By E. O. Wilson, Half-Earth: Our Planet's Fight for Life, May 2016
Editor's note: The following text is from E. O. Wilson's Half-Earth: Our Planet's Fight for Life, chapter 7: "Why Extinction Is Accelerating?" Only a few sentences from the first paragraph were omitted to shorten the text as much as possible. Title and subtitles are mine. I urge the reader to read the entire book.
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Very few people wish to see species disappear, except those occasional pests that attack our bodies and sources of food….Other than still-unknot pathogens among bacteria, microscopic fungi, and viruses, the number of species worthy of extinction, or at least of harmless storage in liquid nitrogen, is probably (my guess( fewer than a thousand…
The millions of other species are beneficial to human welfare, whether directly or indirectly. There are unfortunately almost countless ways humans are hastening their extinction, what ever might be their present or future beneficial roles. The human impact is largely due to the excess of the many quotidian activities we preform just to get on with our personal lives. Those activities have made us the most destructive species in the history of life.
The extinction rate
How fast are we driving species to extinction? For years paleontologists and biodiversity experts have believed that before the coming of humanity about two hundred years ago, the rate of origin of new species per extinction of existing species was roughly one species per million species per year. As a consequence of human activity, it is believed that he current rate of extinction overall is between one hundred and one thousand times higher than it was originally, and all due to human activity.
In 2015 an international team of researchers finished a careful analysis of the prehuman rates and came up with a diversification rate ten times lower in general (groups of closely related species). The data, when translated to species extinctions, suggests species extinction rates at the present time are closer to one thousand times higher than that before the spread of humanity. The estimate is further consistent with an independent study that detected a similar downward shift in the rate of species formation in pre-humans, as well s in their closet relatives among the great apes.
Every expansion of human activity reduces the population size of more and more species, raising their vulnerability and the rate of extinction accordingly. A 2008 mathematical model by a team of botanists predicated that between 37 and 50 percent of rare tree species in the Brazilian Amazon rain forest, “rare” defined as having populations of fewer than ten thousand individuals, will suffer early extinction, caused by contemporary road building, logging, mining, and conversion of land to agriculture. The lower figure, 37 percent applies to areas developed in part but protected by careful management.
It is difficult to make comparisons of origin and extinction rates across different kinds of plants and animals in different parts of the world. But all the available evidence points to the same two conclusions. First, the Sixth Extinction is under way; and second, human activity is its driving force.
Does conservation work?
This grim assessment leads to a second question: How well is conservation working? How much have the efforts of global conservation movements achieved in slowing and halting the devastation of Earth’s biodiversity? Having served on the boards of Conservation International. The Nature Conservancy, and the World Wildlife Fund00U.S., and as advisor to many local conservation organizations, I can testify to the zeal and inspiration, backed by private and public funding, and to the years of sweat and blood in the field, that have gone into conservation efforts around the world during the past half century. How much has this heroic effort accomplished?
In 2010 a survey conducted by close to two hundred experts on vertebrate land animals (mammals, birds, reptiles, amphibians) analyzed the status of all the 25,780 know species. One-fifth were confirmed as threatened with extinction, and of these a fifth had been stabilized as a result of conservation efforts. An independent study in 2006 had already concluded that extinction of bird species in particular had been cut by about 50 percent as a result of conservation efforts during the past century. Thirty-one bird species worldwide still live because of efforts on their behalf. In short, global conservation thus far, when averaged out for land-dwelling vertebrates, has lowered extinction rates of species by approximately 20 percent.
Next, what is the impact of government regulation, in particular the U.S. Endangered Species Act of 1973? A review made in 2005 found that a quarter of the 1,370 American plant and animal species classified earlier as threatened achieved new population growth, while 40 percent declined, with 13 of the listed species improved enough to be taken off the endangered list. The most important statistic is that while 22 spaces had slipped into extinction, 227 had been saved that would likely have otherwise disappeared. Among the more familiar protected species that have climbed back to health are the yellow shouldered blackbird, green sea turtle, and bighorn sheep.
These successes have show that conservation works, but at the level of effort being applied at the present, it falls far short of what is needed to save the natural world. The conservation movement has slowed the species extinction rate but failed to bring it anywhere close to the prehuman level. At the same time the birth rate of species is dropping rapidly. Like and accident patient in emergency care continuing to hemorrhage and with no new supply of blood available, stabilization is out of reach and further decline and death are inevitable. We might be inclined to say to the surgeons and conservationists alike, “Congratulations. You have extended a life, but not by much.”
Of course, not all wild species are threatened by the assault on biodiversity. A few are compatible with a humanized environment. What fraction of the present survivors will last to the end of this century? If present conditions persists, perhaps half. More likely, fewer than one-fourth.
That is my guess. But the fact is that due to habitat loss alone, the rate of extinction is rising in most parts of the world. The preeminent sites of biodiversity loss are the tropical forests and coral reefs. The most vulnerable habitats of all, with the highest extinction rates per unit area, are rivers, streams, and lakes in both tropical and temperate regions.
An established principle in conversation biology for all habitats is that a reduction in art results in a fraction of the species disappearing in time by roughly the fourth root of the area. If 90 percent of a forest is cut, for example, about half of the species will soon disappear that would have been otherwise persisted. And the beginning most of the species may survive for a while, but roughly half will have populations too small to persist for more than a few generations.
Barro Colorado Island in Panama has proved a valuable natural laboratory to study the effect of area on extinction. Covered by rain forest, it was created by the formation of Gatun Lake in 1913 during the construction of the Panama Canal. An ornithologist, John Terborgh, predicated that after fifty years the island would lose seventeen bird species. The actual number was thirteen, representing 12 percent of the one hundred eight breeding species originally present. On the other side of the world, a 0.9-square-kilometer patch of rain forest, the Bogor Botanical Garden of Indonesia, was also isolated, not by water but by clearing of all the forest around it. In the first fifty years it lost twenty of its sixty-two breeding bird species, approximately the number expected.
Causes of extinction
Conservation scientists often use the acronym HIPPO for a quick recall of the most ruinous of our activities, in order of importance:
Habitat destruction. This includes that caused by climate change.
Invasive species. This include plants and animals that crowd out native species and attack crops and native vegetation, as well as microbes causing disease in humans and other species.
Pollution. The effluents from human activity are killers of life, especially in rivers and other freshwater ecosystems, the most vulnerable of Earth’s habitats.
Population growth. Although it is still widely unpopular to say so, we must really slow down. Reproduction is obviously necessary, but it is a bad idea, as Pope Francis I has pointed out, to continue multiplying like rabbits. Demographic projections suggest that the human population will rise to about eleven billion or slightly more before the end of the century, thereafter peak, and begin to subside. Unfortunately for sustainability of the biosphere, per-capita consumption is also destined to rise, and perhaps even more steeply than human numbers. Unless the right technology is brought to bear that greatly improve efficiency and productivity per unit area, there will be a continued increase in humanity’s ecological footprint, defined as the area of Earth’s surface each person on average needs. The footprint is not just local art, but space scattered across land and sea, in pieces for habitation, food, transportation, governance, and all other services down to and including recreation.
Overhunting. Fishing and hunting can be pressed until the target species is driven to extinction or on the way, making the last surviving populations subject to final erasure by disease, competition, changes in weather, and other stresses survived by larger ad more far-ranging populations of the same species.
Single causes in the decline and extinction of species can be identified readily in a few cases. One example is the dietary preferences of the brown tree snake, which is especially skilled in preying on bird nestlings. Another has been identified in the decline of the monarch butterfly in the Midwest, which famously overwinters in masses of millions on pine trees in Mexican state of Michoaćan. By 2014, there was an 81 percent decline of the butterflies in the United States Midwest populations, attributed to a 58 percent decline in milkweeds, the exclusive food plant of the monarch caterpillars. The milkweeds have fallen off in turn because of the increased use of glyphosate weed killer in corn (maize) and soybean fields. The crops have flourished after being genetically modified to resist the weed killer, while the wild milkweed plants have not been so protected. With their food supply unintentionally reduced the migrating monarch butterflies of both the United States and Mexico have declined steeply.
In most extinctions, however, the causes are multiple, linked to one another in some way, all ultimately the result of human activity. In one well-analyzed example of multiple causes, the Allegheny wood rat has vanished or grown endangered through a third of its range. It is considered to have suffered from the extinction of the American Chestnut and hence the loss of its seeds, on which the species partly depend. Also important have been the logging and fragmentation of forests where the wood rats live, plus further reduction of this habitat by the voracity of the invasive European gypsy moth. The coup de gras is the roundworm infection from raccoons, which are animals better suited than the wood rats to live around humans.
Those unimpressed by the fall of a rodent may turn a more caring eye to the songbirds that migrate each yea between their winter grounds in the New World tropics and breeding range in the eastern United States. From data compiled by the federally sponsored North American Breeding Bird Survey along withe Audubon Society Christmas Bird Count, it is clear that populations of more than two dozen species are in a steep descent. Those affected include the wood thrush, Kentucky warbler, eastern kingbird, and bobolink. One, Bachman’s warbler, whose wintering ground was in Cuba, has gone extinct. I have a place in my heart for this little bird. During field trips to the floodplain forests of the U.S. Gulf Coast, which brought me close to canebrakes where the warbler once nested, I’ve often looked and listened for a Backman’s as best I could (admittedly not very skillfully), but to no avail.
It sometimes seems as though the reminder of American native plants and animals are under deliberate assault by everything humanity can throw at them. Leading the list is our deadly arsenal are the destruction o both wintering and breeding habitats, heavy use of pesticides, shortage of natural insect and plant food, and artificial light pollution causing errors in migratory navigation. Climate change and acidification pose newly recognized yet game-changing risks—shifts in all aspects of the rhythms of the environment away from those necessary for wildlife survival and reproduction.
Facts about biodiversity
There exist several facts about global biodiversity to keep in mind while trying to save it. The first is that the human caused agents of extinction are synergistic. As any one of the agents intensifies, it causes others to intensify also, and the sum of the changes is an acceleration of extinction. Clearing a forest for agriculture reduces habitat, diminishes carbon capture, and introduces pollutions that are carried downstream to degrade otherwise pure aquatic habitats en route. With the disappearance of any native predator or herbivore species, the reminder of the ecosystem is altered, sometimes catastrophically. The same is true of the addition of an invasive species.
Another overarching principle of biodiversity is the greater richness of tropical environments over temperate environments, in both number of species and vulnerability. While the variety of aphids, lichens, and conifers increases poleward, a vastly larger number of other kinds of organisms increase in the opposite direction. For example, you can expect to find about fifty species of ants in a square kilometer of New England temperate forest (if you care to look) but up to ten times that number in a comparable area of rain forest in Ecuador or Borneo.
A third principle of biodiversity worth noting is in the relation between its richness and the geographical range of the species comprising it. A large fraction of the species of plants and animals of the temperate NorthAmerican are distributed across most of the content, but very few species range the same way across tropical South America.
When these last two principles are linked, both entailing the numbers of resident species, we find as expected that on average tropical species are more vulnerable than temperate species. They occupy smaller ranges and thereby manage to sustain smaller populations. Furthermore, existing as they do among a greater array of competing species, they tend to be more specialized in where they live, in what they eat, and by the predators that hunt and eat them.
As a general rule to follow in conservation practice is therefore that, while clear-cutting a squat kilometer of old-growth coniferous forest in Canada, Finland, or Siberia will do a lot of environmental damage, cutting the same area of old-growth rain forest in Brazil or Indonesia will do far more damage.
Finally, there is the immense disparity between the 62,839 known species of vertebrates (mammals, birds, reptiles, amphibians, fishes, the total number in 2010) and 1.3 million known invertebrate species (also 2010). Almost all the information on quantitative trends in biodiversity are based on the vertebrates, the big animals with which we have intimate familiarity. There are well-studied groups within the invertebrates, notably mollusks and butterflies, but even these organisms are less well known than the mammals, birds, and reptiles.The great majority of invertebrate species, notably hyper diverse insects and marine organisms, remain to be discovered and made known to science. Nevertheless, of those groups well enough studies to make estimate of their conservation status down to species, such as freshwater crabs, crayfish, dragonflies, and corals, the percentages of vulnerable and endangered species are comparable to those of vertebrates.
Avoiding misconceptions about the Sixth Extinction
In thinking about life and death in the biosphere, it is important to avoid two misconceptions. The first is the a rare declining species is probably senescent. You might think that its time has come so let it go. The contrary, its young are just as vital as the young of the most aggressive expanding species with which they compete. If its population is shrinking in size from vulnerable to endangered to critically endangered (the scale of descent used in the Red List of the International Union of Conservation of Nature)*, the reason is neither age nor destiny of the species. Instead, the reason is the predicament in which the Darwinian process of natural selection has placed it. The environment is changing, and the genes assembled by earlier natural selection are by happenstance not able to adapt quickly enough. The species is a victim of bad luck, rather like a farmer who invests at the start of a ten-year drought. Put some young individuals into an environment where the genes are better adapted, and the species will flourish.
Humanity, keep in mind, is the principal architect of such maladaptive environments. Conservation biology is the scientific discipline by which better environment are identified and protected or restored for species imperiled within them.
Biologists recognize that across the 3.8 billion-year history of life over 99 percent of all species are extinct. This being the case, what, we are often asked, is so bad about extinction? The answer, of course, is that many of the species over eons didn’t die at all, they turned into two or more daughter species. Species are like amoebae; they multiply by splitting, not by making embryos. The most successful are the progenitors of the most species through time, just as the most successful humans are those whose lineages expand the most and persist the longest. The birth and death rates of humans are close to a global balance, with birth having the edge for the last sixty-five thousands years or so. Most important, we, like all other species, are product of a highly successful and potentially important line that goes back all the way to the birth of humanity and beyond that for billions of years, to the time when life began. The same is true of the creatures still around us. They are champions, each and all. Thus far.
* The Red List scale for individual species is the following in descending order: Least Concern (LC), Near Threatened (NT), Endangered (N), Critically Endangered (CR), Extinct in the Wild (EW), Extinct (E).