Sometimes it helps to define terms as you get started on a journey. This allows us to discuss questions within a common framework and avoids unnecessary confusion. In some cases it even helps us to avoid legal hassles. Below are some important definitions.
Extinct (this one seems so obvious, and yet it presents numerous problems):
This is an obvious definition, but how do you apply it? To use an extreme example, the coelacanth (a lobe-finned fish close to the lineage which gave rise to amphibians) was believed to have gone extinct during the Cretaceous (more than 65 million years ago) until a small population of these fish was found off the coast of southeast Africa. Other species believed to be extinct are occasionally sighted (although most of these sightings are unconfirmed). To avoid this problem, the IUCN (International Union for the Conservation of Nature) has come up with the following definition of "extinct":
Note that this slightly legalistic definition refers to animals being located in the wild. What this does is synonymize "extinct" with:
Extinct in the Wild which means that the only members of this species still alive are in captivity or some other human controlled situation.
In either case, the above definitions imply that the species in question is:
Globally Extinct not to be found in the wild anywhere in the world.
This is opposed to a much more common case in which animals are termed:
Locally Extinct - meaning that they are now totally absent from certain portions of what had been their nature range (e.g., The wolf is Locally Extinct in the Adirondacks).
Here we can also use the term Exterpation (the verb form is exterpate), which means that the population of a given species has been exterminated from an area.
Finally, a term which is used informally by some ecologists is Ecologically Extinct. This refers to species whose numbers are so small that they no longer have any significant effect on the community they are (or were) a part of. As an example, consider the role of predators with regard to a herd of herbivorous animals. In many cases the predator acts to cull out the members of the herd who are sick, old or young with bad habits (wandering away from the herd). If the predatory species becomes extremely rare, it no longer affects the members of the herd.
Other terms (from the IUCN) which are often used in discussing the status of a species, and which show both the legalistic and probabilistic approach now necessary in defining terms are:
Critically Endangered species which have a 50% or greater probability of extinction within 10 years or three generations.
Endangered - species which have a 20% or greater probability of extinction within 20 years or five generations.
Vulnerable - species which have a 10% or greater probability of extinction within 100years.
Note the necessity of including both a time range and a number of generations. Remember, we are dealing with organisms whose life expectancy can range from months to 100s of years (some plants).
Definitions Mass Extinctions
Now we move on from consideration of extinction as a single species phenomenon to multi-species events. Over about the past fifty years scientists have come to realize that at times during the history of the earth very large number of species have gone extinct over relatively short periods of time (geologically speaking). The very definition of what exactly constitutes a "mass extinction" has been somewhat controversial, but well start with the definition offered by the late Jack Sepkowski who was one of the leading students of mass extinctions.
Mass Extinction any substantial increase in the amount of extinction (that is lineage termination) suffered by more than one geographically widespread higher taxon during a relatively short interval of geologic time, resulting in at least a temporary decline in their standing diversity.
This definition is admittedly a bit vague, but that is only to be expected when trying to define a highly variable type of event. Let us examine the definition more carefully.
The first item to think about is the phrase " substantial increase in the amount of extinction ." Remember, extinction is to a species what death is to an individual an unfortunate constant in life. There is always a backround extinction rate (remember the graveyard example back in the Introduction) which we can consider the norm for the biosphere. What we are looking for is a significant increase in the rate of extinction relative to that backround rate.
Next we want this event to be geographically widespread not a local phenomenon. When Krakatau (near Java) erupted in 1883, the entire island disappeared (it went from being an island standing 792 meters above sea-level to being a crater 300 meters below sea level). Its safe to assume that any species endemic to the island went extinct very quickly but this was a very localized event which did produce any major effect on the biosphere. Even the extinction of animals on an entire continent would be considered a local event. When Antarctica first rifted from Gondwana it contained a rich fauna and flora. If evolution produced new, endemic species following that rifting, they are extinct now killed off by the movement of Antarctica to its pole position and the resulting frigid environment. Be that as it may, it would still be considered a local event. We only consider those events which are global in their extent to be true mass extinctions.
The term "higher taxa" is significant for a number of reasons. First, since we are dealing with the fossil record when looking at past mass extinctions, we run into the problem of defining species. There is abundant synonymy in descriptions of fossil species, to the point where they would provide a poor numerical database. Even genera are somewhat suspect. Instead, most work is based on the family taxon (Sepkowskis groundbreaking work was done at this level), a level at which the numbers are considered to be more accurate.
ASIDE: For those who dont remember your Linnean classification from intro Bio, we classify organisms using the following hierarchy:
Example 1 Example 2 Example 3
Kingdom Animals Animals Animals
Phylum Vertebrates Vertebrates Vertebrates
Class Mammals Mammals Reptilia
Order Primates Carnivora Theropoda
Family Homonidae Felidae Tyrannosauridae
Genus Homo Felis Tyrannosaurus
Species sapiens felis rex
The Linnean classification starts at the bottom with very small groups of individuals which are capable of reproduction (the species), and then proceeds to group animals into larger and larger groups based on evolutionary relationships (as interpreted by a taxonomist a scientist who studies and classifies plants and animals). Note that the three examples above (a human, a house cat, and a T. rex) all belong to the same Kingdom (theyre all animals) and the same Phylum (all have a backbone). At the level of the Class the T. rex is differentiated as a reptile, while the human and the cat are both considered mammals. We are differentiated from the cat at the level of Order (were grouped with lemurs, apes, etc. within the Primates; while the cats are grouped with other carnivores). Next step down is the Family. At this level we are all alone within the Homonidae (we have lots of fossil relatives whom we might have knocked off); while the house cat is grouped into the Felidae with lions, tigers, bobcats, etc. From the level of genus on up, we think of these categories as higher taxa.
Now the second reason for using the Family should become evident. In order for a Family to become extinct every genus which belongs to that family must become extinct, which means that every species of every genus belonging to that family must become extinct. In general, you need a lot of extinction to knock out a Family [ Note though, that there are Families which contain only one extant genus which has only one extant species. For example, Homo sapiens.]
Next we come to the " relatively short interval of geologic time " phrase. This is extremely important in light of the span of geologic time. Consider the fact that by some estimates roughly 99.9% of all species, which have ever lived are extinct! This is obviously an impressive number (and obviously global), but when spread out over even the last 540 million years what we end up with is our backround extinction rate. So we want to limit ourselves to events, which have occurred over 5 to 10 million years, which is (believe it or not) a relatively short interval of geologic time.
Finally, we have the phrase " resulting in at least a temporary decline in their standing diversity." If you remember your basic ecology, species fill niches. Usually, if a species goes extinct in a "normal" fashion, its niche will be rapidly filled by some already extant or newly evolved species. In cases of mass extinction, the event produces holes in the biosphere, and a drop in the total number of species, which is notable for a few million years.
One of my favorite definitions of mass extinction includes the phrase "leaves an ecological void". Consider the following: from the Ordovician Period to the Devonian Period a reef building fauna made up of corals and sponges evolved to the point where the reefs they built rival the modern Great Barrier Reef of Australia in shear size. This fauna was almost totally wiped out near the end of the Devonian, and a replacement reef building fauna did not appear until the Triassic Period a time gap of a bit more than 100 million years! No real reef building community for 100 million years I guess you could call that an ecological void! Imagine if the "forest building community", currently on earth went extinct tomorrow, and there was nothing to replace it that would also be an ecological void.
Finally, we should consider the word "catastrophe" because the popular media have started to interpret all mass extinctions as catastrophes. Andrew Knoll has defined catastrophe as "a biospheric perturbation that appears instantaneous when viewed at the level of resolution provided by the geological record." The most import point here is that we are dealing with the " level of resolution provided by the geologic record". One recent estimate suggests that we cannot, at present, differentiate between an event which took 100 years, and one which took 100,000 years. In other words, the results of a major event like an asteroid impact cannot be differentiated from the effects of global climate change taking up to 100,00 years without direct evidence for the impact (crater, etc.).
The Big Five
Now that weve defined Mass Extinction we can take a brief look at the data for the Big Five the five definite mass extinction events in earth history.
In the figure below (1.1) we can see how these events stand out as compared to the normal level of backround extinction. Note that the solid line starting at "5" on the Total Extinction Rate axis is a linear regression line. The dashed lines above and below it represent the 95% confidence envelope. In other words, data points falling in between the dashed lines are not statistically different from each other they are all within the normal range of variation to be expected with the data at hand. The five peaks, however, clearly stand out above the backround and are therefore considered to represent significantly greater rates of extinction.
The next figure (1.2) illustrates the number of families of marine invertebrate animals through time, based on the fossil record. From a slow start some 570 million years ago, the biosphere rapidly diversifies reaching its first plateau during the Ordovician, at somewhat more than 400 families. By the end of the Ordovician though, the biosphere experiences its first major crisis. This is the first of the Big Five. As you can see, the second crisis occurs near the end of the Devonian, the third at the end of the Permian, the fourth in the Late Triassic, and the fifth at the end of the Cretaceous (the end of the Dinos). Note that they fulfill one of Sepkowskis requirements in that each event results in a drop in the standing diversity of the biosphere, which lasts for a few millions of years.
The next table (Table 1.1) gives us a better estimate of the total losses for each of the Big Five Extinction events. Jablonski has reworked the data a bit so that his estimates of the percentage of families lost is higher for each event with the exception of the Permo-Triassic. Note that estimates of species loss based on both families and genera are in close agreement.
Since we will be discussing these in more detail later on in the semester, I will not go into any detail here. I only want to point out the Permo-Triassic, which can truly be referred to as "the Mother of All Extinction Events" (a much overused phrase, but it really fits). For this event, the estimates are 95% of species being lost. Try to imagine what it would be like to wake up one morning only to find that while you slept 95% of all species went extinct! The change in the biosphere is so massive that not only does this boundary mark the end of the Permian Geologic Period, it also marks the end of the Paleozoic Era! I cannot imagine even a geologist fresh out with a bachelors degree ever confusing Paleozoic fossiliferous rock for that from any other geologic time period. The difference is that great.
Finally, how does our current Biodiversity Crisis match up against the Big Five? Consider the following two tables (2 and 3). We have looked at them before, but now consider the "expected number of families" column in each table. If you look back to Figure 1 (above), you will note that the Total Extinction Rates for the Big Five range from a low of about 10 families per million years for the Devonian event, to a high of roughly 19 to 20 families per million years for the end Ordovician event. In the tables below, the "Worst-case End Point" estimates a loss of roughly 5 families of birds and 17 families of plants. While this is a worst-case scenario, consider that we are talking about only Amazonia (South American Rain Forest), and New World Tropical Moist Forest there is a lot more world out there. From this limited geographic range we have an estimate of the possible loss of 22 families in roughly 2 to 400 years. Even if these estimates are grossly overblown, and the real loss is 1 family per 400 years, consider what that rate comes out to over 1 million years.