I have often wondered how would it feel if I was aware of being the last, living individual of my human race or, even more profoundly, the entire human species? Being the last of my race or species would constitute a “double extinction”—that of a single, millennial, genetic experiment of me, as an individual, as well as that of a collective, millennial, genetic experiment of humanity as a whole.
Prior to now, there were five geological periods in which “great extinctions” took place: the Ordovician (about 435 million years ago), the Devonian (about 357 million years ago), the Permian (about 250 million years ago), the Triassic (about 198 million years ago), and the Cretaceous (about 65 million years ago). Today, we are facing what some scientist term “the sixth great extinction,” which began about fifty thousand years ago with the undisputed dominance of humanity on the world scene. But what, exactly, does it mean to become extinct—to be the last individual of a species, such as the last “taimen” or giant salmon of the Uur River in Mongolia, or the last of a race of people, such as James Fennimore Cooper’s “Last of the Mohicans?”
WHAT DOES “EXTINCTION” MEAN?
Extinction means that something no longer exists in its living form; its spark of life has died out like embers of a dying fire. Extinction is generally thought of only in terms of the disappearance of a living entity. But, the concept of extinction goes far beyond living things. The disappearance, the irreparable alteration of a nonliving component of the environment is a “hidden extinction” linked inseparably to the extinction of living things. What, you might ask, does he mean by all this.
Well, look at it this way. While I was working in Nepal in the early 1960s, a helicopter crashed and killed two people. A helicopter has a great variety of pieces with a wide range of sizes. The particular problem here was with the engine, which is held together by many nuts and bolts. Each has a small sideways hole through it so that a tiny “safety wire” can be inserted and the ends twisted together to prevent the tremendous vibration created by a running engine from loosening and working the nut off the bolt. The helicopter crashed because a mechanic forgot to replace one tiny safety wire that kept the lateral control assembly together. A nut vibrated off its bolt, the helicopter lost its stability, and the pilot lost control. All this was caused by one missing piece that altered the entire functional dynamics of the aircraft. The engine had been “simplified” by one piece—a tiny, hidden length of wire.
Which piece was the most important part in the helicopter? The point is that each part (structural diversity) has a corresponding relationship (functional diversity) with every other part, and they provide stability only by working together within the limits of their design, whether natural or artificial.
So, one small alteration in the aerial habitat of two unmarried men caused their extinction, as well as that of the unbroken, millennial, genetic experiment that each man represented. In addition, whatever offspring they might have fathered will never materialize, nor will whatever skill or service the offspring might have offered the human ecosystem and the world. Moreover, one unnoticed, forgetful moment caused the absence of a tiny piece of wire to irreparable altered a critical structure that, in turn, caused the extinction of a vital function—lateral control—within the hidden systemic region of their aerial habitat.
It’s these “hidden extinctions,” the ones of which we are almost always unaware, the ones to which we pay no heed, that cause habitats to change. As a habitat changes, so does its ability to function as it once did. In essence, some functional component has become extinct, which in turn affects the species that are dependent on it. If a habitat changes enough, the species that are adapted to that specific habitat in a certain condition become extinct, because the habitat’s ability to fulfill their requirements became extinct. For an example, let’s consider the rain forest in Gabon, Africa, where to biologist Louise Emmons its fascination lies in “its stunning complexity.”
In this forest, says Emmons, “You can stand anywhere and be surrounded by hundreds of organisms that are all ‘doing something,’ going about their living in countless interactions—ants carrying leaves, birds dancing, bats singing, giant blue wasps wrestling with giant tarantulas, caterpillars pretending they are bird droppings, and so on.”1
In Gabon, Emmons found that nine species of squirrels all live together in one forest. Each is a different size; three species have specialized diets or habits, which leaves six that feed on nuts, fruits, and insects and could therefore be potential competitors for food. But a closer look reveals that three of the six species—a large, a medium, and a small—live exclusively in the canopy of the forest, with the largest one, a “giant” squirrel, feeding primarily on very large, hard nuts while the smaller ones eat proportionally smaller fruits and nuts. The other three species—again a large, a medium, and a small—live exclusively on the ground, where they eat the same species of fruits and nuts as do their neighbors in the canopy, except they eat the fruits and nuts after they fall to the ground.
The forest in Gabon is evergreen, and fruit can be found on the trees throughout the year, but any one species of tree produces fruit for only a short period each year. To support three species of squirrels, eight species of monkeys, and eight species of fruit-eating bats (and so on) in the canopy, the Gabon forest must have a wide variety of species of trees and lianas (high-climbing, usually woody vines), each producing fruits and nuts in its own rhythm. The varying sizes of the fruits and nuts can support different sizes of squirrels with different tastes, whereas these same fruits and nuts when they fall to the ground can feed a whole analogous array of species.
But just how rich in species is a tropical rain forest? Alwyn Gentry of the Missouri Botanical Garden had for many years been counting the species of trees and lianas in tropical rain forest. The richest site he found was a plot two and a half acres in extent near Iquitos, Peru, where he counted an amazing 283 species of trees over four inches in diameter. There were 580 trees of this size in the two-and-a-half-acre plot, which means there was an average of only two individual trees per species, and there were an astounding 58 species among the first 65 individual trees that Alwyn counted.
Worldwide, tropical rain forests seem to have from about 90 to 283 species of large trees within every two and a half acres, and this is not counting the other plants and the animals. Even the “poorest” of tropical rain forests have an average of about five individual trees per species every two and a half acres.
In contrast, a dry tropical forest, such as ccurs in northern India, has about half as many species of trees as does a wet, tropical forest. And the richest forests of the United States have about twenty species of trees over four inches in diameter, with an average of about thirty individuals per species, in each two and a half acres of ground. But most temperate forests are much poorer than this.
So it seems clear that tropical rain forests are amazingly rich in species of trees. But not just any trees: especially those trees whose fruits are eaten and dispersed by birds and mammals. Not surprisingly, therefore, tropical rain forests also are rich in species of mammals and birds. But not just any mammals and birds: especially those that eat fruits and disperse their seeds. There are, for example, 126 species of mammals within a single area of forest in Gabon and 550 species of birds within a single lowland site in the Amazon basin of Peru. Further, the life cycle of each species is interdependent on the life cycles of the other species. The enormous number of vertebrate animals appears to be supported by the large number of species of plants acting as sources of food the year round.
If all this biodiversity is to be maintained, each tree must succeed in leaving offspring. Seeds and tender young seedlings are amongst the richest foods available to forest animals, and their succulence greatly increases their chance of begin eaten by the large numbers of hungry animals searching for food around the bases of fruit- and nut-bearing trees. Likewise, such organisms as fungi, worms, and insects soon accumulate where the seeds and seedlings are concentrated, and they spread from one seed or seedling to another.
Under such circumstances, seeds carried away from such concentrations of hungry organisms are more likely to succeed in germinating. Another major benefit of seeds being carried away from the parent tree is the availability of a wide variety of places with different conditions into which a seed is likely to fall. A new condition might offer a pocket of better soil on a mound created by termites, or in a spot where a dead tree has created a hole in the canopy, a hole that emits sunlight.
It is certainly no accident that about eighty to ninety-five percent of the species of trees in tropical rain forests produce fruits that are dispersed by birds and mammals. By dispersing those seeds, the birds and mammals also are maintaining the rich diversity of species of trees, which not only formed their habitat in the first place but also perpetuate it. This is an ideal example of a self-reinforcing feedback loop.
Many species of trees in the tropical rain forests, especially those germinating in the dark understory, have large seeds that carry enough stored energy to put out leaves and roots without much help from the sun. Such fruits and seeds are often so large that only proportionately sized birds and mammals can swallow or carry them. In Gabon, for example, monkeys dispersed sixty-seven percent of the fruits eaten by animals in Emmons’s area of study.
Seed-dispersing animals, like large birds and large monkeys, are the most important animals for replacing the large trees and lianas of the forest canopy and thus helping them survive. Those animals are, however, the first species to disappear when humans hunt them for food. These species, along with elephants, have already been hunted so heavily that have they either been drastically reduced in numbers or eliminated completely over vast areas of African forest, and the situation in the tropical rain forests of Central and South America is much the same.
A male African elephant reaching up to break a branch off for food.
Foresters, for the most part, have overlooked the whole subject of the way the interdependency of plants and animals affect biodiversity of a plant community. Elephants of the Ivory Coast, for example, disperse the seeds of 37 species of trees. Of those, only seven species had alternate ways of being dispersed—by birds and monkeys. In one study area, out of 201 individual trees, 83 species were dispersed by elephants, which are increasingly on the verge of extinction due to illegal hunting for their ivory tusks.
The illegal trade of elephant tusks is primarily driven by a burgeoning demand for ivory in China, where it has played a centuries-old role in Chinese. And, still today it is seen as an important medium in art and as a symbol of wealth. Hence, the rapidly growing Chinese middle class is fueling demand and escalating the price of ivory ever higher.2
East African Ivory trade in the 1880s/1890s.
In one forest where humans had eliminated elephants a century earlier, few juvenile trees of the elephant-dispersed species were left, and the two major species had no offspring at all. One of these two species just happens to be the single most important species for the two largest squirrels that Louise Emmons studied in Gabon—the one that eats the large, hard nuts in the canopy and the other that eats the same nuts once they’ve fallen to the ground.
Once the large species of birds and mammals are gone, the stunningly rich tropical rain forests will change and gradually lose species of trees, lianas, and other plants. Smaller seeds dispersed by wind will replace large seeds dispersed by large animals. Those species of plants whose seeds grow in the shaded understory will not survive, and the land will gradually be forested by fewer, more common species.
As the forests become poorer in species of plants, the number of species of birds, mammals, and other creatures will decline accordingly. The entire complex of interconnected, interdependent feedback loops among plants and animals will gradually simplify. The species of which the feedback loops are composed will be lost forever (= extinction)—and the feedback loops with them. This is how the evolutionary process works. Ecologically, it is neither good nor bad, right nor wrong, but those changes may make the forest less attractive, less usable by species, such as humans, that used to rely on it for their livelihoods and for products. So, if we want to think about the survival of humans, we have to think about all interrelationships of animals with plants.
The same types of self-reinforcing feedback loops that take place in tropical rain forests occur also in the temperate coniferous forests of the world, and they represent the same four basic elements of diversity: genetic, species, structural, and functional. Genetic diversity is the way species adapt to change; it is the hidden diversity that is so often subjected to the “secret extinctions” mentioned earlier. The most important aspect of genetic diversity is that it can act as a buffer against the variability of environmental conditions, particularly in the long term. So healthy environments can act as “shock absorbers” in the face of catastrophic disturbance.
Here looms a critical concept: the past function of an ecosystem determines its present structure, and its present structure determines its future function. This means that structure is defined by function and function is defined by structure! So, as we alter the composition of species in an area (however that is done), so we at the same moment alter its function in time.
Consider, for example, that all white storks in Europe have traditionally flown south to spend the winter in Africa, but in recent decades an increasing number have stayed closer to home, drawn to the food discarded at garbage dumps—a changed in their behavioral patterns due to human influences. This shift in behavior allows birds that stayed north of the Sahara to survived by feeding on human refuse, enabling them to obtain food without the added energy expenditure of long-distance flight.3 However, the ecological services the storks had traditionally performed on their African wintering grounds now go unfulfilled.
European white stork.
Over time, this new arrangement of species will respond to conditions differently than the original arrangement of species would have—often to our human detriment with respect to the ecosystem services we rely on for a good quality of life.
1. The following discussion is based on: Louise H. Emmons. 1989. Tropical rain forests: why they have so many species, and how we may lose this biodiversity without cutting a single tree. Orion, 8:8–14.
2. Wildlife Conservation Network. http://wildnet.org/elephant-crisis-fund?gclid=CPCbiZq_ncoCFc1ffgodjkwIDw
Andrea Flack, Wolfgang Fiedler, Julio Blas, and others. 2016. Costs of migratory decisions: A comparison across eight white stork populations. Science Advances, 2(1) e1500931 DOI:10.1126/sciadv.1500931 (accessed January 23, 2016).
Text © by Chris Maser 2016. Photos gratefully used from Wikimedia Commons. Photograph of elephant eating by Charles J. Sharp. Photographer of ivory trade unknown. Photograph of European white stork by Ron Knight. All rights reserved worldwide.