Extinction, resolved

Editors' Note

Don't worry: we're not extinct yet!

The past two years have been incredibly productive for the editors of and contributors to this blog. We've had a lot of informative discussions, a few great meetings, and some unanticipated opportunities. In order to make the most of that productivity, we'll be changing the publication schedule for Extinct. Instead of the weekly schedule that we've maintained since the blog launched in 2016, we're switching to a monthly schedule in 2018. Look for a new post on the first day of each month!

Even though we're publishing less frequently, we're still on lookout for guest contributors. If you have any ideas or suggestions, don't hesitate to contact the editors.

Want more philosophy of paleontology than a monthly publication schedule can provide? (We know how you feel!) Our very own Adrian Currie is releasing his guide to the historical sciences, Rock, Bone, and Ruin, later this month! Be sure to order a copy. And if you still want more, feel free to peruse our extensive archives!

With all that said, Leonard Finkelman writes...

Introduction: The Thylacine returns

Suppose I offer you a choice between two bets on the year to come. (I mean 2019, of course, given that 2018 is already here.) The first bet is that I'll write an essay about thylacines in 2019. The second bet is that thylacines won’t be rediscovered in the wild in 2019. Which bet should you take?

Your choice is likely informed by your estimation of probabilities. If the probability that I’ll write an essay about thylacines is greater than the probability that there will be a thylacine sighting, then your rational choice would be the first bet. I wrote essays about thylacines in 2016, in 2017, and now in 2018 (and on my first opportunity, to boot!), so you can imagine the probability that I’ll write another in 2019 is pretty close to 100 percent. But is it closer to 100 percent than the probability that thylacines are well and truly extinct?

Comparison between skulls of a thylacine (denoted by a red bar) and a gray wolf (denoted by a green bar). Image from Wikimedia Commons.

Comparison between skulls of a thylacine (denoted by a red bar) and a gray wolf (denoted by a green bar). Image from Wikimedia Commons.

Biologists have developed several models to estimate the probability that a given species is extinct (Solow 1993; Solow & Roberts 2003; Marshall 2010; Bradshaw, et al. 2012; Fischer & Blomberg 2012). According to these models, the probability that any thylacines remain to be discovered is between zero and three percent. The probability of your winning the second bet, then, lies between 97 and 100 percent (and it’s likely much closer to the higher number). Place your bet accordingly.

I like to write about thylacines because the species (Thylacinus cynocephalus) provides a rare opportunity to think very clearly about extinction. As I’ve argued in previous essays about thylacines, the concept of extinction is a difficult and muddled one. This essay will be different from those: drawing from the statistical work I mentioned above, I hope to offer something more constructive. I hope to resolve the problems I raised earlier. The key, I think, is to recast the debate—not in terms of what happened to thylacines themselves, but in terms of how we can study them.

Extinction: The metaphysical problem

Keep three things in mind as we consider different ways to think about extinction:

  1. The last confirmed sighting of a thylacine in the wild was in 1933 and wild populations likely disappeared by 1935.
  2. The last captive thylacine died on 7 September 1936.
  3. As of this writing (30 January 2018), there remain several well-preserved thylacine fetuses from which geneticists can cultivate and have cultivated genetic material. (I recall that the number may be eight, but I can’t find confirmation.)
Preserved thylacine pup and fetuses. Image from National Geographic.

Preserved thylacine pup and fetuses. Image from National Geographic.

Now consider the three different ways to conceive extinction that I’ve discussed in previous essays (see also Delord 2007):

  • A species is functionally extinct if and only if the members of the species are practically incapable of perpetuating any lineages. This is what happens to (say) a sexually reproducing species when its population consists entirely of organisms belonging to a single sex or if it the population has been reduced to an endling.
  • A species is demographically extinct if and only if every member of the species is dead. This has also been called “final extinction” and (presumptuously) “true extinction."
  • A species is substantially extinct if and only if the information necessary to produce new members of the species has disappeared. Technological advances such as those promised by resurrection biology may change the standards for substantial extinction over time.

When did thylacines go extinct? That question is ambiguous: "extinct" might mean any one of the three concepts defined above. The answer is also ambiguous: it depends on the kind of extinction. T. cynocephalus went functionally extinct as late as 1935. The species was demographically extinct on 7 September 1936. By the standards of substantial extinction, the species isn't extinct at all. This ambiguity has the potential to generate a variety of problems, but two in particular strike me as important.

The first problem with the heterogeneity of extinction concepts is a practical one for paleontology. Given the imperfection of the fossil record, the extinction of fossil species is always ambiguous between functional and demographic extinction. This ambiguity produces the Signor-Lipps effect: since the most recent specimen in a fossil species is unlikely to be the biological population's endling, or may not even signal a decline in the population size, paleontologists should assume some lag between the fossil species' latest appearance and the biological population's extinction--which biases our reading of the fossil record against abrupt extinction events (but see below). This makes interdisciplinary work difficult: for example, an inability to compare past and recent extinction rates would complicate conservation efforts.

The second problem with the heterogeneity of extinction concepts is a more metaphysical one. Given differences between definitions for the three extinction concepts, differences between the implications of those definitions, and likely differences in their underlying processes, it's disingenuous to use a single word to describe them all. "Extinction" doesn't seem to have an essence. To say something like, "Thylacines are extinct," would therefore be (strictly speaking) nonsensical because "extinct" per se is not a property of anything at all. In other words, there is nothing in common to all and only the species we describe with that term.

That's a pretty big problem since extinction is supposed to be a concept that unifies almost all of life's history. As paleontologists sometimes point out, more than 99% of all species are extinct, after all.

Extinction: The epistemic problem

As I was writing this post, the U.S. Fish and Wildlife Service declared the eastern cougar (Puma concolor cougar) an extinct subspecies. The last sighting of an eastern cougar was in 1938, five years after humans last laid (reputable) eyes on a wild thylacine. There was also a considerable delay before T. cynocephalus officially joined the ranks of extinct species: the International Union for the Conversation of Nature didn't declare thylacines extinct until 1982.

Caution explains the long wait in both cases. Species are considered endangered, and subject to all relevant protections, before they're declared extinct. That status and those protections are stripped away once the species' extinction is recognized. Declaring a species extinct is therefore an important step with significant consequences for conservation groups. One doesn't want to stop trying to protect an irreplaceable branch of life's proverbial tree until they're sure that there isn't anything left to protect.

The IUCN declared the thylacine extinct after five decades without a sighting, but the USFWS waited an additional three decades to declare the eastern cougar extinct; why the discrepancy? The answer comes down to epistemology. By the time it went extinct the thylacine's range had been reduced to the wilds of Tasmania, and by the time it was declared extinct the species had been the subject of several concerted search efforts. By contrast, the eastern cougar ranged across the eastern seaboard of the United States--a much larger area than Tasmania--and efforts to find remnants over the last eighty years haven't been particularly extensive. By 1982, there was much less reason to hope for thylacine remnants than there was to hope for eastern cougar survivors.

Andrew Solow (1993) statistically formalized this kind of reasoning so that scientists could minimize uncertainty over when a species has gone extinct. Given a sufficiently extensive record of species sightings--that is, an ordered list of confirmed dates on which someone saw a member of the species--one can develop a statistical model that estimates the frequency of sightings and predicts when one should expect to see a member of the species again. As more time passes beyond an expected sighting date without a sighting, the probability that someone will see another member of the species decreases--and once the probability approaches zero, one can be reasonably sure the species is extinct. It's tough to quantify hope, but that's just what Solow's method does.

Using the sighting method, Solow (1993) estimated that the Caribbean monk seal (pictured above) likely went extinct before 1973. Image from Wikimedia Commons.

Using the sighting method, Solow (1993) estimated that the Caribbean monk seal (pictured above) likely went extinct before 1973. Image from Wikimedia Commons.

There are, of course, quirks and kinks to work out. How does a declining population affect the frequency of sightings? How does one factor in the difficulty or infrequency of sighting efforts? What if members of the species are camouflaged or something? Recent work has modified the modeling process to add these subtleties (see, e.g., Solow & Roberts 2003). Fischer & Blomberg (2012), for example, combined the sighting record for thylacines with ecological niche data and population size estimates to infer that the probability of finding a wild thylacine had diminished to near zero by 1935. 

Solow & Roberts (2006) recognized that this method could also be useful in paleontology. Substitute "dated occurrences in the fossil record" for "record of species sightings" and paleontologists can use similar models to estimate true extinction dates for fossil species. After accounting for preservation bias and other taphonomic features, paleontologists can make reasonable inferences about the lag between a fossil species' last appearance in the fossil record and the biological population's extinction, thereby minimizing the Signor-Lipps effect (see also Marshall 2010; Bradshaw, et al. 2012).

Two points are important here. The first point is that the same basic logic governs all estimation of extinction dates, whether those extinctions are ancient or recent. That logic should be familiar to philosophers: it is, essentially, enumerative induction. The second point is that all extinct species do have something in common. It turns out that common feature isn't a metaphysical property of the species; rather, it's an epistemic property of reasonable observers. A species is extinct if and only if we can't reasonably hope to see it again.

Conclusion: What really goes extinct, anyway?

Can we hope to see the thylacine again? People will certainly try. Still, the best answer to that question is one that quotes the wizard Gandalf, by way of Tolkien: "There never was much hope... Just a fool's hope." That's why thylacines are extinct.

One might call this misplaced attribution or affirming the consequent or some other horrifying dereliction of philosophical duty, but I do think there's an important lesson about extinction to be drawn here. One feature common to all extinction concepts is the improbability of observation; where the concepts differ, they differ in the degree of improbability. If one could measure such a thing as the global probability--that is, the probability of any random observer in any random place, quantified over all observers in all places--of encountering a thylacine, then that probability approached zero in 1933, decreased ever so slightly in 1936, and will bottom out if (well: when) de-extinction efforts fail. There may be different underlying processes that account for those probability shifts, but the extremely low probability of encounter is nevertheless common to all extinct species.

What this means is that we can resolve the metaphysical problem of extinction by way of resolving the epistemological problem. Extinction is problematic if conceived as a property of species per se, but it isn't problematic if conceived as a relation between a species and its observers. An extinct species is one that can't be observed. This may raise a host of questions about what constitutes observation, but that's an essay for a different blog--you know, one not named "Extinct."

This resolution suggests a sobering conclusion that's worth bearing in mind as the year 2018 kicks into gear: species aren't what really goes extinct. Our hope does.


Works cited

  • Bradshaw, C.J.A., Cooper, A., Turney, C.S.M., & Brook, B.W. 2012. Robust estimates of extinction time in the geological record. Quaternary Science Reviews 33: 14-19.
  • Fischer, D.O. & Blomberg, S.P. 2012. Inferring extinction of mammals from sighting records, threats, and biological traits. Conservation Biology 26(1): 57-67.
  • Marshall, C.R. 2010. Using confidence intervals to quantify the uncertainty in the end-points of stratigraphic ranges. The Paleontological Society Papers 16: 291-316.
  • Solow, A.R. 1993. Inferring extinction from sighting data. Ecology 74(3): 962-964.
  • Solow, A.R. & Roberts, D.L. 2003. A nonparametric test for extinction based on a sighting record. Ecology 84(5): 1329-1332.
  • Solow, A.R. & Roberts, D.L. 2006. On the Pleistocene extinction of mammoths and horses. Proceedings of the National Academy of Sciences 103(19): 7351-7353.