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.
Humboldt thought that the differences in biological diversity with latitude were due to differences in climate, and in particular temperature, among regions. His hypothesis made a clear, if simplistic, prediction: Not only should diversity increase toward the equator, but for a given latitude, diversity should also increase with temperature, so that a mountain climber moving from slope to summit should see biological diversity decrease in ways similar to the declines seen when an explorer moves toward one of the poles. For the most part, it does. Humboldt’s 200-year-old work came close to explaining the distribution of life on Earth. A bit more work and Darwin’s theory of natural selection should have been enough to complete Humboldt’s picture.
But it wasn’t. Yes, exploring biodiversity has gotten much easier. With vast computer databases on the distributions of species, we can recreate the list of species that Humboldt might have seen as he traveled. Using evolutionary trees built from fossils, genes, and the structures of living creatures, we can trace the diversification of entire groups of organisms. With satellites we can even model patterns of diversity from space. Nevertheless, instead of an explanation for the patterns Humboldt first saw, we have explanations, plural. Lots of them.
The study of biodiversity patterns lay dormant for a hundred years after Humboldt’s death; then, in the 1960s it exploded. Dozens of hypotheses cropped up. Suddenly, everyone seemed to have an idea, and ecologists were seized, says Michael Willig, of the University of Connecticut, with a “lustful preoccupation with high tropical biodiversity.”
The lust was born in part of curiosity. Yet the allure of discovering one of science’s timeless laws is also great. Within the relatively small field of ecology, explaining biodiversity is the primary question. And what accomplishment could exceed producing the answer to a question that has gone unanswered for hundreds of years?
The first response most ecologists give when asked about diversity gradients tends to emphasize the complexity of interacting pressures on diversity. But when pressed, many ecologists will admit they believe in one or at most two main causes of diversity gradients and that we are getting closer to a consensus as to what those are. The trouble is, there is no agreement among those who foresee consensus as to what the consensus will be.
No fewer than six prominent explanations for diversity gradients have found support recently in high-profile journals. Among these more popular arguments, one hypothesis holds that diversity is highest in those regions that have had the longest uninterrupted time for new species to form. Unperturbed by glaciations or other major climatic changes, these evolutionary Edens are the cradles of diversity. Or so that argument goes. A related series of three hypotheses is built on the argument that diversity is highest where speciation rates are highest. And what influences speciation rates? Perhaps they are higher where temperatures are higher or where the geographic areas of biomes are larger, or maybe instead where there is the most plant matter, the most food on which to survive. Another hypothesis holds that the high productivity of the tropics—itself a function of the local plants’ maximal exposure to sunlight—doesn’t boost speciation but instead keeps extinction rates down. If there is enough food, no one starves. Or maybe, one hypothesis suggests, the latitudinal diversity gradient doesn’t deserve explanation at all. Instead, species distributions, like pancakes tossed haphazardly on a plate, pile up in the middle latitudes as a consequence of geometry’s constraints.
The current visibility of these six hypotheses doesn’t mean that more hypotheses aren’t out there. In fact, even this list of six is controversial. There are, in total, nearly three dozen hypotheses still in play. Reputable, reasonable scientists have argued, variously, that diversity is higher in the tropics because of greater environmental predictability, greater environmental stability, reduced climatic harshness, reduced seasonality, greater rates of competition, greater rates of predation, greater population growth rates, smaller geographic range sizes of species, greater numbers of epiphytes, and more.
Many narratives can be constructed to fill out the bits and pieces of the drama we have observed.
Deciding which hypotheses to reject should be straightforward: Test their predictions. If a hypothesis predicts that diversity should be highest where it’s hot, then one should test whether that is the case. If a hypothesis predicts that diversity should be highest where it’s productive, a second test. The problem is that there is just one Earth, and on this Earth heat, productivity, and a suite of the other variables often linked to diversity culminate in the same places. The tropics are warm, wet, and have been less frequently disturbed by changing climates. Which theory does their biological abundance then support? And which diversity are we considering, anyway? There are thousands of groups of organisms whose diversity one might study. For ants productivity seems less important to diversity than does temperature. Aha, it must be support for temperature models. But amphibians tell a different story, as temperature, the passage of time, and the abundance of water have all been implicated in their diversity. Which one matters? Which deviation from expectations should lead to the rejection of a theory? In practice, neither does. Theories have continued to accrue through time, and while this is a problem for the progress of science, for individual scientists it means their theories in can join the rest in an untestable, unrejectable army of theories.
And so although individual scientists believe themselves right, consensus regarding the drivers of diversity patterns eludes us, at least if that consensus needs to involve a broad swath of scientists. Nor is there much changing of minds. And when scientists do change their minds, they often give in to temptation and propose a new theory. How, in their dissatisfaction with those that exist, could they not?
Biodiversity theories remain encumbered by the complexity inherent in the history of an old planet. Roughly 4 billion years ago, life evolved on Earth. Many scenarios might have ensued, but only one did. An incredible diversity of microbial lineages arose, and then, almost as an evolutionary afterthought, so did fungi, plants, and animals. Continents moved past and into each other, buckling like cars colliding slowly but persistently until they were no longer separable. Seas and temperatures rose and fell. Meteorites pocked the ground. Glaciers crawled up and down the continents. All of this has somehow produced the present distribution of species, with vagaries that are both undeniable and unresolvable. Yet there are simple consistencies to the diversity of regions, consistencies that ask for a beautiful story that will unite them. The trouble is that revealing the story of biological diversity is akin to reconstructing a lost novel on the basis of chapter headings and fragments of text. From those pieces, we know that the story is grand. But many narratives can be constructed from the same fragments, many stories made to fill out the bits and pieces of the drama we have observed.
It remains tempting to believe that with enough time, a single, powerful narrative might come. But it might not. There may be questions in biology for which a consensus will never be reached, but instead for which there are multiple serviceable explanations, none of them quite right, but none of them terribly easy to prove wrong either. The early history of life, for example, is muddied by our distance from it, such that for all practical purposes, there may be histories, plural, of microbial life. Perhaps this is also the case with gradients in biological diversity. We have a single planet to observe, a single replicate of a single experiment in the distribution of species. What’s more, our view of the results is cloudy and growing more so. When Humboldt traveled the world, the differences among places were obvious. Today they are less and less so. For example, patterns of diversity along elevational gradients change depending on the intensity of human activity. There remain essentially no undisturbed elevational gradients, and so one window into the past, into how diversity became distributed by climate, history, and evolution on Earth, is already gone. Meanwhile, the distribution of species across the globe is becoming, slowly but surely, more homogeneous: pigeons for example, are as common in Mumbai as the are in London. More pieces of a fragmented text are lost, and the narrative becomes muddier.
One might find it frustrating that we are still so ignorant about what is around us, but there is another possible emotion: awe. If you were to drive from your house toward the equator, even a hundred miles, you would almost certainly see a greater diversity of species than you are familiar with. No one can tell you, for sure, why. Nevertheless, the endeavor to find the single evolutionary story that will unite observations of the diversity of life goes on.
Originally published October 7, 2008