I collected butterflies and other insects as a child. For a couple of years, when we lived in Utah, I mainly collected locusts, the ones with colorful wings. There were many different wing color patterns. Now, fifty years later, I find that both butterflies and colorful locusts (when I visit Utah) are quite a bit scarcer. Where I live now, in the suburbs southwest of Philadelphia, I have seen more butterflies than I did for a long, long time. But nothing matches those young years in Utah and Ohio. I also recall, during high school years in Sandusky, Ohio, recording morning bird song. I wish I still had the tapes! The "morning chorus" that began half an hour before sunrise in the Spring was a rich symphony. I could recognize the calls of 6 or 8 kinds of birds, and heard several calls I didn't know, every time. There is a pretty good morning chorus here, these days, but again, it pales by comparison with what I recall. Two to three kinds of bird calls are the usual fare.
This is not just me, remembering some "golden age" that never existed. Things are dying out, lots of them. I have been hearing about a "sixth extinction" for some years now. This is the title of a sobering, well-researched book by Elizabeth Kolbert, The Sixth Extinction: An Unnatural History.
What is the "normal" rate of species extinction? To jump in with the conclusion, it is probably close to one species yearly. It is closely allied to the normal rate of species turnover. That is, "extinction" can mean one of two things. Firstly, one species changes to another under the pressure of environmental change. When the two species are things like shellfish that leave good fossils, what a geologist would notice is that in rocks of a certain age, only shells of "type 1" are found, and in the next layer, only those of "type 2". An ecologist might notice that certain "recent fossils" are not found among living shellfish. I saw an example of this in Bear Lake, Idaho 60 years ago, snail shells in abundance, but a ranger told us they no longer lived in the lake; there was a different species now.
In geologic terms, the time span across a couple of millimeters of sedimentary rock might be a million years, so the speciation even could be quite gradual as seen from a human perspective. Observations of animals under selection pressure indicate that one may be replaced by another in much less than a million years: 50 to 100 years is sometimes sufficient. Many, many animal species live their life out within one year, so this represents 50-100 generations. Longer-lived creatures are a different matter. Horses, for example, can reproduce as early as two years, but have a fertile lifetime of about ten years, sometimes more. So a "horse generation" is probably about 6-8 years. A human generation is commonly thought of as 25 years, though in very early times it was probably closer to 20. Anyway, species transformation (I dislike the popular conception of "mutation") can occur in hundreds or thousands of years, to perhaps tens of thousands of years. This is synchronous extinction.
The second kind of extinction is that a species dies out when the environment changes to rapidly for it to adapt, and it is no longer suited to it. It may or may not be replaced, in ecological terms, by an unrelated (or more distantly related) species, which may have evolved about that time, or maybe not. This is asynchronous extinction. Depending on the kind of animal, a species that makes fossils is seen to last between one and ten million years, though some that we call "living fossils" are found to have lasted for tens or hundreds of millions of years. Though I wonder if a coelacanth living today could actually be bred with one somehow brought to the present from 300 million years ago; perhaps there have been a hundred synchronous extinctions along the line, as the animal changed in profound ways that did not materially affect what its fossil form would be.
"Mass Extinction" refers to the sudden disappearance of many species over a shorter period of time. A mass extinction is thought to happen because of a great and widespread change in environmental conditions. These are, of necessity, asynchronous extinctions. One thing that can utterly transform the environment worldwide, at least for a time, is the fall of an asteroid a few miles wide. An asteroid impact eliminated the dinosaurs (those that hadn't become birds already), in what is called the end-Cretaceous extinction event. There have been five major mass extinction events in the last 500 million years, and a few dozen lesser ones. Each of the "Big 5" drove at least half of all species out of existence, pretty much overnight. They mark the boundaries between geologic ages. The lesser ones were of less significance only in comparative terms, and also mark the boundaries of geologic ages or significant geologic periods.
The major ages of geologic time are called:
CambrianThe Big 5 ended the Ordovician, Devonian, Permian, Triassic and Cretaceous. Other named ages and periods also ended with lesser mass extinctions.
Mississippian + Pennsylvanian in America, Carboniferous elsewhere
Mesozoic, including Triassic, Jurassic, and Cretaceous
Tertiary (or Paleogene)
Quaternary (or Neogene)
The trouble with geologically sudden events is that, on a human scale, they may not appear sudden at all. While the dinosaur-killing asteroid changed all of Earth's environments in at most a few days, the other mass extinctions seem to have taken more time, in the range of years to centuries, and perhaps tens of millennia. On a scale that considers a thousand-year transformation as "sudden", something that takes only a century is lightning-fast.
That is what we see happening. Synchronous species turnover, and most cases of asynchronous extinction, make up the background rate. Roughly speaking, if species last on average a couple million years, and there are about a million species, then some species or other will go extinct every year or every second year. That's a ballpark estimate of background extinction. One per year or a half that. If the greatest of the Big 5, the Permian catastrophe, took 1,000 years to occur, and nearly a million species were wiped out, that is a rate 1,000 to 2,000 times greater than the background.
The chapters of the The Sixth Extinction each focus on one species, as an example of a group of related species, that are greatly reduced or already extinct. Most are known or strongly suspected to be due to human influence. The first example is a Panamanian tree toad, a "poison dart frog", that is probably already extinct. It represents amphibians in general, that are vanishing at a stunning rate. Of 6,200 species of amphibian (frogs, toads, newts, salamanders, and a couple of similar odd critters), about 1,800, or nearly 30%, are reducing in number rapidly, and at least 440, or 7%, are likely to become extinct within very few years. Just among amphibians, the extinction rate is about 100 times the background rate for all species!
One example cannot possibly be due to human influence (unless you are a strict, young-Earth creationist), the Ammonites. These spiral-shaped critters actually survived the biggest mass extinction, the one at the end of the Permian, 251 million years ago, but were wiped out later on by the end-Cretaceous event, the one that famously ended the "age of reptiles", but let some few mammals and birds (small, feathered dinosaurs) sneak through and repopulate Earth. The chapter focuses on the consequences of an Asteroid Winter, and compares it with other possible causes of mass extinctions. It sets the stage for discussing the massive environmental changes we humans are bringing about. If we are indeed the major actor in the environmental shift called Global Warming, and I think we probably are, that is but one large-scale change in worldwide habitats that we have produced, and the one most likely to kill us along with so many other species.
One chapter dwells on rain forests ("jungles"), and the species-area relationship as determined by counting the number of species to be found in various sized plots of forest. In a portion of a large forest, there is a variation of species quantity with area, which is partly statistical. But in a dissected forest, with forested plots of various sizes surrounded by barren land or farms, there is a similar relationship, though it is steeper. The species of focus for the chapter, a small tree in the genus Alzatea, is not found at all in an isolated plot if its area is below a specific number of acres. When a large forest is broken up into isolated plots, at first, the S/A relationship follows that of the original forest. But over time, species are lost, most rapidly from the smallest plots, until a steeper S/A relationship is developed. Sometimes, keeping plots from total isolation by having forested "highways" between them will preserve some species, but this is not true for all. It is like some species die out if their members cannot get far enough from the forest boundary.
Twelve chapters, twelve species plus a thirteenth, about Homo sapiens, about us. We are very likely to be the ultimate victims of the great mass extinction that we are carrying out. It is not known just how many species go extinct every year. I once read of an experiment with "tree fogging", in which researchers used insecticide fog throughout the canopy of an entire tree, and collected all the insects, particularly beetles, that fell onto sheets spread under the tree. Dozens of new species were found and described. Excitedly, they fogged a second tree. Many more new species were found. But they were sobered that most of the new species from tree #1 were not found at all on tree #2, even though they were within a hundred meters of each other. They concluded that many of those, perhaps 100 species, were endemic not just to that forest, but to that specific tree, and found nowhere else. They had made 100 species extinct in an afternoon! They canceled future experiments of that type.
It is hard to find out everything that exists without going and finding them. But when doing so destroys what you are trying to find, what good is that? I don't how to find out how fast species are going extinct, but I think it very, very likely that this human-induced mass extinction is proceeding at a rate that exceeds that of the Permian event, the biggest of the Big 5, by a large margin. I do believe this needs to be more widely known.