biogeocoenosis

The Grandeur in this View of Life


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One of the realities of rapid climate change is that environmental conditions are changing so fast that many species cannot migrate or adapt quickly enough to keep up with them. This problem is especially present for some of my favorite organisms: trees. Trees, of course, can only move as seeds and their generally long generation times mean that evolution moves at the pace of, well, a tree.
This problem has many ecologists and conservation biologists considering rather extreme measures: helping organisms move around to keep up with climate change. Of course, this sort of action has risks involved, so there is a robust debate going on in scientific and management circles. This post, from the Early Career Ecologists blog, does a great job of laying out the basics of assisted migration.

Early Career Ecologists

By Sarah Bisbing

Trees on the move?! I know you’re thinking, “Come on, Sarah. Trees can’t move.” And, generally, you would be correct in that statement. Tree species are now, however, in a position where movement may be necessary for survival under changing climatic conditions. How trees will move is under debate within the ecological community, but why trees will move is accepted as a survival strategy related to the adaptation of species.

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IBS Meeting, part 2: A Big Question

One of the exciting things about scientific conferences like the International Biogeography Society meeting is that you get exposed to some Big Questions. On my first day at the meeting, I heard a new one.

What are the gifts you can give your readers?

hummingbirdI almost missed the question. It slipped into the workshop like a hummingbird, zipping in between our discussions of How to Expand Your Ideas and Choosing and Using Models of Scientific Writing. The conversation was lively enough that we didn’t stop to answer the question directly, but it hovered in my mind for the rest of the afternoon, buzzing and iridescent.

The question came from Dr. Sarah Perrault, the leader of our workshop on Writing Popular Science. She guided us through an afternoon that mixed active writing with more reflective discussions of the components of various genres, the need to engage their readers in relationships of trust and credibility, and the balance of facts, values, and actions in our narrative ambitions. But in some ways what it all came down to for me was that single passing question.

I thought about Sarah’s question some more on the way back to my hotel. The walk is a little long (North Miami is built for cars, not pedestrians) but on the way to Biscayne Boulevard,  the route cuts through the East Arch Creek Environmental Preserve, a small tangle of woodland and brackish estuaries squeezed in between the Florida International University Biscayne Bay campus and the high rise condominiums that line the bay itself. Walking among the canopies of Australian pine and Brazilian pepper, it struck me that much like the human component of the city, the plants of the preserve are a community of immigrants. These species have tagged along with us humans, transported here from the far sides of the planet. In particular, we planted Australian pine to stabilize beaches and estruary banks, because it is very salt tolerant and forms dense thickets. Unfortunately, like many of our fellow travelers, it has become invasive, displacing large numbers of native species, the collateral damage of our own species’ success.

The Australian pine, Casuarina equisetifolia, is actually not a pine at all. It is not even a conifer. What appear to be evergreen needles are actually small green twigs bearing rings of tiny leaves that fall off during dry periods. Even its fruits look superficially like pinecones, but on closer inspection they actually reflect its closer relationship with flowering plant species in the birch family like ironwood, alder, and hornbeam. Until we humans began transporting it around the world, the Casuarina family was found only in Southeast Asia, India, and Australia, a distribution that reflects its origin on the ancient southern continent of Gondwanaland, sometime around 40-55 million years ago, based on the earliest fossils of those distinctive needle-like twigs and cone-like fruits.

This global re-shuffling of plants (and animals too) signals the emergence of humanity as a geological force, reshaping Earth’s biosphere, atmosphere, and climate in unprecedented and clearly documented ways. Our network of shipping lanes, air traffic, railways, and roads, many of which converge here in Miami, has fostered what biogeographers call a “biotic interchange.” Earlier interchanges include the joining of the Americas by the isthmus of Panama about 3 million years ago. Among many other changes, this event, known as the Great American Interchange, brought hummingbirds into North America for the first time. But our current Global Human Interchange dwarfs all previous events, both in its speed and its extent. Its effects are all around us but often unnoticed, from the zebra mussels choking out the native invertebrates of the Great Lakes to the Casuarina woodlands of the East Arch Creek Environmental Preserve.

Ecology, evolution, and biogeography are sciences of connection, illuminating networks of interactions between species and their environments and tracing those interactions through lineages of descent deep into Earth’s history. This view of life transforms and brightens reality. The tree passed on the trail is no longer just a tree, it’s part of a larger story, a story that ties the present streets of North Miami to the ancient shores of Gondwanaland.

Scientists learn to read these stories through careful observation, measurement, reasoning, and analysis, and they are edited, revised, critiqued, and rewritten collectively via the scientific literature and at conferences like the IBS meeting.

These stories, sometimes complicated, often beautiful, never ending, are the gifts I hope to share with you.

My instructor, Sarah Perrault, is an Assistant Professor in the University Writing Program and an affiliated faculty member with the John Muir Institute of the Environment at UC Davis. Her book on writing about science for general audiences, Communicating Popular Science, is under contract with Palgrave Macmillan. Having taken this workshop with her, I look forward to reading it.

In the writing workshop we also discussed the fact that many press articles about science are actually ghostwritten by writers working in university public affairs offices, because journalists simply attach their bylines to institutional press releases with minimal editing. So, in the spirit of Jonah Lehrer, I just “borrowed” most of Sarah’s blurb above from the conference website (except for the last sentence). Thanks again, Sarah!


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IBS, part 1: Scientific meetings are not just for sun in winter…

There has been a delay in my posting, in part because biogeocoenosis has been on the road. With family in tow, I have traveled to Tucson, AZ, where I will be doing some sabbatical research with members of Brian Enquist’s lab in the Department of Ecology and Evolutionary Biology at the University of Arizona (more on the work we are doing in an upcoming post on biodiversity gradients). After spending the holidays in Tucson (and believe me, there is nothing like xmas eve (or any other day) at the Arizona Sonora Desert Museum! See the photo below…), I’ve come to North Miami, FL, for the meeting of the International Biogeography Society (IBS).

After my last post I got several comments (hooray!) reaffirming my claim that in order to combat science denialism (as well as scientific illiteracy and plain old ignorance and indifference), it is very important for everyday, non-science folk to understand how science is done. And by this, I mean how a reasoned but tentative narrative, supported by verifiable, repeatable observations, arises from the collective efforts of a large community of researchers.

Those comments got me to thinking that it might be useful for me to blog about my experience at the IBS meeting. Of course, the idea of blogging a meeting is nothing new but most cases I am aware of are either blogs targeting other researchers in the same field, so that meeting highlights can be shared with colleagues, or “news” type blogs aimed at bringing the latest scientific breakthroughs to the broader public. My hope is to do a little bit of the latter, but to also reflect on what it is that we are doing as scientists, when we get together at a meeting like this. My hope is that doing so will help to illuminate a little more, what it is that scientists do.

So just as a start, I’ll give you a brief outline of my schedule for the meeting:

Today (Wednesday) I am attending an afternoon pre-conference workshop on “Writing Science for the Public” led by Sarah Perrault from UC Davis. So hopefully, you will see a vast improvement in my posts, starting tomorrow!

Stay tuned!

Thursday is the official opening of the conference. There are two symposia (series of talks dedicated to particular subject areas), on the biogeography of islands and the biogeography of species’ traits, as well as a longer talk on the biogeography of the Caribbean. Ecologists and biogeographers generally like to learn a bit about the particular places they are, even if it takes time away from more academic scientific discussions. There is also a poster session; during lunch and a pre-dinner cocktail hour, researchers gather in a large room with posters describing a research project. Folks circulate among the posters and engage the authors in discussion. My poster, describing research testing the “Tropical Conservatism Hypothesis” is one of 135 presented during that session. I will let you know how it goes.

On Friday there are two more symposia, one on paleontology and biogeography and another looking at biogeographic implications of climate change; so we are looking both deeply into the past as well as into the near future. There will also be more posters, 137 to be exact. There will also be a lecture from an award-winning biogeographer, Miguel Araújo, and, later, a “beach party” at the resort that is hosting the meeting.

Saturday is the last day of the meeting. In the morning and afternoon, there will be four concurrent sessions of contributed talks; 15 minute presentations of research followed by short question periods. Finally, in the evening, there will be a keynote address by James H. Brown, a past president of the IBS, distinguished ecologist and biogeographer, and one of my graduate mentors.

So it’s a busy week coming up. I will learn a lot, meet a lot of new people and see some old friends. I will see old ideas overthrown (or maybe revitalized) and new ideas presented for critical assessment and discussion. For biogeocoenosis, and other scientists like me, this is NERD PARADISE!

desert-museum

 


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Pwnd is Not Enough.

A few days ago, along with almost 7,000 other folks, I shared this figure on Facebook:

climate-pie-chart

The figure is drawn from the wonderful DeSmogBlog, which seeks to eliminate “PR pollution” from the public discussion of climate change. The guest post from which the figure is drawn is by James L. Powell, an emeritus professor of geology from another liberal arts college just a bit north of the one where I teach, a former member of the National Science Board (under Reagan and G.H.W. Bush), and currently executive director of the National Physical Science Consortium. You can read more about the details of Powell’s survey of the scientific literature in his original post, and on his website, he even takes the trouble to list the 24 scientific papers he found that reject anthropogenic climate change.

Like many scientists, I am frustrated by the misinformation-fueled denial of human-caused climate change, and “science denialism” more generally, from climate change to evolution, vaccines, and GMOs. In fact, it is profoundly sad to me that science denialism has become not only a recognizable aspect of our culture, but a potentially lucrative profession. So when I find information like Powell’s figure, I like to spread it around.

But over the past few days, since I shared the pie chart, I have been thinking that, as satisfying as it is to share information like this and say, “The debate is over,*” this sort of action actually does little to build a public consensus on climate change to match the scientific consensus – which is presumably, the underlying goal. Don’t get me wrong – I applaud Powell’s efforts and those of DeSmogBlog. I just feel that it is important to note that it is not enough to slap the deniers with the facts.

The problem is that most people don’t understand how science works. The popular conception of science is generally limited to interesting and unusual collections of facts (think Discovery channel) and perhaps some conception (or misconception) of the “scientific method” that underlies observation and experimentation. But science is not just facts or individual experiments, it is a collective, cultural process that allows humans to constantly revise and refine our ideas about how the world works based on reason, logic, mathematics, and evidence (data). If the results of even the most profound experiment sit mouldering in some notebook (or on some flashdrive), never shared with the community of researchers, they are not really science, because they are not part of the discourse, the narrative of science. I would argue that most non-scientists, even many primary and secondary science teachers, don’t understand this cultural aspect of science; it is simply not part of their education. We are so busy stuffing our students full of the facts and methods of science that we never give them the bigger picture of how all that knowledge manages to fit together.

And if you don’t understand the culture of science, how its stories are written iteratively by generations of researchers, it becomes all to easy to dismiss the findings. Without this knowledge, those 24 contrarian papers might be seen as representing a small number of stalwart researchers courageously challenging “climate change dogma,” as can be seen in the comments on Powell’s post. In reality, scientific discourse always involves contrarian and critical contributions, and if these do indeed demonstrate substantial holes in the developing theory, they end up garnering a lot of attention and precipitating substantial revisions of scientific knowledge. The point is that during the long history of research into anthropogenic climate change, which dates at least to the greenhouse calculations of Svante Arrhenius in 1896, loads of scientists have been contributing to the story, putting forward, confirming, and refuting a variety of hypotheses – and together, through this contentious, argumentative, and incomplete process, they have composed a theory describing the climate system and our interaction with it. “Scientific consensus” is not a matter of researchers lining up behind an idea that they like, it is the outcome of a systematic, but messy collective struggle to understand how nature works.

Individual scientists, like any other human being, may or may not be trustworthy, but the fundamentally skeptical basis of the scientific process gives it additional gravitas. Its claims, from the most mundane to the most outlandish, are always challenged. Powell’s figure is powerful because: 1. it dispels the myth of censorship and publication bias by showing that one can in fact publish an article denying global warming in a peer-reviewed scientific journal, and 2. it demonstrates the hard-won, skeptical scientific consensus that anthropogenic climate change is a well-supported scientific reality. But those points can only be grasped if the person looking at the chart actually understands not just the scientific method, but the culture of science.

As scientists and educators, we have a lot of work to do. I argue that the major challenge is not to convince the people that we are “right” with facts and figures (though I will continue to accost my friends with graphs like this at every opportunity!), but to equip them to understand it for themselves by teaching them how science works.

*Or choose your own exclamation: “We win!”, “Pwnd!”, “Facial!”, etc…


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Eye on Research: Cretaceous Extinction Cascades

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We all know that an asteroid impact* ended the age of the dinosaurs roughly 65.5 million years ago. That event, known as the K-Pg extinction**, marks the cataclysmic demise not just of the non-avian dinosaurs (yes, that was a dinosaur you ate on Thanksgiving!), but of a large number of other living things, including the pterosaurs and pleisiosaurs, as well as many lineages of plants and marine and terrestrial invertebrates like insects, cephalopods, bivalves, and echinoderms. In terms of overall destruction***, the K-Pg extinction was one of the worst extinction episodes in the history of life, second only to the “Great Dying” at the Permian-Triassic boundary. The current consensus is that the asteroid impact threw so much debris and ash into the atmosphere that it greatly reduced the incoming sunlight, which in turn hindered photosynthesis and cooled global climate for years to decades. The resulting declines in plant productivity then cascaded through the food chain, leading to the extinction of herbivores and subsequently to the carnivores that depend on them. So long, T. rex!

But like any other event in life, even the outcome of an asteroid impact may depend in part on context. Did the impact, itself, doom the dinos to extinction, or did the particulars of the interactions among species play some role? Recently, Jonathan Mitchell and his colleagues addressed this question by comparing extensive fossil assemblages from the last few million years of the Cretaceous, just before the impact (the Maastrichtian age) to those of previous Campanian age, thirteen million years earlier. They used a mathematical model to investigate whether the structure of Maastrichtian food-webs made them particularly vulnerable to the kind of disturbance produced by the impact (i.e., declines in plant productivity).

As you can imagine, figuring out who-eats-who is a difficult proposition, even in extant communities, and while gut contents do occasionally occur in fossils, ancient food-webs are even more difficult to ascertain, especially when they involve up to 92 different animal species. But while we can rarely be certain that any extinct species ate any other particular species, we can have more confidence in assigning species to particular feeding guilds, based on their anatomy and their size. For example, while we don’t know exactly which plants it found tasty, Triceratops is definitely a Very Large Herbivore (let’s call it the VLH guild).

Once all of the species at a particular site were assigned to guilds, Mitchell and his colleagues used a computer simulation to assign the feeding connections among the species in the different guilds (see schematic above), based on the “connectedness” of existing food-webs. They dealt with uncertainty in the feeding relationships by repeatedly drawing random connections among the particular species in different guilds to make a large number of sample communities for each site. This approach, which is focused on general aspects of food-web structure rather than a detailed characterization of any particular community, allowed them to meaningfully compare the communities despite uncertainty about the particulars of who-eats-who.

For each randomly sampled food-web, they then simulated the asteroid impact by reducing the productivity of plants and algae and tracked the “cascade” of declines and extinction as they wound their way through the complex network of feeding connections. Their approach is particularly compelling because it is not limited overly simplistic linear chains of causation (e.g., plants decline -> herbivores decline -> carnivores decline) and permits a richer set of indirect interactions (e.g., plants decline -> herbivore A declines -> carnivore B eats more of herbivore C -> herbivore A recovers due to reduced competition from herbivore C.) By compiling sets of simulations from seven different Maastrichtian sites and ten different Campanian sites, they could then ask whether differences in food-web structure affected the robustness of a community in the face of a cataclysmic loss of plant productivity.

They found that the later Cretaceous communities were indeed more fragile, suffering greater degrees of simulated extinction at lower disturbance levels. This is certainly not to say that an asteroid impact earlier in the Campanian would not have resulted in a mass extinction – it certainly would have, but the degree of extinction, and the particular taxa that disappeared, may have been different. Interestingly, they also point out that increases in the average diversity of several guilds from the Campanian to the Maastrichtian, including the dinosaur-dominated VLH guild, was actually associated with a decrease in the robustness of the community. They hypothesize further that the importance of the VLH species was due to the fact that they fed many other guilds as they grew, from the small predators that cracked their eggs to the large carnivores unafraid to confront a fully-grown Triceratops.

This study shows that, even in an apocalypse of planetary proportions, context matters. The structure and diversity of ecological guilds, the numbers and functional types of species present, determines, in part, which species survive and which go extinct. This context-dependent complexity of ecological systems is what makes them so difficult to understand and so deeply fascinating. It also means that as we move forward with an extinction crisis of our own making, we are going to have to consider the interactions among species if we hope to mitigate our own impact.

*The evidence is drawn from the global iridium layer, characteristics of the boundary deposit, and the discovery of the Chicxulub crater in present day Mexico. Other accessory causes might include volcanic activity in the Deccan Traps of present day India.

**K-Pg marks the boundaries between two geological periods (or strata), the Cretaceous (K, C was already taken… not really, it is from the German name, Kreidezeit (chalk-time)) and the Paleogene (Pg). It is also known as the K-T extinction, with the T representing the Tertiary period. However, that nomenclature has been discarded by paleontologists and geologists. Bye, Tertiary! Thanks to the International Commission on Stratigraphy for changing the names of periods to make things extra confusing!

***In terms of the proportion of documented taxa going extinct

The research of Jonathan Mitchell and his colleagues, Peter Roopnarine and Kenneth Angielczyk, “Cretaceous restructuring of terrestrial communities facilitated the end-Cretaceous mass extinction in North America”was published November 13, 2012 in The Proceedings of the National Academy of Science of the United States (2012, vol. 109, pages 18857-18861.) A promotional blurb is also available over at Science Daily.

For an up-to-date review of the strong evidence for the role of the Chicxulub impact, see the paper “The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene Boundary” by Peter Schulte and his colleagues in the journal Science (2010, vol. 327, pages 1214-1218.)


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The Current Readings: Remarkable Creatures

Periodically, Biogeocenosis will share something about the books, papers, films, or music that are making (or have made) a strong impression. This week’s Current Reading is Remarkable Creatures by Tracy Chevalier. This historical novel tells the story of friendship between two 19th century British fossil hunters, Mary Anning and Elizabeth Philpot. As women, neither was admitted to the scientific community to which they both made tremendous contributions, discovering numerous fossil species new to science, including multiple ichthyosaurs and pleisiosaurs.

Anning and Philpot’s discoveries on the beaches and cliffs of Lyme Regis conclusively demonstrated the phenomenon of extinction. Of course, this discovery had manifold scientific, cultural, and even theological implications. Though they are not part of the official scientific discussion, both women struggled to reconcile their worldview with their discoveries. If the world was not simply created as it currently is some 6,000 years ago, if it is in fact almost unimaginably old, what is the status of the account of creation found in the Bible? If large, fantastic creatures could live for a time, then pass from existence, leaving only their mineralized skeletons in stone, how are we to interpret the perfection of God’s creation? Was the pleisiosaurus a mistake, an early error of God’s design, or is there some other unknown order implicit in the suddenly vast history of an Earth ruled by cataclysm and change?

By giving each woman’s narrative voice to alternate chapters, Chevalier animates their intellectual and emotional lives, but she grounds the story firmly in their long and unusual friendship. Philpot was a spinster, living with her sisters in a cottage in the village of Lyme with the support of their brother, a London solicitor. Anning was both 20 years younger than Philpot and from a poor Lyme family. They meet on the beach searching for fossils. The 8-year-old Anning has an incredible knack for finding “curies” as she calls them (short for curiosities), and she sells them to add to the family’s meager income. Philpot, the more mature observer, mentors Anning in the basics of anatomy and geology, and each inspires a passion for knowledge in the other. Over time, despite their society’s rigid norms, they manage to find a few small ways to transcend the lines of class, age, and gender that separate them from one another and from the larger intellectual community, which is open only to men. Still, the story caries a tone of melancholy tragedy, of human potential frustrated by stultifying social norms. Both women would forever be marginalized, unable to openly contribute to discussion of their own discoveries, and they were dependent on men for even the most meager acknowledgement of their invaluable efforts.

Yet the book is not a fiery indictment of past social norms. While Chevalier has each woman struggling in her own way, she also shows us how hard it is for people to see the limits, whether intellectual or cultural, that they place on themselves. In their friendship and their work, the women demonstrate how change in the human condition, whether intellectual or social, comes when our questions and aspirations take us beyond the limits of our own habits of mind. I found it particularly resonant to read this novel in light of recent research suggesting that unconscious bias against women is still part of the culture of science (and even women are biased against women).

With their rich story beautifully rendered by Chevalier’s agile prose, Anning and Philpot are as remarkable as the creatures that they coaxed from stone. Here’s hoping that the film version doesn’t blow it. My vote is to put Jane Campion in the director’s chair.

Remarkable Creatures was published in 2010 by Plume.


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¿Cuántas Especies?

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This roadside sign (“Many Species Live Here”)  is from the Rincon de la Vieja Rainforest Reserve*, in Guanacaste, Costa Rica, where I have been fortunate enough to do a bit of research on forest biodiversity. But it might just as well represent Earth in its entirety.

But how many species do live on Earth?

This is one of those big questions, perhaps the big question of biodiversity. And as with most big questions, it has many answers, the first of which is, quite simply, “We don’t know.”

However, this typical small answer to big questions is also terribly unsatisfying. So if we want to struggle on against uncertainty, how can we go about estimating the number of species on Earth? First, it is important to note that we can’t simply count up all of the names published by expert taxonomists. The named species total up to about 1.5 million, but while such lists are fairly complete for some groups of organisms, like mammals, for others, like fungi or nematode worms, they are woefully spotty. Scientists have simply not had the time and resources necessary to describe and name all of the organisms. In fact, even this year a new species of monkey was described from the central African rain forests, so we’re still getting to know even some of our closest relatives on Life’s family tree.

So where do we start? Sometimes in science, it pays to start with the obvious, and work from there. Back in 1988, a physicist turned ecologist named Robert May (now Lord May – perhaps the most ennobled ecologist) used a paper in the journal Nature to outline the kind of information that would be relevant to making such an estimate. He pointed out that most of the undescribed species tend to be small and/or rare, either in the sense of having low overall abundance or by being endemic to a very particular environment, because small, rare creatures are hard to find. Sounds obvious to me, but it highlights some of the factors we might want to consider.

Focusing on size, May pointed out that overall, there are generally many more small species than large species, but that the pattern shifts, declining for critters under about a centimeter long. If the decline observed among small species was only because of undersampling, May extrapolated that there could be anywhere from 10-50 million terrestrial animal species alone. While May admits the questionable nature of this estimate, the point remains that there may be many, many animal species out there waiting to be described, especially among the smallest critters. Indeed, several recently discovered vertebrates have been record-setting miniatures, including a fly-sized frog from New Guinea that could easily sit on your pinkie nail and four species of similarly sized chameleons from Madagascar.

Another option is to go to a place where many undescribed species are likely to dwell, sample them extensively, then extrapolate to other unsampled regions. This approach was pioneered by the entomologist Terry Erwin, who in the late 1970’s, went down to Panama to study rain forest beetles. (You’re probably starting to see the pattern that most of the undescribed terrestrial species are from the tropics; the reasons for that will be the subject of another post.) Since most of the beetles in a rainforest live high in the canopy, Erwin had to bring them down to the ground for study, so he constructed large tents around 19 trees belonging to a single, relatively common tree species, Luehea seemannii, then fogged the enclosed trees with an insecticide. Combing through the insects that rained down from the canopy, Erwin found about 1,200 different beetle species. But Erwin didn’t just want to know how many beetle species were on one tree species; he wanted to know how many insect species there were in the world’s tropical forests.

So once he’d killed them all and sorted them out, he did some extrapolating. First, he assumed that beetles were about 2/5 of all arthropods, the group that includes insects, spiders, etc.  This is a pretty well-supported number; nature does seem to love beetles. Having a fair amount of experience hunting for tropical beetles, he also estimated that about 2/3 of them lived in the canopy, while the rest lived low on the trunk (or inside it), inside the leaves, or down in the soil among the roots. Being an expert on beetles, he was able to estimate that about 13.5% of the species he found were specialists, living only in the canopy of Luehea seemannii.  Putting these numbers together, and assuming that other insect groups were similar in the diversity of their habitats and their degree of specialization, he estimated that their were about 611 specialist insects on a single tropical rainforest tree species. Taking the next, more daring step, given that there are an estimated 50,000 tree species in Earth’s tropical rain forests, Erwin ended up with a global estimate of about 30 million tropical insects.

Now given all of the attendant assumptions, Erwin’s number was not meant to be a hard-and-fast estimate. Nor, for that matter, was May’s. Instead, they are meant to put some bounds on our thinking, based on a set of rational, and of course debatable, assumptions. Compiling these sorts of estimates for different types of organisms and consulting taxonomic experts for their opinions has lead to a best guess of there being between 5 and 300 million species on Earth. The very broadness of this answer makes it almost as unsatisfying to me as “we don’t know.” In any case, it points to the fact that we still have a wealth of biodiversity for taxonomists to discover. Moreover, we could really use some better methods for narrowing down our estimates.

A recent (2011) paper by Camilo Mora and colleagues attempts to provide such a narrower estimate. Instead of just considering the number of species, Mora’s group took an historical approach and looked at the rates at which new species were described. This angle had been taken on biodiversity estimation before, but they expanded the approach by capitalizing on the fact that while scientists have certainly not described all of Earth’s species, we have a pretty good handle on the number of some “higher taxa,” meaning higher in the taxonomic hierarchy you probably learned in school: kingdom, phylum, class, order, family, genus, species. They examined data on the rates at which new groups were described for each taxonomic level, from 1750 to today. Early in the development of biodiversity classification, new groups were accumulating quickly in the scientific record, but eventually, the numbers of most of the higher taxonomic groups, like phyla and classes, level off. Using these estimates for the number of higher taxonomic groups in a mathematical model, they are able to estimate the number of species. Across all eukaryotes (anything that is not a bacterium, archaean, or virus), they estimate that the Earth harbors between 7.4 and 10 million species, at the lower end of the 5-300 million range, but still a pretty big number. In fact, if they are right, despite 250 years of intensive study, scientists have described less than 15% of the species that call Earth home.

Like May’s and Erwin’s numbers, Mora’s is not a definitive statement, but a step along the way to further scientific insight. In the grand and messy tradition of science, their approach has attracted both support and criticism, with taxonomic, evolutionary, and statistical experts weighing in. In any case, the tall order of describing and quantifying Earth’s existing biodiversity is given added urgency by the current extinction crisis, and scientists are engaged in developing a variety of field, laboratory, and computational methods to better answer the question. So while we don’t actually know how many species there are on Earth, by any estimate, it’s clear that we have a lot of work to do, and a lot of it needs to be done in the tropics.

*The Rincon Reserve is part of the larger Area Conservacion Guanacaste (ACG), which is a UNESCO World Heritage Site. The ACG encompasses about 2% of Costa Rica’s land area, including tropical dry forest, rain forest, and cloud forest, as well as a substantial marine reserve. It is managed by the government of Costa Rica for conservation and scientific research. It houses about 60% of the species that occur in Costa Rica and is one of the most well-surveyed, and beautiful tropical regions on Earth. Find out more about the ACG (in Spanish) or learn why you should donate to the Guanacaste Dry Forest Conservation Fund (which also protects rain forest and cloud forest (in English).