradients of increasing temperature. There is usually also some effect of productivity/resource supply rates, but the quantitatively largest cause is the direct effect of temperature in speeding up biochemical reactions, organism-level metabolic rate and other processes, ecological interactions, and evolutionary rates (including both speciation and extinction rates)."—James Brown, Distinguished Professor, University of New Mexico.

Relevant publication: Brown J.H., Gillooly J.F., Allen A.P., Savage V.M. & West G.B. (2004) Toward a metabolic theory of ecology. Ecology, 85, 1771-1789. PDF

"The hypothesis of effective evolutionary time is best supported by available evidence. It claims that those habitats have the greatest diversity in which evolution has been fastest and which have had the longest undisturbed evolutionary history. Speed of evolution is a result of temperature effects which directly affect mutation rates, generation times and speed of selection. The hypothesis acknowledges that many other factors, such as area, heterogeneity of the habitat, among others, may contribute to diversity, but the most important ones are speed of evolution and a long undisturbed history. The hypothesis predicts the greatest diversity in the tropics (where temperatures are highest and evolution therefore  fastest) and in the deep sea (where habitats have had a long undisturbed history, although temperatures are low). This agrees well with empirical evidence. - As an afterthought, the hypothesis does not imply that an increase in species numbers must lead to a compression of niches (MacArthur's latitude - niche breadth hypothesis). The largely unsaturated niche space permits accumulation of species everywhere, even in species-rich habitats. This also is supported by empirical evidence (e.g., Vazquez, D.P  and Stevens, R.D. 2004, The latitudinal gradient in niche breadth: concepts and evidence. American Naturalist 164, E1-E19). See also http://en.wikipedia.org/wiki/Effective_evolutionary_time." --Klaus Rohde, Professor Emeritus, University of New England, Armidale, Australia.

Relevant publication: Rohde, K. (1992) Latitudinal gradients in species diversity: the search for the primary cause. Oikos, 65, 514-527.

"Spatial gradients exist on multiple scales and so do the forces affecting diversity. On a global scale, I would bet on evolutionary causes, i.e. net diversification is more prominent in some places as either speciation is very rapid (“cradle”) or new clades are very long-lasting (“museum”). There is some evidence for both processes, and I assume that the relative relevance of the cradle and the museum part differs for different biota. On a smaller (say regional) scale, processes of dispersal and spatial heterogeneity are highly relevant, whereas biotic interactions work on even smaller (local) scales." –Helmut Hillebrand, Associate Professor, Institute for Botany, University of Cologne.

Relevant publication: Hillebrand, H. (2004) On the generality of the latitudinal gradient. American Naturalist 163 , 192-211.PDF

"History is obviously important, as all the patterns observed in nature have historical roots. However, according to my personal experience with global bird (and some other) data, diversity patterns closely follow spatial patterns of energy availability, which suggests that historical processes have been modulated by energy so that current diversity patterns represent an "attractor" which would be probably reached by many alternative histories. In this sense history is less important than overall climatic settings of earth surface. It seems that energy shapes biodiversity dynamics by following ways: (1) temperature (a form of kinetic energy) positively affects diversification rates in ectotherms, (2) productivity (potential energy stored in chemical bounds of organic matter) negatively affects extinction rates in all organisms, and (3) productivity (and perhaps also temperature) affects probability of spreading of species ranges (and range dynamics in general). Points 1 and 2 concur with reasonings of "Metabolic Theory of Ecology", but I feel that point 3, i.e. the effects of energy on range dynamics, are really important, and speciation and extinction rates do not represent the whole story - simply because all organisms have capacity to spread, and without constraints on range dynamics they would populate the whole earth surface regardless of any trends in speciation/extinction rates."—David Storch, Associate Professor, Department of Science, Charles University.

Relevant publication: Storch D., Davies R.G., Zajíček S., Orme C.D.L., Olson V., Thomas G.H., Ding T.S., Rasmussen P.C., Ridgely R.S., Bennett P.M., Blackburn T.M., Owens I.P.F. & Gaston K.J. 2006: Energy, range dynamics and global species richness patterns: Reconciling mid-domain effects and environmental determinants of avian diversity. Ecology Letters 9: 1308-1320. PDF

“I [believe] that we now have sufficient evidence to conclude that niche conservatism and time for speciation are the primary drivers of the contemporary diversity gradient. And while it remains true that additional data on geographic variation in speciation/extinction rates and the strength of biotic interactions are needed, these appear to be secondary effects that do not need to be invoked to explain the general patterns we see; that is to say, they are details, and any differences we may find in these processes will not change the overall explanation for why the tropics have more species.”—Bradford Hawkins, Professor, Ecology & Evolutionary Biology School of Biological Sciences, University of California, Irvine.

Excerpt is, per Prof. Hawkins request, from a more extended discussion of the question in “Hawkins, B. A. 2008. Perspectives in biogeography: Recent progress toward understanding the global diversity gradient. IBS Newsletter 6:5-7.”

"This question has to be answered differently at the regional and local scales.

At the regional scale, we want to know why so many species have evolved in the tropics. A parsimonious answer will apply to all groups of plants and animals, living in both terrestrial and marine environments. The answer must have to do with temperature because that is the only thing that differs broadly between low and high latitudes. High temperatures speed up metabolism, which leads to shorter generation times, and probably contribute to elevated mutation rates, which increases genetic diversity. The net result is that evolution proceeds faster in the tropics, so we end up generating more species. Most clades have ecologically important high-latitude species, so evolution can clearly create a successful high-latitude "body plan". The reason we don't see hundreds of variants on these successful plans at high latitudes is just a rate problem.

The second part of the question is how are so many species able to coexist locally (at small spatial scales) in the tropics. I actually think this one is more difficult to solve. Some combination of disturbance, niche differentiation and predation must keep abundances low enough to prevent competitive exclusion. But it is still a puzzle and we don't have a good explanation for high local species coexistence in the tropics."—Nick Gotelli, Professor, Department of Biology, University of Vermont.

"Both history and climate, which themselves are intertwined, are the obvious underlying factors driving geographic patterns of diversity on Earth. Historical patterns of vicariant geography provided opportunities for speciation and isolation, whereas climate determines productivity. Together, these offer ecological opportunities for resource partitioning.  A surprising amount of present day differences in diets among extant lizard species can be traced back to ancient phylogenetic events (Vitt, L. J. and E. R. Pianka. 2005. Deep history impacts present day ecology and biodiversity. Proc. Nat. Acad. Sci. 102: 7877-7881)."—Eric Pianka, University of Texas

"It depends on whether you mean why some localities have fewer species than others (richness) or why species are different at two different localities (species turnover)? Most of the recent research seems to point to two key factors driving richness gradients, productivity and temperature.  The relative importance of these two factors, their exact statistical relationship with diversity, and the processes by which they drive diversity patterns are still fodder for dialogue and research.  In some part the answers depend on spatial scale, habitat, and organisms themselves.  The processes important for plants may not be the same for phytoplankton.  Those that influence vertebrate diversity may be different for invertebrates and different between marine and terrestrial invertebrates.  For example, my Ph.D. advisor Mike Rex found a latitudinal diversity pattern in deep-sea invertebrates.  These findings had major implications because all locations (except for hydrothermal vents) are equally cold (~4 degrees C) in the deep.  That only leaves one factor, food availability.  This same factor drives depth and basin/ocean differences in diversity.  I also remember a study by Hillebrand in 2004 noting that the strength, slope, and variability in latitudinal gradients depended on the body size of the organisms themselves.  Of course this makes sense because both size and temperature dictate metabolic rate and some many other physiological processes which in turn affect ecological and evolutionary processes.  Body size also is related to home range, extinction rate, speciation rates, competition rates, predation, population processes that are also important to diversity patterns.  The role of history is also vital but even harder to quantify and characterize.  Historical 'accidents' can amplify or destroy patterns.  Overall, the answer to your question is complex and could fill books (and has!)." Craig R. McClain, Postdoctoral fellow, Monterey Bay Aquarium Research Institute, California.

[Rob Dunn asked the additional question of whether Prof. Rosenzweig had a sense or feeling of the relative importance of area to patterns of diversity, given current data.] " Better than feeling, the data are absolutely clear. I've got the results from the  world's 50,000 vertebrate species at the level of the biogeographical  province. For the first time, they show the combined effects of area and a climate variable. Two problems remain: the weight of the renascent Tumamoc Hill on my back keeps me from getting this out in the literature. And we do not have a theory to explain the climate connection, at least a theory that holds water. I have a feeling it will take two theories, one for the effect of climate on mutation rates; the other for the effect of climate on extinction rates. But that's all it is-- a feeling."—Michael Rosenzweig, Professor of Ecology and Evolutionary Biology, University of Arizona.

"I think that we have a very clear mixing of ecological and evolutionary aspects, and that for a while the approach was to tease them apart. Although this strategy is clear and is (was?) used in many fields, I think it will not work because there are lots of interactions among these ‘components’. There are also difficulties about our definition of “causes”. For example, diversity gradients are associated with climate, but it is difficult to see how this alone can generate patterns, except assuming strong equilibrium with climate and full dispersal ability. Since this is not the case, it is quite expected that at least part of the patterns we observe are not related to current climate, but instead with past climate, which changes through time. So, this is a historical component in climate driving patterns both in past and future. At deep time scales, we could also argue that speciation or extinction rates are geographically (or climatically) structured, creating higher tropical diversity. In this case, the pattern is generated by ecology or history? At the same time, we can think in deep-time historical contingencies, which are not predictable.

In short, now I think that, although current climate is strongly related to diversity, we must think in evolutionary components that GENERATE this correlation (i.e., the observed correlation is generated by evolutionary components). I’m not convinced that we have geographical structures in speciation or extinction, I think the evidence is controversial because it is difficult to measure these rates. My current analyses suggest that we can have strong patterns of niche conservatism, I’m quite enthusiastic about this approach, as summarized by Wiens & Donoghue (2004)." –Alexandre Felizola Diniz Filho, Professor, Universidade Federal de Goiás, Brazil.

"It would be easy here to give a ‘standard’ answer and say that the LDG is the result of complex, interacting factors.  Somehow we suspect that evolution proceeds faster in the tropics than at the poles, and the greater area of the Tropics, as compared to other major biomes, must be important.  The final part of the ‘Holy Trinity’ is the greater antiquity of the tropics which we believe have always been a part of the Earth’s surface.  Polar ice caps, on the other hand, have come and gone and it might be that for long periods of time the poles were very much warmer than they are today.
But before you solve this difficult (and completely fascinating) problem I believe that you have to bring it into much sharper definition.  Here are some observational features about the LDG that I think are crucial to explaining its ultimate cause:

(i) It is a regional feature picked up at large geographical scales.  If you only measure point diversity you often don’t find it!  And this is important because it brings out the phenomenon of geographical variation.  Taxonomic diversity is a balance between local and regional richness, and the latter indicates the importance of history.
(ii)The gradient occurs in a very wide variety of organisms in both the marine and terrestrial realms.
(iii) But it does not occur everywhere on the surface of the Earth.  The LDG is strongest in the Indonesia – Philippines area and also in the Old World centres of the Caribbean, Amazonia etc.  In between these areas it is much flatter.  The LDG is actually superimposed on equatorial/low-latitude longitudinal gradients.  The latter are intriguing and at least partly due to the influence of tectonics.
(iv) The gradient goes back at least 300 m.y., and probably a lot further than that.  Whenever continents, or continental shelves, have moved into the high latitudes you can find evidence of gradients.
(v) I believe that global taxonomic diversity (i.e. a Sepkoski-type curve) has increased substantially over the last 100 m.y., and in particular over the last 65m.y.  As global taxonomic diversity has increased, so has the steepness of the LDG.  I would emphasize in summary that the LDG fluctuates in space at the present day, as well as through time.

If we’re right in saying that both global taxonomic diversity levels and the latitudinal diversity gradient have steepened sharply over the last 65m.y. then this throws up another conundrum.  Geologists have been able to show, quite convincingly, that the actual area of the tropics has reduced considerably over this time span; in the Late Cretaceous they probably stretched from 45N to 45S, as compared to 23N to 23S at the present day.  And there is a suspicion that they formed a more continuous oceanic belt based on the circum global Tethyan Ocean that fed at either end into the Pacific.  Two things then happened through the Cenozoic: (i) the latitudinal temperature zones became much more pronounced, with the tropics contracting and the cold temperate and polar regions expanding, and (ii) a series of tectonic collisions pinched off the circum tropical ocean to form the more familiar pattern of oceans and islands that we see at the present day.
But if the tropics contracted in size through the Cenozoic how could they have become much more diverse?  This is a key question and one which we do not yet fully understand.  Nevertheless, the new pattern of oceans and islands that formed in some way became the foci for periods of intense speciation.  New shallow water habitats were created in the sea and coral reefs proliferated on these.  On land distinct tracts of tropical rainforest were delineated, one on each of the major continents.  Regional faunas and floras developed and diversity increased at local, intermediate and regional scales.  It should also be emphasized that these Cenozoic bursts of diversification were very much at the species level; this is crown group radiation of well established clades.  I don’t think that truly species-rich clades existed before the Cenozoic."—Alistair Crame, Division Head Geological Sciences Division Cambridge University.

"Well, I think environment and history are both important and have closely intertwining effects, which is part of the reason why it has been so difficult to achieve some kind of consensus on what drives diversity gradients. In terms of pure environment, I do accept that environments rich in both water and energy (within the amplitudes seen on Earth) - i.e., warm-wet environments - are likely to have a greater capacity for sustaining relatively large numbers of species. However, I think it is quite likely that such an environmental capacity is mainly meaningful in relative terms, i.e., that no strict upper limit to the number of coexisting species exist, or that we are generally far below it. This of course opens the door for strong historical effects.

Notably, historical effects are likely to be able to strongly modify the environmental effect, often amplifying it. A key mechanism for amplification reflects that species arise by evolution and the phenomenon of niche conservatism: If warm-wet environment get an environmental head-start on diversity, this will be enhanced as diversity subsequently builds up by speciation and dispersal, as niche conservatism will tend to hinder new species in spreading into other environments. A similar effect works if warm-wet environments get a historical head-start, i.e., that they have existed continuously for a longer time and over a greater area, i.e., as we is true for the Meso- and Cenozoic eras during which most of our current diversity has originated. This is the tropical conservatism hypothesis of Wiens and Donoghue (TREE 2004). i.e, niche conservatism will tend to maintain or even amplify the environmental diversity differences. Of course under these scenarios history and environment is really closely intertwined, so it's not either-or; but the key point is that the environmental effect in this reflects long-term evolution, not short-term ecological maintenance processes.

However, I also think there are important historical effects that are more purely historical. As species arise by speciation and speciation is often a rather slow processes (10^5 or 10^6 yr scale), I think diversity gradients across geographic as well as environmental gradients will often by influenced by time effects, i.e. the time available to build up diversity by speciation and immigration. This will cause regions or environments with a long continuous history to tend to be more species-rich than other environments and regions. This effect is most clearly discernible when comparing regions of similar climate, e.g., the temperate broadleaved forest regions of Europe versus those of East Asia and eastern North America (as done more than 15 years ago by Ricklefs and Latham). However, there is no reason this effect could not occur between different environments, too. This time effect has long been suggested as one explanation for the high tropical diversity and is also part of the Tropical Conservatism hypothesis. In my opinion it also important for the diversity gradients within continents, such in Europe, where postglacial migrational lags in roughly N-S direction enhance environmentally caused diversity differences from the Alps and northwards." –Jens-Christian Svenning, Associate Professor, Department of Biological Sciences, Aarhus Universitet, Denmark.

"Many ecologists who study the Earth’s spatial gradients will advocate the importance of physical, or abiotic, aspects such as solar energy and/or temperature, as well as precipitation, at the exclusion of other potential explanations. I agree that the spatial patterning of these factors and their tremendous physiological importance to organisms makes abiotic effects critically important in biological diversity gradients. For example, in the best studied and most ubiquitous of the gradients, the latitudinal gradient of diversity, the relatively low mean and high variability annually of temperatures at high latitudes may severely limit the number and types of organisms that can colonize and persist in these regions. However, the current lack of a widely accepted, specific mechanism for the latitudinal gradient of diversity suggests that the answer may be more complex, and ecologists have not been persuaded that abiotic factors tell the whole story. I long have suspected that biotic interactions also are crucial in “ratcheting up” diversity in some regions. That is, interactions such as competition, predation, parasitism, and even mutualism may lead to species packing in such areas as the tropics, thus playing a potentially important role in increasing diversity at low latitudes. The idea that biotic interactions may be important to gradients of biological diversity is not new, and may be traced at least as far back as Theodosius Dobzhansky and Robert MacArthur in the mid-20th century. It is true that both physical and biotic factors are important in the creation of an organism’s environment in general, so it also is logical that both might exhibit vital influences on organisms across spatial gradients.  Why might understanding biotic interactions be so critical if they play a part in forming or strengthening biological diversity gradients? First, consider that abiotic “problems” typically will only need to be solved once. For example, species may evolve different physiologies—such as anti-freeze or freeze tolerance—or behaviors—such as hibernation or migration—that enable them to live in extremely cold environments. These adaptations essentially will allow organisms to “conquer” such a physically challenging environment. Now, consider a biotically challenging environment, where the limitations arise mainly from interacting with other species that also have the capacity to evolve. This may result in an evolutionary arms’ race, where species evolve in response to each other, and a species must continually keep solving the “problem” of living in a particular area. Many examples of this type of dynamic exist; like running on a treadmill, species must continue to evolve just to keep up. However, biotic interactions are difficult to measure at large scales and across spatial gradients—unlike abiotic variables—and thus have been much more difficult to test. The inevitable question exists about the role of history. Obviously, if biotic interactions are important, then so likely is evolutionary time. The particular idiosyncrasies of history, such as which species first colonized a continent, may not be particularly important in gradient dynamics (though they may affect the particular species found in current continental biotas). However, continental histories—such as large-scale glaciations—may impact biological gradients and we currently are attempting to better understand these potential effects by studying long-term changes in paleontological gradients." —Dawn Kauffman, Assistant Professor, Joint Science Department, the Claremont Colleges, Claremont, CA

[I asked Marcel an additional question about the role of history in particular.]

"I don’t like using the term “history” because it implies the LDG (and other gradients like altitude) was the result of a series of unique, chance events that could never have been predicted. Of course, a lot of the phylogenetic patterns of diversity that exist today are the result of chance events, but when it comes to the LDG, the scale and ubiquity of the pattern lead me to favour predictable, deterministic explanations. At a broad level, the cause of the LDG is likely to be a faster diversification rate in warmer, wetter climates. This is not to say that speciation is necessarily faster or extinction slower in the tropics, just that the difference between the two is greater. The weight of correlative evidence so far points to climatic factors as the main drivers of faster diversification in the tropics, but as for the particular mechanisms that link climate to diversification rate, the jury is still pretty much out. These kinds of explanations are historical in the sense that they result in the formation of diversity gradients over time, but not in the sense of being contingent on unique events. If we re-ran the tape of life we’d still get a latitudinal diversity gradient.

Perhaps more intriguing than linear diversity gradients are hotspots of diversity and endemism in places like South Africa's Cape Province and Australia's southwest, where few of the environmental factors we normally associate with high diversity apply. These places are dry, unproductive, and at least in the case of southwest Australia, largely flat and featureless. Again, I don't think these hotspots can be accidents of history, because they occur repeatedly in Mediterranean-climate environments on almost every continent. Some general and predictable processes must be at work."— Marcel Cardillo, ARC QEII Fellow, School of Botany & Zoology, ANU

"...there are different ways of measuring or expressing the measurement of biological diversity, and there are different scales and frames of analysis. Which factors are most important depend on what particular set of phenomena you are trying to explain. However, for coarse-scale terrestrial gradients in species richness I think that they are underpinned by coarse-scale gradients in climate, and their patterns of variation in space and through time."

— Robert Whittaker, Professor of Biogeography, School of Geography and the Environment, University of Oxford.