There must be by definition somewhere on Earth a site with the greatest variety of organisms. The Yasuni National Park of Ecuador, which encloses a magnificent rain forest between the Rio Napo and Rio Curaray, is reputed to be that one biologically richest place on Earth. More precisely, its 9,820 square kilometers are believed to contain more species of plants and animals than any other piece of land of comparable area. The known roster supports the claim: recorded in the whole park are 596 bird species, 150 amphibian species (more than the number in all of North America), as many as 100,000 insects, and, growing in just a single average upland hectare, 655 tree species—also more than occur in all of North America. The only question about Yasuni’s supremacy is whether there might exist some other, less explored section of the Amazon and Orinoco Basins that will prove even more diverse. At the very least, the Yasuni National Park is very close to the extreme of its kind. And outside the Amazon-Orinoco region, nothing in the world can approach it.
There is another reason to pay attention, not yet widely recognized even by most biologists: the Yasuni National Park may harbor the highest species numbers that have ever existed. Throughout the entire history of life, from the Paleozoic Era forward, 544 million years, the number of plant and animal species worldwide has been very slowly rising. Thus at the breakout from Africa and worldwide spread of Homo sapiens, beginning about 60,000 years before the present, Earth’s biodiversity was likely at its all-time maximum. Then, extinction by extinction, human activity began to whittle the number down, and today that pace is accelerating. For the time being, Yasuni holds its own, and that is why it is recognized as a world treasure. We know only a fraction of the species of animals, especially the insects, found in the Yasuni, and next to nothing of their biology. We would like to take the full measure of this place, and of others of similar extreme high diversity, and come to understand the reason for its preeminence—before it is ruined by human greed.
In extreme opposition, there exists on Earth a close outward approximation of the lifeless surface of Mars. In its own way it has been worth exploring. The place is the McMurdo Dry Valleys of Antarctica. To casual inspection the land seems as sterile as the surface of autoclaved glassware. But life is there, and it makes up the sparest and most stubborn of all of Earth’s ecosystems outside the open surface of polar ice. Even though nitrogen is at the lowest concentration of any habitat on Earth, and water is almost nonexistent, it is surprising to find bacteria in the soil of the McMurdo Dry Valleys. The rocks scattered about seem lifeless, yet some are etched with almost invisible crevices in which communities of lichens live. These organisms are minute fungi that live symbiotically with green algae. They are concentrated layers just two millimeters beneath the surface of the rocks. Farther in, other such endoliths (“living in rocks”) include bacteria capable of their own photosynthesis.
Scattered about in the McMurdo Dry Valleys are frozen streams and lakes, which contribute a small amount of moisture in the surrounding soil. The free water, which occurs in droplets and films, harbors small numbers of almost microscopic animals: tardigrades, the strange creatures sometimes called “bear animalcules” that I mentioned earlier, rotifers (“wheel animalcules”), and, most abundant of all, nematodes, also called roundworms. Although barely visible to the naked eye, the nematodes are the tigers of the land, the top of the food chain in this quasi-Martian world, and the antelope equivalents on which they feed are bacteria in the soil. In a few places can also be found rare mites and springtails, the latter a primitive form of insect. In all, sixty-seven species of insects have been recorded from the combined habitats of Antarctica, but only a few are free-living. The great majority are parasites that live in and on the warm plumage of birds and the fur of mammals.
As I write, there are many other places on the planet in which biological exploration has only begun. The greatest depths of the ocean, the abyss of eternal dark, consists of great submerged mountain ranges incised by deep unvisited valleys and intervening vast plains. The tips of many of the mountains rise above the water to form the oceanic islands and archipelagoes. Some come close but remain submerged. There are the seamounts. Their peaks are coated with marine organisms, many of whose species are unique to the location. The exact number of seamounts is still unknown. It has been estimated to run in the hundreds of thousands. Imagine the extent of human ignorance! Beneath the surface of the oceans and seas, which cover 70 percent of the Earth’s surface, there exists an all but countless number of lost worlds. Their complete exploration will occupy generations of explorers from every discipline of science.
Life on Earth remains so little known that you can be a scientific explorer without leaving home. We have scarcely begun to map Earth’s biodiversity at any level, from molecule to organism to niche in an ecosystem. Consider the following numbers of known and unknown species among different taxonomic groups of organisms around the world. They are why I like to call Earth a little-known planet. The data were pulled from global surveys made under the auspices of the Australian government in 2009.
The total number of species estimated in 2009 to have been discovered, described, and given a formal Latinized names worldwide was 1.9 million. The true number, both discovered and remaining to be discovered, could easily exceed 10 million. If the single-celled bacteria and archaea, the least known of all organisms, are added, the number might soar past 100 million. Five thousand kilograms of fertile soil contain, by one estimate, 3 million species, almost all unknown to science.
Why haven’t scientists made more progress in exploring the world of bacteria and archaea? (The latter are an important group of single-celled organisms that outwardly resemble bacteria but possess very different DNA.) One reason for our ignorance is that a satisfactory definition of “species” in these organisms remains to be made. An even more important reason is that the different kinds of bacteria and archaea are so diverse in the environments they require in order to grow, and in the food they need to eat. Microbiologists have not learned how to culture the great majority of bacteria and archaea, in order to produce enough cells for scientific study. With the advent of rapid DNA sequencing, however, the genetic code of a strain can be determined with only a few cells. As a result, the exploration of species diversity has increased dramatically.
In citing these remarkable figures on biodiversity, I am not suggesting that you plan to become a taxonomist—although for now and many years ahead that would not be a bad choice. Rather, I wish to stress how little we know of life on this planet. When we also consider that the species is only one level in the hierarchy of biological organizations, molecule to ecosystem, then the immense potential of biology, and of all of the physics and chemistry relevant to biology, becomes immediately apparent.
If scientists know so little of raw biological diversity at the taxonomic level, we know even less of the life cycles, physiology, and niches of each species in turn. And for all but a very few localities on which biologists of diverse training have focused their energies, we are equally ignorant of how the idiosyncratic traits of individual species fit together to create ecosystems. Ponder these questions for a while: How do pond, mountaintop, desert, and rain forest ecosystems really work? What holds them together? Under what pressures do they sometimes disintegrate, and how and why? In fact, many are crumbling. Humanity’s long-term survival depends on acquiring answers to these and many other related questions about our home planet. Time is growing short. We need a larger scientific effort, and many more scientists in all disciplines. Now I’ll repeat what I’ve said when I began these letters: you are needed.
The female gypsy moth, located at the lower point of the active space, releases a pheromone cloud within which is a region of high concentration followed by the male. Drawing by Tom Prentiss (moths) and Dan Todd (active space of gyplure © Scientific American). Modified from “Pheromones,” by Edward O. Wilson, Scientific American 208(5): 100–114 (May 1968).
Seventeen
THE MAKING OF THEORIE
S
THE BEST WAY I can explain the nature of scientific theories to you is not by abstract generalizations but by offering examples of the actual process of making theory. And because this part of science is the product of creative and idiosyncratic mental operations that are seldom put into words, I will stay as close to home as possible by using two such episodes in which I have been personally involved.
The first is the theory of chemical communication. The vast majority of plants, animals, and microorganisms communicate by chemicals, called pheromones, which are smelled or tasted. Among the few organisms that use sight and sound primarily are humans, birds, butterflies, and reef-dwelling fish. Working with the social behavior of ants in the 1950s, I became aware that these highly social insects use a variety of substances that are released from different parts of the body. The information they transmit is among the most complex and precise found in the animal kingdom.
As new information began to pour in, those of us conducting the early research saw that we needed a way to pull together the fragmented data and make sense of them. In short, we needed a general theory of chemical communication.
I was extremely fortunate during this early period to serve as the cosponsor of William H. Bossert, a brilliant mathematician working for a Ph.D. in theoretical biology. After completing his degree requirements in 1963, he was invited to join the Harvard faculty, and in a short time thereafter he received a tenured professorship in applied mathematics. While still a graduate student, he joined me in creating a theory of pheromone communication. The time was right for such an effort, and we were successful. On no other occasion in my scientific career has a project worked out as quickly and as well as did the collaboration with Bill Bossert.
To kick things off, I told him what I knew about the new subject. I laid out the basic properties of chemical communication as I had come to understand them. There was not a great deal of information to go on in this early period. From field and laboratory studies, I said, we knew that a wide variety of pheromones exist. It seemed logical that we should begin with a classification of the roles of all of those known, then try to make sense of each one in turn. The theory should deal not just with form and function of the pheromone molecules, which was the goal of most researchers, but also with their evolution. Put simply, we wanted to know what the pheromones are and how they work, of course, but also why they are one kind of molecule and not another.
Before giving you the theory, here are the specific “why” questions we meant for it to explain. Is the pheromone molecule used the best way possible, or is it one that was selected at random during evolution out of a limited array available for the job? What do the pheromone messages “look like” if you could see them spreading through space? Should the animal emit a lot of the pheromone or just a little in each message? How far and fast do the pheromone molecules travel through air or water, and why?
Here, then, in a nutshell, is the theory. Each kind of pheromone message has been engineered by natural selection—that is, trial and error of mutations that occur over many generations resulting in the predominance of the best molecules, with the most efficient form of transmission allowed by the environment. Suppose a population of ants is started by two ant colonies who compete with each other. The first colony makes one kind of molecule and dispenses it in a certain way, and the second colony makes another kind of molecule that is less efficient, or else is dispensed less efficiently, or both. The first colony will do better than the second, and as a consequence it will produce more daughter colonies. In the population of colonies as a whole, the descendants of the first colony will come to predominate. Evolution has occurred in the pheromone, or in the way it is used, or both.
Bossert and I agreed: “Let’s think about ants and other organisms using pheromones as engineers.” This thought took us quickly to ants recruiting other ants by laying a trail for them to follow. So, at the next picnic (or on your kitchen floor if the house is infested) drop a crumb of cake. It is logical to suppose that the ant scout that finds it needs to dribble out the trail pheromone at a slow rate in order to make the store of the substance she carries in her body last a long time. The piece of cake may be several ant-mile equivalents away. In this function, the ant is like an automobile engine designed for high mileage. In order to achieve such efficiency, the pheromone needs (in theory) to be a powerful odor for the ants following the trail. Only a few molecules should suffice. Also, the pheromone must be specific to the species using it, in order to provide privacy. It is bad for the colony if other ants from other species can pirate the trail, and even dangerous for the colony if a lizard or some other predator can follow the trail back to the nest. Finally, the trail substance should evaporate slowly. It needs to persist long enough for other members of the colony to track it to the end, and start laying trails of their own.
Then there are the alarm substances. When a worker ant or other social insect is attacked by an enemy, whether inside or outside the nest, it needs to be able to “shout” loud and clear, in order to get a fast response. The pheromone must therefore spread rapidly and continuously over a long distance. But it should also fade out quickly. Otherwise even small disturbances, if frequent, would result in constant pandemonium—like a fire alarm that cannot be turned off. At the same time, unlike the case for trail substances, there is no need for privacy. An enemy can gain little by approaching a location teeming with alert and aggressive worker ants.
Let me pause here to describe an easy way for you to smell an alarm pheromone yourself. Catch a honeybee from a flower in a handkerchief or other soft cloth. Squeeze the crumpled cloth gently. The bee will sting the cloth, and as it draws away it will leave the sting (which has reverse barbs) stuck in the cloth. When that happens, the immobile sting pulls out part of the bee’s internal organs. Let the bee move to the side, then crush the sting and the organs between two fingers. You will smell an odor that resembles the essence of banana. Its source is a mixture of acetates and alcohols in a tiny gland located along the shaft of the sting. These substances function as an alarm signal, and they are the reason other bees rush to the same site and add their own stings. Next, if the eviscerated bee hasn’t flown away, crush its head and smell that. The acrid odor you detect is from a second alarm substance, 2-heptanone, emitted by glands at the base of the mandibles. (Don’t feel bad about killing a worker bee. Each has an adult life span of only about a month, and it is only one of tens of thousands that make up a colony. The colony in turn is potentially immortal, since new mother queens replace the old ones at regular intervals.)
The next category of pheromones are the attractants, in particular the sex pheromones, by which females call to males for the purpose of mating. The phenomenon is widespread not only in social insects but also throughout the animal kingdom. Other attractants also include the scent of flowering plants, in which the flowers call to butterflies, bees, and other pollinators. The most dramatic substances of the kind are the sex attractants of female moths, which can draw males upwind for distances of a kilometer or more.
Finally, Bossert and I reasoned in our initial classification, there are the identification substances. An ant, upon smelling these substances, can tell whether another ant is from the same or a different colony. It can also identify a soldier, ordinary worker queen, egg, pupa, or larva, and if the latter, its age. Carrying a chemical badge of this kind with you at all times means wearing the pheromone like a second skin. An identity pheromone is a single substance or, more likely, a mix of substances. It needs to evaporate very slowly and be detectable only at a very close range. If you closely watch one ant or some other social insect approach another, say while running along a trail or entering a nest, you will see the two sweep each other’s body with their two antennae—a movement almost too fast for the eye to catch. They are checking body odor. If they detect the same odor, each will pass the other by. If the body odor is different, they will either fight or else flee from each other.
Reaching this
point in the investigation, Bossert and I left the “adaptive engineering” method of evolutionary biology and passed into biophysics. We needed to envision the spread of the pheromone molecules from the body of the animal releasing them, and as precisely as possible. Obviously, as the pheromone cloud disperses, its density would decline—there would be fewer and fewer molecules in each cubic millimeter of space. Eventually there would be too few to smell or taste. Bossert then devised the crucial idea of “active space,” within which the molecules are dense enough to be detected by the receiving plant, animal, or organism. He constructed models (at last, a place for pure mathematics!) to predict the shape of the active space. We were now in a new phase in creating the theory of pheromone communication.
With the ant or any other broadcasting organism sitting on the ground in still air, the shape of the active space would be hemispherical—one half of a sphere cut in two—with the broadcaster at the center of the flat surface. When an organism releases the pheromone from a leaf or object off the ground and in an air current, the shape of the active space would be an ellipsoid (roughly, shaped like an American football), tapering to a point at each end. The broadcaster would be at one of the points, releasing the pheromone downwind. When a trail is laid on the ground in quantities sufficient for it to be detected over a long period of time, the space would become a very long semiellipsoid, in other words an ellipsoid cut in half lengthwise at ground level.
Next we turned our attention of the design to the molecule itself. Trail substances and identification odors should consist, we reasoned, of either relatively large molecules or mixes of large molecules. They should diffuse slowly. Alarm pheromone molecules should be chosen in evolution to be smaller in size. They should form a more limited active space, and dissipate quickly. The qualities of the active space depend on five variables that can be measured: the diffusion rate of the substance, the surrounding air temperature, the velocity of the air current, the rate at which the pheromone is released, and the degree of sensitivity of the organism receiving it. With these measurable quantities in place, the theory began to take shape in a form that could be taken into the field and laboratory, and used to study animals as they communicated.
Letters to a Young Scientist Page 11